Unusual preference for ketone and reversal of chemoselectivity in Lewis acid-catalyzed aldol reaction of ketene silyl acetal

Ketene silyl acetals undergo facile reaction with ketones in the presence of a Lewis acid such as (C6F5)(2)SnBr2, TMSOTf, or Sc(OTf)(3). This unique reactivity allows ketones to react in preference to aldehydes or acetals in competitive aldol-type reactions. Upon treatment with keto aldehydes or keto acetals, ketene silyl acetals provide single products derived from the exclusive reaction with the ketonic function. Particularly noteworthy is the discrimination of ketones from aldehydes in naked forms. It has been proved that such bias results from the differentiated recognition of ketone and aldehyde carbonyls. The ternary system consisting of ketene silyl acetal, carbonyl, and an appropriate Lewis acid is crucial for achieving the effective differentiation.