Atropo-enantioselective Reduction

The possibihty of chiral modiflcation of NaBH4 for reduction of ketones has been scrutinized. Some success is reported by adding the inexpensive cychc borinate 183 derived from tartaric acid and PhB(OH)2 to the reaction medium. Atropo-enantioselective reduction of biaryl lactones occurs on exposure to NaBH in the presence of the cobalt chelate 184. ... 

In 2003, Molander et al. reported the synthesis of (+ )-isoschizandrin, a natural product displaying anti-ulcer activity in rats. The key step of the synthesis was the atropo-enantioselective reduction of a seven-membered lactone by using a chiral oxazaborolidine, (i )-2-methyl-CBS-oxazaborolidine, which provided the corresponding chiral lactone after recycling (Scheme 5.45). Similar conditions were also applied to the corresponding six-membered lactone, which led to the formation of the corresponding chiral diol in excellent yield and enantioselectivity, as shown in Scheme 5.45. 

Bringmann and co-workers have reported highly efficient atropo-enantioselective reduction using OABs as chiral inducers to give one atropisomer with high enantiose-lectivities [76]. For example, the reduction of biaryl lactones 51 with BH3 THF (4 equiv.) in the presence of 2b (3 equiv.) in THF at 30 °C provided chiral biaryl alcohol 52 with up to 97% ee (Scheme 11.15). 

Scheme11.15 OAB-induced atropo-enantioselective reduction of biaryl lactones.

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