The asymmetricdirect vinylogous aldolreaction of unactivated γ-butenolide with aldehydes has been developed, giving the corresponding 5-(1′-hydroxy)butenolide derivatives in high yields (up to 93%) and enantioselectivities (up to 83% ee) under mild conditions.
Reactions of 2-(trimethylsilyloxy)furan 1 and aldehydes catalyzed by a chiral cationic Cr(salen) complex proceeded with remarkably improved enantioselectivity, when a secondary alcohol was added. The role of the alcohol is acceleration of the conversion of an intermediary 1-aldehyde adduct into the final product.
A vinylogous aldol-type reaction between 4-methyl-2-(trimethylsiloxy)furan and aldehydes catalyzed by cinchonidine-derived quaternary ammonium phenoxide proceeded smoothly to afford the corresponding 5-substituted butenolide derivatives in high yields with good to excellent diastereo- and enantio-selectivities.
A mild method for the diastereoselectiveMukaiyamaaldolreaction is reported. By using a lowloading of the gallium(III) triflatecatalyst (down to 0.01 mol-%), the transformation proceeds efficiently to afford the corresponding β-hydroxy ketones in yields up to 92 %. To the best of our knowledge, this is the first report of a metal triflate acting as a safe, bench-stable, and slow-releasing source