AMINO-ACIDS .17. A NEW SYNTHESIS OF DIDEHYDRO DIPEPTIDES AND DIDEHYDRO TRIPEPTIDES

Esters of N-phthaloyl-protected (R)- or (S)-didehydro dipeptides 5-7 were obtained in good yields by the perrhenate-catalyzed decomposition of 2-azidocarboxylates 1 with N-phthaloyl-protected (R)- or (S)-amino acid chlorides 2 in presence of at least equimolar amounts of N-methyl-2-pyridone (4) as acylation catalyst. Esters of didehydro tripeptides 16 were obtained in a comparable procedure from the optically active amino acid chlorides 2 and 2-(2-azidoacyl)amino acid esters 15, which were prepared from 2-azidocarboxylic acids 12 or the corresponding chlorides 13 with 2-amino acid ester hydrochlorides (S)-14. - As an example, the unprotected (S)-alanyl-didehydrovalin 20 was prepared from the didehydro dipeptide 18a without any racemization by hydrolysis of the benzyl ester and subsequent hydrogenolytic removal of the N-phthaloyl group.