Metallophosphite-Catalyzed Asymmetric Acylation of α,β-Unsaturated Amides

The l-menthone-derived TADDOL phosphite 6b catalyzes highly enantioselective conjugate
additions of acyl silanes to α,β-unsaturated amides. p-Methoxybenzoyl cyclohexyldimethylsilane adds to a
variety of N,N-dimethyl acrylamide derivatives in the presence of the lithium salt of 6b. In many instances
the α-silyl-γ-ketoamide product undergoes facile enantioenrichment (to 97−99% ee) upon recrystallization.
Desilylation with HF·pyr affords the formal Stetter addition products. Baeyer−Villiger oxidation of the
desilylated γ-ketoamides affords useful ester products. An X-ray diffraction study of 6b reveals that the
isopropyl group of the menthone ketal influences the position of the syn-pseudoaxial phenyl group in the
TADDOL structure. Through a crossover experiment, the silicon migration step in the reaction mechanism
is shown to be strictly intramolecular.