Intramolecular Cycloadditions. Studies of Relative Asymmetric Induction

Article abstract of DOI:10.1021/jo00121a028

Treatment of both the E and Z isomers of alkoxyallylic sulfones 20a and 20b with TiCl4 results in the formation of 4 + 3 cycloadducts by intramolecular cycloaddition.The distribution of diastereomers is different from each isomer of educt.This suggests that cycloaddition occurs faster than any isomerization process of the intermediate allylic cations.Both the E and Z isomers of 20c lead to the same 4 + 3 cycloadduct with essentially complete diastereoselectivity.Inherently high simple diastereoselection and a strong conformational bias in the allylic cation intermediates for both isomers account for this selectivity.The importance of allylic cation stereochemistry in these reactions is underscored by the cyclization of 20d.The Z isomer give only 4 + 3 cycloadducts with excellent relative but poor simple diastereoselection, suggestive of a concerted reaction.The E isomer give a <3 + 2> cycloadduct, enol ether 60, as the major cycloadduct as well as 4 + 3 cycloadducts and chloride-trapping product 61.This result is indicative of a stepwise reaction.The data reflect the importance of allylic cation stereochemistry in the regiochemical and stereochemical outcomes of intramolecular 4 + 2 cycloaddition reactions.