Mukaiyama-Michael aldol condensation

During the last decade, use of oxazaborolidines and dioxaborolidines in enantioselective catalysis has gained importance. [1, 2] One of the earliest examples of oxazaborolidines as an enantioselective catalyst in the reduction of ketones/ketoxime ethers to secondary alco-hols/amines was reported by Itsuno et al. [3] in which (5 )-valinol was used as a chiral ligand. Since then, a number of other oxazaborolidines and dioxaborolidines have been investigated as enantioselective catalysts in a number of organic transformations viz a) reduction of ketones to alcohols, b) addition of dialkyl zinc to aldehydes, c) asymmetric allylation of aldehydes, d) Diels-Alder cycloaddition reactions, e) Mukaiyama Michael type of aldol condensations, f) cyclopropana-tion reaction of olefins. 

The chiral acyloxyborane 7 (CAB) has also been found to be an excellent catalyst for asymmetric Mukaiyama-Michael type aldol reaction between silyl enol ethers and aldehydes (Scheme 8). Yamamoto et al. [27] have used 20 mol % of CAB in propionitrile at -78 °C as a highly efficient catalyst for the condensation of several E and Z silyl enol ethers and ketene acetals with a variety of aldehydes (yields 49-97 %, 80-97 % ee). 

Condensation reactions. The title compounds (R2Al-NTf2) are excellent catalysts for Mukaiyama aldol reactions, Michael reactions, and allylation. The allylation is sensitive to steric environments of the substrates, enabling selective attack on a less crowded formyl group. 

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