Electrocyclic processes higher order

It is not possible within the restrictions of this review to treat all carbocyclic frameworks (e.g. spirocy-clics) and a conscious decision was also made specifically to exclude the heterocyclic version of this electrocyclization. The number and variety of heteroatoms and the possible permutations along the hexa-triene system that can a priori be considered would justify a separate treatment. The reader is referred to the excellent monograph by Marvell for references to the treatment of electrocyclizations leading to heterocycles. However, in order to provide at least a few leading references, several selected examples of hetero electrocyclizations are included in this chapter together with comments concerning retro electrocyclizations and higher-order electrocyclic processes. 

The stereochemistry of a thermally induced 10e electrocyclization (predicted to be disrotatory) has not been firmly established and the main synthetic application is found in the formation of azulenes and ring-fused azulenes as in the transformation (452) to (453). Thermolysis of (454) with spontaneous elimination of dimethylamine from intermediate (455) afforded the fused azulene structure (456). The chemistry of even higher order (12e to 20e") pericyclic processes has been recently reviewed. An example of an unusual sequence of pericyclic processes is the transformation of heptahendecafulvadiene (457) to the pentacyclic hydrocarbons (462) and (463) in a 2 1 ratio. The pathway for this transformation can be viewed as an initial conrotatory 20e electrocyclization followed by a cascade of 10e and 6e pericyclic processes. ... 

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