Metallophosphite-catalyzed asymmetric acylation of alpha,beta-unsaturated amides.
The l-menthone-derived TADDOL phosphite 6b catalyzes highly enantioselective conjugate additions of acyl silanes to alpha,beta-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 alpha-silyl-gamma-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 gamma-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.[1]References
- Metallophosphite-catalyzed asymmetric acylation of alpha,beta-unsaturated amides. Nahm, M.R., Potnick, J.R., White, P.S., Johnson, J.S. J. Am. Chem. Soc. (2006) [Pubmed]
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