ORGANIC LETTERS
4-Methoxybenzyloxymethyl Group, a Racemization-Resistant Protecting Group for Cysteine in Fmoc Solid Phase Peptide Synthesis
2012 Vol. 14, No. 7 1926–1929
Hajime Hibino† and Yuji Nishiuchi*,†,‡ SAITO Research Center, Peptide Institute, Inc., Ibaraki, Osaka 567-0085, Japan, and Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
[email protected] Received March 8, 2012
ABSTRACT
The 4-methoxybenzyloxymethyl (MBom) group was introduced for sulfhydryl protection of Cys in combination with Fmoc chemistry. The MBom group proved to substantially suppress racemization of Cys during its incorporation mediated by phosphonium or uronium reagents. Furthermore, this group was found to significantly reduce racemization of the C-terminal Cys linked to a hydroxyl resin during repetitive base treatment, in comparison with the usually used trityl (Trt) and acetamidomethyl (Acm) groups.
Disulfide linkages play an important role in the formation and stabilization of distinct three-dimensional structures in naturally occurring peptides and proteins. For the synthesis of such Cys-rich molecules, native chemical ligation (NCL)1 involving the coupling of a peptide thio ester and a cysteinyl-peptide is an advantageous approach. In the course of peptide assembly, protection of the sulfhydryl group on Cys is indispensable for avoiding side reactions related to its high nucleophilicity, such as acylation, alkylation, or oxidation. For this purpose, many sidechain protecting groups for Cys have been developed.2 Among them, the trityl (Trt) and acetamidomethyl (Acm) groups are widely accepted as a protecting group for Cys in †
Peptide Institute, Inc. Osaka University. (1) (a) Dawson, P.; Muir, T.; Clark-Lewis, I.; Kent, S. Science 1994, 266, 776. (b) Dawson, P. E.; Kent, S. B. H. Annu. Rev. Biochem. 2000, 69, 923. (c) Hackenberger, C. P. R; Schwarzer, D. Angew. Chem., Int. Ed. 2008, 47, 10030. (2) (a) Isidro-Llobet, A.; Alvarez, M.; Albericio, F. Chem. Rev. 2009, 109, 2455. (b) Otaka, A.; Morimoto, H.; Fujii, N.; Koide, T.; Funakoshi, S.; Yajima, H. Chem. Pharm. Bull. 1989, 37, 526. (c) Garcia, O.; Bofill, J.; Nicolas, E.; Albercio, F. Eur. J. Org. Chem. 2010, 3631. (d) Shen, F.; Zhang, Z.; Li, J.; Lin, Y.; Liu, L. Org. Lett. 2011, 13, 568. ‡
10.1021/ol300592w r 2012 American Chemical Society Published on Web 03/27/2012
the Fmoc strategy. During incorporation of these Cys derivatives onto the growing peptide chain, however, considerable base-catalyzed racemization of the Cys residue is known to always occur at the activating and coupling steps with phosphonium or uronium reagents such as PyBOP or HBTU, respectively.3 This may hamper purification of the products including those obtained after the NCL and disulfide formation reactions. Therefore, the carbodiimide-mediated coupling method has been recommended to reduce the racemization rate during Cys incorporation to acceptable levels (
