Mechanistic Studies of Diastereoselective Cyclopropanation via Homochiral Ketals. 2. Studies with Conformationally Restricted 2-Cyclohexen-l-one Ketals

The effect of cyclohexene ring conformation on the diastereoselectivity observed for Simmons-Smith cyclopropanation of 2-cyclohexen-l-one ethylene ketals was examined by using (5S)-5-tert-butyl-2-cyclohexen-l-one 1,2-ethanediol, (2R,3R)-2,3-butanediol, and (2S,3S)-2,3-butanediol ketals. Reagent chelation by the pseudoequatorial dioxolane oxygen atom was shown to result in more effective methylene transfer. This regiochemical preference can either antagonize or reinforce diastereoselectivity due to steric hindrance of the dioxolane oxygen atoms from dissymmetric placement of methyl appendages on the dioxolane ring.