Abstract: Several structural factors are known to restrict the conformational flexibility of peptides, e. g. thioamide groups, a- or N-substitution, etc. For many years we have been interested in the constraints introduced by the presence of a-methylated a-amino acids, and with the 'azirine/oxazolone method' we have developed a convenient synthetic access to such peptides. 3-Amino-2H-azirines proved to be useful synthons for the introduction of a-substituted a-amino acids [1]. Conversion of 3-amino-2H-azirines with thiobenzoic acid, which leads to the corresponding thioamides, has shown the possibility of using a-amino thioacids in the 'azirine/oxazolone method' to sythesize endothiopeptides [2,3].