3-Amino-2H-azirines. Synthons for alpha,alpha-Disubstituted alpha-Amino Acids in Heterocycle and Peptide Synthesis

Abstract

The recent upswing in peptide chemistry has been accompanied by an increasing interest in nonproteinogenic amino acids. These include the alpha,alpha-disubstituted glycines, the best known of which is Aib (2-aminoisobutyric acid, 2-methylalanine). These alpha-amino acids occur in natural oligopeptides such as the peptaibols, a class of membrane-active ionophores that has been isolated from fungal cultures. The twofold substitution at the alpha-C atom of the amino acids severely restricts the conformational freedom of the peptides and causes particular secondary structures to be favored; thus, alpha,alpha-disubstituted alpha-amino acids induce the formation of beta-turns or helices. 3-Amino-2H-azirines are ideal synthons for the construction of oligopeptides, cyclic peptides and depsipeptides (peptolides) containing such alpha,alpha-disubstituted alpha-amino acids. The presence of the ring strain in these molecules means that they can be used in peptide coupling without the need for additional activating reagents. Using 3-amino-2H-azirines a large array of heterocycles containing alpha,alpha-disubstituted alpha-amino acids as structural elements within their skeleton can be synthesized. The driving force in these reactions is the release of the strain on the three-membered ring, which usually takes place in a ring-expansion reaction. The mechanistic elucidation of these reactions, which can be quite complex, contains some surprises.