Claims:We Claim:
1. A method for the preparation of large scale liquid phase synthesis of Octreotide acetate of formula (I)

which comprises :
(i) coupling of formula (A) with formula (B)

in the presence of coupling agent, additive and solvent to obtain protected octapeptide of formula (C);

(ii) deprotection of protected octapeptide of formula (C) with cleaving agent to give deprotected octapeptide
of formula (D);

(iii) disulphide bond formation of linear peptide of formula (D) in the presence of potassium ferricyanide, base
and solvent or mixture of solvents to obtain crude Octreotide of formula (I-1);

(iv) purification of crude Octreotide of formula (I-1) to obtained Octreotide acetate of formula (I)

2. The process as claimed in claim 1, wherein
In step (i), the coupling agent is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl; the additive is selected from the group consisting of HOBt, HOOBt, HOAt or Oxyma; the coupling solvent is selected from the group consisting of DMF, ACN, DMAC, DMF, THF, EtOAc, DCM or NMP;

In step (ii), the cleavage reagent containing the mixture of TFA, TIPS, water and dithiothreitol;

In step (iii), the cyanide is selected from the group consisting of potassium ferricyanide ; the base is selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonium hydroxide and lithium hydroxide; the solvent is selected from the group consisting of ACN, DMAC, DMF, THF, EtOAc, DCM, NMP, water or their mixtures.

3. Compounds of formula (A4), (A5), (A6), (A7), (A8), (A9), (A10), (A11), (A12), (B), (B4) & (C) having
the following structures:

wherein R is C1-4 branched or unbranched alkyl groups.

4. A method for the preparation of compound of formula (A), which comprises :

(i) coupling of formula (A1) with formula (A2) in presence of coupling agent, additive, base and solvent to obtain formula (A3);



(ii) Z-group deprotection of formula (A3) using hydrogen in presence of metal catalyst and solvents followed by reaction with PTSA salt to obtain formula (A4);


(iii) reaction of formula (A4) with Z-(D)-Trp(Boc)-OH in presence of coupling agent, additive, base and solvent to obtain formula of (A5);

(iv) reaction of compound of formula (A5) with 2-methoxy propene, in presence of aromatic sulphonic acid salt, base and solvent to obtain oxazolidine compound of formula (A6);

(v) Z-group deprotection of formula (A6) using hydrogen in presence of metal catalyst and solvent to obtain formula (A7);

(vi) coupling of formula (A7) with Z-Phe-OH in presence of coupling agent, additive, base and solvent to obtain formula (A8);

(vii) Z-group deprotection of formula (A8) using hydrogen gas in presence metal catalyst and a solvent to obtain formula (A9);

(viii) coupling of formula (A9) with Fmoc-cys(Trt)-OH in presence of coupling agent, additive and solvent
to obtain formula (A10);

(ix) Fmoc- group deprotection of formula (A10) with base and solvent to give formula (A11);

(x) coupling of formula (A11) with Boc-D-Phe-OH in presence of coupling agent, additive, base and solvent
to give formula (A12);

(xi) ester hydrolysis of formula (A12) with base and solvent to obtain formula (A).

5. The process as claimed in claim 4, where in

In step (i), the coupling agent is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl; the additive is selected from the group consisting of HOBt, HOOBt, HOAt or Oxyma; the coupling solvent is selected from the group consisting of ACN, DMAC, DMF, THF, EtOAc, DCM, or NMP; the base is selected from the group consisting of NMM, DIPEA, TEA or Collidine;

In step (ii), the metal catalyst is selected from the group consisting of palladium carbon; the solvent is selected from the group consisting of methanol, ethanol, isopropanol dimethyl formamide, acetonitrile, ethyl acetate, N-methyl pyrrollidone, dimethyl amino pyridine or mixtures thereof.

In step (iii), the coupling agent is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl; the additive is selected from the group consisting of HOBt, HOOBt, HOAt, Oxyma potassium salt of Oxyma; the coupling solvent is selected from the group consisting of NMP, DMF, THF, DCM or EtOAc; the base is selected from the group consisting of NMM, DIPEA, TEA or Collidine;

In step (iv) the base is selected from the group consisting of NMM, TEA, DIPEA or collidine; the solvent is selected from the group consisting of ACN, DMAC, DMF, THF, EtOAc, DCM or NMP; the aromatic sulphonic acid salts is selected from the group consisting of benzene sulphonic acid and p-toluene sulphonic acid.

In step (v), the metal catalyst is selected from the group consisting of palladium; The solvent is selected from the group consisting of methanol, ethanol, isopropanol, EtOAc or combination of protic solvents with DMF, NMP, THF or EtOAc

In step (vi), the coupling agent is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl; the additive is selected from the group consisting of HOBt, HOAt, Oxyma or potassium salt of Oxyma; the coupling solvent is selected from the group consisting of NMP, DMF, THF, DCM or EtOAc; the base is selected from the group consisting of NMM, DIPEA, TEA or Collidine;

In step (vii), the metal catalyst is selected from the group consisting of palladium. The solvent used is selected from the group consisting of methanol, ethanol, isopropanol, DMF, EtOAc or combination of protic solvent with DMF, NMP, THF or EtOAc.

In step (viii), the coupling agent is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl; the additive is selected from the group consisting of HOBt, HOOBt, HOAt or Oxyma; the coupling solvent is selected from the group consisting of ACN, DMAC, DMF, THF, EtOAc, DCM, or NMP; the base is selected from the group consisting of NMM, DIPEA, TEA or Collidine;

In step (ix), the base is selected from the group consisting of sodium carbonate, sodium hydroxide, lithium hydroxide, cyclohexylamine, ethanolamine, piperdine, N-methylpiperzine or tert-butylamine; the solvent is selected from the group consisting of ACN, THF, EtOAc, DMF, NMP, DCM, heptane or DMAC;

In step (x), the coupling agent is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl; the additive is selected from the group consisting of HOBt, HOAt or Oxyma; the solvent is selected from the group consisting of ACN, THF, EtOAc, DMF, NMP, DCM or DMAC; the base is selected from the group consisting of NMM, DIPEA, TEA or Collidine;

In step (xi), the base is selected from the group consisting of sodium hydroxide, potassium hydroxide lithium hydroxide, sodium carbonate, cesium carbonate. The solvent used is selected from the group consisting of methanol, ethanol, isopropanol, acetone or combination of protic solvent with DMF, NMP, THF or EtOAc.

6. A method for the preparation of formula (B), which comprises,
(i) conversion of formula (B1) to its mixed anhydride with isobutyl chloroformate in presence of base followed by reduction with metal borohydrides to obtain formula (B2);

(ii) Z-group deprotection from the compound of formula (B2) using hydrogen in presence of metal catalyst and solvents to obtain formula (B3);

(iii) coupling of formula (B3) with Fmoc-Cys(Trt)-OH in the presence of coupling agent, additive, with base or without base and solvent to obtain formula (B4);

(iv) Fmoc- group deprotection of formula (B4) with base and solvent to obtain formula (B).

7. The process as claimed in claim 6, where in
In step (i), the metal borohydrides is selected from the group consisting of sodium borohydride, lithium borohydride ; the base selected from the group consisting of pyridine, NMM, DIPEA, TEA or Collidine; the solvent is selected from the group consisting of methanol, ethanol, isopropanol or combination of protic solvents with DMF, NMP, THF or EtOAc.

In step (ii), the metal catalyst is selected from the group consisting of palladium; the alcohol is selected from the group consisting of methanol, ethanol and isopropanol;

In step (iii), the coupling agent is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl; the solvent is selected from the group consisting of NMP, DMF, THF, DCM or EtOAc; the base is selected from the group consisting of NMM, DIPEA, TEA or Clonidine;
The additive used in the reaction is selected from the group consisting of HOBt, HOAt, Oxyma or potassium salt of Oxyma.

In step (iv), base is selected from the group consisting of sodium carbonate, sodium hydroxide, lithium hydroxide, cyclohexyl amine, ethanolamine, piperdine, N-methylpiperzine or tertiary butylamine,; the solvent used is selected from the group consisting of ACN, DMAC, DMF, THF, EtOAc, DIPE, heptane, DCM, NMP or combination of either of solvents with n-heptane.

8. A method for the preparation of formula (C), which comprises,
(i) coupling of formula (A) and formula (B)

in presence of coupling agent, additive, and a solvent to obtain formula (C);

9. The method as claimed in claim 9, where in
In step (i) the coupling agent is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl; the additive is selected from the group consisting of HOBt, HOAt or Oxyma; the coupling solvent is selected from the group consisting of ACN, DMAC, DMF, THF, EtOAc, DCM or NMP; the base is selected from the group consisting of NMM, DIPEA, TEA or Collidine.

10. A method for the preparation of formula (A6), which comprises :
(i) reacting formula (A4-1) with Z-(D)-Trp (Boc)-OH in presence of coupling agent, additive, base and
solvent to obtain formula of (A6).

, Description:Field of the Invention
The present invention relates to a process for large scale liquid phase synthesis of Octreotide acetate of formula (I).

The invention also relates to process of preparation of novel intermediates of formula (A), formula (B) and formula (C) used in the synthesis of Octreotide acetate of formula (I).

Back ground of the Invention
Octreotide acetate is a cyclic Octa peptide, it is highly potent and pharmacologically selective analog of Somatostatin. The drug substance is chemically designated as (4R,7S,10S,13R,16S,19R)-10-(4-aminobutyl)-19-[[(2R)-2-amino-3-phenyl-propanoyl]amino]-16-benzyl-N-[(2R,3R)-1,3-dihydroxybutan-2-yl]-7-(1-hydroxyethyl)-13-(1H-indol-3-ylmethyl)-6,9,12,15,18-pentaoxo-1,2-dithia-5,8,11,14,17-pentazacycloicosane-4-carboxamide, and is represented by the structural formula (I).

The structure of Octreotide can also be represented as: H-D-Phe1-Cys-Phe-D-Trp-Lys-Thr-Cys7-Thr-CH2OH and one disulphide bond between Cys2 and Cys7; where H-D-Phe is D-phenyl alanine, Cys is cysteine, Phe is phenyl alanine, D-Trp is D-tryptophan, Lys is lysine, Thr is threonine, Thr-CH2OH is threoninol.
Octreotide acetate synthesis was published in several publications and patents. US6476186, US20040039161A1, WO2013046233, KR991096, CN031022390, CN101863961, Tetrahedron Letter, 1998, 39, 1783 -1784 and Journal of Medicinal Chemistry, 1994, 37, 3749 disclosed the preparation of Octreotide acetate by Solid phase synthesis.
US 6346601B1, WO 2005087794A1, WO 2010089757A2 claimed / disclosed the preparation of Octreotide acetate by Hybrid synthesis.
US4395403, EP0029579, WO2013132505A1, US698716, US20050239695A1, CN1355173 claimed / disclosed the preparation of Octreotide acetate by Liquid phase synthesis.

Drawbacks of the prior art process:
Racemized side product along with main product was expected during saponification of hexapeptide methyl ester in presence of sodium hydroxide. Unprotected indole group of Z-D-Trp-OSu self-assembled to give dimer impurity during coupling reaction between H-Lys(Boc)-OH and Z-D-Trp-OSu. Peptide obtained from activate ester of p-Nitro phenol contaminated with the phenol released during coupling and consequently it makes isolation and purification difficult.
The racemized-impurity and dimer were carried forward to the crude Octreotide (which made the purification process difficult and resulted in a low yield)
Hence, there is a need to develop a large scale liquid phase synthesis for Octreotide acetate, which is simple, eco-friendly, inexpensive, reproducible and well suited for commercial scale.

Summary of the Invention
In one aspect, the present invention provides a process for large scale liquid phase synthesis of Octreotide acetate of formula (I).

which comprises :
i) coupling of formula (A) with formula (B)

in the presence of coupling agent, additive and solvent to obtain protected Octapeptide of formula (C);

ii) deprotection of protected Octapeptide of formula (C) with cleaving agent to give linear Octapeptide of
formula (D);

iii) disulphide bond formation of linear Octapeptide of formula (D) in presence of potassium ferricyanide,
base and solvent or mixture of solvents to obtain crude Octreotide of formula (I-1);

iv) purification of crude Octreotide of formula (I-1) by preparative reversed phase high performance liquid
chromatography (Prep-RP-HPLC) to obtain pure Octreotide acetate of formula (I).

In another aspect, the invention provides a novel intermediates of formula (A), (B), (C) which are useful in the preparation of Octreotide acetate of formula (I).

In another aspect, the invention also provides a process for the preparation of novel intermediate of formula (A6).

which comprises :
(i) coupling of formula (A1) with formula (A2) in presence of coupling agent, additive, base and solvent to obtain formula (A3);


wherein, R is C1-4 branched and unbranched alkyl groups.
(ii) Z-group deprotection of formula (A3) using hydrogen in presence of metal catalyst and alcohol followed
by reaction with PTSA salt to obtain formula (A4);

(iii) reaction of formula (A4) with Z-(D)-Trp (Boc)-OH in presence of coupling agent, additive, base and
solvent to obtain formula of (A5);

(iv) reaction of compound of formula (A5) with 2-methoxy propene, in presence of aromatic sulphonic acid
salt, base and solvent to obtain oxazolidine compound of formula (A6);

In another aspect, the invention also provides an alternative process for the preparation of formula (A6),
which comprises:
Reaction of formula (A4-1) with Z-(D)-Trp (Boc)-OH in presence of coupling agent, additive, base and solvent to obtain formula of (A6);

In another aspect, the invention provides a process for the preparation of novel intermediate of formula (A).

(v) Z-group deprotection of formula (A6) using hydrogen gas in presence of metal catalyst and solvent to
obtain formula (A7);

(vi) coupling of formula (A7) with Z-Phe-OH in presence of coupling agent, additive, base and solvent to obtain
formula (A8);

(vii) Z-group deprotection of formula (A8) using hydrogen gas in presence of metal catalyst and solvent to
obtain formula (A9);

(viii) coupling of formula (A9) with Fmoc-cys(Trt)-OH in presence of coupling agent, additive and
solvent to obtain formula (A10);

(ix) Fmoc- group deprotection of formula (A10) with base and solvent to give formula (A11);

(x) coupling of formula (A11) with Boc-D-Phe-OH in presence of coupling agent, additive, base and solvent
to give formula (A12);

(xi) ester hydrolysis of formula (A12) with base and solvent to obtain formula (A); wherein R is C1-4 branched
or unbranched alkyl groups.

In another aspect, the invention provides a process for the preparation of novel intermediate of formula (B), which is useful in the preparation of Octreotide acetate of formula (I).

which comprises :
(i) conversion of formula (B1) to its mixed anhydride with isobutyl chloroformate in presence of base followed
by reduction with metal borohydrides to obtain formula (B2);

(ii) Z-group deprotection from the compound of formula (B2) using hydrogen in presence of metal catalyst and solvents to obtain formula (B3);

(iii) coupling of formula (B3) with Fmoc-Cys(Trt)-OH in the presence of coupling agent, additive, with base or without base and solvent to obtain formula (B4);

(iv) Fmoc- group deprotection of formula (B4) with base and solvent to obtain formula (B).
In another aspect, the invention also provides a process for the preparation of novel intermediate of formula (C).

which comprises, coupling of formula (A) and formula (B)

in presence of coupling agent, additive and solvent to obtain formula (C).

Detailed description of the Invention
Accordingly, the present invention provides a process for the preparation of formula (I).
Scheme-1 illustrates the process for the preparation of formula (I).

Scheme-1: Synthetic scheme for the preparation of Octreotide Acetate of Formula (I)
Step (i): coupling of formula (A) with formula (B) in the presence of coupling agent, additive and solvent to obtain protected Octapeptide of formula (C).
The coupling agent used in the reaction is selected from the group consisting of Benzotriazole-1-yl-oxy-tris-(dimethylamino)-phosphonium hexafluorophosphate (BOP), Benzotriazole-1-yl-oxy-tris-pyrrolidino-phosphonium hexafluorophosphate (PyBOP), O-(IH-Benzotriazol-1-yl)-N,N,N,N- tetramethyluronium tetra fluoroborate (TBTU), O-(7-Azabenzotriazole-1-yl)-N,N,N,N-tetramethyluronium tetra fluoroborate (TATU), O-(1H-Benzotriazole-1-yl)-N, N, N, N-tetramethyluronium hexafluorophosphate (HBTU), 2-(7-Aza-1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU), N,N-dicyclohexylcarbodiimide (DCC), N,N'-Diisopropylcarbodiimide (DIC) or 1-Ethyl-3-(3-dimethyllaminopropyl)carbodiimide hydrochloride (EDC.HCl), preferably using EDC.HCl. The additive used in the reaction is selected from the group consisting of 1-Hydroxybenzotriazole (HOBt), Hydroxy-3,4-dihydro-4-oxo-1,2,3-benzotriazine (HOOBt), 1-Hydroxy-7-azabenzotriazole (HOAt) or Oxyma, preferably using HOBt. The base used in the reaction is selected form the group consisting of N-methylmorpholine (NMM), N,N-diisopropylethylamine (DIPEA), triethylamine or collidine, preferably using NMM. The solvent used in the reaction is selected from the group consisting of acetonitrile (ACN), tetrahydrofuran (THF), ethyl acetate (EtOAc), N,N-Dimethylformamide (DMF), N-Methylpyrrolidone (NMP), dichloromethane (DCM) or dimethyl acetamide (DMAC), preferably using DMF.

Step (ii): deprotection of protected octapeptide of formula (C) with cleavage agent to obtain deprotected peptide of formula (D);
The cleavage reagent containing the mixture of trifluoroacetic acid (TFA), triisopropylsilane (TIPS), water and dithiothreotol.

Step (iii): disulphide bond formation of linear peptide of formula (D) in the presence of potassium ferricyanide, base and solvent or mixture of solvents to obtain crude Octreotide of formula (I-1).
The solvent or combinations of solvents used in the reaction is selected from the group consisting of water, THF, DMF, acetonitrile NMP or DMAc, preferably using water and acetonitrile. The based used in the reaction is selected from sodium carbonate, sodium hydroxide, lithium hydroxide, and ammonium hydroxide. The reaction temperature may range from 20 °C to 40 °C and preferably at a temperature in the range of 25 °C to 35 °C.

Step (iv): purification of crude Octreotide of formula (I-1) to obtained Octreotide acetate of formula (I).
Crude octreotide purified with C-18 reversed phase column pre- coated with proprietary SSP, and purified using 0.1% acetic acid in water (Buffer A), and 0.1% acetic acid in acetonitrile (Buffer B).

In another aspect, the invention provides a novel intermediate of formula (A), which is useful for the preparation of Octreotide acetate of formula (I).

Where in R is C1-4 branched or unbranched alkyl groups.
Scheme-2: Synthetic scheme for preparation of Intermediate A

Step (i): coupling of formula (A1) and formula (A2) in the presence of coupling agent and additive, base and a solvent to obtain formula (A3).
The coupling agent used in the reaction is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl, preferably using TBTU. The solvent used in the reaction is selected from the group consisting of NMP, DMF, THF, DCM or EtOAc, preferably using DMF. The reaction is maintained at ambient temperature. The additive used in the reaction is selected from the group consisting of HOBt, HOAt, Oxyma or potassium salt of Oxyma, preferably using HOBt. The base used in the reaction is NMM, TEA, DIPEA or Collidine, preferably using NMM.

Step (ii): Z-group deprotection of formula (A3) using hydrogen gas in presence of metal catalyst, acid and solvent to obtain formula (A4).
The metal catalyst used in the reaction selected from the group consisting of palladium. The solvents used in the reaction is selected from the group consisting of methanol, ethanol, isopropanol, DMF, NMP, EtOAc or combination of alcoholic solvent with ACN, DMF, NMP, EtOAc or DMAc, preferably using methanol.

Step (iii): reaction of formula (A4) with Z-(D)-Trp(Boc)-OH in presence of coupling agent, additive, base and solvent to obtain formula of (A5).
The coupling agent used in the reaction is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl, preferably using TBTU. The solvent used in the reaction is selected from the group consisting of NMP, DMF, THF, DCM or EtOAc, preferably using DMF. The reaction is maintained at ambient temperature. The additive used in the reaction is selected from the group consisting of HOBt, HOOBt, HOAt, Oxyma or potassium salt of Oxyma, preferably using HOBt. The base used in the reaction is NMM, TEA, DIPEA or Collidine, preferably using NMM.

Step (iv): Oxazolidine ring formation on the compound of formula (A5) with 2-methoxy propene, in presence of aromatic sulphonic acid salt, base and solvent to obtain formula (A6).
The aromatic sulphonic acids salts used in the reaction is selected from benzene sulphonic acid & para- toluene sulfonic acid. The base used in the reaction is selected from the group consisting of NMM, TEA, DIPEA or collidine, preferably using triethyl amine. The solvent used in the reaction is selected from the group consisting of ACN, DMAC, DMF, THF, EtOAc, DCM or NMP.

Step (v) : Z- group deprotection of formula (A6) using hydrogen in presence of metal catalyst and solvent to obtain formula (A7).
The solvent used in the reaction is selected from the group consisting of methanol, ethanol, isopropanol, EtOAc or combination of protic solvents with DMF, NMP, THF or EtOAc, preferably using methanol. The hydrogenation catalyst used in the reaction is Pd/C.

Step (vi): coupling of formula (A7) with Z-Phe-OH in presence of coupling agent, additive, base and a solvent to obtain formula (A8).
The coupling agent used in the reaction is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl, preferably using TBTU. The solvent used in the reaction is selected from the group consisting of NMP, DMF, THF, DCM or EtOAc, preferably using DMF. The reaction is maintained at ambient temperature. The additive used in the reaction is selected from the group consisting of HOBt, HOAt, Oxyma or potassium salt of Oxyma, preferably using HOBt. The base used in the reaction is NMM, TEA, DIPEA or Collidine, preferably using NMM.

Step (vii): Z- group deprotection from the compound of formula (A8) using hydrogen in presence metal catalyst and solvents to obtain formula (A9).
The solvent used in the reaction is selected from the group consisting of methanol, ethanol, isopropanol, DMF, EtOAc or combination of protic solvent with DMF, NMP, THF or EtOAc, preferably using methanol. The hydrogenation catalyst used in the reaction is Pd/C.

Step (viii): coupling of formula (A9) with Fmoc-cys(Trt)-OH in presence of coupling agent, additive, base or without base and solvent to obtain formula (A10).
The coupling agent used in the reaction is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl. Preferably using EDC.HCl. The additive used in the reaction is selected from the group consisting of HOBt, HOOBt, HOAt or Oxyma, preferably using HOBt. The base used in the reaction is selected from the group consisting of NMM, DIPEA, triethylamine or collidine, preferably using NMM. The solvent used in the reaction is selected from the group consisting of ACN, THF, EtOAc, DMF, NMP, DCM or DMAC. preferably using DMF.

Step (ix): Fmoc-group deprotection of formula (A10) with base and solvent to give formula (A11).
Base used in the reaction is selected from the group consisting of sodium carbonate, sodium hydroxide, lithium hydroxide, cyclohexylamine, ethanolamine, piperdine, N-methylpiperzine or tert-butylamine, preferably using tert-butyl amine.
The solvent used in the reaction is selected from the group consisting of ACN, THF, EtOAc, DMF, NMP, DCM, heptane or DMAC, preferably using acetonitrile.
Step (x) : coupling of formula (A11) with Boc-D-Phe-OH in present of coupling agent, additive, base and solvent to give formula (A12).
The coupling agent used in the reaction is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl. Preferably using TBTU. The additive used in the reaction is selected from the group consisting of HOBt, HOAt or Oxyma, preferably using HOBt. The base used in the reaction is selected form the group consisting of NMM, DIPEA, triethylamine or collidine, preferably using NMM. The solvent used in the reaction is selected from the group consisting of ACN, THF, EtOAc, DMF, NMP, DCM or DMAC, preferably using DMF.

Step (xi): ester hydrolysis of formula (A12) with base and solvent to obtain formula (A).
The base used in the reaction is selected from the group consisting of alkali base, sodium hydroxide (NaOH), sodium carbonate (Na2CO3), cesium carbonate (CS2CO3), lithium hydroxide (LiOH), preferably using LiOH or LiOH.H2O. The solvent used in the reaction is selected from the group consisting of methanol, ethanol, isopropanol, acetone or combination of protic solvent with DMF, NMP, THF or EtOAc, preferably using methanol.
In yet another aspect, the invention provides a novel intermediate of formula (B) which is useful for the preparation of Octreotide acetate of formula (I).

Scheme-3: Synthetic scheme for the preparation of Intermediate formula (B)

Step (i): conversion of formula (B1) to its mixed anhydride with isobutyl chloroformate in presence of base followed by reduction with metal borohydrides to obtain formula (B2).
The alkali metal hydrides used in the reaction selected from the group consisting of sodium borohydride, lithium borohydride, preferably using sodium borohydride.
The base used in the reaction is selected form the group consisting of NMM, DIPEA, TEA or collidine, preferably using NMM.
The solvent used in the reaction is selected from the group consisting of methanol, ethanol, isopropanol or combination of protic solvents with DMF, NMP, THF or EtOAc, preferably using THF-methanol.

Step (ii): Z-group deprotection of formula (B2) using hydrogen gas in presence of metal catalyst and solvents to obtain formula (B3).
The solvent used in the reaction is selected from the group consisting of methanol, ethanol, isopropanol or combination of protic solvent with DMF, NMP, THF or EtOAc, preferably using isopropanol. The hydrogenation catalyst used in the reaction is Pd/C.

Step (iii): coupling of formula (B3) with Fmoc-Cys(Trt)-OH in the presence of coupling agent and additive, base or without base and a solvent to obtain formula (B4).
The coupling agent used in the reaction is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl, preferably using EDC.HCl. The solvent used in the reaction is selected from the group consisting of NMP, DMF, THF, DCM or EtOAc, preferably using DMF. the base is selected from the group consisting of NMM, DIPEA, TEA or Clonidine; The additive used in the reaction is selected from the group consisting of HOBt, HOAt, Oxyma or potassium salt of Oxyma, preferably using HOBt.

Step (iv) : deprotection of Fmoc group of formula (B4) with base and solvent to obtain formula (B).
Base used in the reaction is selected from the group consisting of sodium carbonate, sodium hydroxide, lithium hydroxide, cyclohexyl amine, ethanolamine, piperdine, N-methylpiperzine or tertiary butylamine, preferably using sodium hydroxide.
The solvent used in the reaction is selected from the group consisting of ACN, THF, EtOAc, DMF, NMP, DCM or DMAC, preferably using acetonitrile.

In yet another aspect, the invention provides a novel intermediate of formula (C), which is useful for the preparation of Octreotide acetate of formula (I).

Scheme-4: Synthetic scheme for the preparation of Intermediate formula (C)

Step (i) : coupling of formula (A) with formula (B) in the presence of coupling agent, additive, and a solvent to obtain formula (C).
The coupling agent used in the reaction is selected from the group consisting of BOP, PyBOP, TBTU, TATU, HBTU, HATU, DCC, DIC or EDC.HCl, preferably using EDC.HCl. The solvent used in the reaction is selected from the group consisting of NMP, DMF, THF, DCM or EtOAc, preferably using DMF. The additive used in the reaction is selected from the group consisting of HOBt, HOAt, Oxyma or potassium salt of Oxyma, preferably using HOBt.

Example 1: Preparation of Boc-D-Phe-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OH (A) :

Step (i): Preparation of Z-Lys(BOC)-Thr-OMe (A3)
Z-Lys(Boc)-OH (A1) (62 grams, 162.9 mMol) was dissolved in DMF (310 mL) and cooled to 0-5 °C. Next, HOBt (37.4, 244.0 mMol) and TBTU (57.5, 179.0 mMol) and NMM (35.8 mL, 244.0 mMol) were added. The reaction was stirred for 15 minutes. A mixture of solution H-Thr-OMe. HCl (A2) (41.4 grams, 244.0 mMol), DMF (310 mL) and NMM (26.8 mL, 288.0 mMol) was added dropwise. The reaction was maintained for 2-4 hours. After completion of reaction it was quenched with 5% NaHCO3 solution and maintained for 3-5 hours. Precipitated solid was filtered and wash with water and heptane. Further wet compound was purified with DIPE (310 mL). Finally product was dried under vacuum at 40 °C to obtain formula (A3), (66.5 grams, 134.5 mMol).
Yield: 82.5%; Purity: RP -HPLC: 98.6%; [M+H]+) = 495 amu.

Step (ii): Preparation of H-Lys(Boc)-Thr-OMe. PTSA (A4)
Z-Lys(Boc)-Thr-OMe (60.0 grams, 121.0 mMol) was dissolved in methanol (600 mL). The solution was hydrogenated in the presence of 5% Pd/C (6 grams) and p-Toluene sulfonic acid (PTSA, 23.0 grams, 140.9 mM). The reaction was continued for 3 hours at 25-30 °C and progress of the reaction was monitored by HPLC. After completion of reaction the catalyst was filtered through a celite bed, and the celite bed was washed with methanol (140 mL). The solvent was removed under reduced pressure below 40 °C, and co-distilled with DIPE and heptane. The obtained crude was dried under reduced pressure at 25-30 °C to obtain formula (A2), (64.0 grams, 119.9 mM)
Yield: 99.0%; [M+H]+ = 362 amu.

Step (iii): Preparation of Z-D-Trp(Boc)-Lys(Boc)-Thr-OMe (A5)
Z-D-Trp(Boc)-OH (39.0 grams, 88.9 mMol) was dissolved in DMF (195 mL) and cooled to 0-5 °C. Next, HOBt (18.0, 133.5 mMol) and TBTU (31.4, 97.9 mMol) and NMM (24 mL, 218.2 mMol) were added. The reaction was stirred for 15 minutes. A DMF (195 mL) solution of H-Lys(Boc)-Thr-OMe. HCl (54.0 grams, 101.2 mMol) and NMM (10.0 mL, 90.9 mMol) was added dropwise. The reaction was maintained for 2-4 hours. After completion of reaction it was quenched with 2.5% NaHCO3 solution and maintained for 3-5 hours. Precipitated solid was filtered and wash with water and heptane. Further wet compound was purified with acetonitrile (600 mL). Finally product was dried under vacuum at 40 °C to obtain formula (A5), (50 grams, 63.9 mMol).
Yield: 72.0%, Purity by RP -HPLC: 97.6%; [M+H]+ = 781 amu

Alternative process for step (iii) : Preparation of Z-D-Trp(Boc)-Lys(Boc)-Thr-OMe (A5)
Z-Lys(Boc)-Thr-OMe (70.0 grams, 141.3 mMol) was dissolved in DMF (700 mL). The solution was hydrogenated in the presence of 5% Pd/C (7 grams) and p-Toluenesulfonic acid (PTSA, 26.8 grams, 140.9 mM). The reaction was continued for 3 hours at 25-30 °C and progress of the reaction was monitored by HPLC. After completion of reaction the catalyst was filtered through a celite bed, and the celite bed was washed with DMF (140 mL). The combined DMF filtrate used in next step directly without isolation. Z-D-Trp(Boc)-OH (68.1 grams, 155.4 mMol) was dissolved in DMF (210 mL) and cooled to 0-5 °C. Next, HOBt (32.4, 211.9 mMol) and TBTU (54.4, 169.5 mMol) and NMM (31.7 mL, 288.2 mMol) were added. The reaction was stirred for 15 minutes. The above DMF solution of H-Lys(Boc)-Thr-OMe.PTSA and NMM (27.7 mL, 251.8 mMol) was added dropwise at 0-5 °C. The reaction temperature was allowed to raise to ambient temperature and maintained for 2-4 hours. After completion of reaction it was quenched with 2.5% NaHCO3 solution and maintained for 3-5 hours. Precipitated solid was filtered and wash with water and heptane. Further wet compound was purified with IPA and n-heptane and followed by methanol DIPE. Finally product was dried under vacuum at 40 °C to obtain formula (A5), (91.5 grams, 117.0 mMol)
Yield: 82.8%, Purity by RP -HPLC: 96.2%; [M+H]+ = 781 amu

Step (iv): Preparation of Z-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OMe (A6)
Z-D-Trp(Boc)-Lys(Boc)-Thr-OMe (89 grams, 113.8 mMol) was dissolved in mixture of solvents of THF (667.5 mL) and EtOAc (133.5 mL). Next, a salt of benzene sulphonic acid (3.6 grams, 22.8 mMol) and pyridine (1.1 mL, 13.6 mMol) in EtOAc (89 mL) was added and the reaction was maintained for 10-15 minutes. An EtOAc solution of 2-methoxy propene (59.3 mL, 595.3 mMol) was added dropwise and the reaction was heated to 45-50 °C. The reaction was maintained for 2-4 hours and progress of the reaction was monitored by HPLC. After completion of reaction it was cooled and neutralized with TEA (1.0 mL, 9.1 mMol) and water. The compound was extracted with EtOAc and the EtOAc layer was washed with aq. 5% NaHCO3 solution, water, aqueous. 3% citric acid solution, and saturated brine solution. The solvent was removed under reduced pressure to obtain the solid. Finally product was dried under vacuum at 40 °C to obtain formula (A6), (90.5 grams, 110.1 mMol)
Yield: 96.8%, Purity by RP -HPLC: 94.6%; [M+H]+ = 821 amu

Alternative process for step (iv) : Preparation of Z-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OMe (A6)
Z-D-Trp(Boc)-OH (68.1 grams, 155.4 mMol) was dissolved in DMF (210 mL) and cooled to 0-5 °C. Next, HOBt (32.4, 211.9 mMol) and TBTU (54.4, 169.5 mMol) and NMM (31.7 mL, 288.2 mMol) were added. The reaction was stirred for 15 minutes. The above DMF solution of H-Lys(Boc)-Thr(oxa)-OMe.PTSA (89.1 grams, 155.4 mMol) and NMM (27.7 mL, 251.8 mMol) was added dropwise at 0-5 °C. The reaction temperature was allowed to raise to ambient temperature and maintained for 2-4 hours. After completion of reaction it was quenched with 2.5% NaHCO3 solution and maintained for 3-5 hours. Precipitated solid was filtered and wash with water and heptane. Finally product was dried under vacuum at 40 °C to obtain formula (A6), (89.4 grams, 108.8 mMol)
Yield: 70.0%, Purity by RP -HPLC: 92.3%; [M+H]+ = 821 amu

Step (v): Preparation of H-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OMe (A7)
Z-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OMe (87.5 grams, 106.4 mM) was dissolved in methanol (1750 mL). The solution was hydrogenated in the presence of 5% Pd/C (13.1 grams). The reaction was continued for 4-5 hours at 25-30 °C and progress of the reaction was monitored by HPLC. After completion of reaction the catalyst was filtered through a celite bed, and the celite bed was washed with methanol (175 mL). The solvent was removed under reduced pressure below 40 °C, and co-distilled with 175 mL of heptane. The obtained solid was dried under reduced pressure at 35-40 °C to obtain formula (A7), (68 grams, 98.9 mM)
Yield: 92.9%; Purity by RP -HPLC: 93.2%; [M+H]+ = 687 amu.

Step (vi): Preparation of Z-Phe-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OMe (A8)
Z-Phe-OH (30.2 grams, 100.9 mMol) was dissolved in DMF (396 mL) and cooled to 0-5°C. Next, HOBt (22.0 grams, 144.1 mMol) and TBTU (33.9 grams, 105.7 mMol) and NMM (26.4 mL, 240.1 mMol) were added. The reaction was stirred for 15 minutes. A DMF (264 mL) solution of H-Trp(Boc)Lys(Boc)-Thr(oxa)-OMe (66 g, 96.0 mMol) was added dropwise. The reaction was maintained for 1-2 hours and progress of the reaction was monitored by HPLC. After completion of reaction it was quenched with 5% NaHCO3 solution and maintained for 1-2 hours. Precipitated solid was filtered and wash with water and heptane. Further wet compound was purified with the mixture solvents (5% EtOAc in DIPE, 660 mL). Finally product was dried under vacuum at 40 °C to obtain formula (A8), (76 grams, 78.4 mMol).
Yield: 81.6%, Purity by RP -HPLC: 95.1%; [M+H]+) @ 969 amu

Step(vii) : Preparation of H-Phe-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OMe (A9)
Z-Phe-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OMe (70.0 grams, 72.2 mM) was dissolved in methanol (700 mL). The solution was hydrogenated in the presence of 5% Pd/C (7 grams). The reaction was continued for 4-5 hours at 25-30 °C and progress of the reaction was monitored by HPLC. After completion of reaction the catalyst was filtered through a celite bed, and the celite bed was washed with methanol (140 mL). The solvent was removed under reduced pressure below 40 °C, and co-distilled with 400 ml of heptane. The obtained solid was dried under reduced pressure at 35-40 °C to obtain formula (A9), (58 grams, 69.4 mM)
Yield: 96.1 %; Purity by RP -HPLC: 94.9%; [M+H]+ @ 835 amu

Step (viii): Preparation of Fmoc-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-OMe (A10)
Fmoc-Cys(Trt)-OH (39.9 grams; 68.2 mM) was dissolved in DMF (360 mL), and the solution was cooled to 0 to 5 °C. Next, HOBt (11.0 grams, 71.8 mM) and H-Phe-D-Trp(Boc)-Lys(Boc)-Thr(Oxa)-OMe (60 grams, 71.8 mM) were added to the above solution at 0 to 5 °C, and the stirring was continued for 5 minutes. To this solution, DMF (240 mL) solution of EDC.HCl (23.4 grams, 122.2 mM; corrected for assay) was added while maintaining the temperature at 0 to 5 °C. The reaction mixture was stirred for about 30 minutes while allowing the reaction mixture to warm to 25 to 30 °C. The progress of the reaction was monitored by RP-HPLC. After the reaction was complete, it was quenched with 2.5% aqueous NaHCO3 solution, and the solid separated out. The solid was filtered and washed with water and heptane. Wet solid was further purified with heptane (600 mL) and dried under reduced pressure below 40-45 °C to obtain formula (A10), (97 grams, 69.1 mM)
Yield: 96.3%; Purity RP -HPLC: 95.4%; [M+H]+ = 1402 amu

Step (ix): Preparation of H-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-OMe (A11)
Fmoc-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-OMe (96.0 grams; 68.4 mM) was dissolved in acetonitrile (1440 mL) and cooled to 0 -5 °C. It was treated with tertiary butyl amine (124.9 mL, 1231.8 mM). Next, heptane (480 mL) was added to the reaction mixture. The temperature was raised to ambient temperature and maintained the reaction for 18 -24 hours. The progress of reaction was monitored by HPLC. After completion of the reaction, the undissolved material was filtered and cooled to 0 to 5 °C. The pH of the reaction mixture was adjusted to 3 to 4 using 10% KHSO4 solution and separated aqueous layer. The aqueous layer washed with mixture of 30% DIPE in heptane and the organic extracts were discarded. The product was extracted from aqueous layer with mixture of 50% DIPE in EtOAc. The organic layer containing product was washed with brine, water, and dried with anhydrous Na2SO4. The solvent was removed under reduced pressure, and the residue was co-distilled with hexane under reduced pressure to obtain the crude formula (A11), (70 grams, 59.3 mM).
Yield: 86.6%; Purity RP -HPLC: 94.1%; [M+H]+ = 1180 amu

Step (x) : Preparation of Boc-D-Phe-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-OMe (A12)
Boc-D-Phe-OH (16.1 grams, 60.6 mMol) was dissolved in DMF (325 mL) and cooled to 0-5 °C. Next, HOBt (12.6 grams, 82.6 mMol) and TBTU (19.5 grams, 60.6 mMol) and NMM (15.1 mL, 137.6 mMol) were added. The reaction was stirred for 15 minutes. A DMF (325 mL) solution of H-Cys(Trt)-Phe-Trp(Boc)Lys(Boc)-Thr(oxa)-OMe. HCl (65.0 grams, 55.0 mMol) was added dropwise. The reaction was maintained for 2-3 hours and progress of the reaction was monitored by HPLC. After completion of reaction it was quenched with 2.5% NaHCO3 solution and maintained for 1-2 hours. Precipitated solid was filtered and wash with water and heptane. Further wet compound was purified with the mixture solvents (5% DIPE in heptane, 650 mL). Finally product was dried under vacuum at 40 °C to obtain formula (A12), (69.5 grams, 48.7 mMol)
Yield: 88.5%, Purity by RP -HPLC: 94.1%; [M+H]+ =1427 amu.

Step (xi): Preparation of Boc-D-Phe-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-OH (A)
Boc-D-Phe-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-OMe 52.0 grams; 36.4 mM) was dissolved in isopropanol (780 mL) at 25-30 °C, and cooled to 0-5 °C. 0.5 N aqueous lithium hydroxide solution (312 mL, 72.6 mM) was added to the above reaction mass at 0-5 °C over a period of 20 -30 minutes. The reaction was stirred for 1 -2 hours at 0-5 °C and the progress of reaction was monitored by analytical RP-HPLC. The pH of the reaction was adjusted to 3-4 with 10% KHSO4 solution at 0-5 °C and solid was precipitated out. The solid was washed with water and heptane. Further solid was purified using 5% DIPE in heptane and dried to obtain formula (A), (49 grams, 34.6 mM).
Yield: 95.0%; Purity RP -HPLC: 90.9%; [M+H]+ : 1413 amu.

Example-2: Preparation of H-Cys(Trt)-Thr(tBu)-CH2OH (B)
Step (i): Preparation of Z-Thr(tBu)-CH2OH (B2)
Z-Thr(tBu)-OH (50.4 grams, 162.8 mMol) was dissolved in THF (302 mL) at 25-30 °C and cooled to -25° ± 5 °C. Next, NMM (21.5 mL, 195.5 mMol) followed by isobutyl chloroformate (IBCF, 26.7 grams, 195.4 mMol) were added dropwise at same temperature and the reaction was stirred for 50 minutes. Sodium borohydride (18.6 grams, 489.4 mMol) was added portion wise at -25° ± 5 °C and the reaction was stirred for 30 min. Methanol (127.5 mL) was added dropwise at -25° ± 5 °C and the reaction was stirred for 2 hours. The reaction temperature was raised to 25° ± 5 °C and progress of reaction was monitored by HPLC. The reaction was quenched with 25% aqueous ammonium chloride and EtOAc (756 mL). Two layers were separated and EtOAc layer was washed with 5% aqueous NaHCO3, water, 3% aqueous KHSO4, and saturated brine solution. The solvent was removed under reduces pressure, and the residue was co-distilled with heptane under reduced pressure and dried to obtain formula (B2), (44.3 grams, 149.9 mMol)
Yield: 92.0%; Purity RP-HPLC 99.5%; [M+H]+ = 295 amu.

Step (ii): Preparation of H-Thr(tBu)-CH2OH (B3)
Z-Thr(tBu)-CH2OH (16.0 grams, 54.1 mM) was dissolved in isopropanol (192 mL). The solution was hydrogenated in the presence of 5% Pd/C (2.4 grams). The reaction was continued for 4-5 hours at 25-30 °C and progress of the reaction was monitored by HPLC. After completion of reaction the catalyst was filtered through a celite bed, and the celite bed was washed with isopropanol (80 mL). The solvent was removed under reduced pressure below 40 °C, and co-distilled with 160 ml of heptane. The obtained crude was dried under reduced pressure at 35-40 °C to obtain formula (B3), (7.8 grams, 48.3 mM)
Yield: 89.2%; [M+H]+ @ 161 amu.

Step (iii): Preparation of Fmoc-Cys(Trt)-Thr(tBu)-CH2OH (B4)
Fmoc-Cys(Trt)-OH (11 grams; 18.8 mM) was dissolved in DMF (60 mL), and the solution was cooled to 0 to 5 °C. Next, HOBt (4.3 grams, 28.1 mM) and H-Thr(tBu)-CH2OH (4 grams, 24.7 mM) were added to the above solution at 0 to 5 °C, and the stirring was continued for 5 minutes. To this solution, DMF (50 mL) solution of EDC.HCl (5.7 grams, 29.8 mM; corrected for assay) was added while maintaining the temperature at 0 to 5 °C. The reaction mixture was stirred for about 1 hour while allowing the reaction mixture to warm to 10 to 15 °C. The progress of the reaction was monitored by RP-HPLC. After the reaction was complete, it was quenched with water, and the solid separated out. The solid was filtered and washed with water (110 mL) and heptane (110 mL). Wet solid was dried under reduced pressure below 40 °C to obtain formula (B4), (12.9 grams, 17.7 mM)
Yield: 71.6%; Purity RP -HPLC: 97.4%; [M+H]+ = 729.5 amu

Step (iv): Preparation of H-Cys(Trt)-Thr(tBu)-CH2OH (B)
Fmoc-Cys(Trt)-Thr(tBu)-CH2OH (50 grams; 68.55 mM) was dissolved in acetonitrile (750 mL) and cooled to 4 -8 °C. It was treated with 1N NaOH (140 mL, 137.5 mM). Next, heptane (250 mL) was added to the reaction mixture and it was stirred for 20-30 minutes. The temperature was raised to ambient temperature and maintained the reaction for 2-3 hours. The progress of reaction was monitored by HPLC. After completion of the reaction, it was cooled to 4 to 8 °C. The pH of the reaction mixture was adjusted to 4 to 5 using 5% KHSO4 solution and separated aqueous layer. The aqueous layer washed with mixture of 30% DIPE in heptane and the organic extract were discarded. The aqueous layer was neutralized with sodium bicarbonate. The product was extracted from aqueous layer with DIPE. The organic layer containing product was washed with brine, water, and dried with anhydrous Na2SO4. The solvent was removed under reduces pressure, and the residue was co-distilled with heptane to obtain the crude. The product was isolated with heptane and dried under reduced pressure at below 40 °C to obtain formula (B), (25.6 grams, 50.44 mM)
Yield: 73.7%; Purity RP -HPLC: 97.9%; [M+H]+ = 507.5 amu

Example 3: Preparation of Octreotide acetate (I)
Step (i): Preparation of Boc-D-Phe-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-Cys(Trt)-Thr(tBu)-CH2OH (C)
Boc-D-Phe-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-OH (42.0 grams, 29.7 mM) was dissolved in DMF (294 mL). The solution was cooled to 0 to 5 °C. Next, HOBt (4.5 grams, 29.7 mM) and H-Cys(Trt)-Thr(tBu)-CH2OH (15.0 grams, 29.7 mMol) were added to the solution and maintained for 5-10 minutes at 0-5 °C. Next, a solution of EDC.HCl (11.3 grams ; 59.42 mM; assay corrected) in DMF (126 mL) was added to the above solution at 0-5 °C and the reaction was stirred for 30 minutes at 0 to 5 °C. The temperature was raised to ambient temperature and maintained the reaction for 2-3 hours and the progress of reaction was monitored by analytical RP-HPLC. After the completion of reaction, it was quenched with 1% aqueous citric acid solution at 2 to 8 °C. The precipitated solid was filtered and washed with water, heptane, and dried under reduced pressure for 4 hours at 40 °C to obtain formula (C), (54.0 grams, 28.6 mM)
Yield: 96.2%; Purity RP -HPLC: 98.5%; [M+H]+ =1887 amu

Step (ii): Preparation of TFA H-D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-CH2OH (D)
Boc-D-Phe-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(oxa)-Cys(Trt)-Thr(tBu)-CH2OH (37.0 grams, 19.6 mM) was treated with a mixture of trifluoroacetic acid (185 mL ), triisopropylsilane (18.5 mL), water (9.25 mL), and dithiothreotol (9.25 grams) for 0.5 hours at 0 to 5 °C. The reaction mixture was allowed to warm to 25 to 30 °C, and stirring was continued for 1 to 2 hours at 25-30 °C. Next, the reaction was cooled to 0 to 5 °C, and quenched with precooled DIPE (1800 mL) at 0-5 °C, and the suspension was stirred for 1 hour at 0 to 5 °C. The solid was filtered and washed with DIPE (222 mL), and dried under reduced pressure at 40 °C to obtain formula (D), (24.5 grams, 19.6 mM)
Yield: quantative; Purity RP -HPLC: 75.5%; [M+H]+ = 1249 amu

Step (iii): Preparation of Octreotide H-D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-CH2OH (I-1)
TFA H-D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-CH2OH (24.5 grams; 19.6 mM) was dissolved in mixture of water (488 mL) and acetonitrile (366 mL) solvents and cooled to 0 to 5 °C. Next, an aqueous solution of 1N NaOH and followed by aqueous solution of 0.1M potassium ferrocyanide solution (360 mL, 32.2 mMol) were added, and the reaction temperature was allowed to reach 25 to 30 °C and the stirring was continued for an additional 30 minutes. The progress of the reaction was monitored by analytical RP-HPLC. After the completion of reaction, the reaction was treated with di-anion resin and purity of the solution was measured 87.9% by RP-HPLC. The solution was directly purified by RP-Preparative HPLC.

Step (iv): Preparation of Octreotide acetate (I)
Crude Octreotide (21 grams, purity: ~86% Octreotide) was loaded on to a C-18 reversed phase column pre- coated with proprietary SSP, and purified using 0.1% acetic acid in water (Buffer A), and 0.1% acetic acid in acetonitrile (Buffer B). The fractions containing the pure API product were combined and lyophilized to furnish Octreotide acetate as a white powder. (8 grams, Yield: 44.2%; Purity by Analytical RP-HPLC: 99.75%)