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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. |
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