3-amino-2h-azirines in the synthesis of peptides: tetrapeptides with alpha,alpha-disubstituted alpha-amino acids

3-Amino-2,2-dialkyl-2H-azirines 1 are synthons for alpha,alpha-disubstituted alpha-amino acids. Tetrapeptides 15, containing two such amino acids, were synthesized in high yield by application of the "azirine/oxazolone-method". In this versatile approach for the incorporation of di-substituted amino acids into the peptide chain, N-protected amino acids or peptides are coupled with 3-amino-2H-azirines 1. After selective hydrolysis of the newly formed C-terminal amide, condensation with valine-benzylester was achieved via in situ generated oxazole-5(4H)-ones in the presence of hydroxybenzotriazole. The conformational properties of tetrapeptides 15 are discussed on the basis of NMR- and CD-spectra. It is shown that alpha,alpha-disubstitution favours the formation of beta-turns even in very short oligopeptides. The "azirine/oxazolone-method" offers an efficient strategy for the synthesis of peptide-models used for conformational studies