INTRAMOLECULAR [4+3]-CYCLOADDITIONS - STUDIES OF RELATIVE ASYMMETRIC INDUCTION

Treatment df 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 + 3 cycloaddition reactions.