Corey-Bakshi-Shibata

Triphenylphosphine Phosphine, triphenyl- (8,9) (603-35-0) (S)-2-Methyl-CBS-oxazaborolidine (CBS named after Corey, Bakshi, Shibata) ... 

One popular method that has been apphed to industrial processes for the enantio-selective reduction of prochiral ketones, leading to the corresponding optically active secondary alcohols, is based on the use of chiral 1,3,2-oxazaborolidines. The original catalyst and reagent system [diphenyl prolinol/methane boronic acid (R)] is known as the Corey-Bakshi-Shibata reagent. Numerous examples... 

Corey Bakshi Shibata/. Am. Chem. Soc. 1987,109,5551 Corey Bakshi Shibata Chen Singh J Am. Chem. Soc. 1987, 109, 7924 Corey Link Tetrahedron Lett. 1989, 30, 6275 Corey Bakshi Tetrahedron Lett. 1990, 31, 611. 

Scheme 13.2 Examples of successful Corey-Bakshi-Shibata enone reductions. Ar = / -PhCeH4. ...

The pioneering studies by Itsuno [1] and Corey [2] on the development of the asymmetric hydroboration of ketones using oxazaborolidines have made it possible to easily obtain chiral secondary alcohols with excellent optical purity [3]. Scheme 1 shows examples of Corey s (Corey-Bakshi-Shibata) CBS reduction. When oxazaborolidines 1 were used as catalysts (usually 0.01-0.1 equiv), a wide variety of ketones were reduced by borane reagents with consistently high enan-tioselectivity [2]. The sense of enantioselection was predictable. Many important biologically active compounds and functional materials have been synthesized using this versatile reaction [2-4]. 

Numerous theoretical treatments have been carried out to understand the mode of asymmetric induction of the Corey-Bakshi-Shibata (CBS) reduction, more thoroughly.12 Liotta et al. carried out computational studies to identify the transition states for CBS reductions of various ketones13 (Scheme 4.3k).In the asymmetric reduction of acetophenone with the catalyst (R)-28a, four transition states were found. Of the lowest energy is chairlike transition state A, which would lead to formation of the major enantiomer. In transition state A, the phenyl group of acetophenone occupies an equatorial position that is free from any steric interaction, as it is 5.5 A away from one of the two phenyl groups of the diphenylprolinol ring. On the other hand, transition state B, leading to the... 

For aryl ketones the Corey-Bakshi-Shibata (CBS) reduction using oxazaborolidines as catalysts for the boron hydride mediated hydrogenation is particularly useful, with maximum selectivities up to 99 % ee (see Scheme 4) [34]. The excellent review by Corey et al. [35] also shows clearly the power for chemo- and enantioselective reduction of purely aliphatic a,//-enones and -ynones only on the carbonyl group. In the re-... 

Ac = Acetyl acac = Acetylacetonate bda = Benzylidene-acetone BINOL = l,l -bi-2-naphthol Bn = Benzyl brsm = Yield based on recovered starting material Bu = Bntyl CAN = Ceric anunonium nitrate CBS = Corey/Bakshi/Shibata catalyst [(+) or (—)-(S)-2-methyl-oxazaborolidine] COD = Cyclo-l,5-octadiene COT = Cyclooctatetraene Cp = Cyclopentadienyl Cp = Penta-methylcyclopentadienyl Cy = Cyclohexyl DCC = Dicy-clohexylcarbodiimde DMF = Ai,A-dimethylformainide DMPU = l,3-dunethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidin-one DMSO = Dimethylsnlfoxide dppe = Diphenylphosp-hinoethane dr = Diastereomer ratio dppm = Diphenylphos-phinomethane E = Electrophile ee = Enantiomeric excess EHMO = Extended htickel molecular orbital Et =... 

The reduction of ketones by borane catalyzed by chiral oxazaborolidines such as (136), derived from the enantiomeric amino alcohols, has been applied to the synthesis of several drug candidates (127). This system is known as the CBS (Corey, Bakshi, Shibata) reduction (153), and Corey himself has applied it to the synthesis of pharmaceutical compounds (154). A further example is provided by the synthesis of MK-499 (137), a... 

CBS Corey, Bakshi, Shibata propyl) carbodiimide hydro-... 

Corey-Bakshi-Shibata reduction Enantioselective reduction of ketones with BH3 using oxazaborolidines as catalysts. 100... 

Related reactions Corey-Bakshi-Shibata (CBS) reduction, Noyori asymmetric hydrogenation ... 

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