Bakshi-Shibata CBS reagent

The CBS (Corey-Bakshi-Shibata) reagent is a chiral catalyst derived from proline. Also known as Corey s oxazaborohdine, it is used in enantioselective borane reduction of ketones, asymmetric Diels-Alder reactions and [3 + 2] cycloadditions. 

Name Reactions A Collection of Detailed Mechanisms and Synthetic Applications, DOI 10.1007/978-3-319-03979-4 69, Springer International Publishing Switzerland 2014 

Name Reactions, 4th ed., DOI 10.1007/978-3-642-01053-8 62, Springer-Verlag Berlin Heidelberg 2009 

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On the other hand, it was possible to cleave the biaryl lactones with chiral hydride transfer reagents, such as Noyori s BINAL-H, or borane THF in the presence of Corey-Bakshi-Shibata (CBS) reagent, furnishing the corresponding alcohols in excellent yields and enantioselectivities, as shown in Scheme 5.11. 

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]. 

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