Demethoxycarbonylation of methyl 2,5- and methyl 3,6-dialkyl-1H-azepine-1-carboxylates: formation and characterization of 2H-, 3H- and 4H-azepines

Demethoxycarbonylation of methyl 2,5-di-tert-butyl-1H-azepine-1-carboxylate using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gave 3H-azepines. Under similar conditions, methyl 3,6-di-tert-butyl-1H-azepine-1-carboxylate gave not only the 3H-azepine but also the isomerized 2H- and 4H-azepines. Application of the reaction to dimethyl and diisopropyl substituted 1H-azepines showed that bulky alkyl group substitution stabilizes the seven-membered azatriene system. The thermal behaviour of the di-tert-butyl substituted azepines is discussed.