Process for Making Bivalirudin

Abstract

The present invention relates to the efficient commercial synthesis of Bivalirudin.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the efficient commercial synthesis for the making of bivalirudin, a peptide. It is well known that bivalirudin is indicated to reduce the risk of acute ischemic complications, and is an anticoagulant and acts as a direct thrombin inhibitor. The process substantially comprises the syntheses of various fragments of the polypeptide and the coupling of the fragments to produce bivalirudin.

2. Description of the related arts

Thrombin inhibitors are synthesized by various techniques which are well known in the art. These include enzymatic cleavage of natural or recombinant hirudin, recombinant DNA techniques, solid-phase peptide synthesis, solution-phase peptide synthesis, organic chemical synthesis techniques, or a combination of these techniques.

The following references disclose various techniques for producing thrombin inhibitors.

(1) U.S. Pat. No. 5,196,404

(2) U.S. Pat. No. 5,240,913

(3) U.S. Pat. No. 5,425,936

(4) U.S. Pat. No. 5,433,940

(5) U.S. Pat. No. 5,514,409

(6) U.S. Pat. No. 5,691,311

(7) US 2007093423

(8) US 2008051558

(9) US 2008287648

(10) US 2008287650

(11) US 20090062511

(12) WO9850563

The disclosure of the above references are herein incorporated in their entirety by reference. Certain of the references disclose the solid-phase peptide synthesis for making Bivalirudin, as the synthesis can be rapidly conducted. However, this synthesis method results in low yield and high cost of manufacture.

SUMMARY OF THE INVENTION

The present invention provides for an efficient process of making Bivalirudin in solution that is high in yield and scalable for commercial production. The process comprises the stepwise synthesis of amino acid segments S1, S2, S3, S4, and the coupling together of these segments to produce Bivalirudin. The process of the present invention provides for the making of Bivalirudin that is in high yield and of high purity compared to the solid-phase peptide synthesis method.

The appended claims are directed to process for making Bivalirudin and for making various novel intermediates. Recitation within these claims to first organic solvent, second organic solvent, and so forth, is meant to indicate that the organic solvents may be different or the same within the same claimed process; and that recitation of the same term for the organic solvent from one claimed process to another different claimed process does not necessarily indicate that the solvents are the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the synthesis of the peptide Segment 1 (S1) as disclosed in Examples 1, 2 and 3.

FIG. 2 depicts the synthesis of the peptide Segment 2 (S2) as disclosed in Examples 4 and 5.

FIG. 3 depicts the synthesis of the peptide Segment 3 (S3) as disclosed in Examples 6, 7 and 8.

FIG. 4 depicts the synthesis of the peptide Segment 4 (S4) as disclosed in Examples 9 and 10.

FIG. 5 depicts the synthesis of the Bivalirudin as disclosed in Examples 11, 12 and 13.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The identifying reference codes of the intermediates used in the present description and schemes are defined as follows.

S1M1 (M1 of segment 1)—BocArg(HCl)ProOBn

S1M2 (M2 of segment 1)—HCl Arg(HCl)ProOBn

S1M3 (M3 of segment 1)—Boc-D-PheProOBn

S1M4 (M4 of segment 1)—Boc-D-PheProOH

S1M5 (M5 of segment 1)—Boc-D-PheProArg(HCl)ProOBn

S1 (Segment 1)—Boc-D-PheProArg(HCl)ProOH

S2M1 (M1 of segment 2)—FmocAsn(Trt)GlyOtBu

S2M2 (M2 of segment 2)—Asn(Trt)GlyOtBu

S2M3 (M3 of segment 2)—FmocGlyAsn(Trt)GlyOtBu

S2M4 (M4 of segment 2)—GlyGlyAsn(Trt)GlyOtBu

S2M5 (M5 of segment 2)—FmocGlyGlyGlyGlyAsn(Trt)GlyOtBu

S2 (Segment 2)—FmocGlyGlyGlyGlyAsnGlyOH

S3M1 (M1 of segment 3)—BocileProOBn

S3M2 (M2 of segment 3)—HCl lIeProOBn

S3M3 (M3 of segment 3)—FmocGlu(tBu)IleProOBn

S3M4 (M4 of segment 3)—Glu(tBu)IleProOBn

S3M5 (M5 of segment 3)—FmocGlu(tBu)Glu(tBu)IleProOBn

S3M6 (M6 of segment 3)—Glu(tBu)Glu(tBu)IleProOBn

S3M7 (M7 of segment 3)—FmocPheGlu(tBu)Glu(tBu)IleProOBn

S3M8 (M8 of segment 3)—H-PheGlu(tBu)Glu(tBu)IleProOBn S3 M9 (M9 of segment 3)—FmocAsp(tBu)PheGlu(tBu)Glu(tBu)IleProOBn

S3 (Segment 3)—Asp(tBu)PheGlu(tBu)Glu(tBu)IleProOBn

S4M1 (M1 of segment 4)—FmocTyr(tBu)LeuOtBu

S4M2 (M2 of segment 4)—Tyr(tBu)LeuOtBu

S4M3 (M3 of segment 4)—FmocGlu(tBu)Tyr(tBu)LeuOtBu

S4M4 (M4 of segment 4)—Glu(tBu)Tyr(tBu)LeuOtBu

S4M5 (M5 of segment 4)—FmocGlu(tBu)Glu(tBu)Tyr(tBu)LeuOtBu

S4 (Segment 4)—Glu(tBu)Glu(tBu)Tyr(tBu)LeuOtBu

M1 Fmoc GlyGlyGlyGlyAsnGlyAsp(tBu)PheGlu(tBu)Glu(tBu)IleProOBn

M2—GlyGlyGlyGlyAsnGlyAsp(tBu)PheGlu(tBu)Glu(tBu)IleProOBn

M3—Boc-D-PheProArg(HCl)ProGlyGlyGlyGlyGlyAsnGlyAsp(tBu)—PheGlu(tBu)Glu(tBu)IleProOBn

M4—Boc-D-PheProArg(HCl)ProGlyGlyGlyGlyGlyAsnGlyAsp(tBu)—PheGlu(tBu)Glu(tBu)IleProOH

M5—Boc-D-PheProArg(HCl)ProGlyGlyGlyGlyGlyAsnGlyAsp(tBu)—PheGlu(tBu)Glu(tBu)IleProGlu(tBu)Glu(tBu)Tyr(tBu)LeuOtBu

The abbreviations used in the present description are defined as follows.

Boc—tert-Butyloxycarbonyl

Bn—benzyl

Fmoc—9-Fluorenylmethyloxycarbonyl

Trt—Trityl

tBu—tert-Butyl

HOBt—N-hydroxybenzotriazole

EDCl—Ethyl(3-dimethylaminopropyl)carbodiimide hydrochloride

EA—Ethyl acetate

DIC—N,N′-Diisopropylcarbodiimide

DCM—Dichloromethane

DMF—N,N-Dimethylformamide

DMSO—Dimethyl sulfoxide

DBU—1,8-Diazobicyclo[5,4,0]undec-7-ene

DEA—Diethanolamine

DIEA—N,N-Diisoproylethylamine

MTBE—Methyl tert-butyl ether

NMM—N-Methylmorpholine

The following examples are provided for the purpose of further illustration only and are not intended to be limitations on the disclosed invention.

Example 1

Preparation of HCl Arg(HCl)ProOBn (S1M2)

The protected amino acid BocArgOH.HCl.H2O (1.00 Kg) and ProOBn.HCl (0.77 kg) were dissolved in DMF (3 L) and HOBt (0.45 kg) was added. DIC (0.57 L) and NMM (0.37 L) were added and the reaction was stirred for another 15 h. The reaction mixture was filtered and the intermediate was precipitated out by addition of Acetone (30 L). The precipitated intermediate (S1M1) was separated by filtration and washed with Acetone (2×6 L). The precipitated intermediate (S1M1) was dissolved in HCl(g)/IPA (13%, 5 L) and the reaction was stirred for 6 h. The product was precipitated out by added into pre-cooled methyl-t-butyl ether (MTBE, 40 L). The precipitated product was separated by filtration and washed with MTBE (2×20 L). Yield: 1.06 Kg

Example 2

Preparation of Boc-D-PheProOH (S1M4)

The protected amino acid Boc-D-PheOH (1.00 Kg) and ProOBn.HCl (0.93 kg) were dissolved in DCM (8 L) and HOBt (0.56 kg) was added. DIEA (0.72 L) and EDCl (0.87 kg) were added and reaction was stirred for 2 h. The reaction mixture was washed with 5% sodium bicarbonate solution (10 L), 2.5% Citric acid solution (5 L) and 5% sodium bicarbonate solution (5 L) and was concentrated to get intermediate of yellow oil (S1M3). The intermediate (S1M3) was cooled to 20° C. and 1N NaOH (aq. 7.5 L) was added. The reaction mixture was allowed to assume room temperature for another 3 h. The reaction mixture was neutralized with 1 N HCl (aq.), followed by concentrated to about ⅗ volume. The product in aqueous layer was precipitated out by adjusting pH to about 3 with 1N HCl (aq.). The precipitated product was separated by filtration and washed with water (2×10 L). Yield: 1.12 Kg

Example 3

Preparation of Boc-D-PheProArg(HCl)ProOH (S1)

The compounds from Example 1 (1.00 Kg) and Example 2 (1.32 kg) were dissolved in DMF (5 L) and HOBt (0.41 kg) was added. NMM (0.41 L) and EDCl (0.63 kg) were added and the reaction was stirred for 2 h. The reaction mixture was diluted by DCM (10 L), washed with water (10 L) and concentrated to about ⅕ volume. The concentrated mixture was added into methyl-t-butyl ether (MTBE, 40 L) for intermediate precipitation. The precipitated intermediate was separated by filtration and washed with MTBE (2×20 L) to get intermediate (S1M5). The intermediate (S1M5) was dissolved in MeOH (6 L) followed by added into mixed solvents of MeOH (6 L) and 1 N NaOH (aq. 5.5 L). The reaction was allowed stirring for 5 h, then neutralized with HCl (aq.), followed by concentrated. The concentrated residue was extracted with DCM (15 L) after pH adjusted to about 3. The organic layer was concentrated and added into MTBE (40 L) for product precipitation. The precipitated product was separated by filtration and washed with MTBE (2×20 L). Yield: 1.48 kg

Example 4

Preparation of GlyGlyAsn(Trt)GlyOtBu (S2M4)

The protected amino acid GlyOtBu.HCl (0.30 kg) and FmocAsn(Trt)OH (1 kg) and HOBt (0.24 kg) were stirred in DCM (5 L). NMM (0.19 L) and EDCl (0.48 kg) was added to the mixture. The reaction mixture was reacted for 3 h to give the DCM solution of intermediate (S2M1). DBU (1 L) was added to the DCM solution of intermediate (S2M1) and reacted for 3 h. The reaction mixture was washed with water (3×3.5 L) to give DCM solution of intermediate (S2M2, about 5L). Protected amino acid FmocGlyGlyOH (0.59 kg) and HOBT (0.14 kg) were added to the DCM solution of intermediate (S2M2) and EDCl (0.48 kg) was added. The reaction mixture was reacted for 6 h to give solution of intermediate (S2M3). DBU (1 L) was added to the solution of intermediate (S2M3) and reacted for 1 h. The reaction mixture was washed with water (3×15 L). The organic layer was concentrated and the product was precipitated out by adding MTBE (40 L). The precipitated product was separated by filtration and washed with MTBE (2×7 L). Yield 0.71 kg

Example 5

Preparation of FmocGlyGlyGlyGlyAsnGlyOH (S2)

The compounds from Example 4 (1.00 Kg) and protected amino acid FmocGlyGlyOH (0.59 kg) were dissolved in mixed solvent of DMF/DCM (3 L/10 L) and HOBt (0.14 kg) was added. EDCl (0.35 kg) was added to the mixture and reacted for 16 h. The intermediate (S2M5) was precipitated out by adding mixed solvent of n-heptane/MTBE (30 L/30 L) and separated by filtration, then washed with MTBE (2×5 L). The precipitated intermediate (S2M5) was stirred in mixed solvents of water (0.39 L), TIS (0.39 L) and TFA (14.8 L) and reacted for 2 h. The reaction mixture was precipitated out by adding MTBE (80 L) and separated by filtration, washed with MTBE (2×8 L). Yield: 0.72 kg

Example 6

Preparation of HCl.IleProOBn (S3M2)

The protected amino acids ProOBn.HCl (1.00 kg) and BocIleOH 0.5 H₂O (1.04 Kg) were dissolved in DCM (6 L) and HOBt (0.56 kg) was added. NMM (0.84 L) and EDCl (1.19 kg) were added and the reaction was allowed to stir for 4 h. The reaction mixture was washed with water (total 9 L) and NaHCO3 (aq. 3 L) followed by cocentrated to give the intermediate (S3M1, Foam). The intermediate (S3M1) was dissolved into HCl(g)/IPA (about 13%, 3 L) and stirring continued for another 3 h. The product was precipitated out by solvent replacement with n-Heptane and adding MTBE (20 L).The precipitated product was separated by filtration, washed with MTBE (total 16 L). Yield: 1.4 kg

Example 7

Preparation of FmocGlu(tBu)Glu(tBu)IleProOBn (S3M7)

The compounds from Example 6 (1.00 Kg), protected amino acid FmocGlu(tBu)OH (0.87 kg) and HOBt (0.28 kg) were dissolved in DCM (7 L). NMM (0.35 L) and EDCl (0.60 kg) were added and the reaction was allowed to stir for 1 h to give DCM solution of intermediate (S3M3, about 7 L). DBU (0.47 L) was added into the DCM solution of intermediate (S3M3) and was allowed react for 1 h. The reaction was washed with water (total 9 L) and 5% Na2CO3 (aq. 3 L) to give DCM solution of intermediate (S3M4, about 7 L). The DCM solution of intermediate (S3M4) was mixed with protected amino acid FmocGlu(tBu)OH (0.86 kg) and HOBt (0.28 kg). EDCl (0.60 kg) was added and was allowed to react for 1 h to give DCM solution of intermediate (S3M5, about 7 L). DBU (0.47 L) was added into the DCM solution of intermediate (S3M5) and was allowed react for 1 h. The reaction mixture was washed with water (total 9 L) and 5% Na2CO3 (aq. 3 L) to give DCM solution of intermediate (S3M6, about 7 L). The DCM solution of intermediate (S3M6) was mixed with protected amino acid FmocPheOH (0.78 kg) and HOBt (0.28 kg). EDCl (0.60 kg) was added and was allowed to react for 1 h. The product was precipitated out by slowly adding MTBE (20 L) and was separated by filtration and washed with MTBE (3 L). The intermediate (S3M1) was dissolved into HCl(g)/IPA (about 13%, 3 L) and stirring continued for another 3 h. The product was precipitated out by solvent replacement with n-Heptane and adding MTBE (20 L).The precipitated product was separated by filtration, washed with MTBE (total 16 L). Yield: 1.84 kg

Example 8

Preparation of AspPheGlu(tBu)Glu(tBu)IleProOBn (S3)

The compound from Example 7 (1.00 Kg) was dissolved in DCM (10 L). DBU (0.22 L) was added and the reaction was allowed to react for another 1 h. The reaction mixture was washed with water (total 9 L) and 5% Na2CO3 (aq. 3 L) to give DCM solution of intermediate (S3M8, about 10 L). The DCM solution of intermediate (S3M8) was mixed with protected amino acid FmocAsp(tBu)OH (0.38 kg) and HOBt (0.13 kg). EDCl (0.27 kg) was added into the mixture and was allowed to react for 1 h to give DCM solution of intermediate (S3M9, about 10 L). DBU (0.22 L) was added into the DCM solution of intermediate (S3M9) and was allowed to react for another 1 h. The reaction mixture was washed with water (total 9 L) and 5% Na2CO3 (aq. 3 L). The product was precipitated out by slowly adding mixed solvent of n-Heptane/MTBE =1/1(35 L). The precipitated product was separated by filtration and washed with MTBE (6 L). Yield: 0.90 kg

Example 9

Preparation of Glu(tBu)Tyr(tBu)LeuOtBu (S4M4)

The protected amino acids FmocTyr(tBu)OH (1.00 kg) and LeuOtBu.HCl (0.50 kg) were dissolved in DCM (8 L) and HOBt (0.33 kg) was added. DIEA (0.44 L) and EDCl (0.50 kg) were added and the reaction mixture was stirred for 2 h. DEA (2.27 L) was added and the reaction mixture was allowed to react for 2 h. The reaction mixture was neutralized by 1 N HCl (aq.) and the resulting mixture was further washed by 5% Na2CO3 (aq. about 5.3 L) to give DCM solution of intermediate (S4M2, about 8 L). The DCM solution of intermediate (S4M2) was mixed with protected amino acid FmocGlu(tBu)OH (0.93 kg) and HOBt (0.32 kg). EDCl (0.63 kg) was added and the reaction mixture was allowed react for 2 h. DEA (2.27 L) was added to the reaction and allowed to react for another 3 h followed by washed with water (2×15 L). The organic layer was diluted by n-Heptane (30 L) followed by concentrated to about 25 L and the product was precipitated out by adding another parts of n-Heptane ( 5 L). The precipitated product was separated by filtration and washed with n-Heptane (10 L). Yield: 1.09 kg

Example 10

Preparation of Glu(tBu)Glu(tBu)Tyr(tBu)LeuOtBu (S4)

The compounds from Example 9 (1.00 Kg), protected amino acid FmocGlu(tBu)OH (0.72 kg) and HOBt (0.25 kg) were dissolved in DCM (8 L). EDCl (0.39 kg) was added and the reaction mixture was allowed to react for 2 h. DEA (1.77 L) was added and the reaction mixture was allowed to react for 3 h followed by washed with water (2×15 L). The organic layer was diluted by n-Heptane (30 L) followed by concentrated to about 25 L and the product was precipitated out by adding another parts of n-Heptane (5 L). The precipitated product was separated by filtration and washed with n-Heptane (10 L). Yield: 1.11 kg

Example 11

Preparation of GlyGlyGlyGlyAsnGlyAsp(tBu)PheGlu(tBu)Glu(tBu)IleProOBn (M2)

The compounds from Example 8 (1.00 Kg) and Example 5 (0.70 kg) were dissolved in DMSO (7 L) and HOBt (0.16 kg) was added. EDCl (0.48 kg) was added and the reaction mixture was allowed stirring for another 1 h, then Piperidine (1 L) was added and the reaction was allowed stirring for another 2 h. The product was precipitated out by adding water (35 L). The precipitated product was separated by filtration and washed with water (2×10 L) and MTBE (3×10 L). Yield: 1.26 kg

Example 12

Preparation of Boc-D-PheProArgProGlyGlyGlyGlyAsnGlyAsp(tBu)PheGlu(tBu)Glu(tBu)IleProOH (M4)

The compounds from Example 11 (1.00 Kg) and Example 3 (0.49 kg) were dissolved in DMSO (5.5 L) and HOBt (0.12 kg) was added. EDCl (0.16 kg) was added and the reaction mixture was allowed stirring for another 1 h. The intermediate (M3) was precipitated out by adding water (28 L) and separated by filtration and washed with water (2×12 L). The precipitated intermediate (M3) was stirred in 66% ACN (aq, about 20 L). Pd/C (10%, 0.13 Kg) was added into the mixture, followed by Hydrogen gas was introduced and reaction was allowed to agitate vigorously for 16 h. The reaction mixture was filtered and the product in the filtrate was precipitated out by azeotrope water with ACN to dryness. Yield: 1.04 Kg

Example 13

Preparation of: D-PheProArgProGlyGlyGlyGlyAsnGlyAspPheGluGluIleProGluGluTyrLeuOH.nTFAn (Crude Bivalirudin)

The compounds from Example 12 (1.00 Kg) and Example 10 (0.42 kg) were dissolved in DMSO (5.8 L) and HOBt (0.08 kg) was added. EDCl (0.15 kg) was added and the reaction mixture was allowed stirring continued for another 1 h. The intermediate (M5) was precipitated out by adding water (24 L) and was separated by filtration and washed with water (2×6 L). The precipitated intermediate (M5) was dissolved in mixed solvents of water (0.08 L), TIS (0.33 L) and TFA (7.92 L) and the mixture was allowed to react for 1 h. The product was precipitated out by slowly adding MTBE (29.2 L) and was separated by filtration, washed with MTBE (2×8.3 L) and THF (2×8.3 L). Yield: 1.12 kg

While the invention has been described and illustrated with reference to certain particular embodiments thereof, those skilled in the art will appreciate that various adaptations, changes, modifications, substitutions, deletions, or additions of procedures and protocols may be made without departing from the spirit and scope of the invention. For example, reaction conditions other than the particular conditions as set forth herein above may be applicable as a consequence of variations in the reagents or methodology to prepare the compounds from the processes of the invention indicated above. It is intended, therefore, that the invention be defined by the scope of the claims which follow and that such claims be interpreted as broadly as is reasonable.

Claims

1. A process for the preparation of Bivalirudin comprising the steps of: a) condensing the segment AspPheGlu(tBu)Glu(tBu)IleProOBn (SEQ ID NO: 5) (S3) and the segment FmocGlyGlyGlyGlyAsnGlyOH (SEQ ID NO: 2) (S2) in a first organic solvent;b) deprotecting the product of step a);c) condensing the product of step b) and the segment Boc-D-PheProArg(HCl)ProOH (S1) in a second organic solvent;d) deprotecting the benzyl group of the product of step c);e) condensing the product of step d) and the segment Glu(tBu)Glu(tBu)Tyr(tBu)LeuOtBu (SEQ ID NO: 6) (S4) in a third organic solvent;f) deprotecting the product of step e) to obtain Bivalirudin;

2. The process according to claim 1, wherein the first, second and third organic solvents are independently selected from DCM, DMF, and DMSO.

3. The process according to claim 1, wherein the deprotection of the product of step b) is performed by using a nucleophilic base.

4. The process according to claim 1, wherein the deprotection of the product of step d) is by hydrogenation in presence of a catalyst.

5. The process according to claim 1, wherein the deprotection of the product of step f) is by reacting with a mixture of TFA/TIS/H2O to obtain Bivalirudin.

6. A compound of Segment 1, Boc-D-PheProArg(HCl)ProOH.

7. A process, wherein the preparation of Segment 1 Boc-D-PheProArg(HCl)ProOH (S1) of claim 6 comprises the steps of: a) condensing HCl Arg(HCl)ProOBn (S1M2) and Boc-D-PheProOH (S1M4) in a first organic solvent;b) deprotecting the benzyl group of the product of step a) under alkaline condition;c) precipitating the segment Boc-D-PheProArg(HCl)ProOH (S1) in a second organic solvent.

8. The process according to claim 7, wherein HCl Arg(HCl)ProOBn (S1M2) is prepared by: a) condensing HCl ProOBn and BocArgOH HCl in a third organic solvent;b) deprotecting the product of step a) in an acidic solution to obtain HCl Arg(HCl)ProOBn (S1M2).

9. The process according to claim 7, wherein Boc-D-PheProOH (S1M4) is prepared by: a) condensing HCl ProOBn and Boc-D-PheOH in a fourth organic solvent;b) deprotecting the product of step a) in an alkaline solution to obtain Boc-D-PheProOH (S1M4).

10. A compound of Segment 2, FmocGlyGlyGlyGlyAsnGlyOH (SEQ ID NO: 2).

11. A process, wherein the preparation of Segment 2 FmocGlyGlyGlyGlyAsnGlyOH (SEQ ID NO: 2) (S2) of claim 10 comprises the steps of: a) condensing HCl GlyOtBu and Fmoc Asn(Trt) in a first organic solvent;b) deprotecting the product of step a) in presence of a base;c) condensing the product of step b) and FmocGlyGlyOH in a second organic solvent;d) deprotecting the product of step c) in presence of a base;e) condensing the product of step d) and FmocGluGluOH in a third organic solvent;f) deprotecting the product of step e) to obtain the segment FmocGlyGlyGlyGlyAsnGlyOH (SEQ ID NO: 2) (S2).

12. A compound of Segment 3, Asp(tBu)PheGlu(tBu)Glu(tBu)IleProOBn (SEQ ID NO: 5).

13. A process, wherein the preparation of Segment 3 AspPheGlu(tBu)Glu(tBu)IleProOBn (SEQ ID NO: 5) (S3) of claim 12 comprising the steps of: a) condensing HCl ProOBn with BocIleOH in a first organic solvent;b) deprotecting the product of step a) under acidic condition;c) condensing the product of step b) and FmocGlu(tBu)OH in a second organic solvent to obtain a solution of intermediate;d) deprotecting the intermediate of step c) in presence of a base;e) condensing the product of d) and FmocGlu(tBu)OH in a third organic solvent;f) deprotecting the product of e) in presence of a base;g) condensing the product of f) and FmocPheOH in a fourth organic solvent;h) deprotecting the product of step g) in presence of a base;i) condensing the product of h) and FmocAsp(tBu)OH in a fifth organic solvent;j) deprotecting the product of step i) to obtain the segment AspPheGlu(tBu)Glu(tBu)IleProOBn (SEQ ID NO: 5) (S3).

14. A compound of Segment 4, Glu(tBu)Glu(tBu)Tyr(tBu)LeuOtBu (SEQ ID NO: 6).

15. A process, wherein the preparation of Segment 4 Glu(tBu)Glu(tBu)Tyr(tBu)LeuOtBu (SEQ ID NO: 6) (S4) of claim 14 comprises the steps of: a) condensing FmocTyr(tBu)OH with HCl LeuOtBu in a first organic solvent;b) deprotecting the product of step a) in presence of a base;c) condensing the product of step b) and FmocGlu(tBu)OH in a second organic solvent;d) deprotecting the product of step c) in presence of a base;e) condensing the product of step d) and FmocGlu(tBu)OH in a third organic solvent;f) deprotecting the product of step e) to obtain the segment Glu(tBu)Glu(tBu)Tyr(tBu)LeuOtBu (SEQ ID NO: 6) (S4).