3‐Amino‐2 H ‐Azirines. Synthons for α,α‐Disubstituted α‐Amino Acids in Heterocycle and Peptide Synthesis [New Analytical Methods (43)]

The recent upswing in peptide chemistry has been accompanied by an increasing interest in nonproteinogenic amino acids. These include the α,α‐disubstituted glycines, the best known of which is Aib (2‐aminoisobutyric acid, 2‐methylalanine). These α‐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 α‐C atom of the amino acids severely restricts the conformational freedom of the peptides and causes particular secondary structures to be favored; thus, α, α‐disubstituted α‐amino acids induce the formation of β turns or helices. 3‐Amino‐2 H ‐azirines are ideal synthons for the construction of oligopeptides, cyclic peptides and depsipeptides (peptolides) containing such α,α‐disubstituted α‐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‐2 H ‐azirines a large array of heterocycles containing α, α‐disubstituted α‐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.