EFFICIENT CONTROL OF THE STEREOSELECTIVITY IN REACTIONS OF 2-OXY-SUBSTITUTED BENZYLIC RADICALS

The stereoselectivity in reactions of 2-oxy-substituted radicals of type B was investigated. As expected, minimization of allylic 1,3-strain was the major controlling factor. Under standard conditions, only a modest level of stereoselectivity was observed. E.g., deuteration of the benzylated radical (R(1) = benzyl) gave diastereoisomer ratios less than or equal to 2:1. Use of a bulky protective group on the O-atom (R(1) = (t-Bu)Ph(2)Si) enhanced slightly the selectivity (ratio 4.1:1). However, a dramatic increase of the stereoselectivity (ratio 13:1) was obtained, when the reaction was performed with the free alcohol after treatment with bulky methylaluminium bis(phenoxide) derivatives (methylaluminium bis[2,6-di(tert-butyl)-4-methylphenoxide] (MAD) and methylaluminium bis(2,6-diphenyiphenoxide) (MAPH)).