Abstract
Oxazaborolidine catalyst (CBS catalyst) has been extensively used for catalytic borane reduction with a predictable absolute stereochemistry and high enantioselectivity. However, the use of isolated CBS catalyst sometimes has the drawback of low reproducibility due to the aging of the CBS catalyst during storage. Therefore, we investigated a more reliable and practical method for the reduction of a variety of ketones including challenging substrates, primary aliphatic ketones, α,β-enones, and trifluoromethyl ketones. This review surveys the developments in borane reduction using oxazaborolidine catalysts generated in situ from chiral lactam alcohols and borane.
Highlights
Asymmetric reduction of prochiral ketones is one of the most important methods for the synthesis of chiral secondary alcohols and constitutes a valuable step in the synthesis of a variety of natural products and several medicinally important compounds
The oxazaborolidine-catalyzed asymmetric borane reduction of prochiral ketones (CBS reduction) using chiral amino alcohols [1,2,3,4] has been extensively investigated, since the stoichiometric reductions were reported by Itsuno et al [5,6,7] and the catalytic versions were reported by Corey et al [8]
We investigated investigated the enantioselective reduction of α,β-enones as challenging substrates using the oxazaborolidine generated in situ from the chiral lactam alcohol 3 to expand its scope and generality the enantioselective reduction of from α,β-enones as lactam challenging substrates using the oxazaborolidine oxazaborolidine generated in situ the chiral alcohol
Summary
Asymmetric reduction of prochiral ketones is one of the most important methods for the synthesis of chiral secondary alcohols and constitutes a valuable step in the synthesis of a variety of natural products and several medicinally important compounds. The oxazaborolidine-catalyzed asymmetric borane reduction of prochiral ketones (CBS reduction) using chiral amino alcohols [1,2,3,4] has been extensively investigated, since the stoichiometric reductions were reported by Itsuno et al [5,6,7] and the catalytic versions were reported by Corey et al [8]. The B-Me oxazaborolidine 1b formed by the reaction of 1 with methylboronic acid has been developed as an air- and moisture- stable catalyst that catalyzes the borane reduction of ketones with an excellent enantioselectivity [11], there is the requirement of complete removal of water to avoid undesired effects [12]. Modifications of the method are described for the borane reduction of challenging substrates, i.e., primary aliphatic ketones, α,β-enones and trifluoromethyl ketones.
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