Mechanism and origin of stereoselectivity in Lewis acid catalyzed [2 + 2] cycloadditions of ketenes with aldehydes
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abstract
Divergent rate-limiting steps for the formation of diastereomeric products! Evidence for this unexpected finding comes from experimental and theoretical studies of the title reaction. The predicted differing rate-determining steps (rds) in the formation of cis- and trans--lactones (see scheme; cy = cyclohexyl, TBS = tert-butyldimethylsilyl) are used to explain the stereoselectivity of these reactions and suggest that a broader range of mechanistic possibilities should sometimes be considered in reactions leading to diastereomeric products.