Boron-Trifluoride-Catalyzed Reactions of 3-Amino-2H-azirines with Amino-acid Esters and Amines After activation by protonation or complexation with BF3, 3-amino-2H-azirines 1 react with the amino group of alpha-amino-acid esters 3 to give 3,6-dihydro-5-aminopyrazin-2(1H)-ones 4 by ring enlargement (Scheme 2, Table 1). The configuration of 3 is retained in the products 4. With unsymmetrically substituted 1 (R1 # R2), two diastereoisomers of 4 (cis and trans) are formed in a ratio of 1:1 to 2:1. With beta-amino-acid esters 5 and 7, only openchain alpha-amino-imidamides 6 and 8, respectively, are formed, but none of the seven-membered heterocycle (Scheme 3). Primary amines also react with BF3-complexed 1 to yield alpha-amino-imidamides of type 9 (Scheme 4, Table 2). Compound 9b is characterized chemically by its transformation into crystalline derivatives 10 and 12 with 4-nitrobenzoyl chloride and phenyl isothiocyanate, respectively (Scheme 5). The structure of 12 is established by X-ray crystallography. Mechanisms for the reaction of activated 1 with amino groups are proposed in Schemes 6 and 7.