Silyloxyfurans Mukaiyama aldol reaction

Bismuth-Catalyzed Mukaiyama Aldol Reaction of Silyloxyfurans. 102... 

SCHEME 52 Regioselectivity of the vinylogous Mukaiyama aldol reaction using 2-silyloxyfuran. 

M. Woyciechowska, G. Forcher, S. Buda, J. Mlynarski, General switch in regioselectivity in the Mukaiyama aldol reaction of silyloxyfuran with aldehydes in aqueous solvents, Chem. Commun. 48 (2012) 11029-11031. 

Alternatively, the iminium-activation strategy has also been apphed to the Mukaiyama-Michael reaction, which involves the use of silyl enol ethers as nucleophiles. In this context, imidazolidinone 50a was identified as an excellent chiral catalyst for the enantioselective conjugate addition of silyloxyfuran to a,p-unsaturated aldehydes, providing a direct and efficient route to the y-butenolide architecture (Scheme 3.15). This is a clear example of the chemical complementarity between organocatalysis and transition-metal catalysis, with the latter usually furnishing the 1,2-addition product (Mukaiyama aldol) while the former proceeds via 1,4-addition when ambident electrophiles such as a,p-unsaturated aldehydes are employed. This reaction needed the incorporation of 2,4-dinitrobenzoic acid (DNBA) as a Bronsted acid co-catalyst assisting the formation of the intermediate iminium ion, and also two equivalents of water had to be included as additive for the reaction to proceed to completion, which... 

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