Addition Reactions of 2-Amino-1-azetines with Cyclopropenones; Formation of Azepine Derivatives by Ring Expansion Reactions. The reaction of 2-amino-1-azetines of type 6 with 2,3-diphenylcyclopropenone (1a) in acetonitrile leads to azeto[1,2-a]pyrroles (cf: 7 and 9, Schemes 3 and 4) in good yield. It is remarkable that in the reaction of 6a with 1a only endo-7 is formed. With silicagel in ether endo-7 isomerizes to the thermodynamically more stable exo-7 (Schemes 3 and 6). The crystal structure of the latter compound has been established by X-ray crystallography. The reaction of 6a and 2-isopropyl-3-phenyl- cyclopropenone (1b) yields only one product, which isomerizes with silicagel in ether to exo-10 (Scheme 4). The structure of exo-10 has been determined by NMR- spectroscopy. It seems reasonable that this structure results from a nucleophilic attack of the four-membered amidine to the phenyl-substituted C-atom of 1b.