Polyalkenes cyclization

In this chapter emphasis will be placed on preparatively useful procedures, the general rules which pertain in polyene cyclization, and the circumstances under which the Stork-Eschenmoser hypothesis is valid. The success, or otherwise, of polyalkene cyclization depends on a complex interplay between the... 

Initiation of polyalkene cyclization by an acid chloride and termination by an alkenylsilane have been combined in a synthesis of an octahydronaphthalene as part of studies towaids the synthesis of dihydro-compactin, where the silane can be viewed as controlling the Friedel-Crafts acylation of the disubstituted double bond (equation 10). In this case, antimony pentachloride gave superior yields compared to a wide variety of commonly used Lewis acid catalysts. 

Lewis acids are also effective in activating a-diazo ketones towards intramolecular nucleophilic attack by alkenes and arenes. The reaction has been used effectively for the synthesis of cy-clopentenones (eq 27) starting with 8, /-unsaturated diazo ketones derived from the corresponding acid chloride and diazomethane. It has also been used to initiate polyalkene cyclizations (eq 28). Typically, boron trifluoride etherate is used as the Lewis acid, and electron-rich alkenes are most effective providing the best yields of annulation products. 

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