Synthesis of N,N'-unsymmetrical diacylcystines and cystine-containing dipeptides

N,N'-Unsymmetrical diacylcystines ((RR)-1 and (SS)-1) were prepared in good yields by three-step reaction sequences starting from cystine diethyl esters ((RR)-5 and (SS)-5). The key step reaction for diacylcystine synthesis, cleavage of diacylcystine diethyl esters ((RR)-2 and (SS)-2) proceeded with no detectable racemization of the products ((RR)-1 and (SS)- 1). Cystine-containing dipeptides ((RRS)-3, (RRR)-3, and (SSR)-3) were also prepared in high yields and in high optical purity.