Diene-transmissive hetero Diels–Alder reaction of cross-conjugated azatrienes with ketenes: a novel and efficient, stereo-controlled synthetic method for hexahydroquinolinones

The cross-conjugated azatrienes, N-aryl-, N-alkyl- or N-dimethylamino-diβ-styrylmethanimines, reacted with diphenylketene and dimethylketene at room temperature to afford [2+2] cycloadducts, while the reaction with dichloroketene produced [4+2] cycloadducts. Upon heating, the [2+2] cycloadducts underwent a [1,3] sigmatropic rearrangement giving the formal [4+2] cycloadducts. The second Diels–Alder reaction of the [4+2] mono-adducts with electron-deficient dienophiles such as tetracyanoethylene, N-phenylmaleimide and methyl vinyl ketone gave hexahydroquinolinone derivatives stereoselectively. The cross-conjugated azatriene bearing different substituents at β- and β′-positions also underwent the diene-transmissive hetero Diels–Alder reaction in a highly site-, regio- and stereo-selective manner.