Reaction of pyrylium salts with hydrazine and methylhydrazine. Concerning the scope of the 1,2(4H)-diazepine synthesis

The reaction of pyrylium salts 1a–e with methylhydrazine gives pyrazolines (2), pyrazoles (3), pyridines (4), 1,2-diazepine (5), and N-methylaminopyridinium salts (6). Product composition has been studied as a function of pyrylium salt counter anion, concentration of methylhydrazine, and conditions of the reaction (Table 1). Pyridines (4) and 1,2-diazepines (5) are minor components, the major products being the pyrazolines (2) and pyrazoles (3). The reactions of more unusual pyrylium salts (1f–i, 8a, 9a, 9c, and 11a) with hydrazine have been shown to provide 1,2(4H)-diazepines (10, 12–15) and N-aminopyridinium salts (8b, 9b) (Table 2), the nature of the product apparently depending on the absence or presence of 2-methyl substitution respectively in the starting pyrylium salts.