Valinol 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. 

One of the most active areas of organoborane chemistry this year has been the application of boron enolates to enantioselective aldol condensations. Thus the enolate (82), derived from (5)-valinol, reacts with aldehydes R CHO to give, after cleavage of the oxazolidinone residue with R OH, the alcohols (83) with an erythro threo selectivity greater than 140 1, whereas the enolate (84), obtained from (IS, 2/ )-norephedrine, gives alcohols (85) with a selectivity of at least 500 1. Somewhat lower levels of selectivity are observed with the azaenolates (86) and (87) which give predominantly threo- and erythro-alcohols (88) and (89), respectively. ... 

From the illustrated (S)-valinol imide (175), the derived dibutylboryl enolates undergo condensation with a broad range of aldehydes in greater than 99% asymmetric induction for both newly formed asymmetric centers 180). Evans et al. have shown that the propionyl sidely chain in (175) may be replaced by other alkanoyl substituents without loss of stereoselectivity in the aldol type reaction 180). 

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