Indian Journal of Chemistry Vol. 43B, June 2004, pp. 1282-1287

Synthesis of NU-protected peptide acids by the N~ C chain extension employing O,N-bis-trimethylsilyl-amino acids using the mixed anhydride method

Subramanyam J Tantry & Vommina V Suresh Babu*

Department of Studies in Chemistry Central College Campus, Bangalore University, Dr B R Ambedkar Veedhi, Bangalore 560 001

Received II October 2002; accepted (revised) 21 July 2003

Synthesis of NQ-protected peptide acids employing N--C extension strategy usi ng ill situ generated X-NH-CHR'-CO­O-CO-iBu and O,N-bis-trimethylsilyl-amino acids has been accomplished. The coupling is very rapid and efficient. The yield and purity of the peptide acids obtained are good. The coupling, as determined by the HPLC analysis of the two di­astereomeric dipeptide acids Fmoc-L-Phg-Phe-OH and Fmoc-D-Phg-Phe-OH prepared by this method is found to be free from racemization . The same strategy has been further extended to the synthesis of the known a-helical peptide segment H­Val-Ala-Leu-Val-Ala-Leu-OH. During its synthesis all the intermediate peptide fragments have been isolated and character­ized by the IH NMR and mass spectra.

IPC: Int.C1.7 C 07 KlIOO

For the synthesis of a dipeptide in solution, in a di­rected manner, it is obvious that certain functional groups must be protected. In order to couple a-amino acids, the functional groups that are not directly in­volved in the amide bond formation have to be pro­tected. Thus, the NU-amino group of carboxyl compo­nent and the C-terminaI carboxyl group of the amino component are blocked before the activation of the carboxyl groupl-5. As practiced routinely now, the at­tachment of subsequent amino acids in the C-tN ter­minal convergent chain extension strategy involves the deprotection of NU-amino protecting group of NU_ protected dipeptide ester and subsequent coupling with NU-protected third amino acid after activation of its a­carboxyl group. Therefore the protection scheme adopted and the method to be executed for the depro­tection of the semipermanent protection group is of the utmost importance. It must be possible to introduce, and more importantly to remove, all the protection groups under conditions that do not affect the integrity of the peptide (including its stereochemical purity) be­ing synthesized. In addition, the yields in all the chemi­cal modifications during the building of the peptide chain should be as high as possible. All these are strin­gent requirements, but the field by now has been exten­sively worked and a whole range of protection groups employable in peptide synthesis is available6-9. How­ever, new chemical approaches to the synthesis of pep­tides which reduce the total number of steps is desir­able. This will result in limited exposure of the care­fully build peptide to drastic conditions and to obtain the final peptide with high yield and without loss of its

optical integrity. This paper describes the synthesis of hexamer by the N-tC terminal extension strategy em­ploying only NU-terminal amino acid as its NU_ FmoclBoc-protected one and O,N-bis-trimethylsilyl­amino acids by the mixed anhydride method using iso­butyl chloroformate (IDC-CI)IN-methyl morpholi ne (NMM) for activation.

Meienhoffer' s group has explored the utility of hy­drolytically labiie trimethylsilyl (TMS) group for the introduction of Fmoc group onto a-amino acids to obtain a whole range of NU-carbamates lo. A similar approach for the facile synthesis of six NU_ protected dipeptide acids employing the pentafluorophenyl es­ters is also known II . In the present study, a series of NU-Fmoc protected dipeptide acids by the coupling of Fmoc-amino acid and O,N-bis-tri nethylsilyl-amin o acid (Scheme I) using IDC-CI has been accom­plished. In a typical procedure, a-amino acid 1 was suspended in dichloromethane (OCM) and N­ethyldiisopropylamine (OlEA) and freshly distilled TMS-CI was added. The mixture was gently heated to about 40°C for 2 hr and then cooled to room tempera­ture to obtain O,N-bis-trimethylsilyl··amino acid 2. To a solution of X-NH-CHR'-CO-O-CO-iBu 3, generated in situ at 0 DC by stirring a mixture of Fmoc-amino acids, NMM and IBC-CI in THF was added the solu­tion containing O,N-bis-trimethylsilyl-amino acid 2 at once. The resulting mixture was continued to stir at the same temperature. The coupling, as monitored by TLC using chloroform-methanol-acetic acid (40:2: 1), was found to be complete in about 20 min. A simple work-up of the reaction mixture lead to the isolation

TANTRY et al.: SYNTHESIS OF NU-PROTECfED PEPTIDE ACIDS 1283

Ii R

H2N~OOH 1

i) TMS-CI •

ii) DIEA

')zR /~~OOH X-HN Y ~""""I o "H

4 3 x = Fmoc or Boc group; 3, in situ generated NQ-protected amino acid anhydride

using X-NH-CHR'-COOH, NMM, mC-Cl, 0 °c, THF.

Scheme 1 - Synthesis of NQ-protected peptide acid

Table 1-Physical constants of NU-protected peptide acids

Compd Peptide [aJD25

(cl, DMF)

4a Fmoc-PhgN-Phe-OH -44.1

4b Fmoc-o-Phg-Phe-OH +43.5

4c Fmoc-Leu-Aib-OH -10.5

4d Fmoc-Val-Aib-OH -12.7

4e Fmoc-Val-Sar-OH -11.7

4f Boc-Val-Pro-OH -1.5

5 Fmoc-Val-Ala-OH -10.5

6 Fmoc-Val-Ala-Leu-OH -22.5

7 Fmoc-Val-Ala-Leu-Val-OH -5.5

8 Fmoc- Val-Ala-Leu-Val-Ala-OH -8.9

9 Fmoc- Val-Ala-Leu-Val-Ala-Leu-OH -10.2

# Phg, 2-amino-2-phenylacetic acid (phenyl glycine)

of Fmoc-/Boc-protected peptide acid 4 in 78-83% yield (Table I). All the peptide acids 4a-f made by this method have been fully characterized by 'H NMR and mass spectral studies. The HPLC analysis of the pure and crude diastereomeric dipeptide acids Fmoc­L-Phg-Phe-OH (Figures la & Ib : R, value for pure 30.192 min.) and Fmoc-D-Phg-Phe-OH (Figures lc & Id : R, value for pure 31.300 mjn.) prepared by this method revealed that the coupling is free from race­mization.

This method is then extended for the synthesis of the hexapeptide H-Val-Ala-Leu-Val-Ala-Leu-OH, a known strandard a-helical peptide segment'2 (Scheme II). The intermediate peptide acid fragments dipeptide

MALDI-TOF [M+NaJ mp Yield Crystallization

Observed Theoretical °c (% ) Solvent

543.3 543.6 170-73 80 EtOAc

543.9 543.6 175-78 82 EtOAc

461.2 461.5 173-76 80 EtOAc : hexane

447.3 447.5 195-97 87 EtOAc : hexane

432.8 433.3 Oil 78

338.0 337.4 103-06 90 EtOAc : hexane

432.9 433.3 205-08 90 EtOAc

546.8 546.5 178-80 85 EtOAc

647.0 645.9 198-200 84 CHCl3 : MeOH

717.7 717.0 >265 80 CHCl3 : MeOH

830.6 830.2 255-58 79 CHCI3 : MeOH

acid 5 (Fmoc-Val-Ala-OH), tripeptide acid 6 (Fmoc­Val-Ala-Leu-OH), tetrapeptide acid 7 (Fmoc-Val­Ala-Leu-Val-OH), pentapeptide acid 8 (Fmoc-Val­Ala-Leu-Val-Ala-OH), and hexapeptide acid 9 (Fmoc-Val-Ala-Leu-Val-Ala-Leu-OH) have been isolated and are obtained in about 79-83% yield, and the purity of crude peptide 9 as checked by HPLC was found to be about 90% pure (Figure 2 : R, value for crude 6.750 min.) and are fully characterized.

In summary, the synthesis of NU-protected (Fmoc and Boc) peptide acids by the extension of the peptide chain from N~C terminal end using the mjxed anhy­dride method for coupling and O,N-bis-trimethylsilyl­amjno acids as amino components has been achieved.

1284 [NOl AN 1. C HEM ., SEC B, JUNE 2004

26 28 30

.... ... ... o ..,

32

Minutes

36

Figure III - Analytical RP-HPLC of pure Fmoc- Phg- Phe-OH:

Waters C- IS deltapak colu mn (3.9 x 300mm. 15/1); Flow rate 1.5 mLlmin; e luant Illethanol and H20; (60: 40); isocratic; contain­ing 1% TFA; monitoring at 215 nm.

'" .., .. ~

'"

f f

211 211

'" III

o ...

~ 32

Minu ies

'" ... ~ ... ...

f f -r-. 34 38 :se

Figure Ib - Analyti cal RP-HPLC 01 crude Fmoc-Phg-Phe-Oll: Waters C- IS c!eltapak coluilln (3 .9 x 300mm, 15/1) ; Flow rate 1.5 mIJlllin; eluant methanol anc! 1-1 -0: (60 : 40) isocratic; contain­Ing 1% TFA; monitoring at 2 15 11 m.

26 28 30

o o .., ... '"

32

Minutes 36

Figure ie - Analytical RP-HPLC of pure FIllOc-D-Phg- Phe-OH:

Waters C- IS de ltapak coluilln (3. 9 x 300mm, 15/1); Flow rate 1.5 mLlmin; el uant methanol and H20 (60: 40) lsocrati c; containin o I% TFA; monitoring at 215 nm . b

211 ~

III

'" ... ... '"

I

32

Minutes

34

... .. r: III ... • 0

'"

f

3& :se

Figure Id - Analyticai RP-HPLC of crude Fmoc-D-Phg-Phe-O ll Waters C- IS c!eltapak co lumn (3.9 x 300mm, I Su); Flow rate 1.5 mLllllin ; el uant methanol and H20 (GO : 40) i ·ocratic: cont ainillg 1% TFA mon itoring at 2 15 nm.

TANTRY et al.: SYNTHESIS OF NU-PROTECTED PEPTIDE ACIDS 1285

Val Ala Leu Val Ala Leu

I-OH ~MS-A

c- -TM§ Fmo

Fmo c

Fmo v

Fmo c

Fmo ,~

Fmo 'v

~ ... ..

0 10 0 . .. . 0 ~ 10 ..

"! :r . .. .. ..

A B

OHfrMS- f-TM~ A

B OH~MS--TM~

A B

OHrrMS- -TM~ A

B OH~MS- ---' TM~

B OH

A. H2N-CHR-COOH, OlEA, TMS-CI, DCM. B. Fmoc-NH-CHR'-COOH, NMM , IBC-CI , 0 °c, THF.

Scheme II - Synthesis of Fmoc-Val-Ala-Leu-Val-Ala-Leu-OH

... .. . ...

, 10

Minutes

.. .. ..

, 12

, , 1. 1.

This approach has resulted in obtaining the hexapep­tide Fmoc-Val-Ala-Leu-Val-Ala-Leu-OH 9 in fewer steps than the conventionally followed routes with good yield.

Experimental Section

All amino acids except glycine unless mentioned were of L-configuration. Melting points were deter­mined using Leitz-Wetzlar melting point apparatus and are uncorrected. Optical rotations were deter­mined with an automatic digital AA-lO polarimeter (Optical activity, U.K.). Analytical HPLC was per­formed on a Waters 3000 LC system. IH NMR was recorded on a Bruker AMX 400 MHz instrument with Me4Si as an internal standard.

Synthesis of O,N-bis-trimethylsilyl-amino acids 2: General procedure. To a suspension of amino acid 1 (l mmole) in DCM (5 mL) was added DIEA (0.35 mL, 2 mmoles) and TMS-Cl (0.15 mL, 2 rrunoles) and refluxed under N2 for 2 hr. The resulting solution was cooled to r.t. and used directly.

Figure 2 - Analytical RP-HPLC of crude Fmoc- Val-Ala-Leu­

Val -Ala-Leu -OH ; Column: Merck RP- 18, (4.6 x 250 mm, 51l); Flow rate 1.5 mLllllin ; elu ant : acetonitrile: water (60 : 40) iso­cratic : pH adj u ' tcd to 3.5 wi th TFA; monitoring at 250 nm.

Synthesis of NU-protected peptide acids 4. Gen­eral procedure. To a chilled solution of Fmoc amino acid (1 mmole) and NMM (0.11 mL, 1 mmoJe) in dry THF (5 mL) was added lEC-Cl (0.135 mL, I mmole)

1286 INDIAN 1. CHEM., SEC B, JUNE 2004

and stirred at O°C for 20 min. Then, the solution con­taining O,N-bis-trimethylsilyl-amino acid 2 was added directly in one portion through syringe. The stirring was continued till the completion of the reac­tion. The organic layer was evaporated and the result­ing residue was partitioned between 10 % aqueous NaHC03 solution (10 mL) and diethyl ether (10 mL). The aqueous layer was washed further with diethyl ether (5 mL) twice and acidified using 5% HCI (10 % aqueous citric acid solution in case of Boc protected peptide acid). The precipitated solid was filtered, washed twice with water and recrystallized using suit­able solvent to obtain the peptide acid 4 as a crystal­line solid.

Fmoc-Phg-Phe-OH 4a. To a solution of Fmoc­Phg-OH (700 mg, 1.87 mmoles), NMM (0.2 mL, I.S7 mmoles) and IBC-CI (0.25 mL, 1.S7 mmoles) at 0 °C was added the solution containing [O,N-bis-TMS­Phe] prepared by using Phe (340 mg, 2.05 mmoles), TMS-CI (0.32 mL, 4.1 mmoles) and OIEA (0.71 mL, 4.1 mmoles) to obtain 770 mg (SO %) of the peptide acid 4a. IH NMR (OMSO) : J 2.9 (2H, d), 3.1 (1H, t), 4.25 (3H, t), 4.5 (1H, d) , 5.3 (1H, d) , 7.1-7 .9 (ISH, ArH), S.O (1H, d) .

Fmoc-D-Phg-Phe-OH 4b. To a solution of Fmoc­D-Phg-OH (700 mg, 1.S7 mmoles), NMM (0.2 mL, 1.S7 mmoles) and IBC-CI (0.25 mL, I.S7 mmoles) at o °C was added the solution containing [O,N-bis­TMS-D-Phe] prepared by using D-Phe (340 mg, 2.05 mmoles), TMS-CI (0.32 mL, 4.1 mmoles) and OIEA (0.71 mL, 4.1 mmoles) to obtain 770 mg (SO %) of the peptide acid 4b. IH NMR (OMSO) : J 2.9 (2H, d), 3.1 (1H, t), 4.25 (3H, m), 4.5 (1H, d), 5.3 (1H, d) , 7.1-7.9 (ISH, ArH), S.O (l H, d).

Fmoc-Leu-Aib-OH 4c. To a solution of Fmoc­Leu-OH (700 mg, 2.0 mmoles), NMM (0.2 mL, 2.0 mmoles) and IBC-CI (0.27 mL, 2.0 mmoles) at 0 °C was added the solution containing [O,N-bis-TMS­Aib] prepared by using Aib (210 mg, 2.05 mmoles), TMS-CI (0.34 mL, 4.2 mmoles) and OIEA (0.76 mL, 4.2 mmoles) to obtain 700 mg (SO %) of the peptide acid 4c. IH NMR (OMSO) : J 0.S5 (6H, d), 1.31 (3H, s), 1.33 (3H, s), 1.5 (3H, m), 4.1 OH, m), 4.2 (3H, m), 7.3-7.9 (SH, ArH), S.05 (lH, d).

Fmoc-Val-Aib-OH 4d. To a solution of Fmoc­Val-OH (675 mg, 2.0 mmoles) , NMM (0.2 mL, 2.0 mmoles) and IBC-CI (0.27 mL, 2.0 mmoles) at 0 °C was added the solution containing [O,N-bis-TMS­Aib] prepared by using Aib (210 mg, 2.05 mmoles) , TMS-CI (0.34 mL, 4.2 mmoles) and OIEA (0.76 mL, 4.2 mmoles) to obtain 750 mg (S9 %) of the peptide

acid 4d. IH NMR (OMSO) : J 0.S5 {6H, d), 1.3 (6H, s), 1.95 (lH, m), 3.S6 (lH, m), 4 .2 (2H, d), 4.5 (lH, t), 5.3 (1H, d), 7.3-7 .9 (SH, ArH), S.l (lH, d).

Fmoc-Val-Sar-OH 4e. To a solution of Fmoc-Val­OH (675 mg, 2.0 mmoles), NMM (0.2 mL, 2.0 mmoles) and IBC-CI (0.27 mL, 2.0 mmoles) at 0 °C was added the solution containing [O,N-bis-TMS-Sar] prepared by using Sar (190 mg, 2.1 nunoles), TMS-CI (0.34 mL, 4.2 mmoles) and OIEA (0.76 mL, 4.2 mmoles) to yield 630 mg (78 %) of the peptide acid 4e. IH NMR (OMSO) : J O.S5 (6H, d) , 1.95 (1H, m), 2.9 (2H, s), 4.2 (2H, d), 4.5 (lH, t) . 4.7 (3H, s), 5.3 (lH, d), 7.3-7.9 (SH, ArH).

Roc-Val-Pro-OH 4f. To a solution of Boc-Val-OH (430 mg, 2.0 mmoles) , NMM (0.2 mL, 2.0 mmoles) and IBC-CI (0.27 mL, 2.0 IllJTIoles) at 0 °C was added the solution containing [O,N-bis-TMS-Pro] prepared by using Pro (240 mg, 2.1 mmoles) , TMS-CI (0.34 mL, 4.2 mmoles) and OIEA (0.76 mL, 4.2 IllJTIoles) to obtain 560 mg (90 %) of the peptide acid 4f. IH NMR (OMSO) : J 0.S5 (6H, d), 1.3 (9H, s), 1.4-1.6 (SH, m), 4 .2 (2H, d), 4.5 (lH, t) , 4.7 (3H, s), 7.3-7 .9 (SH, ArH), S.l (lH, d) .

Synthesis of Fmoc-Val-Ala-Leu-Val-Ala-Leu­OH (9).

Fmoc-Val-Ala-OH 5. To a solution of Fmoc-Val­OH (675 mg, 2.0 mmoles), NMM (0.2 mL, 2.0 mmoles) and IBC-CI (0.27 mL, 2.0 mmoles) at 0 "c was added the solution containing [O,N-bis-TMS­Ala] prepared by using Ala (190 mg, 2.1 mmoles), TMS-CI (0.34 mL, 4.2 mmoles) and DIEA (0.76 mL, 4.2 mmoles) to obtain 730 mg (90 %) of the peptide acid 5. IH NMR (OMSO) : J 0.S5 (6H, m), 1.3 (3H, d), 2.0 (1H, m), 3.9 (lH, t), 4.3 (4H, Ill), 4.5 (lH, t), 7.3-7.5 (5H, m) .

Fmoc-Val-Ala-Leu-OH 6. To a solution of Fmoc­Val-Ala-OH 5 (610 mg, 1.5 IllJTIoles), NMM (0.175 mL, 1.57 mmoles) and IBC-CI (0.21 mL, 1.57 mmoles) at 0 °C was added the solution containing [O,N-bis-TMS-Leu] prepared by using Leu (210 mg, 1.65 mmoles), TMS-CI (0.27 mL, 3.5 mmoles) and OIEA (0.61 mL, 3.5 mmoles) to obtain 660 mg (S5 %) of the peptide acid 6. IH NMR (DMSO) : J O.S5 (l2H, m), 1.1 (2H, d), 1.2 (3H, d) , 1.4 (2H, t) , 1.6 (lH, m), 1.9 (2H, m), 3.9 (1H, t), 4.1 (lH, m), 4.2-4.4 (4H, m), 7.3-7.5 (5H, t), 7.7-S.1 (6H. Ill) .

Fmoc-Val-Ala-Leu-Val-OH 7. To a solution of Fmoc-Val-Al~-Leu-OH 6 (525 mg, 0.96 mmoles), NMM (0.11 mL, 1.0 mmole) and IBC-Cl (0.135 mL, 1.0 mmole) at 0 °C was added the solution containing [O,N-bis-TMS-Val] prepared by using Val (120 mg,

TANTRY et al.: SYNTHESIS OF Na-PROTECTED PEPTIDE ACIDS 1287

2.0 mmoles), TMS-Cl (0.156 rnL, 2.0 mmoles) and DIEA (0.3 mL, 2.0 mmoles) to obtain 520 mg (84 %) of the peptide acid 7. IH NMR (DMSO) : b 0.85 (l8H, m), 0.91 (7H, m), 1.2 (3H, d), 1.5 (2H, t) , 1.6 (lH, m), 2.0 (2H, m), 3.9 (lH, t), 4.2 (lH, m), 4.3-4.5 (5H, m), 7.3 (2H, t), 7.4 (3H, t), 7.7-8 .1 (7H, m) .

Fmoc-Val-Ala-Leu-Val-Ala-OH 8. To a solution of Fmoc-Val-Ala-Leu-Val-OH 7 (500 mg, 0.8 mmole), NMM (0.09 rnL, 0.9 mmole) and IBC-Cl (0.11 mL, 0.9 mmole) at 0 DC was added the solution containing [O,N-bis-TMS-Ala] prepared by using Ala (80 mg, 0.9 mmole), TMS-Cl (0.15 mL, 1.9 mmoles) and DIEA (0.3 rnL, 1.8 mmoles) to obtain 440 mg (80 %) of the peptide acid 8. IH NMR (DMSO) : b 0.8 (l8H, m), 0.91 (7H, m), 1.2 (6H, d) , 1.5 (2H, t) , 1.6 (lH, m), 1.9(2H), 3.9 (lH, t) , 4.1 (lH, m), 4.2-4.4 (6H, m), 7.3 (2H, t) , 7.4 (3H, m), 7.7-8.1 (8H, m).

Fmoc-Val-Ala-Leu-Val-Ala-Leu-OH 9. To a so­lution of Fmoc-Val-Ala-Leu-Val-Ala-OH 8 (400 mg, 0.57 mmole) and NMM (0.065 rnL, 0.6 mmole) dissloved in THF : DMF (1:1, 10 rnL) was added IBC-Cl (0.08 rnL, 0.6 mmole) at 0 DC. To this solution was added the solution containing [0, N-bis-TMS­Leu] prepared by using Leu (80 mg, 0.65 nuTIole), TMS-Cl (0.10 rnL, 1.36 mmoles) and DIEA (0.23 mL, 1.36 mmoles). After work up it resulted in 360 mg (79 %) of the peptide acid 9. IH NMR (DMSO) : b 0.8 (24H, m), 1.2 (6H, d), 1.5 (2H, t) , 1.6 (2H, m),1.9 (4H, m), 3.9 (l H, t), 4.1 (l H, m), 4.2-4.4 (7H, m), 7.3 (2H, t), 7.4 (3H, m), 7.7-8.1 (9H, m).

Acknowledgement

Authors thank Professors K M Sivanandaiah and B S Sheshadri for useful discussions. Authors also thank CSIR, New Delhi for financial assistance. Also help from SIF, lISc, Bangalore is gratefully acknowledged for getting the IH NMR spectra done.

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