10.1021/ol302550p r 2012 American Chemical Society Published on Web 10/17/2012 ORGANIC LETTERS 2012 Vol. 14, No. 21 5472–5475 Acid-Labile Cys-Protecting Groups for the Fmoc/tBu Strategy: Filling the Gap Miriam G  ongora-Benı ´tez, †,‡ Lorena Mendive-Tapia, †,‡ Iv  an Ramos-Tomillero, †,‡ Arjen C. Breman, † Judit Tulla-Puche,* ,†,‡ and Fernando Albericio* ,†,§, ) Institute for Research in Biomedicine, 08028-Barcelona, Spain, CIBER-BBN, 08028-Barcelona, Spain, Department of Organic Chemistry, University of Barcelona, 08028-Barcelona, Spain, and School of Chemistry, University of KwaZulu Natal, 4001-Durban, South Africa albericio@irbbarcelona.org; judit.tulla@irbbarcelona.org Received September 16, 2012 ABSTRACT To address the existing gap in the current set of acid-labile Cys-protecting groups for the Fmoc/tBu strategy, diverse Fmoc-Cys(PG)-OH derivatives were prepared and incorporated into a model tripeptide to study their stability against TFA. S-Dpm proved to be compatible with the commonly used S-Trt group and was applied for the regioselecive construction of disulfide bonds. Since the early days of peptide chemistry, the effective synthesis of natural or non-natural isomers, analogues, or de novo designed peptides with complex disulfide bridge patterns has been a demanding task. The oxidative folding 1 of fully deprotected linear peptides is a desirable and commonly applied approach for the synthesis of complex Cys-rich peptides. However, achievement of the desired disulfide bond connectivity through this approach is not always affordable. To overcome these challenging syn- theses, a myriad of protecting groups for the β-thiol group of Cys, along with efficient regioselective protection schemes, have been developed. 2 In recent years, several acid-labile Cys-protecting groups have been developed for the Fmoc/tBu strategy (Figure 1). 3 Most of these are highly sensitive to acid, the S-Trt group being one of the most commonly used in the Fmoc/tBu approach. In contrast, the S-Mob group re- quires a high TFA concentration and harsh conditions (high temperature and long reaction times) to be fully removed. In this regard, the current gap between S-Trt and S-Mob groups captured our attention and prompted us to browse through acid-labile protecting groups to find Cys-protecting groups that, ideally, could be quantita- tively removable under mild acidic conditions and, simul- taneously, show compatibility with S-Trt for their further application in synthetic strategies for the preparation of Cys-rich peptides. Thus, three distinct scaffolds, namely diphenylmethyl, biphenylmethyl, and benzyl groups, were selected and finely tuned for this purpose. Twelve Fmoc- Cys(PG)-OH (1al) were prepared and incorporated into the model tripeptide Fmoc-Ala-Cys(PG)-Leu-NH 2 (2al), † Institute for Research in Biomedicine. ‡ CIBER-BBN. § University of Barcelona. ) University of KwaZulu Natal. (1) (a) Moroder, L.; Besse, D.; Musiol, H. J.; Rudolph-B€ ohner, S.; Siedler, F. Biopolymers (Pept. Sci.) 1996, 40, 207. (b) Narayan, M.; Welker, E.; Wedemeyer, W. J.; Scheraga, H. A. Acc. Chem. Res. 2000, 33, 805. (c) Anfinsen, C. B. Biochem. J. 1972, 128, 737. (2) (a) Barany, G.; Merrifield, R. B. Solid-phase peptide synthesis. In The Peptides; Gross, E., Meienhofer, J., Eds.; Academic Press: New York, 1979; Vol. 2, pp 184. (b) Andreu, D.; Albericio, F.; Sol e, N. A.; Munson, M. C.; Ferrer, M.; Barany, G. In Methods in Molecular Biology: Peptide Synthesis Protocols; Pennington, M. W., Dunn, B. M., Eds.; Humana Press Inc.: Totowa, NJ, 1994; Vol. 35, pp 91169. (3) (a) Isidro-Llobet, A.;  Alvarez, M.; Albericio, F. Chem. Rev. 2009, 109, 2455–2504. (b) Boul  egue, C.; Musiol, H. J.; Prasad, V.; Moroder, L. Chem. Today 2006, 24, 24.