Microwave irradiated high-speed solution synthesis of peptide acids employing Fmoc-amino acid pentafluorophenyl esters as coupling agents
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1 Indian Journal of Chemistry Vol. 44B, ovember 2005, pp Microwave irradiated high-speed solution synthesis of peptide acids employing Fmoc-amino acid pentafluorophenyl esters as coupling agents Vommina V Suresh Babu* & R V Ramana Rao Department of Studies in Chemistry, Central College Campus, Dr. B. R. Ambedkar Veedhi, Bangalore University, Bangalore , India hariccb@rediffmail.com Received 23 ovember 2004; (revised) aceepted 24 May 2005 A high-speed solution phase synthesis of peptide acids employing commercially available Fmoc-amino acid pentafluorophenyl esters as coupling agents has been demonstrated. The coupling has been found to be fast and completed in sec. A simple work-up of the reaction mixture has resulted -protected peptide acids in good yield. Utilizing the present method, the coupling of difficult sequences containing highly hindered α, α-dialkyl amino acids has also been demonstrated. Further, the synthesis of diastereomeric dipeptides, Fmoc-Phg-Phe-Me and Fmoc-D-Phg-Phe-Me revealed that the coupling is free from racemization. Keywords: Peptide acids, microwave, Fmoc, pentafluorophenyl esters, coupling agents IPC: Int.Cl. 7 C 07 K Despite the dominance of the solid phase method, the synthesis of peptides in solution remains one of the major chemical approaches to the peptide and protein synthesis. The principal advantage of synthesis in solution is that intermediates can be isolated and characterized at every step 1. This approach is particularly important in the large-scale synthesis of peptides, the synthesis of peptides composed of unusual amino acids, and in the synthesis of cyclic peptides and depsipeptides, etc. The chemical synthesis of peptides by the active ester method employing mainly halogenated (chloro or fluoro compounds) phenyl and succinamidyl esters is a time tested and well-known method. Their solubility in dimethylformamide and 1-methylpyrrolidine, storage, easy preparation as well as chiral stability under the usual conditions of coupling in peptide synthesis are some of the reasons for their popularity 2,3. However, they have low level of activation compared to several coupling reagents like -(benzotriazol-1-yl)-,, ', '-tetramethyluronium hexaflurophosphate (HBTU), -benzotriazol-1-yl-,, ', '-tetramethyluronium tetrafluoroborate (TBTU) and (benzotriazol-1-yloxy)-tris (dimethylamino)phosphonium hexaflurophosphate (BP) 4, etc. Consequently, using an equimolar quantity of HBt and in some cases, a tertiary amine, as well, generally enhances the rate of aminolysis. At ambient temperature, the completion of the coupling using active esters or carbodiimides requires about 2 to 3 hr. n the other hand, the scope of the benefits of microwave irradiation in organic synthesis is being explored rapidly 5-9. There are a few reports describing the utility of microwave irradiation in solid phase peptide synthesis The coupling efficiency using 9-flurenylmethyloxycarbonyl (Fmoc) amino acids in solid phase method employing symmetrical anhydrides or performed HBt esters (prepared using DCC and HBt) 13, PyBP, TBTU, HATU 14, has been studied. The synthesis of peptides assisted by microwave irradiation is not only efficient but also results in high purity as well as yield 15. Recently, our group has explored the utility of microwave irradiation in the preparation of several building blocks useful in peptide synthesis. These include amino acid benzyl esters 16, β-amino acids 17, Fmoc-/Z- /Boc-5-oxazolidinones 18 and isocyanates of Fmocamino acid azides 19. Herein, we describe a microwave enhanced high-speed route for the solution synthesis of protected peptide acids employing Fmoc-/Boc-/Zamino acid pentafluorophenyl esters. In a typical experiment, amino acid was dissolved in dioxane-10% a 2 C 3 and Fmoc-amino acid pentafluorophenyl ester in dioxane was added. The
2 BABU et al.: SYTHESIS F PEPTIDE ACIDS FMC-AMI ACID PETAFLURPHEYL ESTERS 2329 reaction was then exposed to microwave irradiation (Scheme I). The microwave oven is 1200 W oven and the reaction was speciphically carried out at 60% of the total power output which would correspond to an average power of 720 W. The formation of the peptide, as monitored by TLC using the solvent system chloroform-methanol-acetic acid (40:2:1) as well as by HPLC analysis, has been found to be complete in about 30 to 45 sec. After the simple regular work-up of the reaction mixture, all the peptide acids 2a-q (Table I) synthesized have been isolated as pure solids in about 60 to 90% yield. They have been completely characterized by 1 H MR as well as mass spectrometry. The procedure is then applied with suitable modifications for the synthesis of protected peptide esters also 4a-f (Table II) 20, 21. In this case, the C- protected amino acid ester salt and - methylmorpholine (MM) were dissolved in DMF and then Fmoc-/Boc amino acid pentafluorophenyl ester was added (Scheme II). The resulting solution was subjected to microwave irradiation. After the completion of the coupling, it was subjected to usual work-up and resulting peptide ester was isolated in good yield. The coupling is found to be fast and complete within 1 min. Finally, in order to evaluate the efficacy of the present procedure, the synthesis of peptide acids containing highly hindered amino acid like α- aminoisobutyric acid was chosen 22. The synthesis of the model dipeptide Fmoc-Aib-Aib-H 2o was carried out employing six of its active esters (trichlorophenyl, pentachlorophenyl, o-nitrophenyl, p- nitrophenyl, succinamidyl and pentafluorophenyl esters) and the results are summarized in Table III. These studies revealed that the coupling of Fmoc- Aib to Aib to obtain Fmoc-Aib-Aib-H employing Fmoc-Aib-Pfp was found to be the most efficient R 5 R 6 R 1 R 4 CH H Fmoc- Pfp a 2 C 3 /MW R 2 R 3 R 1 Fmoc- R 5 R 6 H R 2 R3 R Compd R 1 R 2 R 3 R 4 R 5 R 6 2a H H CH 2 Ph H H H 2b H H CH (CH 3 ) 2 H H H 2c H H CH 3 H H CH 3 2d H H CH 2 Ph H H CH 2 C 6 H 4 (ph) 2e H H CH 2 CH 2 CHTrt H H CH(H)CH 3 2f H H H 2 C Trt H H CH 2 H 2g H H CH 3 H H CH 2 CH (CH 3 ) 2 2h H H CH (CH 3 ) 2 H H H 2i H H CH 2 Ph R 4 & R 5 =(CH 2 ) 3 H 2j H H CH (CH 3 )CH 2 CH 3 H H CH 2 Ph 2k H H H R 4 & R 5 =(CH 2 ) 3 H 2l R 1 & R 2 =(CH 2 ) 3 H R 4 & R 5 =(CH 2 ) 3 H 2m H H CH 2 CH (CH 3 ) 2 H H CH 3 2n H H CH (CH 3 ) 2 H H CH 3 2o H H CH 3 H H CH 3 2p H R 2 & R 3 = (CH 2 ) 6 H H CH 2 H 2q H H CH 3 H H CH 3 Scheme I Synthesis of peptide acids by pentafluorophenyl ester method
3 2330 IDIA J. CHEM., SEC B, VEMBER 2005 one. Thus, with the duration of the coupling of about 45 sec, 82% of the pure peptide was isolated. This procedure was then extended to the synthesis of Fmoc-Aib-Aib-Aib-H 2q. Table I The Fmoc-/Boc-/Z-dipeptide acids synthesized employing Fmoc-amino acid pentafluorophenyl ester under microwave irradiation Compd Dipeptide acid Time m.p. Yield (sec) o C (%) 2a Fmoc-Phe-Gly-H b Fmoc-Val-Ala-H c Fmoc-Ala-Ala-H d Fmoc-Phe-Tyr-H e Fmoc-Gln(Trt)-Thr-H f Fmoc-His(Trt)-Ser-H g Fmoc-Val-Ala-Leu-H h Z-Val-Gly-H i Z-Phe-Pro-H j Z-Ile-Phe-H k Boc-Gly-Pro-H l Boc-Pro-Pro-H m Fmoc-Leu-Aib-H n Fmoc-Val-Aib-H o Fmoc-Aib-Aib-H p Fmoc-Ac 6 c-ser-h q Fmoc-Aib-Aib-Aib-H Table II The Fmoc-/Boc-dipeptide esters synthesized employing Fmoc-amino acid pentafluorophenyl ester under microwave irradiation Compd Dipeptide ester Time Yield m.p (sec) (%) o C 4a Fmoc-Tyr(Bzl)-Phe-Me b Fmoc-Ile-Gly-Et c Boc-Ile-Val-Me d Fmoc-Aib-Ala-Bzl e Fmoc-Phg-Phe-Me (194-96) 25 4f Fmoc-D-Phg-Phe-Me (192-94) 25 In summary, we developed a high-speed solution synthesis of peptide acids employing commercially available pentafluorophenyl esters as coupling agents. The coupling was found to be complete in sec. At ambient temperature the rate of acylation reactions using Fmoc-amino acid pentafluorophenyl esters has been increased by carrying out the coupling in the presence of an equimolar quantity of 1- hydroxybenzotriazole and a base (-ethyldiisopropyl amine) 23. In the present study no coupling additives were necessary. All the peptide acids made have been obtained in good yield. The coupling of α, α- dialkylaminoacids employing several coupling routes is known to be slow and results in poor yields. The microwave irradiation technique employing the coupling agents namely HBTU/HBt and PyBP/HBt also required 30 min irradiation for such couplings 24. In our study, it has been found that such couplings can also be accomplished within one minute with good yield. Many of the peptides made by the present procedure have been carried out on 10 mmoles scale. Consequently, the adaptation of this procedure will certainly speed up the preparation of small peptides in any research laboratory. Experimental Section Melting points were determined using capillary method and are uncorrected. IR spectra were recorded Table III Yield and duration of coupling employing several active esters of Fmoc-Aib for the synthesis of Fmoc-Aib-Aib- H Active ester m.p. Time Yield o C (sec) (%) Fmoc-Aib-Tcp Fmoc-Aib-Pcp Fmoc-Aib-p Fmoc-Aib-pp Fmoc-Aib-Su Fmoc-Aib-Pfp HCl.H 2 CHR 2 CR 3 Fmoc-H Pfp R 2 Fmoc-H MM/MW R 1 R 1 H R R 3 = methyl, ethyl and benzyl moiety Scheme II Synthesis of peptide ester by pentafluorophenyl ester method
4 BABU et al.: SYTHESIS F PEPTIDE ACIDS FMC-AMI ACID PETAFLURPHEYL ESTERS 2331 on a icolet model impact 400D FT-IR spectrometer (KBr pellets, 3 cm -1 resolution); mass spectra on a MALDI-TF (KRATS); and 1 H MR spectra on a Bruker AMX-400 MHz spectrometer using TMS as internal standard. LG domestic microwave oven operating at 2450 MHz frequency at its 60 % power was used for microwave irradiation. Amino acids were purchased from Sigma Aldrich Co., USA. Several active esters [2, 4, 5-trichlorophenyl (Tcp), pentachlorophenyl (Pcp), o-nitrophenyl (o-p), p- nitrophenyl (p-p), succinamidyl (Su), pentafluorophenyl (pfp) ] of Fmoc-Aib have been prepared following the reported procedures 26 and their physical constants are listed in the Table III. General procedure for the synthesis of dipeptide acids 2a-q. Amino acid (12 mmoles) dissolved in 10% a 2 C 3 (10 ml) and dioxane (10 ml) was added to the solution of Fmoc-/Boc-/Z-amino acid pentafluorophenyl ester (10 mmoles) in dioxane (10 ml) in a closed Teflon vessel (Volume 250 ml) was exposed to microwave irradiation operating at its 60% power. After completion of the reaction, it was diluted with water and washed with ether. The aqueous layer was acidified by using 10% HCl (10% citric acid solution in the case of Boc protected peptide esters) and extracted with ethyl acetate. The organic layer was given water wash followed by brine and dried over anhydrous a 2 S 4. The solvent was removed under vacuum and the resulting residue was recrystallized using hexane-ethyl acetate. All the peptide acids (Table I) made were obtained as crystalline solids in good yield. General procedure for the synthesis of dipeptide esters 4a-f. To a glass beaker containing a mixture of Fmoc-/Boc-amino acid pentafluorophenyl ester (10 mmoles) and amino acid ester hydrochloride (12 mmoles) in dry DMF (10 ml), MM (1.3 ml, 12 mmoles) was added. The reaction mixture was exposed to microwave irradiation operating at its 60% power. After completion of the reaction, the organic layer was diluted with water (50 ml), extracted into dichloromethane (DCM, 30 ml), washed with 5% HCl or10% citric acid (5 ml 2), 10 % aqueous ahc 3 (5 ml 2) and water (5 ml 2). The organic layer was dried over anhydrous a 2 S 4 and evaporated under reduced pressure. The resulting residue was crystallized from n-hexane-ethyl acetate (3:1) to obtain the peptides as crystalline solids. All the peptide esters (Table II) made were obtained as crystalline solids in good yield. Fmoc-Phe-Gly-H 2a: 1 H MR (DMS): δ 1.4 (2H, d), 3.2 (2H, d), 5.5 (1H, br), 4.14 (1H, t), 4.44 (2H, d), 6.3 (1H, d), (13H, m), 8.0 (1H, d); MS (MALDI-TF) m/z observed: Fmoc-Val-Ala-H 2b: 1 H MR (DMS): δ 0.85 (6H, m), 1.3 (3H, d), 2.0 (1H, m), 3.9 (1H, t), 4.3 (4H, M), 4.5 (1H, t), (5H, m); MS (MALDI-TF) m/z observed: Fmoc-Ala-Ala-H 2c: 1 H MR (DMS): δ 1.2 (6H, d), 4.14 (1H, t), 4.45 (2H, d), 5.8 (1H, m), 6.7 (1H, br), 7.3 (2H, t), 7.4 (2H, t), 7.58 (2H, d), 7.76 (2H, d); MS (MALDI-TF) m/z observed: Fomc-Phe-Tyr-H 2d: 1 H MR (DMS): δ 2.7 (2H, d), 3.0 (2H, d), 3.9 (1H, m), 4.02 (1H, m), 4.14 (1H, t), 4.45 (2H, d), 5.7 (1H, Br), 6.3 (1H, Br), (17H, m), 9.1 (1H, Br); MS (MALDI-TF) m/z observed: Fmoc-Gln(Trt)-Thr-H 2e: 1 H MR (CDCl 3 ): δ 1.1 (3H, d), (m, 2H), (m, 2H), 4.39 (1H, dd), 4.1 (1H, dd), 3.5 (2H, m), 4.13 (1H, t), 4.45 (2H, d), 5.9 (1H, d), 6.0 (1H, d), (23H, m); MS (MALDI-TF) m/z observed: Fmoc-His(Trt)-Ser-H 2f: 1 H MR (CDCl 3 ): δ 2.5 (2H, d), 3.1 (d, 2H), 3.9 (1H, dd), 4.03 (1H, dd), 4.1 (1H, t), 4.4 (2H, d), 5.7 (1H, d), 5.9 (1H, d), (25H, m); MS (MALDI-TF) m/z observed: Fmoc-Val-Ala-Leu-H 2g: 1 H MR (DMS): δ 0.85 (12H, m), 1.1 (2H, d), 1.2 (3H, d), 1.4 (2H, m), 1.6 (1H, m), 1.9 (2H, m), 3.9 (1H, t), 4.1 (1H, m), (4H, m), (5H, t), (6H, m); MS (MALDI-TF) m/z observed: Z-Val-Gly-H 2h: 1 H MR (CDCl 3 ): δ 0.87 (6H, d) 1.5 (1H, m), 2.54 (2H, d), 3.79 (1H, m), 4.5 (2H, s), 4.8 (1H, t), 5.18 (2H, s), 5.5 (1H, d), 5.9 (1H, d), 7.36 (5H, s); MS (MALDI-TF) m/z observed: Z-Phe-Pro-H 2i: 1 H MR (CDCl 3 ): δ (6H, m), 3.0 (2H, d), 3.9 (1H, m), 4.0 (1H, m), 5.17 (2H, s), (10H, m); MS (MALDI-TF) m/z observed: Z-Ile-Phe-H 2j: 1 H MR (CDCl 3 ): δ 0.88 (6H, m), (2H, m), 1.8 (1H, m), 3.0 (2H, d), 3.9 (1H, m), 4.1 (1H, m), 5.1 (1H, d), 5.12 (2H, s), 5.4 (1H, Br), (10H, m); MS (MALDI-TF) m/z observed: Boc-Gly-Pro-H 2k: 1 H MR (CDCl 3 ): δ 1.13 (9H, s), (6H, m), 2.6 (2H, d), 4.0 (1H, m), 5.7 (1H, d); MS (MALDI-TF) m/z observed: 301.3
5 2332 IDIA J. CHEM., SEC B, VEMBER 2005 Boc-Pro-Pro-H 2l: 1 H MR (CDCl 3 ): δ 1.1 (9H, s), (12H, m), 4.0 (2H, m); MS (MALDI-TF) m/z observed: Fmoc-Leu-Aib-H 2m: 1 H MR (CDCl 3 ): δ 0.94 (6H, q), (3H, m), 1.53 (6H, s), 4.1 (1H, m), 4.14 (1H, t), 4.44 (2H, d), 4.8 (1H, d), 4.9 (1H, d), (8H, m); MS (MALDI-TF) m/z observed: Fmoc-Val-Aib-H 2n: 1 H MR (CDCl 3 ): δ 0.85 (6H, s), 1.3 (6H, s), 1.95 (1H, m), 3.86 (1H, m), 4.2 (2H, d), 4.5 (1H, t), 5.3 (1H, d), (8H, m), 8.1 (1H, d); MS (MALDI-TF) m/z observed: Fmoc-Aib-Aib-H 2o: 1 H MR (CDCl 3 ): δ 1.53 (6H, s), 1.49 (6H, s), 4.14 (1H, t), 4.44 (2H, d), 5.33 (1H, s), 6.9 (1H, s), 7.31 (2H, t), 7.4 (2H, t), 7.58 (2h, d), 7.76 (2H, d); MS (MALDI-TF) m/z observed: Fmoc-Ac 6 c-ser-h 2p: 1 H MR (CDCl 3 ): δ (10H, m), 4.2 (1H, d), 4.4 (2H, t), 6.1 (1H, s), (8H, m), 6.9 (1H, m), 3.6 (2H, d), 3.4 (1H, t); MS (MALDI-TF) m/z observed: Fmoc-Aib-Aib-Aib-H 2q: 1 H MR (CDCl 3 ): δ 1.53 (6H, s), 1.49 (6H, s), 1.47 (6H, s), 4.14 (1H, t), 4.44 (2H, d), 5.33 (1H, s), 5.3 (1H, s), 6.9 (1H, s), 7.31 (2H, t), 7.4 (2H, t), 7.58 (2H, d), 7.76 (2H, d); MS (MALDI-TF) m/z observed: Acknowledgement Authors thank the Department of Biotechnology, Govt. of India for financial support. ne of the authors (R V R R) thanks the CSIR, Govt. of India, for award of junior research fellowship (ET). References 1 Llyod-Williams P, Albericio F & Giralt E, Chemical Approaches to the Synthesis of Peptides and Proteins, (CRC Press, ew York), 1997, Bodanszky M, Principles of Peptide Synthesis, revised edn, (Springer-Verlag, Berlin), 1993, Benoiton L, Methods of organic chemistry, Synthesis of peptides and peptidomimetics, edited by M Goodman, A Felix, L Moroder & C Toniolo, (Houben-weyl, ew York), 2002, E 22a, Albericio F, Chinchilla R, Dodsworth D J & ajera C, rg Prep Proc Int, 33, 2001, Abramovitch R A, rg Prep Proc Int, 23, 1991, Caddick S, Tetrahedron, 51, 1995, Amy l, Peter K, Matthew E H & Richard Chaberlin A, J Combinatorial Chem, 4, 2002, Galema S, Chem Soc Rev, 26, 1997, Ahluwalia V K & Aggarwal Renu, rganic synthesis-special techniques, (arosa Publishing House, ew Delhi), 2001, Andre Loupy, Microwave in rganic Synthesis, (Wiley- VCH), 2002, Brittany L H, Aldrichimica Acta, 37, 2004, Grieco P, Chemistry Today, 2004, Yu H-M, Chen S-T & Wang K-T, J rg Chem, 57, 1992, Erdelyi M & Gogoll A, Synthesis, 2002, livos H J, Alluri P G, Reddy M M, Salony D & Kodadek T, rg Lett, 4, 2002, Patil B S, Vasanthakumar G-R & Suresh Babu V V, Lett Peptide Sci, 9, 2003, Vasanthakumar G R, Patil B S & Suresh Babu V V, Lett Peptide Sci, 9, 2002, Tantry S J, Kantharaju & Suresh Babu V V, Tetrahedron Lett, 43, 2002, Patil B S, Vasanthakumar G-R & Suresh Babu V V, J rg Chem, 68, 2003, As illustrative examples, four peptide esters have been made and their physical as well as spectral data given. (a) Fmoc-Tyr (Bzl)-Phe-Me: 1 H MR (CDCl 3 ): δ 2.9 (2H, d, ), 3.0 (2H, d), 3.7 (3H, s), (2H, m), (4H, m), 4.7 (1H, t), 5.2 (1H, d), 6.8 (1H, d), (22 H, ArH); MS (MALDI- TF) m/z observed: (b) Fmoc-Ile-Gly-Et: 1 H MR (CDCl 3 ): δ 0.8 (3H, t), 0.9 (3H, d), 1.1 (2H, m), 1.2 (3H, t), 1.8 (1H, m), 2.6 (2H, d), 3.9 (1H, m), 4.1 (2H, q), 4.2 (1H, t), 4.5 (2H, d), 5.9 (1H, d), 6.2 (1H, d), (8H, ArH); MS (MALDI-TF) m/z observed: (c) Boc-Ile-Val-Me: 1 H MR (CDCl 3 ): δ (21H, m), 1.2 (2H, m), 1.3 (1H, m), 1.8 (1H, m), 3.6 (3H, s), 3.9 (1H, m), 4.0 (1H, t), 5.2 (1H, br), 5.8 (1H, br); MS (MALDI-TF) m/z observed: (d) Fmoc-Aib-Ala-Bzl: 1 H MR (CDCl 3 ): δ 0.85 (3H, s), 1.3(6H, s), 3.9 (1H, m), 4.3 (2H, d) 4.5 (1H, t), 5.3 (1H, d), 5.4 (1H, s), 5.13 (2H, s), (13H, m); MS (MALDI- TF) m/z observed: The coupling has been found to be free from recemization. The data for the two-diastereomeric dipeptides made by the present conditions has been furnished. (a) Fmoc-L-Phg-Phe- Me; [α] 25 D 22.2 (c1, DMF); 1 H MR (CDCl 3 ): δ 2.9 (2H, d), 3.16 (1H, t), 3.64 (3H, s), 4.25 (1H, t), 4.5 (2H, d), 5.2 (1H, d), 6.2 (1H, d), 6.95 (1H, d), (18H, m); MS (MALDI-TF) m/z observed: (b) Fmoc-D-Phg-Phe- me:; [α] 25 D (c 1, DMF); 1 H MR (CDCl 3 ): δ 3 (2H, d), 2.98(1H, t), 3.73 (3H, s), 4.25 (1H, t), 4.5 (2H, d), 5.2 (1H, d), 6.2 (1H, d), 6.95 (1H, d), (18H, m); MS (MALDI- TF) m/z observed: Formaggio F, Broxlerman Q B & Toniolo C, Methods of organic chemistry. Synthesis of peptides and peptidomimetics, edited by, M Goodman, A Felix, L Moroder & C Toniolo, (Houben-Weyl, ew York), 2002, E 22c, Carpino L A, Chao H G, Beyermann M & Bienert M, J rg Chem, 56, 1991, Santagada V, Fiorino F, Perissutti E, Severino B, De Filippis V, Vivenzio B & Calieendo G, Tetrahedron Lett, 42, 2001, Carpino L A, J rg Chem, 53, 1998, Bodanszky M & Bodanszky A, The practice of peptide synthesis, (Springer, Berlin), 1984, 114.
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