Stereoselective oxazaborolidine-borane reduction of biphenyl methyl diketones: influence of biphenyl substitution pattern

The stereoselective oxazaborolidine-borane reduction of biphenyl methyl diketones with different substitution patterns on the biaryl unit was investigated, with the aim to better rationalize the factors influencing the diastereo- and enantioselectivity of the reaction for this class of compounds. The observed stereoselectivity in the reduction of diketones 6 and 7, possessing a methoxy group in the meta position with respect to the carbonyl group, is clearly dependent on the experimental conditions. The presence of an asymmetric centre in the intermediate hydroxyketone influences the global stereoselectivity in the reduction of 5 and 6, as indicated from the different selectivity ratios determined for each step of reaction.