Mechanistic Studies of Diastereoselective Cyclopropanation via Homochiral Ketals. 1. Dioxolane Structural Effects1

Compounds structurally related to 2-cyclohexen-l-one 1,4-di-O-benzyl-L-threitol ketal were prepared and subjected to the Simmons-Smith cyclopropanation. From these experiments a mechanistic model for diastereoselective cyclopropanation of common ring systems (five-, six-, and seven-membered) has been formulated. Diastereo-selectivity is thought to result from preferential chelation of the Simmons-Smith reagent at the least sterically hindered lone pair of electrons on the dioxolane oxygen proximal to the alkene. It is found that the presence of oxygen atoms in the dioxolane appendages provide sites for competitive chelation of the reagent, which can antagonize the diastereoselection due to chelation at dioxolane oxygen. That chelation by dioxolane oxygen does occur and is responsible for diastereoselectivity is inferred from studies with a hydrocarbon model system. Surprisingly, both dioxolane appendages are shown to be necessary for optimum diastereoselection since, under the conditions of the Simmons-Smith cyclopropanation, 2-cycloalken-l-one ethylene ketals are reversibly ring opened to zwitterionic intermediates.