Synthesis of optical isomers of cystinylvaline

Abstract
The synthesis of L-cystinyl-L-valine, L-cystinyl-D-valine and DL-cystinyl-D-valine by routes using the mixed-anhydride and dicyclohexylcarbodiimide-coupling methods is described. The problems of racemization during the removal of the protecting groups and removal of salt from the free peptides are discussed. The synthesis of L-cystinyl-L-[carboxy-C14]-valine and L-cystinyl-D-[carboxy-Cl4]valine is reported.