Cyclic peptides

Four cyclic dehydrodipeptides, cyclo (‐Δaminoacyl‐L‐Ala‐), in which aminoacyl is Phe, Apb (2‐amino‐4‐phenylbutanoic acid), App (2‐amino‐5‐phenylpentanoic acid) and Aph (2‐amino‐6‐phenylhexanoic acid), were prepared by condensation of cyclo (‐ N ‐Ac‐Gly‐ N ‐Ac‐L‐Ala‐) with the corresponding aldehydes. Among them, the yield of cyclo (‐ΔApb‐L‐Ala‐) was exceptionally low. Each compound was hydrogenated in the presence of Pd black at various temperatures and chiral induction in hydrogenation was evaluated. Low chiral induction at high temperature (50d̀) was observed in the case of cyclo (‐ΔPhe‐L‐Ala‐). Optically pure L‐Apb, L‐App and L‐Aph were obtained from the corresponding cyclic dehydrodipeptides, respectively, by hydrogenation and subsequent acid hydrolysis.