Selenium dioxide oxidations of dialkyl-3H-azepines: The first synthesis of 2-azatropone from oxidation of 2,5-di-tert-butyl-3H-azepine

Oxidation reactions of 2,5- and 3,6-di-tert-butyl-3H-azepines (1 and 2) with selenium dioxide (SeO₂) were performed. The oxidation of 1 with SeO₂ gave 3-tert-butyl-7,7-dimethyl-4-oxo-octa-2,5-dienal 3 in 36% yield, 4-tert-butyl-5-(3,3-dimethyl-2-oxo-butylidene)-1,5-dihydro-pyrrol-2-one 4 in 13% yield, 2,6-di-tert-butyl-2-pyridinecarbaldehyde 5 in 12% yield, and 4,7-di-tert-butyl-2H-azepin-2-one (2-azatropone) 6 in 6% yield, respectively. Oxidation of 2 with SeO₂ gave 2,2-dimethyl-1-[2-(5-tert-butyl)-pyridyl]propanol 7 in 55% yield, and 3,6-di-tert-butyl-2H-azepine 8 in 5% yield, respectively. We found that selenium dioxide oxidation of 1 affords 4-oxo-octa-2,5-dienal 3 by a new ring cleavage reaction of 1, and we described the first synthesis of 2-azatropone 6 from this oxidation of 1. In the case of 2, pyridylpropanol 7 was obtained as the major product. We now report in detail result of these oxidation reactions, which have led to the synthesis of a novel azatropone derivative.