Facial Selectivity in Conjugate Addition Reactions

Control of Facial Selectivity in Conjugate Addition Reactions 

As is the case for aldol addition, chiral auxiliaries and catalysts can be used to control stereoselectivity in conjugate addition reactions. Oxazolidinone chiral auxiliaries have been used in both the nucleophilic and electrophilic components under Lewis acid-catalyzed conditions. (V-Acyloxazolidinones can be converted to nucleophilic titanium enolates with TiCl3(0-/-Pr).320 

Reactions of Carbon Nucleophiles with Carbonyl Compounds 

Unsaturated acyl derivatives of oxazolidinones can be used as acceptors, and these reactions are enantioselective in the presence of chiral to-oxazoline catalysts.321 Silyl ketene acetals of thiol esters are good reactants and the stereochemistry depends on the ketene acetal configuration. The Z-isomer gives higher diastereoselectivity than the Zf-isomer. 

The above examples contain an ester group that acts as a second activating group. The reactions are also accelerated by including one equivalent of (CF3)2CHOH. This alcohol functions by promoting solvolysis of a dihydropyran intermediate that otherwise inhibits the catalyst. 

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