Convenient access to β-substituted cysteines and β- and γ-mercapto prolines

Herein we describe a convenient method for the synthesis of the ,8-substituted Cys analogs 1a, b and trans-,8-mercapto proline 1c, from their corresponding a ,,8-unsaturated-a-amino esters using p-methoxybenzyl mercaptan as the sulfur source. The intermediate ,8-thioethers are stable toward chromatographic purification, and after global deprotection the ,8-mercapto amino acids 1a-c are obtained in good to high yields (64%-99%).