1-azatriene intermediate

Sol 3. (a) Electrocyclic ring opening of 1,2-dihydropyridine derivative (I) gives 1-azatriene intermediate (II), which then undergoes [1,7] hydrogen shift to give III. Electrocyclization of III then affords IV. 

Cheng and co-workers reported a one-pot synthesis of substituted pyridines through a rhodium-catalyzed C-H alkenylation of a,(3-unsaturated ketoximes 287 with symmetrical alkyne substrates 288. 67t-Electro-cyclization of the azatriene intermediates 289 and subsequent loss of water afforded the desired pyridines 290 in moderate to good yields. ... 

The condensation of iV-triphenylphosphoraniliden-benzamidine with acyclic a,P-unsaturated aldehydes produces dihydropyrimidines in good to high yields. The reaction mechanism probably involves an aza-Wittig reaction followed by a 6e-n -electrocyclic ring closure of the azatriene intermediate to give dihydropyrimidines, which is oxidized to the corresponding pyrimidines. 

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