Enatioselective reduction

Since the seminal work by Tatchell s group, many efforts have been directed to developing a highly enatioselective reduction of ketones.11 One successful... 

In a totally different approach, Noyori et al. have used binaphthol-modifled aluminum hydride reagent for enatioselective reduction of alkynyl ketones. Suitably modified boranes can be used for stereoselective reduction of ketones. Along these same lines. Midland" has developed Alpine borane (1, Scheme 21.5), which is excellent for several acetylenic ketones but has been found inefficient for hindered ot,p-acetylenic ketones. To overcome this problem, Brown et al." have introduced P-chlorodiisopinocamphenyl borane 2(-)-DIP-Cl (2, (Scheme 21.5), which reacts well with hindered ketones to provide the corresponding propargyl alcohols in 96 to 99% e.e. 

Optically active /3-ketoiminato cobalt(III) compounds based on chiral substituted ethylenedi-amine find use as efficient catalysts for the enatioselective hetero Diels Alder reaction of both aryl and alkyl aldehydes with l-methoxy-(3-(t-butyldimethylsilyl)oxy)-1,3-butadiene.1381 Cobalt(II) compounds of the same class of ligands promote enantioselective borohydride reduction of ketones, imines, and a,/3-unsaturated carboxylates.1382... 

Various polymer-bound (polystyrene-bound) oxazaboroHdine catalysts for the reduction of secondary alcohols were reported [128]. These can simply be prepared by condensation of the resin-bound boronic acid with chiral 1,2-amino alcohols. The best results as far as enatioselectivity is concerned were obtained with oxaza-borohdine (59) (Scheme 4.36). 

Enatioselectivity for the reduction improves as the size differential of the groups on the carbonyl increases. 

Scheme 9 Enzyme-mediated chemical transformations. (A) Enatioselective enzymatic oxidation and lactonization (B) enzyme reduction with baker s yeast and enantioselective rule and (C) enzymatic hydrolytic desymmetrization.

COREY Enatioselective borane reduction Enantioselective reduction ot ketones by borane or catecholborane catalyzed by oxazaborolidine 3... 

Transfer hydrogenation of ketones is quite general with regard to the molecular complexity of substrates and tolerates many functional groups. Recently, enatioselective transfer hydrogenation has been successfully applied to imine reduction [39]. From a synthetic perspective the principal virtue of this approach is the ease of generation of alcohols and amines in excellent yields with high enantiomeric excess. 

Reactions of phosphonic acids and their derivatives A novel approach to cyclic phosphonates (296) has been developed via a quinine thiourea catalysed, enatioselective Michael addition of quaternary a-nitroallq lphosphonates (293) to enones (294). Selected nitrophos-phonates (295) have been conveniently transformed to the phosphonates (296) and (297) via in situ reduction-intramolecular cyclisation or Baeyer-Villiger oxidation followed by in situ reduction-intramolecular cyclisation, respectively (Scheme 99). ... 

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