Tai et al. could obtain asymmetric cystine peptides from (Boc-Cys-OH)₂ by enzymatic catalysis with immobilized papain. Oxidation of Boc-cystine with hydrogen peroxide in the presence of Na wolframate yielded the sodium salt of Boc-cysteic acid. Boc cystine benzyl ester was used as educt for obtaining S-isoacyl dipeptide building blocks.