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Scheme 1. On-resin peptide cyclization.excellent yields in less than 15 min using microwave.Head-to-tail on-resin cyclization. Introduction of a conformational restraint in peptidesthrough cyclization increases their receptor affinity. Cyclic peptides exhibit improvedmetabolic stability, and increased potency and bioavailability as compared to their linearcounterparts [7]. Head-to-tail on-resin cyclization strategy is an important tool in SPPS thattakes advantage of the resin induced pseudo-dilution effects. However, such cyclizations oftenrequire long reaction times under conventional conditions and result in a low crude purity ofthe cyclized peptide.The present work describes optimized microwave reaction conditions for each step in thehead-to-tail on-resin cyclization method (Scheme 1). All of the synthesis steps were carriedout in a fully automated fashion using the CEM Liberty microwave peptide synthesizer. Thetest sequence (Gly-Val-Tyr-Leu-His-Ile-Glu) for the cyclization studies was synthesized onFmoc-Glu(Wang resin)-ODmab (0.32 mmol/g) in which the side chain γ-carboxyl group isanchored to the resin and the α-carboxyl is protected by Dmab orthogonal protecting group. Asmall amount of peptidyl resin was cleaved at the end of each step to assess the purity. Thus,Fmoc deprotection with 20% piperidine in DMF for 0.5 min and 3 min at 75 o C and couplingwith Fmoc-AA-OH/HBTU/DIEA for 5 min at 75 o C assembled the backbone 1 in 91% crudepurity (Scheme 1). Selective on-resin removal of Dmab protection was effected by treatmentwith 5% hydrazine in DMF (2 x 3 min at 75 o C) to give the linear precursor 2 in 91% crudepurity. Head-to-tail cyclization of resin bound peptide 2 was accomplished using DIC/HOBt(3 x 10 min at 75 o C); cleavage of the cyclic peptide 3 from the resin followed by LC-MSanalysis of the crude product indicated 77% purity.In summary, we have developed efficient microwave assisted methods for the N-terminalmodifications and head-to-tail on-resin cyclization of peptides. Microwave synthesis allowsthe completion of these transformations in high yields and purities in a fraction of the timecompared to conventional peptide synthesis.AcknowledgmentsWe thank the Bioscience Division of CEM Corporation for providing the research facilities.References1. Quibell, M., Johnson, T. In Fmoc Solid Phase Peptide Synthesis. A Practical Approach; Chan,W.C., White, P.D., (Eds.), Oxford University Press, New York, 2000.2. Loupy, A. Microwaves in Organic Synthesis Wiley-VCH, Weinheim, 2002.3. (a) Yu, H.-M., et al. J. Org. Chem. 57, 4781-4784 (1992); (b) Erdelyi, M., Gogoll, A. Synthesis 11,1592-1596 (2002); (c) Collins, J.M., et al. Poster Presentation at the 18 th American PeptideSymposium, Boston, MA. (2003); (d) Matsushita, T., et al. Org. Lett. 7, 877-880 (2005); (e) Bacsa,B., et al. J. Pept. Sci. 12, 633-638 (2006); (f) Fara, M.A., et al. Tetrahedron Lett. 47, 1011-1014(2006); (g) Tantry, S.J., et al. ARKIVOC I, 21-30 (2006); (h) Grieco, P., et al. J. Med. Chem. 51,2701-2707 (2008); (i) Santagada, V., et al. Mini Rev. Med. Chem. 9, 340-358 (2009); (j) Galanis,A.S., et al. Biopolymers 92, 23-34 (2009).4. Palasek, S.A., Cox, Z.J., Collins, J.M. J. Pept. Sci.13, 143-148 (2007).5. Chicharro, C., et al. Antimicrob. Agents Chemother. 43, 1267-1269 (1999).6. Winkler, D.F.H., McGeer, P.L. Proteomics 8, 961-967 (2008).7. Rovero, P. In Solid Phase Synthesis. A Practical Guide; Kates, S.A., Albericio, F., (Eds.), MarcelDekker, New York, 2000.169