Electrocyclization of Heteroatomic Trienes

Electrocyclization can also occur when heteroatoms are incorporated into diene, triene, or polyene systems. Most attention has focused on 1-azatriene and 1-oxatrienes, which lead to dihydropyridines and pyrans, respectively. 

Comparison of the energy requirements of these reactions with the all-carbon system indicates reduced barriers for the aza and oxa systems, but because of the loss of the C=0 bond, the 1-oxahexatriene electrocyclization is slightly endothermic. Marvell and co-workers estimated the acceleration as being a factor of 10 to 10 . This result suggests a change in mechanism for the heteroatom cases. 

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