Easily available 3-amino-2H-azirines proved to be useful synthons in peptide chemistry. The smooth coupling with acids together with the selective hydrolysis of the C-terminal amide group present a novel strategy for the synthesis of peptides containing 2,2-disubstituted glycines ('Azirine/Oxazolone Method'). As a result of the gem-disubstitution, these conformationally restricted peptides preferentially form turns and helical structures. The scope of this methodology for the synthesis of linear and cyclic peptides is demonstrated by the preparation of poly-Aib-peptides, segments of peptaibols, and cyclopeptides. A variation of the 'Azirine/Oxazolone Method', using thiocarboxylic acids in the reaction with aminoazirines and a novel acid-catalyzed isomerization of the resulting thioamides, offers a convenient access to endothiopeptides. The specifically thionated products are of special interest as they contain two elements of conformational restriction.