Oxazaborolidine prolinol-derived

Borane reduction catalyzed by chiral oxazaborolidines (CBS reduction, CBS = Corey, Bakshi, and Shibata) exhibits excellent enantio- and chemoselectiv-ity for a wide variety of ketonic substrates (Figure 1.27). This reaction was originally developed as a stoichiometric system consisting of diphenylvalinol and borane, ° but was later extended to a useful catalytic method. Because of the high efficiency of this reaction, many chiral oxazaborolidines have been synthesized from p-amino alcohols.Among them the prolinol-derived oxazaboro-lidine is one of the most widely used catalysts. ... 

This reduction method has a number of advantages that include wide scope, predictable absolute stereochemistry, ready availability of the chiral catalyst in both enantiomeric forms, high yields, experimental ease, recovery of the catalyst (as the amino alcohol), and low cost of goods. The most common form of the chiral oxazaborolidine is derived from prolinol and has a methyl substituent on the boron atom (B-Me-CBS) 1. When one conducts a reduction on a novel system for the first time, this catalyst provides a good compromise of cost, enantioselectivity, and experimental ease. If sufficient control is not observed with this reagent, one can then systematically evaluate the numerous variations of this framework. 

Oxazaborolidines (70) have been used extensively in organic synthesis and their preparation from substituted prolinol derivatives and alkyl boronic acids is well documented (Equation (7)) <88JOC2861, 9lJOC75l>. Earlier workers prepared related compounds starting from bis(dimethylamino) phenylborane <85JOM(294)l> but the boronic acid route has now superseded this. 

Not only polystyrene supports, also other polymer supports were used in the preparation of polymeric amino alcohol ligands for dialkylzinc alkylation. For example, a vinylferrocene derivative with A,N -disubstituted norephedrine was copolymerized with vinylferrocene [60]. This polymeric chiral ligand (53) was used in the ethylation of aldehydes with moderate activity. Brown has reported that chiral oxazaborolidines have catalytic activity in the addition of diethyl zinc to aldehydes [61]. Polymers bearing chiral oxazaborolidines 37 were also active in the reaction and result on moderate enantioselectivity (<58 % ee) [62]. Enantiopure a,a -diphenyl-L-prolinol coupled to a copolymer prepared from 2-hydroxyethylmethacrylate and octadecyl methacrylate... 

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