Stereoselective Nazarov cyclizations of bridged bicyclic dienones.

[reaction: see text] Bridged bicyclic dienones underwent silyl-directed Nazarov cyclization with generally very high diastereoselectivity. In most cases, a strong preference for cyclization to the product with an exo-disposed cyclopentenone was seen. However, the presence of additional unsaturation in the three-carbon bridge of a bicyclo[3.2.1]octadiene system caused complete reversal in selectivity with exclusive formation of the endo-cyclopentenone. The observed selectivities are believed to result from a combination of steric and electronic effects.