FIELD OF THE INVENTION
The present invention relates to a process for the preparation of Plecanatide, which comprises preparation of three Fragments such as Fragment A (7 amino acids), Fragment B (3 amino acids), Fragment C (6 amino acids) and coupling the Fragments to provide Plecanatide followed by purification using buffer system comprising Tris hydrochloride (or) Triethylammonium phosphate.
BACKGROUND OF THE INVENTION
Plecanatide is an agonist of the guanylate cyclase type-C receptor ("GCC agonists"). Plecanatide is a 16 amino acid peptide with the following chemical name: L-Leucine, L-asparaginyl-L-a-aspartyl-L-a-glutamyl-L-cysteinyl-L-a-glutamyl-L-leucyl-L-cysteinyl-L-valyl-L-asparaginyl-L-valyl-L-alanyl-L-cysteinyl-L-threonylglycyl-Lcysteinyl-, cyclic (4—»12).(7—*15)-bis(disulfide). The amino acid sequence for plecanatide is shown below:
Plecanatide is approved in the United States under the trade name TRULANCE™ for treatment of Chronic Idiopathic Constipation (CIC) in adult patients.
Plecanatide is described in US patent No. 7,041.786. The US '786 patent discloses that the peptide including plecanatide are synthesized and purified (>95% purity) using a published procedure of Klodt, et al., J. Peptide Res. 50:222-230 (1997). The article discloses general solid-phase synthesis, deprotection of Fmoc groups with 20% piperidine in NMP, deprotection of dry peptidyl resins by using a mixture of TFA/EDT/HhO, formation of disulfide bond by air oxidation and with iodine, acidification with TFA. purification using preparative CI8-HPLC column (buffer A: 0.1%TFA; buffer B: 0.1% TFA in MeCN/water, 80:20).

US Patent No. US 9,580,471 describes a process for preparation of plecanatide using combination of solid and solution phase synthesis. Further, US '471 describes purification process on RP-HPLC column with mobile phase A (1.0% TEA, 0.5% H3PO4 in H20, pH=7) and mobile phase B (acetonitrile) to obtain peptide, which is then purified on RP-HPLC column with ammonium acetate, acetic acid (AcOH) and acetonitrile (ACN) to obtain pure Plecanatide (^95%). which is then desalination on RP-HPLC column with ammonium acetate/acetonitrile/water buffer system, and elute with aqueous alcohol, concentration of peptide alcohol solution and precipitation with diethyl ether or methyl tert-butyl ether (MTBE) to obtain Plecanatide.
The inventors of the present invention developed an improved process for the preparation of pure Plecanatide. which is simple, cost-effective, and avoids or reduces content of impurities and makes the process robust.
OBJECTIVE OF THE INVENTION
The objective of the present invention is to provide a process for the preparation of Plecanatide.
Another objective of the present invention is to provide a process for the purification of Plecanatide.
SUMMARY OF THE INVENTION
In an aspect, the present invention provides a process for the preparation of Plecanatide of formula I:

d) oxidizing the linear 1-16 peptide to obtain Plecanatide.
In another aspect, the present invention provides a process for the purification of Plecanatide, which comprises:
a) purification of crude plecanatide onto preparative HPLC column with buffer system comprising Tris hydrochloride or Triethylammonium phosphate;
b) desalination of pure Plecanatide; and
c) isolation of Plecanatide.

The present invention relates to a process for the preparation of Plecanatide by coupling of two or more protected fragments either by solution phase or SPPS methods.
In an aspect, the present invention provides a process for the preparation of Plecanatide of formula I:
Wherein Y represents amino protecting group. X represents carboxyl, phenol and alcoholic protecting group, Z-represents thiol protecting group.
The side chain protecting groups for a hydroxyl group in an amino acid include, but are not limited to, benzyl (Bzl). tert-butyl (tBu), acetamidomethyl (Acm), and trityl (Trt). tetrahydropyranyl. Cbz. and 2.5-dichlorobenzy) (Deb). The suitable

side chain protecting groups for a thiol group include, but are not limited to, acetamidomethyl (Acm), trityl (Trt), Bzl, tBu, tert-butylthio (tButhio), p-methoxybenzyl (pMeoBzl). and 4-methoxytrityI (Mmt). The side chain protecting groups for a carboxylic acid include, but are not limited to benzyl. 2,6-dichlorobenzyl. tBu. and cyclohexyl. The side chain protecting groups for amide group include, but are not limited to, Xan, Trt; and the side chain protecting groups for alpha amino protecting group include, but are not limited to Fmoc.
In an embodiment, the Fragment A and Fragment C are prepared in solid phase synthesis; Fragment-B is prepared in solution phase; and coupling of ihe fragments is performed in solution phase.
The solid phase synthesis comprises elongation of peptide sequence by coupling of protected amino acids onto a peptide resin, cleaving amino protecting group, coupling of second protected amino acid via peptide linkage to the carboxyl group of a second protected amino acid and repeating the cycle till to obtain protected peptide intermediates.
The resin is selected from the followings: wang resin. TentaGel S, chlorotrityl resin (CTC), 4-methytrity] chloride resin, TentaGel TGA, Rink acid resin, NovaSyn TGT resin, HMPB-AM resin.
The process for the preparation of linear 1-16 peptide involves coupling of Fragment A (7 amino acids) with Fragment B (3 amino acid) in solution phase in the presence of a coupling reagent to produce a Fmoc-protected Decapeptide (Fragment C), which is subjected for Fmoc deprotection in the presence of base to produce a decapeptide. The obtained decapeptide is coupled with a Fragment D (6 "aininoacids)'by solution phase in presence of a coupling agent to give a protected linear 1-16 peptide, which is then deprotected with a cocktail mixture to produce linear 1-16 peptide.

In an embodiment, the Fragment A is Fmoc-Cys(Acrn)-Val-Asn(Trt)-Val-Ala-Cys(Trt)-Thr(tBu)-OH, the Fragment B is H-Gly-Cys(Acm)-Leu-OtBu, the Fragment C is H-Cys(Acm)-Val-Asn(Trt)-Val-Ala-Cys(Trt)-Thr(tBu)-Gly-Cys(Acm)-Leu-OtBu and the Fragment D is Boc-Asn(Xan)-Asp(OtBu)-Glu(OtBu)-Cys(Trt)-Glu(OtBu)-Leu-OH.
The coupling reagent is used in presence or absence of additive. The coupling reagents includes but are not limited to DIC, DCC, HATU, HBTU, TBTU, BOP.. BOP-C1, PyBOP, PyBrOP, JBCF, WSCD1, EEDQ, IPCF; TNTU, PPAA, TSTU, PyClOP, Oxyma pure, TCTU, COMU, HOSu, The additive includes but are not limited to HOBt, HODhbt, HOAts 6-CF3- HOBt 6-N02-HOBt , ethyl-2-cyano-2-(hydroxyimino) acetate (Oxyma) or mixture thereof. In one embodiment, the coupling agent and additive is of HATU/HOAt.
The base used for coupling is organic or inorganic base. The inorganic base
comprises potassium carbonate, lithium carbonate, sodium carbonate, sodium
bicarbonate, potassium bicarbonate, sodium hydroxide, potassium hydroxide,
ammonium hydroxide, and mixtures thereof; the organic base comprises t-
butylamine, 4-Dimethylaminopyridine (DMAP), diisopropylamine, N,N-
diisopropylethylamine triethylamine, dimethylamine, trimelhyl amine, isopropyl
ethylamine, pyridine, N-methyl morpholine, piperidine, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), and/or mixtures thereof. The solvent may be used for coupling reaction that comprises dimelhylformamide (DMF), dimethylsulfoxide (DMSO), N-Methyl pyrrolidine (NMP), Dimelhylacetamide (DMAC). dichloromethane (DCM), methanol, isopropanol. dichloroethane, 1,4-dioxane, tetrahydrofuran (THF), 2-melhyl tetrahydrofuran, ethyl acetate, acetonitrile, acetone or mixtures thereof.
The base used for Fmoc deprotection includes but are not limited to l-butylamine, piperidine. diethyl amine. DBU, piperazine, pyrrolidine, derivatives of piperadine,

piperazine and pyrrolidine in presence of solvent comprises alcohol, amide or ether.
In one embodiment, the base and solvent used for Fmoc deprotection are t-butylamine and DMF.
In an embodiment, the cleavage and deblocking is performed in presence of cocktail mixture i.e. TFA: TIPS: DTT: solvent (or) TFA: TIPS: DTT: water: solvent (or) TFA: TIS: solvent. The solvent comprises water, dimethyl sulfide, alcohol solvent selected form methanol, ethanol. 1-propanaol, isopropanol, n-butanol; chlorinated solvent is selected form dichloromethane, dichloroethane, chlorobenzene; ether solvent selected form diethyl ester, tetrahydrofuran. diisopropyl ether and the or combination thereof. The additional cocktail reagents is selected from 1, 2-ethanedithiol (EDT), dimethyl sulfide (DMS), thioanisole, phenol, anisole etc. In an embodiment, the present invention provides deblocking of protected linear peptide using a mixture of TFA: TIPS: DTT: water: DMS.
In one embodiment, the cleavage and deblocking is performed in presence of cocktail mixture is Cocktail of 84% TFA (Trifluoroacetic acid): 5% TIPS (Triisopropylsilane): 5% H20: 5% DTT (Dithiothreitol): 5% DMS.
After completion of cleavage and deprotection. the linear peptide or its TFA salt is precipitated by using suitable solvent, which is selected from ether solvent like diethyl ether, diisopropyl ether, methyl tertiary butyl ether, ethyl acetate and tetrahydrofuran.
After completion of cleavage and deprotection. the linear peptide is oxidized with
'■' molecular oxygen and/or oxidizing agent comprises hydrogen peroxide, dimethyl
sulfoxide in presence or absence of solvent, which is selected from water, nitrile,
alcoholic solvent or combination, thereof.„The_obtained peptide oplionally treated

with Iodine in a solvent, which is selected from nitrile to provide dicyclised peptide. The oxidation may be performed at a pH range from 3 to 4.
The oxidation of open-chain peptide containing two free and/or two protected sulfhydryl groups with hydrogen peroxide. The protected/non-protected linear peptide may be subjected for pH adjustment of 3 to 4 or 6 to 7 by using acid/base followed by air oxidation using hydrogen peroxide and compressed air to afford mono-cyclized 1-16 peptide. The mono-cyclized peptide may be further treated with iodine in a solvent comprising nitrile solvent to provide disulfide 1-16 peptide i.e. Plecanatide.
In another embodiment, the present invention provides a process for the preparation of Fragment A is shown in the below Scheme-1.
In yet another embodiment, the present invention provides a process for the preparation of Fragment B is shown in the below Scheme-2.

In yet another embodiment, the present invention provides a process for the process for the preparation of Fragment C is shown in the below Scheme-3.
The crude Piecanalide obtained by the process of present invention can be purified using preparative column chromatography or reverse phase column chromatography (RPHPLC).
In another aspect of the present invention, there is provided a process for the purification of piecanalide of formula I. which comprises:

a) purification on preparative HPLC column with Tris hydrochloride (buffer A) and acetonitrile (buffer B) to obtain plecanatide having purity >95%;
b) second purification of plecanatide obtained from step a) on preparative HPLC column with Tris hydrochloride (buffer A) and acetonitrile (buffer B) to obtain pure plecanatide (>99%); or
c) purification of plecanatide obtained from step a) on preparative HPLC column with Triethylammonium phosphate (buffer A) and acetonitrile (buffer B) to obtain pure plecanatide (>99%);
d) desalting of plecanatide obtained from step b) or step c) on preparative HPLC column with acetic acid in water and acetonitrile; and
e) isolation of pure Plecanatide.
The present application relates to a purification process of crude Plecanatide or a reaction mixture containing Plecanatide comprising preparative reverse phase column chromatography. The column used for purification of Plecanatide may be conventional column known in the art.
The column in preparative HPLC may be packed with reverse phase CI8 hydrid silica. Suitable silica gel types: which can be selected from, but are not limited to the following silica gel sorbents: Kromasil™C18 100 - 16, Kromasil™C18 100 -10.. Kromasil™C8 100 - 16, Kromasil™C4 100 - 16, Kromasil™ Phenyl 100 - 10, Kromasil™ CI 8 Eternity 100 - 5, Kromasil™ C4 Eternity 100-5, Chromalorex™ CI 8 SMB 100-15 HE, Chromatorex™ C8 SMB 100-15 HE, Chromalorex™ C4 SMB 100-15 HE, Daisopak™ SP 120-15 ODS-AP, Daisopak™ SP 120-10-C4-Bio? Daisopak™ SP 200-10-C4-Bio, Zeosphere™ CI 8 100-15, Zeosphere™ C8 100-15, Zeosphere™ C4 100-15, SepTech ST 150-10 C18, Luna C18 100:10, Gemini C18 110-10, YMC Triart C18 120-5 and YMC TriartC8 200-10.

The column is packed with silica using Tris HC1 having pH of 6 to 8 or pH of 7 and nitrile solvent such as acetonitrile, as buffers to obtain fractions having purity of about 94% by HPLC. The obtained fractions may be re-purified using column by packing with reverse phase CI 8 hybrid silica using Tris HCI.
The Plecanatide obtained according' to the present invention has purity greater than 95% and preferably greater than 99% (by HPLC). The yields of the Plecanatide obtained according to present invention are consistent.
The plecanatide. as produced by any one of the reaction conditions and purification process described above, undergoes desalting process by ion-exchange/preparative column chromatography. The resultant pure Plecanatide is subjected for precipitation, lyophilization or spray drying techniques to provide amorphous or crystalline Plecanatide.
The Plecanatide prepared using above described process of the present invention results in high yield, particularly consistent with high purity. In the process according to the present invention, one or more of the different factors relating to the reagents and their quantities which are used in carrying out the various steps to prepare Plecanatide; the manner in which the steps are carried out; the methods used; and the various process parameters like temperature, pH. concentration, etc. were optimized and controlled in proper manner so as to obtain the desired product in a consistent manner.
Having described the invention with reference to certain aspects and embodiments, which will become apparent to one skilled in the art from consideration of the specification. The invention is further defined by reference to the following examples. Jt will be apparent to those skilled in the an that many modifications, both to materials and methods, may be practiced without departing from the scope of the invention.

EXAMPLES
EXAMPLE 1: Synthesis of H-G1y-Cys(Acm)-Leu-Otbu (Fragment B)
Step-I: Synthesis of H-Cys(Acm)-Lue-Otbu.
H-Leu-(Otbu).HCl (356.27gm,l.leq), HOBT(222.97gm,1.0eq) were added to a solution of Fmoc-Cys(Acm)-OH (600.0 g, I.Oeq) in DMF (3.0L) and then cooled to 5-10°C. HBTU (603.9gm: 1.1 eq) and D1PEA(883.4 ml, 3.5eq) were added to the reaction mass. After completion of the reaction, the product was extracted with ethyl acetate (6.0V) and then washed with HC1. 5% aq sodium bicarbonate soln, 10%NaCl solution and water. The organic layer was collected, filtered and the filtrate was concentrated to obtain Fmoc-protected dipeplide. The obtained product was proceeded for next step without any further purification. Yield: 945 g Fmoc-Cys (Acm)-Leu-Otbu (945 g, leq) was taken in flask containing DMF (1.89 L) and cooled the solution to 5-10°C. Tertiary butyl amine (255 ml 1.5eq) was added slowly to the solution and stirred for 15min. After completion of reaction, the reaction mass was cooled to 5-10°C. Water and IN HC1 were added to the solution and the obtained aqueous solution was washed with hexane. The pH of product was adjusted to 8 to 8.5 with saturated sodium bicarbonate solution and the product was extracted with ethyl acetate. The ethyl acetate layer was washed with 10% NaCl solution and water and then filtered. The obtained filtrate was evaporated completely to obtained light yellow thick residue H-Cys(Acm)-Leu-Otbu. Yield: 450 g.
Step —II: Synthesis of H-Gly-Cys (Acm)-Lue-Otbu.
H-Cys (Acm)-Leu-Olbu (450.0 g; I.leq) was taken in flask containing DMF (1.68 L). Fmoc-Gly-OH (336.45g, l.0eq): HOBT (174.2gs I.Oeq) were added to the solution and then cooled to 5-)0°C. HBTU (472g; l.leq) and D1PEA (414.15ml: 2.leq) were added and stirred. After completion of the reaction, ethyl acetate was added to the above reaction mass and washed with pre-cooled 0.5N HCl,-5% aqueous NaHCOs solution (pre-cooled). 10%NaCl solution and water. The resultant organic layer was evaporated completely to give Fmoc-Gly-Cys (Acm)-Lcu-Otbu. Yield: 825 g.

Fmoc-Gly-Cys(Acm)-Leu-Otbu (825g, leq) was taken in flask containing THF (1650ml) and cooled to 5-10° C. Tertiary butyl amine (338 ml 2.5eq) was added slowly to the solution. After completion of reaction, the solution was cooled to 5-10°C and then water (2.48L) was added to the reaction mass. The aqueous reaction mass was washed with hexane. 50% ethyl acetate in hexane, then the product was extracted with dichloromethane. The dichloromethane layer was washed with 10%NaCl solution and water. The (collected) organic layer was dried with anhydrous sodium sulphate and then filtered. The obtained filtrate was evaporated completely to obtain H-Gly-Cys (Acm)-Leu-Otbu as light yellow semi solid. Yield: 392 g; HPLC purity: -92%.
EXAMPLE 2: Synthesis of Fmoc-Cys(Acm)-Val-Asn(Trt)-Val-Ala-Cys (Trt)-Thr (otbu)-2CTC. (Fragment-A) Step A:
CTC resin (750gm) was taken in a SPPS reactor, 7.5L of dry dichloromethane (DCM) was added and allowed it to swell for 20min and drained. Step B:
A solution of Fmoc-Thr(otbu)-OH (954gm. 2eq) and D1EA (627.6ml, 3eq) in dry dichloromethane (3.75L) were added to the resin at step A and stirred for at room temperature and drained.
The resin was then capped with DIEA (1%) solution in DCM: methanol (1:1)) and then drained. Thereafter, washed the resin with one bed volume of DMF (2 times); DCM (2 limes) and MTBE (2 times), isolated and dried. Yield: 1150gm The above resin was deblocked with 20% piperidine in DMF and washed with DMF (2times), 1PA (2 times) and DMF (2 limes). Step C: - To a solution of Fmoc-Cys (Trt)-OH' (808g: l.5eq.) and HOBT (212.5g, 1.5eq) in DMF, DIC (323ml, 2.25eq) was added. The obtained reaction mixture was added to the resin in Step B and stirred. After completion of the reaction the resin was

drained and washed with DMF (2 times) and then resin was deblocked with 20% piperidine in DMF and then washed with DMF, IPA and DMF. Step-D:
Followed by sequential coupling of Fmoc-Ala-OH. Fmoc-Val-OH, Fmoc-Asn (Trt)-OH, Fmoc-Val-OH similar to the procedure described in step-C. Step E:
To a solution of Fmoc-Cys (acm)-OH (572g, 1.5eq.) and HOBT (212.5g, 1.5eq) in DMF. DIC (323ml. 2.25eq) was added. The obtained reaction mixture was added to the resin in Step D and stirred. After completion of the reaction, the resin was drained and washed with DMF (2 times) DCM (2 times) and MTBE (2 times). It was isolated and dried to give FMOC-Cys (Acm)-Val-Asn (Trt)-Val-Ala-Cys (Trt)-Thr(otbu)-2CTC Resin. Yield: 2.17 kg. Step F:
Selective cleavage of 2-chloro trityl resin from the Fmoc-Cys(acm)-Val-Asn(Trt)-VaJ-Ala-Cys(Trt)-Thr(otbu)-2CTC Resin was performed with a mixture 1%TFA in dichloromethane and then above peptidyl resin was taken in SPPS reactor and treated with a solution of 1% TFA in DCM and drained. The filtrate was immediately neutralized with precooled saturated NaHCCb Solution to precipitate the product. The same process was repeated 3 more times and dried to obtain off-white solid. The obtained solid was further purified by treating with MTBE (2ml/g of product) to give Fmoc-Cys(Acm)-Val-Asn(Trt)-Val-Ala-Cys(Trt)-Thr(tbu)-OH, Yield: 1133gm; Purity: 94.29%.
EXAMPLE-3: Synthesis of Boc-Asn(Xan)-Asp(Otbu)-Glu(Otbu)-Cys(Trt)-GIu(Otbu)-Leu-2CTC (Fragmenr-C) Step A
CTC resin (445gm) was taken in a SPPS reactor, 4.45L of dry dichloromethane
■■ was added and allowed it to swell for 20min and drained. ■ * ■■
StepB

A solution of Fmoc-Leu-OH (525gm, 2eq) and DIEA (388.6ml, 3eq) in dry
dichloromethane (2.22L) was added to the resin at step A and stirred at room
temperature and drained.
The resin was then capped with DIEA (1%) solution in DCM: methanol (1:1)).
Thereafter, washed the resin with one bed volume of DMF (2 times), DCM (2
times) and MTBE (2 times), isolated and dried. Yield: 700gm.
The above resin was deblocked with 20% piperidine in DMF for 10 and 15
minutes and washed with of DMF (2times), IPA (2 times) and DMF (2 times).
StepC
To a solution of Fmoc-Glu(Otbu)-OH (553gm, 2.0eq.) and HOBT (200.5gms
2.0eq) in DMF. DIC (305ml. 3.0 eq) was added. It was added to the resin in Step
B. After completion of the reaction, the resin was drained and washed with DMF.
The above resin was deblocked with 20% piperidine in DMF for 10 and 15
minutes and washed with of DMF (2times), IPA (2 times) and DMF (2 times).
Step D:
Sequential coupling of Fmoc-Cys(Trt)-OH. Fmoc-Glu(Otbu)-OH. Fmoc-
Asp(Otbu)-OH, Fmoc-Asp(Otbu)-OH similar to the procedure of step-C.
Stcp-E:
To a solution of Boc-Asn(xan)-OH (537gm, 2.0eq.) and HOBT (200.5gm, 2.0eq)
in DMF. DIC (305ml, 3.0eq) were added and stirred. It was added to the resin in
Step F and stirred. After completion of the reaction, the resin was drained and
washed with DMF (2 times), DCM (2 times) and MTBE (2 times). It was isolated
and dried to give Boc-Asn(Xan)-Asp(Otbu)-Glu(Otbu)-Cys(Trt)-GIu(Otbu)-Leu-
2CTC resin. Yield: 1.37kg.
StepH
Selective cleavage of 2-chloro trityl resin from the Boc-Asn(Xan)-Asp(Otbu)-
Glu(Otbu)-Cys(Trt)-Glu(Otbu)-Leu-2CTC resin was performed with a mixture
1%TFA in dichloromethane.
The above peptidyl resin was taken in SPPS reactor and treated with a solution of
1% TFA in DCM. The filtrate was neutralized with precooled saturated NaHCCb
solution. The-same process was repeated 3 more times, the collected organic

solution was washed with water, dried with sodium sulphate and evaporated to obtain off-white solid. The obtained solid was further purified by treating with MTBE (2ml/g of product) to give Boc-Asn(Xan)-Asp(Otbu)-Glu(Otbu)-Cys(Trt)-Glu(Otbu)-Leu-OH. Yield: 720g; Purity: 90.74%.
EXAMPLE 4: Synthesis of H-Cys(Acm)-VaI-Asn(Trt)-Val-Ala-Cys(Trt)-Thr(tbu)-Gly-Cys(Acm)-Leu-Otbu.
Fmoc-Cys(Acm)-Val-Asn(Trt)-Val-Ala-Cys(Trt)"Thr(tbu)-OH (Fragment-A)
(500.0 g_. l.Oeq) was taken in a round bottom flask containing DMF (2.5L). H-Gly-Cys (Acm)-Leu-Otbu (Fragment-C) (209g, 1.5eq) was added to the reaction mass and cooled to -10 to -15°C. HOAT (50g; 1.1 eq) and HATU (151.75g , 1.2 eq) were added to reaction mass and then DIPEA (116ml. 2eq) was added drop wise while stirring at -10 to-15°C. After completion of the reaction, methanol (15.0L) was added to the above reaction mass (solid formation was observed), then pH of the reaction mixture was adjusted with IN HC1 up to pH 3. stirred for one hour, filtered and dried to give decapeptide. Yield: 700g. The above obtained decapeptide (700g, l.Oeq) was taken in a round bottom flask containing DMF (4.2 L) and cooled the solution to 5-10°C. Tertiary butyl amine (77.30ml 2.0eq) was added and stirred the reaction mass for 15min at 5-10°C and then stirred at R.T. After completion of the reaction, methanol (25.20L) was added to precipitate the product. The obtained solid was Altered and dried to give NH2-Cys (Acm)-Va!-Asn(Trt)-Val-Ala-Cys(Trt)-Thr(otbu)- Gly-Cys (Acm)-Lue-Otbu. Yield: 437g: HPL-C purity: -97%
EXAMPLE 5: Synthesis of Boc-Asn(Xan)-Asp(Otbu)-Glu(Otbu)-Cys(Trt)-
Glu(Otbu)-Lcu-Cys(Acm)-VaI-Asn(Trt)-Val-A1a-Cys(Trt)-Thr(otbu)-Gly-
Cys(Acm)-Leu-Otbu.
NH2-Cys(Acm)-Val-Asn(Tn)-Val-Ala-Cys(Trt)-Thr(otbu)-Gly-Cys-(Acm)-Lue-Otbu, (435.0g, 1 .Oeq) and Boc-Asn(Xan)-Asp(Otbu)-Glu(Olbu)-Cys(Trt)-Glu(Otbu)-Leu-OH (Fragment C):(365.0 g; 0.9eq) were taken in a round bottom

flask containing DMF (2.61 L). The solution obtained was cooled to -10 to -15°C and then HOAT (42.0g, 1.2eq) and HATU (118g, 1.2eq) were added. DIPEA (90 ml. 2.0eq) was added to the reaction mass and stirred at -10 to-15°C. Methanol (13.0L) was added to the above reaction mass to precipitate the product. The obtained solid was filtered and dried to give Boc-Asn(Xan)-Asp(Olbu)-GIu(Otbu)-Cys(Trt)-Glu(Otbu)-Leu-Cys(Acm)-Val-Asn(Trt)-Val-Ala-Cys(Trt)-Thr(otbu)-Gly-Cys(Acm)-Leu-Otbu as off-white solid. Yield: 750g.
EXAMPLE 6: Synthesis of H-Asn-Asp-Glu-Cys-Glu-Leu-Cys(Acm)-Val-Asn-Val-A1a-Cys-Thr-Gly-Cys(Acm)-Leu-OH.
Protected peptide (500.0 g) obtained in example 5 was treated with a pre-cooled solution of 84% TFA (4200 ml), 5% TIPS (250 ml), 5% H20 (250 ml), 5% DTT (250 gm): 1%DMS (50ml) for 2 hrs at R.T. The product was precipitated by the addition of reaction mass to the pre-cooled MTBE. filtered the product under nitrogen and washed with MTBE and dried. Yield: 310.0g: Purity: 73 %.
EXAMPLE 7: PREPARATION OF CRUDE PLECANATIDE
Linear 1-16 peptide obtained from example 6 was dissolved in degassed 0.015 M ammonium hydroxide solution at a concentration of lg /0.75 L, pH was adjusted between 8 .5to 9.0 by using ammonia solution. After dissolution of compound. H2O2 (200 ul / g) was added. After completion of oxidation, the pH was adjusted between 3 to 4 by IN HC1 to obtain mono cyclized 1-16 peptide solution and then solution was treated with 5% iodine in acetonitrile. till the yellow color persist. The reaction mixture with excess of iodine was quenched with 0.1M aqueous ascorbic acid solution and the pH was adjusted between 6.5 and 7 by using ammonia solution. The solution was filtered through 2.4 micron. The obtained filtrate was used as such for next stage purification.
EXAMPLE 8: PURIFICATION OF PLECANATIDE
Stage 1: Crude Plecanalide solution obtained from example 7 was purified on preparative HPLC. column was packed with reverse phase CI 8 hybrid silica using

Tris HC1 pH 7(as buffer A) and 100% acetonitrile (as buffer B).The fractions were collected and purity of fractions were monitored by analytical HPLC. Fractions containing > 94% pure Plecanatide were pooled as main pool; and fractions not meeting the pooling criteria were re-processed in a similar manner.
Stage 2: The main pool obtained from stage 1 purification were diluted with equal amount of purified water and re-purified on preparative HPLC. column was packed with reverse phase CI 8 hybrid silica using Tris HC1 pH 7(as buffer A) and 100% acetonitrile (as buffer B).The fractions were collected and purity of fractions were monitored by analytical HPLC. Fractions containing > 98.5% pure Plecanatide were pooled as main pool for desalting.
(OR) The main pool obtained from stage-1 purification were diluted with equal amount of purified water and re-purified on preparative HPLC, column was packed with reverse phase CI8 hybrid silica using TEAP (as buffer A) and acetonitrile (as buffer B).The fractions were collected and purity of fractions were monitored by analytical HPLC. Fractions containing > 98.5% pure Plecanatide were pooled as main pool for desalting.
EXAMPLE 9: DE-SALTING AND LYOPHIUZATION
The main pool obtained from the purification were diluted with equal amount of purified water and loaded on preparative HPLC, column was packed with reverse phase C18 hybrid silica.
De-salting was done by passing 5 void volume of 0.1% acetic acid in purified water fallowed by elulion of product from the column by using 30% acetonitrile (HPLC grade) in purified water containing 0.1% acetic acid. The fractions were collected and purity of fractions were monitored by analytical HPLC. ■'-. The fractions containing pure Plecanatide (>98.5%) were pooled; the organic modifier was removed under reduced pressure and filtered through 0.2 micron filter. The resulting peptide solution was freeze-dried to isolate Plecanatide.

After completion of lyophilization cycle, the compound was unloaded and dissolved in purified water at a concentration Of 70 g /L, filtered through 0.2 micron filter. The resulting peptide solution was freeze-dried to obtain white solid lyophilized powder as Plecanatide. Purity: 99.1%
EXAMPLE 10: PREPARATION OF MONO CYCUZED PLECANATIDE
Linear 1-16 peptide obtained from example 6 was dissolved in degassed 0.015 M ammonium hydroxide solution at a concentration of lg /0.75 L, the pH was adjusted between 8 .5to 9.0 by using ammonia solution. After dissolution of compound. H2O2 (200ul / g) was added and stirred for 30 minutes. The progress of oxidation was monitored by analytical reverse phase HPLC & Ellman's test. After completion of oxidation, the pH was adjusted between 6.5 and 7 by using IN HC1 to obtain mono cyclized 1-16 peptide solution.
EXAMPLE 11: PURIFICATION OF MONO CYCLIZED PLECANATIDE
Mono cyclized solution obtained from example 10 was purified on preparative HPLC, column packed with reverse phase CI 8 hybrid silica using Tris HC1 pH 7(as buffer A) and 100% acetonitrile (as buffer B).The fractions were collected and purity were monitored by analytical HPLC.
Fractions containing > 95% pure Plecanatide were pooled as main pool; and fractions not meeting the pooling criteria were re-processed in a similar manner.
EXAMPLE 12: PREPARATION OF CRUDE PLECANATIDE
The resultant purified solution was diluted with equal amount of water, adjusted pH 3.5 with IN HCI and treated with 5% iodine in acetonitrile, till the yellow color persist and the reaction mass was stirred for two hours. The completion of oxidation was monitored by analytical reverse phase HPLC; quenched-the excess iodine with 0.1 M aqueous ascorbic acid soiutionand then--pH was adjusted between 6.5 and 7 by using ammonia solution. The solution was filtered through 2.4 micron filter and used as such for next stage purification.

EXAMPLE 13: PURIFICATION OF PLECANATIDE
The main pool obtained from stage 1 purification were purified on preparative HPLC, column was packed with reverse phase C18 hybrid silica using Tris HC1 pH 7 (as buffer A) and 100% acetonitrile (as buffer B).
The fractions were collected and purity of fractions were monitored by analytical HPLC. Fractions containing > 98.5% pure Plecanatide were pooled as main pool for desalting.
(OR) The main pool obtained from stage 1 purification further purified on preparative HPLC. the column was packed with reverse phase CI 8 hybrid silica using TEAP (as buffer A) and acetonitrile (as buffer B). Fractions containing > 98.5% pure Plecanatide were pooled as main pool for desalting.
EXAMPLE 14:
DE-SALTING AND LYOPHILIZATION
The main pool obtained from the purification were diluted with equal amount of
purified water and loaded on preparative HPLC, column packed with reverse
phase CI 8 hybrid silica.
De-salting was done by passing 5 void volume of 0.1% acetic acid in purified
water fallowed by elulion of product from the column by using 30% acetonitrile
(HPLC grade) in purified water containing 0.1 % acetic acid.
The fractions were collected and purity of fractions were monitored by analytical
HPLC.
The fractions containing pure Plecanatide (>98.5%) were pooled; the organic
modifier was removed under reduced pressure and filtered through 0.2 micron
filler. The resulting peptide solution was freeze-dried to isolate Plecanatide.
After completion of lyophilization cycle, unload the compound and dissolve in
purified water at a concentration Of 70 g /L. filtered through 0.2 micron filler. The
resulting peptide solution was freeze-dried to obtain white solid lyophilized
powder as Plecanatide.
Purity: 99% by HPLC

WE CLAIM:
1. A process for the preparation of Plecanatide of formula 1:
which comprises the following steps:
a) coupling of Fragment A with Fragment B to provide Fragment C;
b) coupling of the Fragment C with Fragment D: to provide protected linear peptide;
Protected linear peptide
c) deprotecting protected linear peptide to obtain linear 1-16 peptide; and
d) oxidizing the linear 1-16 peptide to obtain Plecanatide.

2. The process as claimed in claim 1, wherein the Fragment A and Fragment C are prepared in solid phase synthesis, and Fragment-B is prepared in solution phase.
3. The process as claimed in claim 1. wherein the X represents carboxyl, phenol and alcoholic protecting group comprises benzyl (Bzl), tert-butyl (t'Bu). acetamidomethyl (Acm). and triiyl (Trt), tetrahydropyranyl. Cbz. and 2;5-dichlorobenzyl (Deb), benzyl. 2.6-dichlorobenzyl, tBu. and cyclohexyl. Fmoc; the Y represents amino protecting group comprises Xan. Trt: and the Z-represents

thiol protecting comprises acetamidomethyl (Acm), trityl (Trt), Bzl, tBu. tert-butylthio (tButhio), p-methoxybenzyl (pMeoBzl), and 4-methoxytrityl (Mmt).
4. The process as claimed in claim 1, wherein the coupling reagent used in step a) comprises DIC, DCC, HATU, HBTU, TBTUS BOP, BOP-C1, PyBOP, PyBrOP, IBCF, WSCD1, EEDQ, 1PCF, TNTU, PPAA, TSTU and PyClOP.
5. The process as claimed in claim 1. wherein the additive reagent used in step a) comprises HOBt, HODhbt, HO At, 6-CF3- HOBt 6-N02-HOBt, ethyl-2-cyano-2-(hydroxyimino) acetate (Oxyma) or mixture thereof.
6. The process as claimed in claim 1, wherein the base used in step a) for Fmoc deprotection comprises t-butylamine, piperidine, diethyl amine, DBU, piperazine. pyrrolidine, derivatives of piperadine. piperazine and pyrrolidine
7. The process as claimed in claim 1. wherein the solvent used in step a) for Fmoc deprotection comprises alcohol, amide and ether.
8. The process as claimed in claim 1, wherein the cleavage and deblocking in step c) is performed in presence of cocktail mixture comprising TFA: TIPS: DTT: DMS, TFA: TIPS: DTT: water: DMS, TFA: T1S: DMS.
9. A process for the purification of Plecanatide of formula I having a purity of >98%: which comprises:

a) purification on preparative HPLC column with Tris hydrochloride (buffer A) and acetonitrile (buffer B) to obtain plecanatide having purity >95%;
b) second purification of plecanatide obtained from step a) on preparative HPLC column with Tris hydrochloride (buffer A) and acetonitrile (buffer B) to obtain pure plecanatide (>99%); or

c) purification of plecanatide obtained from step a) on preparative HPLC column with Triethylammonium phosphate (buffer A) and acetonitrile (buffer B) to obtain pure plecanatide (>99%);
d) desalting of plecanatide obtained from step b) or step c) on preparative HPLC column with acetic acid in water and acetonitrile; and
e) isolation of pure Plecanatide.
10. The process as claimed in claim 9, wherein the buffer A is Tris HC1 and buffer B is acetonitrile of step b) to obtain purity of >95%.