Synthesis of Cyclic Depsipeptides and Peptides via Direct Amide Cyclization

The 2,2-disubstituted 2H-azirin-3-amines 7 (2,2-disubstituted 3-amino-2H-azirines) were used as amino-acid synthons in the preparation of medium-sized cyclic depsipeptides and peptides derived from salicylic acids 6 and anthranilic acid 19 , respectively (Schemes 2--4 and 5, resp.). The combination of the ‘azirine/oxazolone method’ for the synthesis of linear peptides containing α,α-disubstituted α-amino acids and the acid-catalyzed amide cyclization in DMF at 60° proved to be an excellent preparative route to ten-membered cyclic depsipeptides and peptides. In the case of the anthranilic-acid derivative, a transannular ring-closure reaction was observed ( 24 → 25 ). Larger rings proved to be extremely sensitive to hydrolysis.