CBS reagents

In the synthesis of (—)-hennoxazole (37), Wipf and Lim used a CBS reagent to prepare the chiral allylic alcohol 3918 (Scheme 4.3n). The enantioselective reduction of the enone 38 using a catalytic amount of the oxazaborolidine 28b... 

More effective is the chiral borohydride analogue developed by Corey, Bakshi, and Shibita. It is based upon a stable boron heterocycle made from an amino alcohol derived from proline, and is known as the CBS reagent after its developers. 

Reductions withNature s CBS reagent—NABH—are discussed In Chapter 51.. ... 

In fact, the enantiomer of the CBS reagent can be made by a resolution strategy. 

These reactions are quite messy, and are best done on a large scale Notice how the selectivity of baker s yeast is die reverse of that of the CBS reagent with respect to the large and small ketone substituents. This is most useful, since (i )-pro line is expensive, and an enantiomeric yeast cell would be a rarity indeed. 

Several novel catalysts in which borohydride is complexed with a difiinctional chiral ligand have been developed and used for the enantioselective reduction of prochiral ketones to chiral alcohols. Corey-Bakshi-Shibatareduction (CBS reduction) is an organic reaction which reduces ketones enantioselectively into alcohols by using chiral oxazaborolidines and BHs-THF or catecholborane as stoichiometric reductants (CBS reagent, 1.64) (also see Chapter 6, section 6.4.2). 

The (S)-CBS reagent delivers hydride (H from the front side of the C=0. This generally affords the R alcohol as the major product. 

In this example, the (R)-CBS reagent adds the new H atom from behind, the same result observed with acetophenone and propiophenone. In this case, however, alcohol A has the R configuration using the rules for assigning priority in Chapter 5. 

Enantioselective reduction of an aldehyde or ketone with the chiral CBS reagent (20.6)... 

CBS reagent (Section 20.6A) A chiral reducing agent formed by reacting an oxazaborolidine with BH3. CBS reagents predictably give one enantiomer as the major product of ketone reduction. 

Reductions with Nature s CBS reagent—NADH—are discussed in Chapter 51. 

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