Diketopiperazine forming dipeptidyl linker The invention relates to a method for homogeneous solution phase peptide synthesis (HSPPS) of a N-terminal peptide fragment PEP-N and a C-terminal peptide fragment C-PEP, with CPEP carrying a specific diketopiperazine (DKP) comprising C-terminal protecting group, which contains a handle group HG, with HG being connected to the C-terminus of the peptide fragment; thereby this specific DKP comprising C-terminal protecting group can be selectively cleaved from the peptide as a conventionally used C-terminal protecting group. By the use of this DKP and HG comprising C-terminal protecting group, certain process steps in convergent peptide synthesis based on a combination of HSPPS and solid phase peptide synthesis (SPPS) can be avoided. The invention relates further to a method for the preparation of such specifically protected fragment C-PEP by SPPS by using a linker comprising a specific dipeptide and HG for connecting the growing peptide chain to the resin, which linker forms said DKP group, when the peptide fragment C-PEP is cleaved from the supporting resin; and further to the intermediates of the preparation method. In this text, the nomenclature of amino acids and of peptides is used according to "Nomenclature and symbolism for amino acids and peptides", Pure & Appl. Chem., Vol. 56, No. 5, pp. 595-624, 1984, if not otherwise stated. The following abbreviations have the meaning as given in the following list, if not otherwise stated: CTC chlorotrityl chloride Alloc allyloxycarbonyl Boc tert-butoxycarbonyl Bsmoc 1,1-dioxobenzo[b]thiophen-2-ylmethyloxycarbonyl Bzl or Bn benzyl cHx cyclohexyl Ct C terminal Dpr 2,3-diaminopropanoic acid Dde N-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl ivDde l-(4,4-dimethyl-2,6-dioxocyclohexylidene)3-methylbutyl Ddz alpha,alpha-dimethyl-3,5-dimethoxybenzyloxycarbonyl DKP 2,5-diketopiperazine Dmab dimethylaminoborane Fm 9-Fluorenylmethyl Fmoc N-(fluorenyl-9-methoxycarbonyl) Hpr piperidine-2-carboxylic acid, homoproline HSHSPPS hybrid solid and homogenous solution phase peptide synthesis HSPPS homogenous solution phase peptide synthesis Hyp trans-4-hydroxyproline M t 4-methoxytrityl Mpe 3-methylpent-3-yl, Mtt 4-methyltrityl Orn ornithine Pbf 2,2,4,6,7- pentamethyldihydrobenzofuran-5-sulfonyl PG protecting group 2-PhiPr 2-phenylisopropyl Pmc 2,2,5,7,8-penta- methylchroman-6-sulfonyl pN0 2Z nitrobenzyloxycarbonyl Py Pyridine SPPS solid phase peptide synthesis tBu tert-butyl TES SiEt3, Triethylsilyl TFA Trifluoroacetic acid Tfac trifluoroacetyl Trt or Tr triphenylmethyl or trityl Z benzyloxycarbonyl The terms "fragment" and "peptide fragment" are used synonymously, if not otherwise stated. The terms "handle" and "handle group", e.g. "Fmoc-Rink amide handle group", "Rink amide handle" or "Rink amide handle group", and the term "linker", e.g. "Fmoc-Rink amide linker" or Fmoc-Rink-OH, are often used synonymously, if not stated otherwise. Peptides are often prepared by hybrid solid and homogenous solution phase peptide synthesis HSHSPPS: firstly two or more peptide fragments are prepared by solid phase peptide synthesis SPPS, which are thereafter coupled in solution phase by homogenous solution phase peptide synthesis HSPPS to provide for the desired target peptide. This approach is particularly attractive for the commercial scale preparation of large peptides as it combines the advantages of both the SPPS and the HSPPS. In particular, the SPPS of fragments can be developed and scaled-up rapidly and avoids many of the solubility problems often encountered in HSPPS of relatively long fragments. Production cycle times are short compared to solution phase methodologies. In addition, yields and purities are often higher because of the use of excess reagents, especially during the coupling reactions, which often results in intermediates that do not require purification. After optimization of selection of the sequences of the fragments made by SPPS, the final stages of the process can be scaled-up by conventional HSPPS methodologies. These final stages of the process are the fragment coupling and the final deprotection of the amino acid residues, i.e. the deprotection of the side chains and of the N- and C-terminus, both being performed in solution. Thus, when applying the HSHSPPS synthesis, the advantages of the SPPS, i.e. rapid synthesis of fragments with high purities, and the advantages of solution-phase synthesis, i.e. full monitoring of coupling reactions and isolation and optional purification including full characterization of the formed intermediate fragments, can be exploited in order to efficiently produce peptides, especially on commercial scale. In HSHSPPS, always at least two fragments PEP-N and C-PEP prepared by SPPS are coupled in solution phase to provide the desired peptide PEP, which is either the final peptide or again an intermediate peptide fragment, which again thereafter is coupled with a third peptide fragment, and so on. Fragment PEP-N presents herein the N-terminus of peptide PEP, fragment C-PEP presents the C-terminus of peptide PEP, and therefore the C-terminus of fragment PEP-N is coupled with the N-terminus of fragment C-PEP to provide peptide PEP. It is necessary, that the N-terminus of fragment PEP-N is protected as well as the C-terminus of fragment C-PEP is protected during solution phase coupling in order to avoid undesired coupling of fragment PEP-N with fragment PEP-N, of fragment C-PEP with fragment C-PEP, or of fragment C-PEP with fragment PEP-N in the wrong direction. This N-terminally protected peptide fragment PEP-N is in the following also called PEP-N, if not otherwise stated. The fragment C-PEP, prepared by SPPS on a supporting resin, will carry an Nterminal protecting group after the addition of the last amino acid residue, and will then be cleaved from the supporting resin in a final step. This cleavage results usually in a fragment C-PEP with an unprotected C-terminus, which must be protected in a separate step, before fragment C-PEP can be coupled in HSPPS with fragment PEP-N. Actually, this necessary protection of the C-terminus of fragment C-PEP comprises not only one step, but several steps such as reaction, purification and isolation, possibly with another subsequent purification and isolation. In case the target peptide PEP to be prepared is a peptide amide PEP-NH2, i.e. with the Cterminus being a carboxamide group, the C-terminus of the respective fragment C-PEP-NH2 normally does not need to be protected during fragment coupling in HSPPS, since the carboxamide group itself acts as a protecting group. While a fragment C-PEP-OH, with the Cterminus being the carboxylic acid, can be easily obtained after SPPS by use of a resin which forms the carboxylic acid group after cleavage, the use of a resin which forms the carboxamide group after cleavage, e.g. the Sieber amide resin, causes problems due to partial side chain deprotection of the fragment C-PEP-NH2 during cleavage, since cleavage from amide resins typically requires acidic conditions, such as the use of 3 to 5 % by weight of TFA in a solvent, and side chain protecting groups, such as Trt in case of Fmoc/Trt SPPS (for example His(Trt)) or such as acetale in pseudo-proline derivatives (i.e. Fmoc-Ser(tBu)- Thr(psi e,Mepro)-OH), are not completely stable under such cleavage conditions, which results in partial loss of the side chain protecting groups. Therefore for preparing fragment C-PEP-NH2, it is common to start the SPPS with the amino acid second in position from the C-terminal amino acid residue of the desired fragment C-PEP-NH2 and not with the Cterminal amino acid itself of fragment C-PEP-NH , and with a resin which affords a carboxylic acid as C-terminus after cleavage. Cleavage from the resin affords therefore a fragment C-OH without the C-terminal amino acid of the desired fragment C-PEP-NH2, and with the C-terminus of this fragment C-OH being the amino acid of the second position from the C-terminus of the finally desired fragment C-PEP-NH2 and bearing a carboxylic acid group. The missing C-terminal amino acid of fragment C-PEP-NH2 is then separately coupled to the fragment C-OH in form of its amide H-Xaa-NH2 in solution phase. WO 90/09395 discloses the use of a cleavable linker between peptide and the supporting resin, which forms a diketopiperazine (D P) linker group when cleaved from the resin, wherein the DKP group is connected to the peptide via an amide bond between the epsilon amino group of a Lys in the linker group and the C-terminus of the peptide. This DKP linker group cannot be removed selectively from the peptide at a later stage. Thus, it does not allow for the preparation of fully protected C-terminal fragments with unprotected N-terminus suitable in HSPPS. Furthermore, the linker group of WO 90/09395 does not allow for the preparation of natural or unmodified peptides. It is only suitable for the synthesis of permanently C-terminally modified peptides, since any peptide cleaved from the resin always carries a DKP linker group at its C-terminus, which is not cleavable without cleaving the other peptide bonds of the peptide. Another disadvantage is the restriction of its cleavage to the use of trifluoroacetic acid (TFA) during the cleavage step, which implies the partial or total removal of any tBu, Boc, Trt or Acetale based protecting groups of the side chains of the amino acid residues of the peptide, thereby restricting its use to the preparation of either peptides with unprotected side chains or to side chain protecting groups other than tBu, Boc, Trt and Acetale. There was a need to simplify the procedure of HSHSPPS by reducing the number of steps in the reaction sequence. Surprisingly, this can be achieved by using a specific diketopiperazine group forming dipeptidyl linker in the SPPS used to prepare the fragment C-PEP, which carries a specific diketopiperazine comprising C-terminal protecting group, together with an appropriate combination of the different types of protecting groups and a specific chemical nature of the connection of the linker to the fragment C-PEP providing specific cleavage possibility of the linker from the fragment. Protecting groups (PG), be it for protecting functional groups in side chains of amino acid or for the protection of N-termina! amino groups or C-terminal carboxy groups of amino acids or peptides, are for the purpose of this invention classified into four different groups: 1. basic cleavable type protecting groups, in the following called "basic type PGs", 2. strong acid cleavable type protecting groups, in the following called "strong type PGs", 3. weak acid cleavable type protecting groups, in the following called "weak type PGs", and 4. reductively cleavable type protecting groups in the following called "reductive type PGs", with the two groups "strong type PGs" and "weak type PGs" also collectively called "acid cleavable type protecting groups" or "acid type PGs". Within the meaning of this invention, any PG is classified by the following four classification reaction conditions. The classification is done using a CTC resin with a loading capacity of .5 to 1.7 mmol per g resin, the resin being loaded with only one amino acid carrying the respective PG which is to be classified. The term "part" in the following four classification procedures is meant to be a factor of the parts by weight of the loaded CTC resin starting material, if not otherwise stated. 1. Classification reaction conditions for basic type PG, in the following text called "basic classification conditions": Treatment for 25 +/ 5 min at 25 +/- 5 °C of the resin loaded with the basic type PG carrying amino acid with 7 +/- 1 parts of a cleaving solution, the cleaving solution consisting of 22.5 +/- 2.5 % by weight solution of piperidine in dimethylformamide (DMF), the % by weight being based on the total weight of the cleaving solution. 2. Classification reaction conditions for strong type PGs, in the following text called "strong classification conditions": Treatment for 25 +/ 5 min at 25 +/- 5 °C of the resin loaded with the strong type PG carrying amino acid with 7 +/- 1 parts of a cleaving solution, the cleaving solution consisting of 85 +/- 5 % by weight solution of trifluoro acetic acid (TFA) in dichloromethane (DCM), the % by weight being based on the total weight of the cleaving solution. 3. Classification reaction conditions for weak type PGs, in the following text called "weak classification conditions": Treatment for 25 +/ 5 min at 25 +/- 5 °C of the resin loaded with the weak type PG carrying amino acid with 7 +/- 1 parts of a cleaving solution, the cleaving solution consisting of 2 +/-1 % by weight solution of TFA in DCM, the % by weight being based on the total weight of the cleaving solution. 4. Classification reaction conditions for reductive type PGs, in the following text called "reductive classification conditions": Treatment for 30 +/ 5 min at 25 +/- 5 °C of the resin loaded with the reductive type PG carrying amino acid with 7 +/- 1 parts of DMF, with 0.1 mol equivalent of a soluble organic Pd(0) catalyst, preferably Pd[PPh 3]4, dissolved in the DMF, the mol equivalent being based on the mol of cleavable groups loaded on the resin. PGs and typical reaction conditions and parameters and reagents for cleaving PGs, which are conventionally used in peptide chemistry, are known in the art, e.g. T.W. Greene, P.G. M. Wuts "Protective Groups in Organic Synthesis" John Wiley & Sons, Inc., 1999; or LloydWilliams, P., Albericio, F., Giralt, E., "Chemical Approaches to the Synthesis of Peptides and Proteins" CRC: Boca Raton, Florida, 1997. Basic type PGs are preferably cleaved under following possible reaction conditions, in the following text called "basic cleaving conditions": Basic cleaving conditions involve treatment of the respective material with a basic cleaving solution. The basic cleaving solution comprises a basic reagent and a solvent. Preferably, the basic cleaving solution consists of a basic reagent and a solvent. If the basic reagent is liquid at the temperature, at which the basic cleaving is done, the basic reagent can also act simultaneously as the solvent, i.e. no solvent different from the basic reagent is used. Basic reagents are preferably secondary amines, more preferably the basic reagent is selected from the group consisting of piperidine, 4-(aminomethyl)piperidine, tris(2-aminoethyl)amine, morpholine, dicyclohexylamine, 1,3-cyclohexanebis(methylamine)piperazine, l,8-diazabicyclo[5.4.0]undec-7-ene and mixtures thereof. Even more preferably, the basic reagent is piperidine. The basic cleaving solution can also comprise an additive, the additive preferably selected from the group consisting of 6-chloro-l-hydroxy-benzotriazole, 2,4-dinitrophenol, picric acid, 1-hydroxy-7-azabenzotriazole, 1-hydroxy-benzotriazole and ethyl 2-cyano-2- hydroxyimino-'acetate and mixtures thereof. Preferably, the solvent is selected from the group consisting of dimethylsulfoxide (DMSO), dioxane, tetrahydrofuran (THF), l-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), pyridine, dichloromethane (DCM), dichloroethane (DCE), chloroform, dioxane, tetrahydropyran, ethyl acetate, toluene, acetonitrile and mixtures thereof; more preferably the solvent is l-methyl-2-pyrrolidone (NMP), N,Ndimethylformamide (DMF) or a mixture thereof. The term "part" in this description of basic cleaving conditions is meant to be a factor of the parts by weight of the treated material carrying the basic type PG(s). Preferably, of from 5 to 20 parts, more preferably of from 5 to 15 parts of basic cleaving solution are used. Preferably, the amount of basic reagent is of from 1 to 30 % by weight, more preferably of from 10 to 25 % by weight, even more preferably of from 15 to 20 % by weight, with the % by weight being based on the total weight of the basic cleaving solution. Preferably, basic cleaving is done at a temperature of from 10 to 50 °C, more preferably of from 10 to 30 °C, even more preferably of from 15 to 25 °C. Preferably, basic cleaving is done at atmospheric pressure. Preferably, the reaction time for basic cleaving is of from 5 min to 2 h, more preferably of from 10 min to lh, even more preferably of from 15 min to 30 min. Strong type PGs are preferably cleaved under the following possible reaction conditions, in the following text called "strong cleaving conditions": Strong cleaving conditions involve treatment of the respective material with a strong cleaving solution. The strong cleaving solution comprises an acidolytic reagent. Acidolytic reagents are preferably selected from the group consisting of hydrogen acids, such as trifluoroacetic acid (TFA), hydrochloric acid (HC1), aqueous hydrochloric acid (HC1), liquid hydrofluoric acid (HF) or trifluoromethanesulfonic acid, Lewis acids, such as trifluoroborate diethyl ether adduct or trimethylsilylbromid, and mixtures thereof. The strong cleaving solution preferably comprises one or more scavengers, the scavengers being selected from the group consisting of dithiothreitol (DTT), ethanedithiol (EDT), dimethylsulfide (DMS), triisopropylsilane (TIS), triethylsilane (TES), 1,3-dimethoxybenzene (DMB), phenol, anisole, p-cresol and mixtures thereof. The strong cleaving solution can also comprise water, a solvent or a mixture thereof, the solvent being stable under strong cleaving conditions. Preferably, solvents are selected from the group consisting of dichloromethane, dichloroethane, acetonitrile, toluene, tetrahydrofurane, TFA, dioxane and mixtures thereof. More preferably, the acidolytic reagent acts simultaneously as solvent, so no further solvent is needed. The term "part" in this description of strong cleaving solution is meant to be a factor of the parts by weight of the treated material carrying the strong type PG(s). Preferably, of from 10 to 30 parts, more preferably of from 15 to 25 parts, even more preferably of from 19 to 2 1 parts of strong cleaving solution are used. Preferably, the amount of acidolytic reagent is of from 30 to 100 % by weight, more preferably of from 50 to 100 % by weight, even more preferably of from 70 to 100 % by weight, especially of from 80 to 100 % by weight, with the % by weight being based on the total weight of the strong cleaving solution. Preferably, of from 1 to 25% by weight of total amount of scavenger is used, more preferably of from 5 to 5 % by weight, with the % by weight based on the total weight of the strong cleaving solution. Preferably, strong cleaving is done at a temperature of from -10 to 30 °C, more preferably of from -10 to 30 °C, even more preferably of from 5 to 15 °C. Preferably, strong cleaving is done at atmospheric pressure. Preferably, the reaction time for strong cleaving is of from 30 min to 20 h, more preferably of from 1 h to 10 h, even more preferably of from 1 h to 5 h. Weak type PGs are preferably cleaved under the following possible reaction conditions, in the following text called "weak cleaving conditions": Weak cleaving conditions involve treatment of the respective material with a weak cleaving solution. The weak cleaving solution comprises an acidolytic reagent. The acidolytic reagent is preferably selected from the group consisting of hydrogen acids, such as trifluoroacetic acid (TFA), trifluoroethanol (TFE), hydrochloric acid (HCl), acetic acid (AcOH), mixtures thereof and/or with water. The weak cleaving solution also comprises water, a solvent or a mixture thereof, the solvent being stable under weak cleaving conditions. Preferably, solvents are selected from the group consisting of dichloromethane, dichloroethane, acetonitrile, toluene, tetrahydrofurane, TFA, dioxane and mixtures thereof. The term "part" in this description of weak cleaving solution is meant to be a factor of the parts by weight of the treated material carrying the weak type PG(s). Preferably, of from 4 to 20 parts, more preferably of from 5 to 10 parts, of weak cleaving solution are used. Preferably, the amount of acidolytic reagent is of from 0.01 to 5 % by weight, more preferably of from 0.1 to 5 % by weight, even more preferably of from 0.15 to 3 % by weight, with the % by weight being based on the total weight of the weak cleaving solution. Preferably, weak cleaving is done at a temperature of from 10 to 50 °C, more preferably of from 20 to 40 °C, even more preferably of from 25 to 35 °C. Preferably, weak cleaving is done at atmospheric pressure. Preferably, the reaction time for weak cleaving is of from 5 min to 2 h, more preferably of from 10 min to 1 h, even more preferably of from 10 min to 30 min. The weak type PG can be subclassified into further groups, these groups being differentiated from one another and can be aligned consecutively according to the amount of acid necessary for cleavage. According to above definition of weak classification conditions, all weak type PGs can be cleaved using 2 +/- 1% by weight solution of TFA in DCM, the % by weight being based on the total weight of the cleaving solution. A weak type PG, which is only cleaved by a solution of at least 1% by weight of TFA in DCM, but not by a solution with less amount of TFA, is called "weak 1 type PG" and the cleaving conditions are called "weak 1 cleaving conditions"; a weak type PG, which is cleaved already by a solution of at least 0.1% by weight of TFA in DCM, but not by a solution with less amount of TFA, is called "weak 2 type PG" and the cleaving conditions are called "weak 2 conditions"; a weak type PG, which is cleaved already by a solution of at least 0.01% by weight of TFA in DCM, is called "weak 3 type PG" and the cleaving conditions are called "weak 3 conditions"; the % by weight being based on the total weight of the cleaving solution. Reductive type PGs are preferably cleaved under the following possible reaction conditions, in the following text called "reductive cleaving conditions": Reductive cleaving conditions involve treatment of the respective material with a reductive cleaving solution. The reductive cleaving solution comprises a catalyst, an additive and a solvent. The catalysts are preferably selected from the group consisting of organic derivatives of Pd(0) and organic derivates of Pd(II), more preferably selected from the group consisting of Pd[PPh3]4, PdCl2[PPh3]2, Pd[OAc]2[P(2,4-xyloyl)3]2, Pd[OAc]2[P(ortho-tolyl)3]2, in situ prepared Pd(0) catalysts, prepared by mixing less stably coordinated Pd-complexes with ligands, such as PdCl2(PPh3)2 / PPh3, PdCl2(PPh3)2 / P(ortho-tolyl)3, Pd(DBA)2 / P(ortho-tolyl)3 or Pd[P(ortho-tolyl)3]2, Pd(OAc) / triethyl-phosphite, Pd(OAc)2 / PPh3 or Pd(OAc) / P(ortho-tolyl)3, and mixtures thereof; even more preferably selected from the group consisting of Pd[PPh3]4, PdCl2[PPh3]2, Pd[OAc]2[P(2,4-xyloyl)3]2, Pd[OAc]2[P(ortho-tolyl)3] and mixtures thereof. The additive is preferably selected from the group consisting of dimethylbarbituric acid, thiosalicylic acid, N-methylaniline, NH3BH , Me2NHBH3, tBu-NH2BH3, Me3NBH3, PyBH3, HCOOH/DIEA, diethydithiocarbamate sodium, dimedone, morpholine, AcOH/NMM, phenylsilane, sulfinic acids comprising PhS0 2H, tolS0 2Na, sodium 2-ethylhexanoate (SEH), sodium 2-thiophenesulfinate (STS), sodium 4-chloro-3- nitrobenzenesulfinate (SCNBS) and i-BuS0 2Na, and mixtures thereof; more preferably the additive is tolS0 2Na. Preferably, the solvent is selected from the group consisting of dimethylsulfoxide (DMSO), dioxane, tetrahydrofuran (THF), l-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), pyridine, dichloromethane (DCM), dichloroethane (DCE), chloroform, dioxane, tetrahydropyran, ethyl acetate, toluene, acetonitrile and mixtures thereof; more preferably the solvent is l-methyl-2-pyrrolidone (NMP), N,Ndimethylformamide (DMF) or a mixture thereof. Preferably, the catalyst is dissolvable in the solvent and is dissolved in the solvent. The term "part" in this description of reductive cleaving conditions is meant to be a factor of the parts by weight of the treated material carrying the reductive type PG(s). Preferably, of from 4 to 20 parts, more preferably of from 5 to 10 parts, of reductive cleaving solution are used. Preferably, 0.001 to 1 mol equivalents, more preferably 0.01 to 0.05 mol equivalents, of catalyst are used, the mol equivalent being based on the mol of reductively cleavable groups loaded on the resin. Preferably, 1 to 10 mol equivalents, more preferably 1.5 to 5 mol equivalents, of additive are used, the mol equivalent being based on the mol of reductively cleavable groups loaded on the resin. Preferably, reductive cleaving is done at a temperature of from 10 to 60 °C, more preferably of from 30 to 50 °C, even more preferably of from 35 to 45 °C. Preferably, reductive cleaving is done at atmospheric pressure. Preferably, the reaction time for reductive cleaving is of from 15 min to 10 h, more preferably of from 30 min to 4 h, even more preferably of from 30 min to 2h. Preferably, the reductive cleaving solution has to be protected form the light. Preferably, reductive cleaving is done in a container made of metal. The basic type PGs are not cleavable by strong or weak cleaving conditions. Preferably, the basic type PGs are not cleavable by strong, weak or reductive cleaving conditions. The strong type PGs are not cleavable by weak or basic cleaving conditions. Preferably, the strong type PGs are not cleavable by weak, basic or reductive cleaving conditions. The weak type PGs are not cleavable by basic cleaving conditions, but they are cleavable by strong cleaving conditions. Preferably, the weak type PGs are not cleavable by basic or reductive cleaving conditions, but they are cleavable by strong cleaving conditions. The weak 1 type PGs are not cleavable by weak 2 or weak 3 cleaving conditions; the weak 2 type PGs are cleavable by weak 1 cleaving conditions, but not by weak 3 cleaving conditions; the weak 3 type PG are cleavable by weak 1 and by weak 2 cleaving conditions. preferably, the weak 1, 2 and 3 type PGs are also not cleavable by basic or reductive cleaving conditions. Preferably, reductive type PGs are not cleavable by strong, weak and basic cleaving conditions, these are called "exclusively reductive type PGs". Reductive type PGs, which are not cleavable by weak and basic cleaving conditions, but which are cleavable by strong cleaving conditions; these PGs are called "mixed type PGs". The connection of the linker to the peptide can also be classified to be cleavable under one of these four cleaving conditions. The connection of an amino acid to a resin can also be classified to be cleavable under one of these four cleaving conditions. Preferably, a basic type PG is selected from the group consisting of Fmoc, Bsmoc, Tfac, Dde, Dmab and cHx. Preferably, a strong type PG is selected from the group consisting of Boc, tBu, Pmc, Mpe, Pbf, Z, Bzl, cHx, pN0 2Z and Ddz. Preferably, a weak type PG is selected from the group consisting of Trt, Mmt, Mtt, acetale and 2-PhiPr. If a weak type PG is actually a weak 1 type PG or a weak 2 type PG, depends on the side chain group which it protects. Preferably, a reductive type PG is selected from the group consisting of Alloc, Allyl, ivDde and Z. More preferably, a basic type PG is Fmoc. More preferably, a strong type PG is Boc. More preferably, a weak type PG is Trt. More preferably, a reductive type PG is Alloc. In conventional SPPS, the peptide is cleaved from the resin after the SPPS is finished, the cleavage resulting in a peptide with a C-terminus in form of a free carboxylic acid group or in form of a carboxamide, depending on the resin and a possible handle used in SPPS. If this peptide with a free carboxylic group at its C-terminus is to be used in HSPPS as the Cterminal peptide fragment C-PEP, this free carboxylic acid group has firstly to be protected, before the peptide can be used in HSPPS. This protection of the free C-terminus needs several process steps (reaction, isolation, perhaps purification). The instant invention discloses a method for reducing these steps necessary for protecting a free carboxylic acid at the C-terminus of the fragment resulting from cleavage of the fragment from the resin after SPPS. This is achieved by using said diketopiperazine forming dipeptidyl linker to couple in the SPPS the first amino acid XaaC 1 of the desired fragment C-PEP via said linker onto the resin support, which linker forms a diketopiperazine residue comprising C-terminal protecting group, when the SPPS is finished and the synthesized fragment C-PEP is being cleaved from the resin. The diketopiperazine forming dipeptidyl linker comprises a dipeptide moiety, whose first amino acid Xaal is via its carboxylic acid group connected to the resin, and whose second amino acid Xaa2 is via its side chain connected a handle group HG, which handle group HG is connected to the peptidyl radical, and Xaa2 carries an Nterminal protecting group PG2. The formation of said diketopiperazine residue comprising C-terminal protecting group is achieved by cleaving the protecting group PG2 of Xaa2, thereby making an intramolecular ring closure between Xaa2 and Xaal possible, which ring closure forms said diketopiperazine residue and simultaneously cleaves Xaal from the resin. This diketopiperazine residue comprising C-terminal protecting group, formed by the cleavage from the resin, remains connected to the C-terminus of the fragment C-PEP after cleavage from the resin and acts thereby as a protecting group of the C-terminus of the fragment C-PEP, which can therefore directly be used in HSPPS. After coupling of this Cterminal fragment C-PEP with an N-terminal peptide fragment PEP-N by HSPPS to yield the peptide PEP, the diketopiperazine residue comprising C-terminal protecting group is cleaved from the peptide PEP, preferably simultaneously with the deprotection of any protected side chain in fragment PEP. The diketopiperazine forming dipeptidyl linker and the resulting diketopiperazine residue comprising C-terminal protecting group comprise the handle group HG, which makes the cleavage of the peptide PEP from the diketopiperazine residue comprising C-terminal protecting group possible. To make this desired function possible, said diketopiperazine forming dipeptidyl linker is constructed in such a way, that the four principal cleavage steps 1. the cleavage of each N-terminal protecting group of the amino acids during the cycles of SPPS, 2. the cleavage of the fragment C-PEP from the resin by cleaving the protecting group PG2 from Xaa2, and then cleaving Xaal from the resin by forming the diketopiperazine residue comprising C-terminal protecting group, and 3. the cleavage of the diketopiperazine residue comprising C-terminal protecting group from the peptide PEP, which is the cleavage of the peptide from HG in the diketopiperazine residue comprising C-terminal protecting group; 4. cleavage of any side chain PG; can be done under different reaction conditions, therefore each cleavage can be done separately and independently from the other cleavage at the appropriate point of time in the reaction sequence. To achieve this function, the chemical nature of the various PGs involved in the reaction strategy and the chemical nature of the connection of the handle group HG of the linker to the peptide is chosen in such a way, that any PG belongs to one of the four types of PGs in such a way, and that the connection of the handle group HG to the peptide is cleavable under such reaction conditions, that this grouping of the protecting groups and this selection of the nature of the connection of the handle group HG to the peptide allow for the desired and necessary separate and stepwise cleavage. If there are one or more side chain PGs in the desired peptide C-PEP, one preferred embodiment is, that . any side chain protecting group is a strong, reductive or mixed type PG; and 2. any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is a basic type PG, or any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is a basic, a reductive or a mixed type PG, if any side chain protecting group is not a reductive or mixed type PG; and 3. PG2 is a weak, a reductive or a mixed type PG, if any side chain protecting group and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is not a reductive or mixed type PG, or PG2 is a weak type PG; and 4. the N-terminal PG of the last amino acid used in SPPS in the synthesis of C-PEP, i.e. of the N-terminal amino acid of the peptide C-PEP, is a basic or a weak type PG, or the N-terminal PG of the last amino acid used in SPPS in the synthesis of C-PEP, i.e. of the N-terminal amino acid of the peptide C-PEP, is a basic, a weak, a reductive or a mixed type PG, if any side chain protecting group is not a reductive or mixed type PG; and 5. the diketopiperazine residue comprising C-terminal protecting group of PEP or C-PEP is cleavable from the peptide PEP or C-PEP in strong cleaving conditions, or in strong or reductive cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not reductive or mixed type PGs, or in weak cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not weak type PGs, or in weak 1 cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not weak 1 type PGIf there are one or more side chain PGs in the desired peptide C-PEP, one more preferred embodiment is, that 1. any side chain protecting group is a strong type PG; and 2. any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is a basic type PG; and 3. PG2 is a weak, a reductive or a mixed type PG, if any side chain protecting group and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is not a reductive or mixed type PG; and 4. the N-terminal PG of the last amino acid used in SPPS in the synthesis of C-PEP, i.e. of the N-terminal amino acid of the peptide C-PEP, is a basic, a weak, a reductive or a mixed type PG, if any side chain protecting group is not a reductive or mixed type PG; and 5. the diketopiperazine residue comprising C-terminal protecting group of PEP or C-PEP is cleavable from the peptide PEP or C-PEP in strong cleaving conditions, or in strong or reductive cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not reductive or mixed type PGs, or in weak cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not weak type PGs, or in weak 1 cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not weak 1 type PG. If there are one or more side chain PGs in the desired peptide C-PEP, another more preferred embodiment is, that 1. any side chain protecting group is a strong, reductive or mixed type PG; and 2. any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is a basic type PG; and 3. PG2 is a weak type PG; and 4. the N-terminal PG of the last amino acid used in SPPS in the synthesis of C-PEP, i.e. of the N-terminal amino acid of the peptide C-PEP, is a basic or a weak type PG; and 5. the diketopiperazine residue comprising C-terminal protecting group of PEP or C-PEP is cleavable from the peptide PEP or C-PEP in strong or reductive cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not reductive or mixed type PGs, or in weak 1 cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not weak 1 type PG. If there are no side chain PGs in the desired peptide C-PEP, one preferred embodiment is, that 1. any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is a basic, a reductive or a mixed type PG; and 2. PG2 is a strong, weak, a reductive or a mixed type PG, if any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is not a reductive or mixed type PG, or PG2 is a strong, a weak or a mixed type PG; and 3. the N-terminal PG of the last amino acid used in SPPS in the synthesis of C-PEP, i.e. of the N-terminal amino acid of the peptide C-PEP, is a basic, a strong, a weak, a reductive or a mixed type PG; and 4. the diketopiperazine residue comprising C-terminal protecting group of PEP or C-PEP is cleavable from the peptide PEP or C-PEP in strong cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not strong type PGs, or in strong or reductive cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not strong, reductive and mixed type PGs, or in weak cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not strong or weak type PGs, or in weak 1 cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not strong or weak 1 type PGs. If there are no side chain PG in the desired peptide C-PEP, one more preferred embodiment is, that 1. any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP except for the last one, i.e. except for the N-terminal PG of the N-terminal amino acid of the peptide C-PEP, is a basic type PG; and 2. PG2 is a strong, weak, a reductive or a mixed type PG, or 3. the N-terminal PG of the last amino acid used in SPPS in the synthesis of C-PEP, i.e. of the N-terminal amino acid of the peptide C-PEP, is a basic, a strong, a weak, a reductive or a mixed type PG; and 4. the diketopiperazine residue comprising C-terminal protecting group of PEP or C-PEP is cleavable from the peptide PEP or C-PEP in strong cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not strong type PGs, or in strong or reductive cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not strong, reductive and mixed type PGs, or in weak cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not strong or weak type PGs, or in weak 1 cleaving conditions, if PG2 and any N-terminal PG of the amino acids used in SPPS in the synthesis of C-PEP are not strong or weak 1 type PGs. Subject of the invention is a method(C-PEP) for the preparation of a peptide C-PEP, C-PEP comprises a peptidyl radical PEP-C, the C-terminus of PEP-C is protected by a protecting group DKP-PG, DKP-PG comprises a handle group HG, optionally a spacer group SG, and a diketopiperazine residue DKP; SG is a spacer group conventionally used in peptide chemistry; DKP is a diketopiperazine residue derived from a dipeptide residue DPR; DPR comprises alpha amino acid residues Xaal and Xaa2; Xaal is the C-terminal amino acid residue of DPR; Xaa2 is the N-terminal amino acid residue of DPR, and Xaa2 has a side chain, said side chain is substituted by a functional group FG; PEP-C is connected via XaaC 1 to HG; XaaC is an amino acid residue of PEP-C; index (1) in XaaC(1) denotes the C-terminal position of PEP-C; XaaC 1 is the C-terminal amino acid residue of PEP-C; HG is a handle group conventionally used in solid phase peptide synthesis SPPS for connecting the C-terminus of a peptide to the solid phase, which allows for cleavage of the C-terminus from HG under conditions, which do not cleave an amide bond connecting two amino acid residues in a peptide; HG is either directly connected to FG, or, if a SG is present, HG is connected to SG and SG is connected to FG; method(C-PEP) comprises a step (iii); step (iii) comprises a reaction(INRIFO); reaction(INRIFO) is a reaction which comprises an intramolecular ring formation and a simultaneous cleavage reaction in a peptide PEP-C-DKP-L-ResinA; PEP-C-DKP-L-ResinA is the precursor of C-PEP and comprises PEP-C and a resin DK.P-LResinA, with PEP-C being connected to DKP-L-ResinA; DKP-L-ResinA comprises a ResinA and a DKP-PG forming linker DKP-L, with ResinA being connected to DKP-L, ResinA is a resin used conventionally as solid phase in SPPS, DKP-L comprises HG, optionally SG, and DPR, with the C-terminal carboxylic acid group of DPR, which is the carboxylic acid group of Xaal, being connected to ResinA; the intramolecular ring formation in reaction(INRIFO) is a reaction of the N-terminal amino group of DPR, which is the alpha amino group of Xaa2, with the C-terminal carboxylic acid group of DPR, thereby forming DKP, thereby Xaal is simultaneously cleaved from ResinA and DKP-PG is formed; HG is chosen in such a way, that the bond between HG and XaaC is not cleaved during reaction(INRIFO). By the use of HG, a cleaving site between XaaC 1 and ResinA is provided which can be selectively cleaved without cleaving any amide bond between two amino acids in the peptide; the cleaving site being the bond between XaaC 1 and HG. By this cleavage, the C-terminus of XaaC 1 is set free, either in form of a unprotected, free carboxylic acid group, or the Cterminal carboxylic acid group is set free in form of an amide group, preferably as C(0)NH 2, depending on the chemical nature of HG. By the use of HG, this specific DKP comprising C-terminal protecting group acts as a conventionally in peptide chemistry used C-terminal protecting group. Preferably, PEP-C is prepared prior to the reaction (INRIFO) by a solid phase peptide synthesis SPPS(PEP-C), more preferably the SPPS(PEP-C) uses DKP-L-ResinA as solid phase. Therefore further subject of the invention is the method(C-PEP), with the method(C-PEP) as defined above, also with all its preferred embodiments, wherein PEP-C is prepared prior to step (iii) by a solid phase peptides synthesis SPPS(PEP-C), more preferably the SPPS(PEP-C) uses DKP-L-ResinA as solid phase. In SPPS(PEP-C), PEP-C is built by coupling the XaaC consecutively, first to the solid phase, then to the growing peptide chain. The various XaaC can be coupled individually and sequentially, but two or more of them can also be coupled e.g. as dipeptides, tripeptides or oligopeptides to the solid phase or to the growing peptide chain. ResinA is chosen in such a way, that the bond between ResinA and Xaal is not cleaved during SPPS(PEP-C). Preferably, SPPS(PEP-C) comprises further a step (i) and a step (ii); in step (i) XaaC(1) is attached to DKP-L-ResinA; in step (ii) the further amino acids XaaC according to the sequence of PEP-C are consecutively connected by SPPS(PEP-C) initially to XaaC(1) and then to the Nterminus of the growing peptidyl chain, which is bound via DKP-L to the ResinA; HG is chosen in such a way, that the bond between HG and XaaC 1 is not cleaved during SPPS(PEP-C); and that the bond between HG and XaaC 1 is not cleaved during reaction(INRIFO); with C-PEP, ResinA, DKP-PG, HG, SG, DPR, DKP, Xaal, XaaC l , XaaC, PEP-C, SC-PG, reaction(INRIFO) as defined above, also with all their preferred embodiments; and with the connectivities between PEP-C, HG, SG and DPR and ResinA as defined above, also with all their preferred embodiments. Prior to reaction(INRIFO), the N-terminus of DPR, which is alpha amino group of Xaa2, is protected by a protecting group PG2. Therefore further subject of the invention is a method(C-PEP), with the method(C-PEP) as defined above, also with all its preferred embodiments, wherein step (iii) comprises cleavage of the protecting group PG2; PG2 is an N-terminal protecting group conventionally used in peptide chemistry and is selected from the group consisting of basic cleavable type protecting groups, acid cleavable type protecting groups and reductively cleavable type protecting groups. PG2 is cleaved from Xaa2 before the reaction(INRIFO) in step (iii). Preferably, PG2 is cleaved from Xaa2 after SPPS(PEP-C). Preferably, PG2 is cleaved from Xaa2 after the addition of the N-terminal amino acid residue of PEP-C in step (ii). The cleavage of PG2 from Xaa2 and the reaction(INRIFO) can occur consecutively or simultaneously. PG2 is chosen in such a way, that the bond between PG2 and Xaa2 is not cleaved during SPPS(PEP-C). PG2 and HG are chosen in such a way, that the bond between HG and XaaC 1 is not cleaved during the cleavage of PG2 from Xaa2. Any side chain of C-PEP or PEP-C can be protected independently from any other side chain of C-PEP or PEP-C by a protecting group SC-PG, in case of more than one SC-PG being present in C-PEP or PEP-C, these SC-PG are independently from each other identical or different. Any SC-PG is a protecting group which is conventionally used in peptide chemistry for protecting side chains of amino acid residues of a peptide or for protecting side chains of amino acids during SPPS or during HSPPS. Preferably, any SC-PG is chosen in such a way, that no SC-PG is cleaved during SPPS(PEPC). Preferably, any SC-PG is chosen in such a way, that no SC-PG is cleaved during reaction(INRIFO). Preferably, PG2 and any protecting group SC-PG are chosen in such a way, that no SC-PG is cleaved during the cleavage of PG2 from Xaa2. In order to avoid complexity of the description, the abbreviation XaaC is used either for the amino acids used to synthesis PEP-C and C-PEP, or it is used for the amino acid residues of PEP-C and C-PEP, or PEP-N respectively; and likewise XaaN is used either for the amino acids used to synthesis PEP-N, or it is used for the amino acid residues of PEP-N. Therefore these abbreviations do not differentiate between amino acids and amino acid residues. The skilled person can unambiguously distinguish from the context, whether an amino acid or an amino acid residue is meant. To summarize the connectivities: PEP-C is connected via XaaC 1 to HG. HG is either directly connected via FG to Xaa2, or, if a SG is present, HG is connected to SG and SG is connected via FG to Xaa2. Xaa2 is connected with Xaal via a peptide bond, Xaa2 is the N-terminal and Xaal the Cterminal amino acid in DPR. In PEP-C-DKP-L-ResinA, the carboxylic acid group of Xaal is connected to ResinA. ResinA is chosen in such a way, that the bond between ResinA and Xaal is not cleaved during SPPS (PEP-C). HG is chosen in such a way, that the bond between HG and XaaC 1 is not cleaved during reaction(INRIFO). HG is chosen in such a way, that the bond between HG and XaaC 1 is not cleaved during SPPS(PEP-C). PG2 is chosen in such a way, that the bond between PG2 and Xaa2 is not cleaved during SPPS(PEP-C). PG2 and HG are chosen in such a way, that the bond between HG and XaaC(1 is not cleaved during the cleavage of PG2 from Xaa2. Preferably, any SC-PG is chosen in such a way, that no SC-PG is cleaved during SPPS(PEPC). Preferably, any SC-PG is chosen in such a way, that no SC-PG is cleaved during reaction(INRIFO). Preferably, PG2 and any protecting group SC-PG are chosen in such a way, that no SC-PG is cleaved during the cleavage of PG2 from Xaa2. Preferably, C-PEP is PEP-C, whose C-terminus is protected by DKP-PG. Preferably, DPR consists of the amino acid residues Xaal and Xaa2. Xaal and Xaa2 are chosen in such a way, that they allow the formation of DKP by reaction(iNRIFO). Preferably, any side chain of C-PEP is protected by a protecting group SC-PG If any SC-PG is present in C-PEP, then preferably HG is chosen in such a way, that HG, and thereby DKP-PG, is cleaved from XaaC 1 simultaneously in the reaction which cleaves SC-PG, preferably all SC-PGs. Preferably, SC-PG is selected from the group consisting of basic cleavable type protecting groups, acid cleavable type protecting groups and reductively cleavable type protecting groups. More preferably, any SC-PG is a strong type PG. Preferably, FG, when connected to HG or to SG, is present as a connecting group CG. Preferably, FG is selected from the group consisting of COOH, NH2, OH and SH, more preferably consisting of NH and OH; therefore CG is preferably selected from the group consisting of -C(0)0-, -N(H)-, -O- and -S-, more preferably consisting of - N(H)- and -0-. The bond between HG and FG, or, if a SG is present in DKP-PG, the bonds between HG and SG and between SG and FG, are chosen to be of such a chemical nature, that they are not cleaved during SPPS (PEP-C); and that they are not cleaved during reaction(INRIFO), step (i), step (ii) or step (iii); preferably, they are also not cleaved during any cleavage of any protecting group. Preferably, the bond between HG and FG, or, if a SG is present in DKP-PG, the bonds between HG and SG and between SG and FG, are amide or ester bonds, more preferably amide bonds. Especially, these bonds are of similar nature or stability as a conventional amide bond between two amino acid residues in a peptide. The N-terminus of C-PEP can be protected by a protecting group N-PG, N-PG is an Nterminal protecting group conventionally used in peptide chemistry. Preferably, N-PG is selected from the group consisting of basic cleavable type protecting groups, acid cleavable type protecting groups and reductively cleavable type protecting groups. Therefore, C-PEP comprises both the N-terminally unprotected embodiment and the embodiment, wherein the N-terminus of PEP-C is protected by N-PG. Further subject of the invention is a method(C-PEP) for the preparation of C-PEP, characterized by the steps (i), (ii) and (iii), which steps comprise a solid phase peptides synthesis SPPS(PEP-C) and a subsequent reaction(INRIFO); the SPPS(PEP-C) is done on a resin DKP-L-ResinA as solid support, the DKP-L-ResinA is a ResinA, which carries as a functional group a DKP-PG forming linker DKP-L, DKP-L comprises HG, optionally SG, and DPR, with the Xaal of the DPR being connected via its C-terminal carboxylic acid group to ResinA, reaction(INRIFO) is a intramolecular ring formation reaction of the N-terminal amino group of DPR with the C-terminal carboxylic acid group of DPR, thereby forming DKP; and by reaction(INRIFO) Xaal is simultaneously cleaved from ResinA and DKP-PG is formed; in step (i) XaaC(1) is attached to DKP-L-ResinA; in step (ii) the further amino acids XaaC according to the sequence of PEP-C are consecutively connected by SPPS(PEP-C) initially to XaaC 1 and then to the Nterminus of the growing peptide chain, which is bound via DKP-L to the ResinA; in step (iii), which is done after the addition of the N-terminal amino acid residue of PEP-C in step (ii), C-PEP is formed by reaction(INRIFO), ResinA is chosen in such a way, that the bond between ResinA and Xaal is not cleaved during SPPS(PEP-C); HG is chosen in such a way, that the bond between HG and XaaC 1 is not cleaved during SPPS(PEP-C); and that the bond between HG and XaaC is not cleaved during reaction(INRIFO); any SC-PG protecting a side chain of C-PEP is chosen in such a way, that SC-PG is not cleaved during SPPS(PEP-C); and that SC-PG is not cleaved during reaction(INRIFO); with C-PEP, ResinA, DKP-PG, HG, SG, DPR, DKP, Xaal, XaaC 1), XaaC, PEP-C, SC-PG, reaction(INRIFO) as defined above, also with all their preferred embodiments; and with the connectivities between PEP-C, HG, SG and DPR and ResinA as defined above, also with all their preferred embodiments. Further subject of the invention is a method(DKP-L) for preparation of a DKP-PG forming linker DKP-L, method(DKP-L) comprises a step (DKP-L-i), a step (DKP-L-iii) and optionally a step (DKPL- ii); in step (DKP-L-i) Xaa2 is coupled to Xaal ; in optional step (DKP-L-ii) SG is coupled to Xaa2, if SG is present in DKP-L; in step (DKP-L-iii) HG is coupled either to SG, if SG is present in DKP-L, or to Xaa2; with DKP-PG, DKP-L, DKP, Xaa2, Xaal, HG and SG as defined above, also with all their preferred embodiments. The steps (DKP-L-i), (DKP-L-iii) and the optional step (DKP-L-ii) can be done in any order. Preferably, at first the step (DKP-L-i) is done, then the optional step (DKP-L-ii) is done, if SG is present in DKP-L, and the step (DKP-L-iii) is done as the last step. Further subject of the invention is a method(DKP-L-ResinA) for preparation of DKP-LResinA, method(DKP-L-ResinA) is a method(Xl ) or a method(X2); method(Xl) comprises a step (Xl-i), a step (XI -ii), a step (XI -iv) and optionally a step (XIiii); in step (Xl-i) the amino acid Xaal is coupled to ResinA; in step (XI -ii) the amino acid Xaa2 is coupled to Xaal ; in the optional step (XI -iii) SG is coupled to the side chain of Xaa2, if SG is present in DKP-L-ResinA; in step (XI -iv) HG is coupled either to SG, if SG is present in DKP-L-ResinA, or to Xaa2; method(X2) comprises a step (X2-i); in step (X2-i) DPK-L is coupled to ResinA; with DKP-L-ResinA, ResinA, DKP-PG, DKP-L, DKP, Xaa2, Xaal, HG and SG as defined above, also with all their preferred embodiments. In method(Xl), the steps (Xl-i), (XI -ii), (XI -iv) and the optional step (Xl-iii) can be done in any order. Preferably, at first the step (Xl-i), then the step (XI -ii) is done, then the optional step (Xl-iiii) is done, if SG is present in DKP-L-ResinA, and the step (XI -iv) is done as the last step. HG, any SG, Xaa2 and Xaal, when used as building blocks in method(DKP-L-ResinA) or in method(DKP-L), can carry a protecting group: Xaal, used as building block in method(Xl) or method(DKP-L), is used as a conventionally C-terminally protected amino acid, the protecting group being a protecting group CPG. The alpha amino group of Xaal is unprotected and is the coupling site in the respective coupling reaction. Xaa2, used as building block in method(Xl) or method(DKP-L), is used as a conventionally N-terminally protected amino acid, the protecting group being a protecting group NPG. The 1-carboxylic acid group of Xaa2 is unprotected and is the coupling sites in the respective coupling reaction. Any side chain of Xaal or Xaa2 is preferably also protected by a SC-PG. C-PG is a protecting group conventionally used in peptide chemistry for protecting the carboxylic acid group of an amino acid or for protecting the C-terminus of a peptide. Preferably, C-PG is selected from the group consisting of basic cleavable type protecting groups, acid cleavable type protecting groups and reductively cleavable type protecting groups. Each HG and SG, in form of individual building blocks used in the respective coupling reactions, has at least two reactive functional groups. The first reactive functional group is used as a functionality resembling the alpha amino group of an amino acid building block in peptide synthesis and can be protected by a suitable protecting group, preferably by a protecting group N-PG; preferably, this functional group is OH or NH2 and is present in the protected state as -O- or -N(H)-. The other reactive functional group of HG and SG is used as a functionality resembling the carboxylic acid group of an amino acid building block in peptide synthesis and is usually unprotected and is the coupling site in the respective coupling reaction. Preferably, this unprotected site is a carboxylic acid group. After this coupling reaction, any protecting group of the first reactive functional group, preferably said N-PG, can be cleaved in order to make this first functional group available for the next coupling reaction. The DKP-PG forming linker DKP-L, obtainable by method(DKP-L), usually still carries any protecting group of HG in order to be usable in the coupling to ResinA in method(X2). Prior to the coupling in method(X2), a C-PG of Xaal must be cleaved off. Preferably, method(DKP-L) comprises this cleaving of C-PG from Xaal . Therefore, DKP-L comprises both embodiments, one embodiment with a protecting group C-PG on Xaal, the other embodiment without a protecting group C-PG on Xaal . In DKP-L-ResinA, HG can still carry a protecting group which was present in the building block HG used for preparing DKP-L-ResinA. Any protecting group on HG must be cleaved prior to step (i) in method(C-PEP). Preferably, method(DKP-L-ResinA) comprises this cleaving of any protecting group from HG. Therefore, DKP-L-ResinA comprises both embodiments, one with any protecting group on HG, the other without any protecting group on HG. Further subject of the invention is a method(PEP-HSPPS) for the preparation of a peptide PEP, method(PEP-HSPPS) comprises a step (i-pep) and a step (ii-pep), in step (i-pep) a peptide C-PEP is prepared according to method(C-PEP); then in step (ii-pep) C-PEP obtained in step (i-PEP) is coupled with an N-terminally protected amino acid or with an N-terminally protected peptide PEP-N by homogeneous solution phase peptide synthesis HSPPS; with method(C-PEP), C-PEP and PEP-N being as defined above, also with all its preferred embodiments. Method(PEP-HSPPS) is a method(C-PEP) comprising the further step (ii-pep). Any side chain of PEP-N can be protected independently from any other side chain of PEP-N by a protecting group SC-PG, in case of more than one SC-PG being present in PEPN, these SC-PG are independently from each other identical or different; with SC-PG being as defined above, also with all its preferred embodiment. C-PEP in method(PEP-HSPPS) is used N-terminally unprotected. Therefore, any protecting group N-PG, which protects the N-terminus of C-PEP, is cleaved prior to the coupling reaction of method(PEP-HSPPS). This cleaving reaction is preferably comprised in method(C-PEP). Since PEP-C is made by SPPS(PEP-C), the N-terminal amino acid of PEP-C used in SPPS(C-PEP) is usually used with a protected amino group N-PG on its alpha amino group. Depending on the nature of this protecting group N-PG of the N-terminus of PEP-C this N-PG can be cleaved from the N-terminus simultaneously under the condition of the ring formation in reaction(INRIFO), or it can be cleaved from the N-terminus simultaneously with the cleaving of PG2 from Xaa2 prior to the reaction (INRIFO). Further subject of the invention are following methods: 1. a method(PEP-HSPPS), wherein the DKP-L-ResinA of method(C-PEP) has been prepared by the method(DKP-L-ResinA); 2. a method(PEP-HSPPS), wherein the DKP-L-ResinA of method(C-PEP) has been prepared by method(X1) of the method(DKP-L-ResinA); 3. a method(PEP-HSPPS), wherein the DKP-L-ResinA of method(C-PEP) has been prepared by method(X2) of the method(DKP-L-ResinA); and wherein the DKP-L of method(DKP-L-ResinA) has been prepared by the method(DKP-L); 4. a method(C-PEP), wherein the DKP-L-ResinA has been prepared by the method(DKP-L-ResinA); 5. a method(C-PEP), wherein the DKP-L-ResinA has been prepared by method(Xl) of the method(DKP-L-ResinA); 6. a method(C-PEP), wherein the DKP-L-ResinA has been prepared by method(X2) of the method(DKP-L-ResinA); and wherein the DKP-L of method(DKP-L-ResinA) has been prepared by the method(DKP-L). Further subject of the invention is a compound selected from the group consisting of C-PEP, PEP-C-DKP-L-ResinA, DKP-L-ResinA and DKP-L, with C-PEP, PEP-C-DKP-LResinA, DKP-L-ResinA and DKP-L as defined above, also with all their preferred embodiments. Further subject of the invention is the use of C-PEP, with C-PEP being as defined above, also with all its preferred embodiments, in homogeneous solution phase peptide synthesis HSPPS for the preparation of a peptide PEP by a coupling reaction of C-PEP with an N-terminally protected amino acid or with an N-terminally protected peptide PEP-N. Further subject of the invention is the use of a compound selected from the group consisting of C-PEP, PEP-C-DKP-L-ResinA, DKP-L-ResinA and DKP-L; or the use of DKP-L as a DKP-PG forming linker, in peptide chemistry; or for the preparation of a peptide; or in a method for the preparation of a peptide; or in a step of a method for the preparation of a peptide; or in a peptide coupling reaction; or in SPPS for the preparation of a peptide; or in HSPPS for the preparation of a peptide; with C-PEP, PEP-C-DKP-L-ResinA, DKP-L-ResinA, DKP-L and DKP-PG as defined above, also with all their preferred embodiments. Any of the following embodiments of the invention are comprised in the hitherto described embodiments of the invention. Further subject of the invention is a method(A) for the preparation of a compound of formula (III-PEP-PG) PGIII PEP HG SG n (III-PEP-PG) Xaa2 Xaal —I by homogenous solution phase coupling of an amino acid, which is N-terminally protected by a protecting group PGIII, or of an N-terminally protected peptide fragment PEP-N, the N-terminally protected peptide fragment PEP-N being a compound of formula (III-PEP-N-PG), PGIII (XaaN('P")) pn (III-PEP-N-PG) a compound of formula (III-H); (1) H PEP-C HG SG n (III-H) Xaa2 Xaal HG is a handle group conventionally used in solid phase peptide synthesis SPPS for connecting the C-terminus of a peptide to the solid phase, which allows for cleavage of the C-terminus from HG under conditions, which do not cleave an amide bond connecting two amino acid residues in a peptide; n is 0 or 1; SG is a spacer group conventionally used in peptide chemistry; Xaal is an alpha amino acid residue; Xaa2 is a 2-(Ci -5 alkyl)-alpha amino acid residue, wherein the Ci-5 alkyl group is substituted by a functional group FG, FG is selected from the group consisting N¾, OH, SH and COOH; FG is bonded with SG when n is 1; FG is bonded to HG when n is 0, and therefore FG is present in the compound of formula (III-PEP-PG) as a connecting group CG selected from the group consisting -N(H)-, -0-, -S- and -C(0)0-; PEP-C is a peptidyl radical of formula (XaaC ( p )pc; the hydrogen H denoted with (1) in formula (III-H) is a hydrogen of the unprotected Nterminus of PEP-C; XaaC is an amino acid residue of the peptidyl radical PEP-C; in XaaC , , (ipc) signifies the index of XaaC of PEP-C at the position ipc, with the position count starting from the C-terminus of PEP-C, pmax is 502; pc is an integer of from 2 to (pmax-2) and represents the total number of amino acid residues in PEP-C; ipc is an integer of from 1 to pc; PGIII in formulae (III-PEP-PG) and (III-PEP-N-PG) are identical and is an N-terminal protecting group commonly used in peptide chemistry; PEP is a peptidyl radical of formula (XaaP i )p; pn is an integer of from 2 to (pmax-pc) and represents the total number of amino acid residues in PEP-N; p is pc + pn; XaaN is an amino acid residues of the peptide fragment PEP-N; in XaaN pn , (ipn) signifies the index of XaaN of PEP-N at the position ipn, with the position count starting from the C-terminus of PEP-N; XaaP is an amino acid residue; in XaaP ip , (ip) signifies the index of XaaP of PEP at the position ip, with the position count starting from the C-terminus of PEP; ipn is an integer of from 1 to pn; ip is an integer of from 1 to p; with the proviso, that XaaP ,p is identical with XaaC ip for ip having a value from 1 to ipc; and XaaP p is identical with XaaN pn for ip having the value (pc + ipn); with pmax, pc, XaaC, XaaC i c , ipc and compound of formula (III-H) being as defined above, also with all their preferred embodiments; XaaC in formula (III-H) and XaaN are independently from each other identical or different. Therefore, PEP-C is a peptidyl radical having pc amino acid residues XaaC. Preferably, the alpha amino group of Xaal is coupled to the 1-carboxy group of Xaa2 by a peptide bond. Compound of formula (III-H) is an embodiment of the above defined C-PEP. HG, SG, Xaa2 and Xaal are embodiments of the respective HG, SG, Xaa2 and Xaal of the above defined peptide C-PEP. The cyclic dipeptide in e.g. formula (III-H) is one embodiment of the above mentioned DKP, i.e. the diketopiperazine residue derived from DPR. Preferably, PEP-C is prepared by SPPS. The SPPS, by which PEP-C is prepared, is called above SPPS(PEP-C). Preferably, PEP-C is a peptidyl radical of formula (XaaC i )pc, which has been synthesized by SPPS using amino acids of formula PGXaaC(ip - XaaC(ipc) - OH. PGXaaC is an N-terminal protecting group conventionally used in SPPS and is selected from the group consisting of basic cleavable type protecting groups, acid cleavable type protecting groups and reductively cleavable type protecting groups. In PGXaaC ip , the index (ipc) defines PGXaaC i as the protecting group of the amino acid PGXaaC i )-XaaC(ipc)-OH, with each PGXaaC(ipc) being independently from each other identical or different from another PGXaaC p . Preferably, PGXaaC and PGXaaC i respectively is an N-terminal protecting group conventionally used in SPPS to protect the alpha amino group of any amino acid PGXaaCXaaC- OH and PGXaaC ipc -XaaC( -OH respectively used in the SPPS to synthesize PEP-C. In order to avoid complexity of the description, the abbreviations PGXaaC in the text is used either for the protecting group of the amino group of the amino acids used to synthesis PEP-C and C-PEP, or it is used for the N-terminal protecting group of the N-terminal amino acid residues of PEP-C and C-PEP at the various stages during SPPS. The skilled person can unambiguously distinguish from the context, which of the two meanings is meant. Therefore, PEP-N is a peptidyl radical having pn amino acid residues XaaN. Therefore, PEP is a peptidyl radical having p amino acid residues XaaP. The residue of formula (Ill-res) SG (Ill-res) n Xaa2 Xaal which appears e.g. in the formulae (III-PEP-PG) and (III-H), is an embodiment of the DKPPG mentioned above; with HG, SG, n, Xaa2 and Xaal being as defined above; with Xaa2 and Xaal forming the D P mentioned above; and with (8) denoting the bond between the peptidyl radical PEP-C and HG. Compound of formula (III-PEP-PG) is an embodiment of above defined peptide PEP and can also be an embodiment of above defined C-PEP. PEP-N can be prepared by conventional peptide synthesis, either by SPPS, by HSPPS or by a combination of SPPS and HSPPS, preferably by SPPS. In case that the compound of formula (III-PEP-PG) shall be used in a next HSPPS coupling according to method(A) as a next fragment C-PEP with a next fragment PEP-N, only the Nterminal protecting group PGIII of said compound of formula (III-PEP-PG) needs to be removed, to provide for said next fragment C-PEP, i.e. for the next compound of formula (IIIH), for said next HSPPS coupling according to method(A). Since both the compound of formula (III-H) and PEP-N may themselves have been prepared by method(A) in one of their preparation steps, they can have practically any number of amino acids as long as the solution phase coupling still works in a reasonable time. Preferably, pmax is 500, more preferably pmax is 400 or 402, even more preferably 300 or 302, especially 200 or 202, more especially 150 or 152, even more especially 100 or 102, particularly 80 or 82, more particularly 50 or 52, even more particularly 25 or 27. Preferably, the peptidyl radical PEP-C is a linear peptidyl radical, and preferably PEP-N is a linear peptide, resulting in a linear peptidyl radical PEP. Preferably, if PEP-C in compound of formula (III-H) and/or PEP-N have been prepared using SPPS, they have independently from each other of from 2 to 100, more preferably of from 2 to 50, even more preferably of from 2 to 40, especially preferably of from 2 to 25 amino acid residues. Preferably, if PEP-C in compound of formula (III-H) and/or PEP-N have been prepared using HSPPS, they have independently from each other from of 2 to 250, more preferably of from 2 to 200, even more preferably of from 2 to 100, especially preferably of from 2 to 50, in particular of from 2 to 25 amino acid residues. Any functional groups on the side chains of the individual amino acid residues of peptidyl radical PEP-C and of PEP-N are independently from each protected or unprotected by protecting groups SC-PG; preferably, all functional groups on the side chains of the individual amino acid residues of peptidyl radical PEP-C and of PEP-N are protected by protecting groups SC-PG or unprotected; more preferably, all functional groups on the side chains of the individual amino acid residues of peptidyl radical PEP-C and of PEP-N are protected during the solution phase coupling according to method(A) of fragment PEP-N with compound of formula (IIIH), even more preferably, all functional groups on the side chains of the individual amino acid residues of peptidyl radical PEP-C and of PEP-N are protected by strong type PG. Preferably, the C terminus or, in case that the C-terminal amino acid residue has a side chain, the side chain of the C-terminal amino acid residue of the peptidyl radical PEP or PEP-C respectively, is bonded to HG; more preferably, the C terminus of the peptidyl radical PEP or PEP-C respectively, is bonded to HG. Preferably, HG is a handle group conventionally used in solid phase peptide synthesis SPPS to connect an amino acid, which will become the C-terminal amino acid residue of a peptide, which is to be synthesised by SPPS, via said HG to a solid phase, preferably to a ResinA. HG allows for cleavage of the C-terminal amino acid residue from HG under conditions, which do not cleave an amide bond connecting two amino acid residues in a peptide. More preferably, HG is a handle group selected from the group consisting of handle group of formula (HGF-I), handle group of formula (HGF-II), handle group of formula (HGF-III), handle group of formula (HGF-IV), handle group of formula (HGF-V) and handle group of formula (HGF-VI), Rl wherein (*) denotes the bond between the C atom of the C terminus of the respective peptidyl radical, e.g. of PEP for formulae (III-PEP-PG), of PEP-C for formula (III-H) or of PEP-C in method(C-PEP), and HG, or denotes, in case that the C-terminal amino acid residue of the respective peptidyl radical, e.g. of PEP for formulae (III-PEP-PG), of PEP-C for formula (III-H) or of PEP-C in method(C-PEP), has a side chain and is connected via this side chain to HG, the bond between the side chain of the C terminal amino acid residue of the respective peptidyl radical, e.g. of PEP for formulae (III-PEP-PG), of PEP-C for formula (III-H) or of PEP-C in method(C-PEP), and HG, (**) denotes the bond between HG and SG when n is 1, or denotes the bond between HG and FG when n is 0, with SG and FG as defined above, also with all their preferred embodiments; Rl , R2, R3, R4, RIO and Rl 1 are identical or different and independently from each other selected from the group consisting of hydrogen and O-C1-4 alkyl, sl-1, s2, s3, s4 and s6 are identical or different and independently from each other selected from the group consisting of 1, 2, 3 and 4, s5-l is 0, 1, 2, 3 or 4, si -2, s5-2 and s5-3 are identical or different and independently from each other 0 or 1, Tl-1 is O or NH, Tl-2 and T5-l are O, with n, SG, FG, PEP-C and method(C-PEP) as defined above, also with all their preferred embodiments. Preferably, (*) denotes the bond between the C atom of the C terminus of the respective peptidyl radicals, e.g. of PEP for formulae (III-PEP-PG), of PEP-C for formula (III-H) or of PEP-C in method(C-PEP), and HG. Preferably, SG is a spacer group conventionally used in SPPS, preferably comprising one or more, more preferably 1 to 500, ethylenoxide units. More preferably, SG is a spacer group selected from the group consisting of spacer group of formula (SGI), spacer group of formula (SG-II), spacer group of formula (SG-III), spacer group of formula (SG-IV) and spacer group of formula (SG-V); ml, m5, m6, m7, m9, ml 0, ml 1 and ml 2 are identical or different and independently from each other an integer of 1to 500; m2, m3 and m4 are identical or different and independently from each other 1, 2, 3 or 4, (***) is the bond from SG to HG when n is 1, (****) is the bond between SG and Xaa2 when n is 1. (***) is the bond denoted by (**) in the respective embodiments of HG, when n is 1. (****) s the bond between SG and FG, when n is 1; with HG, Xaa2 and n as defined above, also with all their preferred embodiments. Preferably, XaaC and XaaN are alpha amino acid residues. More preferably, XaaC and XaaN are naturally occurring alpha amino acid residues. If XaaC or XaaN carries a side chain with a functional group, this functional group of the side chain of XaaC or XaaN is either protected or unprotected, preferably it is protected. More preferably, XaaC and XaaN are identical or different and are independently from each other selected from the group consisting of Ala, Aib, Cys, Asp, Glu, Phe, Gly, His, e, Lys, Leu, Met, Asn, Pro, Gin, Arg, Ser, Thr, Val, Trp, Tyr, Asp, Asn, Glu and Gin; where any functional group in the side chain is either protected or unprotected, preferably protected. Preferably, PGIII is selected from the group consisting of basic type PGs, strong type PGs, weak type PGs and reductive type PGs. If all functional groups on the side chains of the individual amino acid residues of peptidyl radical PEP-C and of PEP-N are protected by strong acid cleavable type protecting groups, and in case that the compound of formula (III-PEP-PG) shall be used in a next HSPPS coupling according to method(A) as a next fragment C-PEP with a next fragment PEP-N, then PGIII is preferably selected from the group consisting of basic type PGs, weak type PGs and reductive type PGs. Handle groups of formula (HGF-I) and handle groups of formula (HGF-IV) are cleavable from the PEP-C by strong cleaving conditions, handle groups of formula (HGF-II) are cleavable by strong cleaving conditions, handle groups of formula (HGF-III) are cleavable by weak or by strong cleaving conditions, handle groups of formula (HGF-V) are cleavable by reductive cleaving conditions, and handle groups of formula (HGF-VI) are cleavable by weak or by strong cleaving conditions Preferably, HG is a handle group selected from the group consisting of handle group of formula (HGF-I), handle group of formula (HGF-IV) and handle group of formula (HGF-VI). Preferably, Rl, R2, R3, R4, R10 and Rl 1 are identical or different and independently from each other selected from the group consisting of hydrogen and 0-CH 3. More preferably, Rl and R2 are identical and selected from the group consisting of hydrogen and 0-CH 3. More preferably, R3, R4, R10 and Rl 1 are 0-CH 3. Preferably, sl-1 and s6 are 1. Preferably, si -2, s5-l, s5-2 and s5-3 are independently from each other 0 or 1. Preferably, s2 and s3 are 4. Preferably, s4 is 1 or 2. Preferably, Tl-1 is NH, sl-1 is 1 and sl-2 is 1. Preferably, Tl-1 is O, sl-1 is 1 and si -2 is 0. Preferably, Tl-1 is O, sl-1 is 1 and sl-2 is 1. Especially, HG is a handle group selected from the group consisting of handle group of formula (HG-la), handle group of formula (HG-lb), handle group of formula (HG-Ic), handle group of formula (HG-Id), handle group of formula (HG-II), handle group of formula (HGIII), handle group of formula (HG-IVa), handle group of formula (HG-IVb), handle group of formula (HG-Va), handle group of formula (HG-Vb) and handle group of formula (HG-VI), (**) (HG-VI) wherein (*) is as defined above, also with all its preferred embodiments, (**) is as defined above, also with all its preferred embodiments. More especially, HG is a handle group selected from the group consisting of handle group of formula (HG-Ia), handle group of formula (HG-lb), handle group of formula (HG-Ic), handle group of formula (HG-Id), handle group of formula (HG-IVa), handle group of formula (HGIVb), and handle group of formula (HG-VI). Even more especially, HG is a handle group of formula (HG-Ia), a handle group of formula (HG-IVa) or a handle group of formula (HG-VI). The various handle groups HG are known handle groups or are structurally closely related derivatives of known handle groups. The reaction conditions necessary for cleaving any of these handle groups HG from a peptidyl radical connected to the respective handle group HG, are also known in peptide chemistry. The handle group of formula (HG-Ia) is derived from the Rink amide handle, (HG-lb), (HGIc) and (HG-Id) from benzhydryl handles, (HG-II) from the PAL handle, (HG-III) from the Sieber handle, (HG-IVa) from the HMPA(-Wang) handle, (HG-IVb) from the HMPP(-Wang) handle, and (HG-Va) and (HG-Vb) from allyl handles, (HG-VI) from Ramage handle. Preferably, ml, m5, m6, ml, m9, mlO, ml 1 and ml2 are identical or different and independently from each other 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30; more preferably, ml, m5, m6, m7, m9, mlO, ml 1 and ml2 are identical or different and independently from each other 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 23 or 27; even more preferably ml, m5, m6, m7, mlO, ml 1 and ml2 are identical or different and independently from each other 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10; m9 is 4, 8, 12 or 27; especially ml is 3; m5 is 1 or 2; m6 and m7 are 2; m9 is 4, 8, 12 or 27; mlO is 1; ml 1 is 3. Xaal is preferably selected from the group consisting of non-naturally occurring alpha amino acids, naturally occurring alpha amino acid residues; more preferably selected from the group consisting of naturally occurring alpha amino acid residues, alpha-N-methylamino acid residues, L-Hpr residue, D-Hpr residue, DL-Hpr residue, 2-(Ci -5-alkyl)-D-amino acid residues, 2-(Ci -5-alkyl)-L-amino acid residues, 2- (Ci - -alkyl)-DL-amino acid residue and a residue derived from compound of formula (HypX); wherein X is O, S or C(R13)R14; R5, R7, R12, R13 and R14 are identical or different and independently from each other selected from the group consisting of hydrogen, alkyl and 0-R8; R8 is a protecting group conventionally used for side chain protection in peptide chemistry, or a substituent of formula (Sub-R8); wherein m8 is 1, 2 ,3 ,4 ,5 ,6 ,7 ,8 ,9 or 10; R9 is C alkyl. Preferably, X is C(R13)R14; R5, R7, R12 and R14 are hydrogen; R13 is O-R8; R8 is a protecting group conventionally used for side chain protection in peptide chemistry. The alpha-N-methylamino acid residues is preferably selected from the group consisting of Lalpha- N-methylamino acid residues, D-alpha-N-methylamino acid residues and DLalpha- N-methylamino acid residues; more preferably selected from the group consisting of N-methylglycine residue (sarcosine), LN- methylphenylalanine residue, D-N- methylphenylalanine residue, DL-Nmethylphenylalanine residue, L-N-methylalanine residue, D-N-methylalanine residue, DL-N-methylalanine residue, L-N-methylvaline residue, D-N-methylvaline residue, DL-N-methylvaline residue, L-N-methyltryptophane residue, D-N-methyltryptophane residue, DL-N-methyltryptophane residue. The naturally occurring alpha amino acid residue is preferably selected from the group consisting of Pro residue and Gly residue; more preferably selected from the group consisting of L-Pro residue, D-Pro residue, DL-Pro residue and Gly residue. Preferably, compound of formula (HypX) is derived from L-Hyp, D-Hyp or DL-Hyp, more preferably from L-4Hyp, D-4Hyp or DL-4Hyp. Especially, Xaal is selected from the group consisting of L-N-methylglycine residue, D-Nmethylglycine residue, DL-N-methylglycine residue, L-N-methylphenylalanine residue, D-Nmethylphenylalanine residue, DL-N- methylphenylalanine residue, L-Pro residue, D-Pro residue, DL-Pro residue, side chain protected L-Hyp residue, side chain protected D-Hyp residue, side chain protected DL-Hyp residue, L-Hpr residue, D-Hpr residue and DL-Hpr residue; with Hyp being preferably 4Hyp. More especially, Xaal is L-N-methylphenylalanine residue, D-N- methylphenylalanine residue, DL-N- methylphenylalanine residue, L-Pro residue, D-Pro residue, DL-Pro residue, side chain protected L-Hyp residue, side chain protected D-Hyp residue, side chain protected DL-Hyp residue; with Hyp being preferably 4Hyp. Even more especially, Xaal is D-Pro residue, D-N- methylphenylalanine residue or side chain protected D-Hyp residue; with Hyp being preferably 4Hyp. FG is bonded with SG via the bond (***) in the respective embodiments of SG, when n is 1, or FG is bonded to HG via the bond (**) in the respective embodiments of HG. Preferably, Xaa2 is selected from the group consisting of L-Lys residue, D-Lys residue, DLLys residue, L-Orn residue, D-Orn residue, DL-Orn residue, L-4-aminoproline residue, D-4- aminoproline residue, DL-4-aminoproline residue, L-alpha,gamma-diamino _'butanoic acid residue, D-alpha,gamma-diaminobutanoic acid residue, DL-alpha gamma-diamino butanoic acid residue, L-alpha,beta-diaminopropanoic acid residue, D-alpha,beta-diamino-propanoic acid residue, DL-alpha,beta-diaminopropanoic acid residue, L-Ser residue, D-Ser residue, DL-Ser residue, L-Thr residue, D-Thr residue, DL-Thr residue, L-Cys residue, D-Cys residue, DL-Cys residue, L-homocysteine residue, D-honiocysteine residue, DL-homocysteine residue, L-Asp residue, D-Asp residue, DL-Asp residue, L-Glu residue, D- Glu residue and DL- Glu residue. Preferably, FG is NH2 or OH, more preferably NH2; therefore CG is preferably -N(H)- or -0-, more preferably -N(H)-; therefore Xaa2 is preferably selected accordingly. More preferably, Xaa2 is selected from the group consisting of L-Lys residue, D-Lys residue, DL-Lys residue, L-alpha,beta-diamino-propanoic acid residue, D-alpha,betadiamino^ propanoic acid residue and DL-alpha beta-diamino-p ropanoic acid residue. Even more preferably, Xaa2 is L-Lys residue or L-alpha beta-diamino- propanoic acid residue. A preferred embodiment is the combination, wherein the Xaa2 is an L-alpha amino acid residue and Xaal is a D-alpha amino acid residue, or alternatively Xaa2 is a D- and Xaal is a L-alpha-amino acid residue, with Xaal and Xaa2 as defined above, also with all their preferred embodiments. More preferably, Xaal is selected from the group consisting of L-Pro residue, D-Pro residue, DL-Pro residue, L-N-methylphenylalanine residue, D-N- methylphenylalanine residue and DL-N- methylphenylalanine residue; and Xaa2 is selected from the group consisting of L-Lys residue, D-Lys residue, DL-Lys, L-alpha beta-diamino- propanoic acid residue, D-alpha,beta-diamino-propanoic acid residue and DL-alpha,betadiamino- propanoic acid residue. Even more preferably, Xaal is D-Pro or D-N- methylphenylalanine residue, and Xaa2 is LLys or L-alpha,beta-diamino-propanoic acid residue; or Xaal is L-Pro or L-Nmethylphenylalanine residue, and Xaa2 is D-Lys or D-alpha beta-diamino- p ropanoic acid residue. Especially, Xaa2 is of L- and Xaal is of D-configuration, with Xaal and Xaa2 as defined above, also with all their preferred embodiments. More especially, Xaal is D-Pro or D-N- methylphenylalanine residue, and Xaa2 is L-Lys or L-alpha,beta-diamino-propanoic acid residue. The homogenous solution phase coupling in method(A), i.e. HSPPS, is carried out using conventional process parameters and reagents typical for HSPPS. HSPPS is conventionally done in a solvent and using one or more coupling reagents, and is done preferably in the presence of one or more coupling additives, and preferably in the presence of one or more tertiary bases. Preferable coupling reagents used in HSPPS are phoshonium or uronium salts and carbodiimide coupling reagents. Phosphonium and uronium salts are preferably derivatives of benzotriazol; more preferably Phosphonium and uronium salts are selected from the group consisting of BOP (Benzotriazole-l-yl-oxy-tris-(dimethyl amino)-phosphonium hexafluorophosphate), PyBOP (Benzotriazol- 1-yl-oxy-trispyrrolidinophosphonium hexafluorophosphate), HBTU (0-( 1H-benzotriazole- 1-yl)- 1,1,3,3 -tetramethyluronium hexafluorophosphate), HCTU (0-( 1H-6-chloro-benzotriazole- 1-yl)- 1,1,3,3- tetramethyluronium hexafluorophosphate), TCTU (0-( 1H-6-chlorobenzotriazole- 1-yl)- 1,1,3,3-tetramethyluronium tetrafluoroborate), HATU (0-(7-azabenzotriazol- 1-yl)- 1,1,3 ,3-tetramethyluronium hexafluorophosphate), TATU (0-(7-azabenzotriazol-l-yl)-l,l,3,3-tetramethyluronium tetrafluoroborate), TBTU (0-(benzotriazol-l -yl)-l ,1,3,3-tetra-Tnethyluronium tetrafluoroborate), TOTU (0-[cyano(ethoxycarbonyl)methyleneamino]- 1,1,3,3-tetramethyluronium tetrafluoroborate) , HAPyU (0-(benzotriazol- 1-yl)oxybis-(pyrrolidino)-uronium hexafluorophosphate, PyAOP (Benzotriazole- 1-yl-oxy-tris-pyrrolidino-phosphonium hexafluorophosphate), COMU (l-[(l-(cyano-2-ethoxy-2-oxoethylideneaminooxy)-dimethylaminomorpholinomethylene)] methanaminiumhexafluorophosphate), PyClock (6-chloro-benzotriazole- -yl-oxy-tris-pyrrolidino-phosphonium hexafluorophosphate), PyOxP (O-[(1-cyano-2-ethoxy-2-oxoethylidene)amino] -oxytri(pyrrolidin- 1-yl) phosphonium hexafluorophosphate) and PyOxB (0-[(l -cyano-2-ethoxy-2-oxoethylidene)amino]-oxytri(pyrrolidin- 1-yl) phosphonium tetrafluoroborate) . Carbodiimide coupling reagents are preferably selected from the group consisting of diisopropyl-carbodiimide (DIC), dicyclohexyl-carbodiimide (DCC) and water-soluble carbodiimides (WSCDI) such as l-ethyl-3-(3-dimethylaminopropyl)- ,carbo_,diimide (EDC) Other coupling techniques use pre-formed active esters, such as hydroxysuccinimide (HOSu) and p-nitrophenol (HONp) esters, pre-formed symmetrical anhydrides, non-symmetrical anhydrides such as N-carboxyanhydrides (NCAs) and acid halides, such as acyl fluoride or acyl chloride. Preferred coupling reagents are phoshonium or uronium coupling reagents, especially TCTU, TOTU or PyBop. Preferably, the conjugated acid of said tertiary base used in HSPPS has a pKa value of from 7.5 to 15, more preferably of from 7.5 to 10. Said tertiary base is preferably trialkylamines, such as diisopropylethylamine (DIEA) or triethylamine (TEA), further N,N' alkylanilines, such as N,N-diethylaniline, 2,4,6-tri-Ci.4 alkylpyridines, such as collidine (2,4,6-trimethylpyridine), or N -C alkyl-morpholines, such as N -methylmorpholine, with any C alkyl being identical or different and independently from each other straight or branched C alkyl. A coupling additive is preferably a nucleophilic hydroxy compound capable of forming activated esters, more preferably having an acidic, nucleophilic N-hydroxy function wherein N is imide or is N-acyl or N-aryl substituted triazeno, the triazeno type coupling additive being preferably a N-hydroxy-benzotriazol derivative (or 1-hydroxy-benzotriazol derivative) or a N-hydroxybenzotriazine derivative. Such coupling additives have been described in WO 94/07910 and EP 410 182. Since they also act as scavengers, they are also called scavengers. Preferred coupling additives are selected from the group consisting of N-hydroxy-succinimide (HOSu), 6-Chloro-l-hydroxy-benzotriazole (Cl-HOBt), N-hydroxy- 3,4-dihydro-4-oxo-l,2,3-benzotriazine (HOOBt), 1-hydroxy-7-azabenzotriazole (HOAt), 1- hydroxy-benzotriazole (HOBt) and ethyl 2-cyano-2-hydroxyimino _ ,acetate (CHA). CHA is available under trade name OXYMAPURE®. CHA has proved to be an effective scavenger as racemization is more suppressed compared to benzotriazole-based scavengers. In addition, CHA is less explosive than e.g. HOBt or Cl-HOBt, so that its handling is advantageous, and, as a further advantage, the coupling progress can be visually monitored by a colour change of the reaction mixture. Preferably, HOBt or CHA, more preferably HOBt is used. In a preferred embodiment, the combination of reagents in the HSPPS reaction is selected from the group consisting of TCTU/Cl-HOBt/DIPEA, TOTU/CHA/DIPEA and PyBop/HOBt/DIPEA. As solvent, any inert liquid solvent, which can dissolve the reactants, may be used in HSPPS. Preferred solvents are selected from the group consisting of dimethyl sulfoxide (DMSO), dioxane, tetrahydrofuran (THF), l-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), pyridine, dichloromethane (DCM), dichloroethane (DCE), chloroform, dioxane, tetrahydropyran, ethyl acetate, toluene, acetonitrile and mixtures thereof. More preferred solvents are NMP, DMF and mixtures thereof. Preferably, HSPPS is done at a temperature of from 0 to 50 °C, more preferably of from 5 to 30 °C, even more preferably of from 15 to 25 °C. Preferably, HSPPS is done at atmospheric pressure. Preferably, the reaction time for HSPPS is of from 15 min to 20 h, more preferably of from 30 min to 5 h, even more preferably of from 30 min to 2 h. The term "part" in this description of reaction conditions of HSPPS is meant to be a factor of the parts by weight of the combined peptide material, if not otherwise stated. Preferably, of from 1 to 30 parts, more preferably of from 5 to 10 parts, of solvent are used. Preferably, of from 0.9 to 5 mol equivalents, more preferably of from 1 to 1.5 mol equivalents, of coupling reagent is used, the mol equivalent being based on the mol of reactive C-terminal carboxy groups. Preferably, of from 0.1 to 5 mol equivalents, more preferably of from 0.5 to 1.5 mol equivalents, of coupling additive is used, the mol equivalent being based on the mol of coupling reagent. Preferably, of from 1 to 10 mol equivalents, more preferably of from 2 to 3 mol equivalents, of tertiary base is used, the mol equivalent being based on the mol of coupling reagent. If the N-terminally and C-terminally protected PEP, which was prepared according to method(A), represents the target peptide, preferably the N-terminal protecting group and the C-terminal protecting group and any side chain protecting group are removed after the preparation according to method(A), to provide for the unprotected peptide PEP. This is usually called global deprotection. The global deprotection conditions, which need to be applied, depend on the nature of the chosen PG. Preferably, the involved PGs are selected to allow global deprotection under weak, strong or reductive cleaving conditions, as defined above, depending on the nature of PGs. The C-terminal protecting group of PEP, i.e. the DKP-PG, can be cleaved by conditions applicable for cleaving the respective handle group HG from the peptidyl radical, these conditions are known in peptide chemistry. Usually, the conditions are either reductive, weak or strong cleaving conditions, as defined above. Preferably, the handle group HG is chosen to be cleavable under acidic conditions from the peptidyl radical PEP, and in this case, if the N-terminal protecting group of fragment PEP-N is a basic cleavable type protecting group or a reductively cleavable type protecting group, the N-terminal protecting group and the C-terminal protecting group and any side chain protecting group are removed preferably after the preparation according to method(A) in two steps; but if the N-terminal protecting group of fragment PEP-N is acid type removable protecting group, the N-terminal protecting group and the C-terminal protecting group and any side chain protecting group are removed preferably after the preparation according to method(A) in one step. Any side chain protecting groups are typically retained until the end of the HSPPS. This deprotection reaction can be carried out under conditions applicable for the various side chain protecting groups, which have been used, and these conditions are known in peptide chemistry. In the case that different types of side chain protecting groups are chosen, they may be cleaved successively. Advantageously, the side chain protecting groups are chosen, so that they are cleavable simultaneously, and more advantageously concomitantly with Nterminal protecting group of PEP. Usually, side chain PGs are cleaved by strong, weak or reductive cleaving conditions as defined above. Further subject of the invention is the use (A) of compound of formula (III-H), with the compound of formula (III-H) being as defined above, also with all its preferred embodiments, for the preparation of a peptide PEP; preferably the use (A) of compound of formula (III-H), with the compound of formula (III-H) being as defined above, also with all its preferred embodiments, in homogeneous solution phase peptide synthesis for the preparation of a peptide PEP by a coupling reaction of the compound of formula (III-H) with an N-terminally protected amino acid or with an Nterminally protected PEP-N, with PEP-N being as defined above, also with all its preferred embodiments. Use (A) is an embodiment of the above defined use of C-PEP in HSPPS. Further subject of the invention is a method(B) for the preparation of a compound of formula (III-H), with the compound of formula (III-H) being as defined above, also with all its preferred embodiments, characterized by cleaving a protecting group PGXaaC p from a compound of formula (III-PGXaaC pc ); PGXaaC

PEP-C HG SG (III-PGXaaC(P >) wherein PGXaaC is an N-terminal protecting group conventionally used in SPPS and is selected from the group consisting of basic cleavable type protecting groups, acid cleavable type protecting groups and reductively cleavable type protecting groups; pc, XaaC, PEP-C, HG, n, SG, Xaal and Xaa2 have the same definition as above, also with all their preferred embodiments, in PGXaaC( , the index (pc) defines PGXaaC(p as the N-terminal protecting group of PEP-C; with the proviso, that PGXaaC p is chosen to be of such a cleavable type protecting group, that PGXaaC(p can be cleaved from PEP-C without cleaving PEP-C from HG. Preferably, if PEP-C carries side chain PGs, PGXaaC p is chosen to be of such a cleavable type protecting group, that PGXaaC p can be cleaved from PEP-C without cleaving any side chain PGs from PEP-C. PGXaaC p therefore is the protecting group of the N-terminal amino acid residue of PEP-C, which is an embodiment of N-PG. Method(B) is comprised in above defined method(C-PEP). Compound of formula (III-PGXaaC pc ) is an embodiment of the above defined C-PEP. If PGXaaC is a basic type PG, it is preferably Fmoc. If PGXaaC( is a strong type PG, it is preferably Boc. If PGXaaC(p is a weak type PG, it is preferably Trt. If PGXaaC( ) is a reductive type PG, it is preferably Alloc. PGXaaC p is, depending on its type, cleaved by strong, weak, basic or reductive cleaving conditions, these conditions being as defined above. Further subject of the invention is a method(C) for the preparation of a compound of formula (III-PGXaaC(p ), with the compound of formula (III-PGXaaC (p ) being as defined above, also with all its preferred embodiments, method(C) comprises the consecutive steps a) and b), wherein in step a) a protecting group PG2 is cleaved from a compound of formula (II-PG2) PGXaaC^ ) PEP-C HG SG (II-PG2) PG2 Xaa2 Xaal ResinA wherein in formula (II-PG2) PGXaaC(p , PGXaaC, pc, PEP-C, HG, n, SG, Xaal and Xaa2 have the same definition as above, also with all their preferred embodiments; PG2 is an N-terminal protecting group conventionally used in peptide chemistry and is selected from the group consisting of basic cleavable type protecting groups, acid cleavable type protecting groups and reductively cleavable type protecting groups; the alpha amino group of Xaa2 is protected by PG2, ResinA being a resin used conventionally as solid phase in SPPS; the 1-carboxy group of Xaal is coupled to a functional group of ResinA; to provide the compound of formula (II-H); PGXaaC P PEP-C HG SG (II-H) H Xaa2 Xaal ResinA wherein in formula (II-H) PGXaaC p , PGXaaC, pc, PEP-C, HG, n, SG, Xaal, Xaa2 and ResinA have the same definition as above, also with all their preferred embodiments; the hydrogen H denoted with (2) is a hydrogen of the unprotected alpha amino group of the amino acid residue Xaa2; with the proviso, that PG2 is chosen to be of such a cleavable type protecting group, that PG2 can be cleaved from Xaa2 without cleaving PEP-C from HG; and in step b) the ResinA is cleaved from Xaal by an intra molecular ring formation reaction reaction(INRIFO) between the alpha amino group of Xaa2 and the carboxylic group of Xaal of compound of formula (II-H), reaction(INRIFO) forms a cyclic dipeptide of Xaal and Xaa2, to provide the compound of formula (III-PGXaaC p ); with the proviso, that the connection between ResinA and Xaal is chosen to be cleavable under such cleaving condition, that ResinA can be cleaved from Xaal by said reaction(INRIFO) without cleaving PEP-C from HG. This means, that PG2 is chosen to be of such a cleavable type protecting group and HG is chosen to be cleavable under such cleaving condition different from those cleaving conditions needed to cleave PG2 from Xaa2, that PG2 can be cleaved from Xaa2 without cleaving PEPC from HG; and this means also, that the connection between ResinA and Xaal is chosen, i.e. ResinA is chosen, to be cleavable under such cleaving condition, that ResinA can be cleaved from Xaal by reaction(INRIFO) without cleaving PEP-C from HG. Preferably, if PEP-C carries side chain PGs, PG2 is chosen to be of such a cleavable type protecting group, that PG2 can be cleaved from Xaa2 without cleaving any side chain PGs from PEP-C. Preferably, if PEP-C carries side chain PGs, the connection between ResinA and Xaal is chosen to be of such a cleavable type protecting group, that ResinA can be cleaved from Xaal by reaction(INRIFO) without cleaving any side chain PGs from PEP-C. Compound of formula (II-PG2) is an embodiment of the above defined PEP-C-DKP-LResinA. The dipeptide in e.g. formula (II-H) is one embodiment of the above mentioned DPR, which forms the D P, i.e. the diketopiperazine residue. PGXaaC(pc) and PG2 can be different protecting groups, which are cleaved under different reaction conditions; in this case, deprotection of the N-terminus of PEP-C and deprotection of Xaa2, i.e. method(B) and method(C) are done consecutively. But preferably, PGXaaC pc and PG2 are identical or at least are such different protecting groups, which are cleavable under the same reaction conditions; in this case, deprotection of the N-terminus of PEP-C and deprotection of Xaa2, i.e. cleavage of PGXaaC from the compound of formula (III-PGXaaC (p ), which is method(B), and cleavage of PG2 from the compound of formula (II-PG2), which is step (a) of method(C), can be done simultaneously in one step. PG2 is, depending on its type, cleaved by strong, weak, basic or reductive cleaving conditions, these conditions being as defined above. Preferably, PG2 is selected from the group consisting of Fmoc, Alloc, Boc, Trt, Mtt, Mmt and Ddz. If PG2 is a strong type PG it is preferably Boc. If PG2 is a weak type PG it is preferably Trt. If PG2 is a reductive type PG it is preferably Alloc. If PG2 is a basic type PG it is preferably Fmoc. ResinA is a resin conventionally used as solid phase in SPPS and the bond between ResinA and Xaal can be cleaved under conditions, which do not cleave an amide bond between two amino acid residues of a peptide. Preferably, ResinA is a resin with functional groups, which is conventionally used as solid support in SPPS, the functional groups being N¾ or OH. Preferably, ResinA is coupled to the 1-carboxylic acid group of Xaal by an ester or amide bond. More preferably, ResinA is chosen to be such a resin, that ResinA is coupled to the CI-atom of the carboxy group of Xaal by an ester or amide bond, neither of the bonds being cleavable under basic, weak or reductive cleaving conditions. Preferably, ResinA is selected from the group consisting of hydroxymethylpolystyrene (HMPS) resins, polyethylenglycol (PEG) based resins, resins, wherein PEG is grafted on a resin different from a PEG resin, polystyrene resin, p-benzyloxybenzyl alcohol resins, chloromethyl polystyrene-divinylbenzene resins, poly(vinyl alcohol)-graftpoly( ethylene glycol) (PVA-g-PEG) resins. Resins, wherein PEG is grafted on a resin different from a PEG resin, are preferably PEG grafted on polystyrene resin, on p-benzyloxybenzyl alcohol resin or on chloromethyl polystyrene-divinylbenzene resin. More preferably, ResinA is a HMPS resin or a chloromethyl polystyrene-divinylbenzene resin. HydroxyChemMatrix® resins have a ChemMatrix® support, which is a polyethylene glycol (PEG) support, and are an example for polyethylene glycol based resins. HydroxyTentagel® resins have a Tentagel® support, which is a grafted copolymer consisting of a low cross-linked polystyrene matrix on which polyethylene glycol (PEG) is grafted, and are an example for polystyrene based resins. p-Benzyloxybenzyl alcohol resins are called Wang resins. Chloromethyl polystyrene-divinylbenzene resin are called Merrifield resins. Step (a) and step (b) may require different reaction condition, i.e. step (a) and step (b) can be done consecutively. Preferably, step (a) and step (b) require the same reaction conditions, i.e. step (a) and step (b) are done simultaneously in one step. Preferably, method(B), i.e. cleavage of PGXaaC p from the compound of formula (III-PGXaaC( ), step (a) of method(C), i.e. cleavage of PG2 from the compound of formula (II-PG2), and step (b) of method(C), i.e. the reaction(INRIFO), require the same reaction conditions and therefore can be done simultaneously in one step. Preferably, step (b), the reaction(INRIFO), which cleaves Xaal from the ResinA, is done in a solvent (b). Step (b) preferably is done at conditions, which afford for the alpha amino group of Xaa2 to be in a deprotonated state as an unprotonated amino group, i.e. not to be present as an ammonium ion. More preferably, step (b) is done by the addition of at least one base (b). If step (a) was done in acidic conditions, the pH is preferably neutralised by the addition of a base, preferably a tertiary base, more preferably the tertiary base is one of those used in HSPPS as mentioned above. To induce the reaction(INRIFO) of step (b), a base (b) is added, preferably the base (b) is a secondary amine, more preferably the conjugated acid of said secondary amine has a pKa value of from 5.0 to 15, more preferably of from 7.5 to 10. Said secondary amine is preferably a dialkylamine, more preferably it is selected from the group consisting of dimethylamine, di-n-propylamine, diethylamine, alpha-(p-tolyl)pyrroline, pyrrolidine, alpha-ethylpyrrolidine, alpha-benzylpyrrolidine, alpha-cyclohexylpyrrolidine, morpholine, piperidine, 2-methylpiperidine, N,N-dimethylhydroxylamine and N-Ci-4 alkylanilines, with the C1-4 alkyl in the N-Ci^ alkylanilines being linear or branched and selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl and isobutyl, more preferably said C alkyl being ethyl. As solvent (b), any inert solvent, which can dissolve the reactants, may be used. Preferably, solvent (b) is selected from the group consisting of dimethyl sulfoxide (DMSO), dioxane, tetrahydrofuran (THF), 1-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), pyridine, dichloromethane (DCM), dichloroethane (DCE), chloroform, dioxane, tetrahydropyran, ethyl acetate, toluene, acetonitnle and mixtures thereof. More preferably, solvent (b) is selected from the group consisting of NMP, DMF, THF and mixtures thereof. Preferably, step (b) is done at a temperature of from 0 to 50 °C, more preferably of from 5 30 °C, even more preferably of from 15 to 25 °C. Preferably, step (b) is done at atmospheric pressure. Preferably, the reaction time for step (b) is of from 1 min to 1 h, more preferably of from 1 min to 30 min, even more preferably of from 5 min to 15 min. The term "part" in this description of step (b) is meant to be a factor of the parts by weight of the treated material, which is the compound of formula (II-H), if not otherwise stated. Preferably, of from 5 to 20 parts, more preferably of from 5 to 5 parts of solvent are used. Preferably, the amount of base (b) is of from 30 to 1% by weight, more preferably of from 15 to 2 % by weight, even more preferably of from 10 to 5 % by weight, with the % by weight being based on the total weight of the compound of formula (II-H). Further subject of the invention is a method(D) for the preparation of a compound of formula (II-PG2), with the compound of formula (II-PG2) being as defined above, also with all its preferred embodiments, characterized by the sequential addition of the amino acids of PEP-C of compound of formula (II-PG2), except for the C-terminal amino acid of PEP-C, to a compound of formula (IIXaaC 1 ) by conventional solid phase peptide synthesis SPPS methodology, comprising the necessary and conventional steps of repetitive SPPS cycles such as deprotecting the Nterminus of the C-terminal amino acid attached to the resin, coupling the next amino acid, deprotecting, if more amino acids have to be coupled, the N-terminus of the thus coupled amino acid and so on, starting the SPPS with deprotecting the N-terminus of Xaa( and coupling of the amino acid of the second position from the C-terminus of PEP-C, said amino acid of the second position from the C-terminus of PEP-C having the formula PGXaaC( - XaaC(2 - OH; and, continuing the SPPS, in case that pc is 3 or greater, consecutively with any next amino acid of formula PGXaaC(iippcc) - XaaC(iippc - OH according to the sequence of PEP-C, with iippcc being an integer of from 3 to (pc-1); and ending the SPPS with the addition of the N-terminal amino acid of PEP-C, said N-terminal amino acid having the formula PGXaaC(pc) - XaaC( c - OH; PGXaaC XaaC ) SG n (II-XaaCO) PG2 Xaa2 Xaal ResinA wherein PGXaaC(pc), PGXaaC, XaaC, pc, PEP-C, HG, n, SG, PG2, Xaal, Xaa2 and ResinA have the same definition as above, also with all their preferred embodiments; in PGXaaC(1), PGXaaC(2 and PGXaaC(i pp , the indices (1), (2) and (iippcc) define the respective PGXaaC as the protecting group of the amino group of the respective amino acid residue XaaC of PEP-C; in XaaC( , XaaC(2 and XaaC ipp , the indices (1), (2) and (iippcc) define the respective XaaC as the respective amino acid residue XaaC of PEP-C; with the proviso, that PGXaaC , PGXaaC(2) and any PGXaaC pp are protecting groups different from PG2 and that they are cleavable under reaction conditions different from those needed to cleave PG2 from Xaa2, and with the further proviso, that the reaction conditions used to cleave PGXaaC , the reaction conditions used to cleave the N-terminal protecting group PGXaaC of said second amino acid and the reaction conditions used to cleave any N-terminal protecting group PGXaaC(iippcc) of said next amino acids, do not cleave PG2 from Xaa2; and with the further proviso, that the bond between Xaal and ResinA is of such a type, that is it not cleaved during the SPPS. Preferably, if any XaaC carries side chain PGs, any PGXaaC is chosen to be of such a cleavable type protecting group, that any PGXaaC can be cleaved from its amino acid XaaC without cleaving any side chain PGs from any side chain protected XaaC. We claim: 1. A method(C-PEP) for the preparation of a peptide C-PEP, C-PEP comprises a peptidyl radical PEP-C, the C-terminus of PEP-C is protected by a protecting group DKP-PG, DKP-PG comprises a handle group HG, optionally a spacer group SG, and a diketopiperazine residue DKP; SG is a spacer group conventionally used in peptide chemistry; DKP is a diketopiperazine residue derived from a dipeptide residue DPR; DPR consists of alpha amino acid residues Xaal and Xaa2; Xaal is the C-terminal amino acid residue of DPR; ^ ^ Xaa2 is the N-terminal amino acid residue of DPR, and Xaa2 has a side chain, said side chain is substituted by a functional group FG; Xaal is selected from the group consisting of naturally occurring alpha amino acid residues, alpha-N-methylamino acid residues, L-Hpr residue, D-Hpr residue, DL-Hpr residue, 2-(Ci.s-alkyl)-D-amino acid residues, 2-(Ci.s-alkyl)-L-amino acid residues, 2-(Ct.5- alkyl)-DL-amino acid residue and a residue derived from compound of formula (HypX); R7 X ^ wherein X isO,SorC(R13)R14; R5, R7, R12, R13 and R14 are identical or different and independently from each other selected from the group consisting of hydrogen, CM alkyl and 0-R8; R8 is a protecting group conventionally used for side chain protection in peptide chemistry, or a substituent of formula (Sub-R8); CH2 CHj O R9 (Sub-R8) ^ -^ m8 mi wherein mS is 1,2,3 ,4,5 ,6,7,8 ,9or 10; R9 is Ci.4 alley]; Xaa2 is selected from the group consisting of L-Lys residue, D-Lys residue, DL-Lys residue, L-Orn residue, D-Orn residue, DL-Orn residue, L-4-aminoproline residue, D-4-aminoproline residue, DL-4-aminoproHne residue, L-alpha,gamma-diamino-