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Since small peptides do not tend to exist as a single conformation in solution and possess a large amount of flexibility, determination of the preferred peptide structure should be approached by chemical modifications designed to decrease conformational flexibility. The flexibility of a peptide can be limited by utilization of local constraints and cyclization of peptide strand. Also, the incorporation of conformationally constrained amino acids into peptides has been extensively used in the design of conformationally restricted peptides. In order to retain biological activity of parent peptides, the constrained analogs should accommodate the peptide backbone conformation and frequently sustain the crucial functionalities. 1 As a part of our program for targeting zinc-containing enzyme, we designed a series of cysteine-incorporated small peptidomimetic analogs. It is well known that cysteine is the most frequent residue in the catalytic zinc sites of metalloenzymes. 2 In general, zinc ion is likely to coordinate with the cysteine sulfur on the protein/peptide substrate and participate in catalytic and structural sites. 3 This strongly suggested that the cysteine thiol group is one of key elements for enzyme activity. The incorporation of conformationally constrained amino acids into peptides is a powerful approach for generating structurally defined peptides as conformational probes and bioactive agents. Particularly, we were interested in prolines as conformationally constrained cysteine residues. During the synthesis of 3-mercaptoprolines, we found a serious of acyl transfers. Here, we wish to report the observation of a series of S→N→N acyl transfer in the deprotection of the Sacetate in 3-mercaptoproline systems.

An Observation of a Series of S→N→N Acyl Transfers in 3-Mercaptoproline