Corey-Bakshi-Shibata reagents

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... 

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

Addition of alkenylzinc 322 to the aldehyde 321 resulted in a diastere-omer mixture (1 1) of allylic alcohol, which was oxidized to afford ketone 303. Although Terashima s asymmetric aluminum reagent did not give the desired alcohol, the asymmetric borohydride reduction catalyzed by the Corey-Bakshi-Shibata reagent gave a 5 1 mixture of separable diastereomers, in favor of the (17J )-alcohol 323. Finally, protective group manipulation and oxidations led to a seco-acid, which was subjected to Yonemitsu-modified Ya-maguchi macrolactonization to yield the macrocycle (201) (Scheme 68). 

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]. 

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. 

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