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Ketosteroids absorbed into various insoluble supports (powdered polyethylene, microporous 2% crosslinked polystyrene beads, macroporous highly crosslinked polystyrene beads, silica gel and alumina) can be reduced using aqueous potassium borohydride. The use of phase transfer catalysts generally raises yields. In the case of 6-ketosteriods the supported reactions often follow a stereochemical course significantly different from that of analogous reactions in solution. This is attributed to the adsorbtion of the steroid onto the inner surfaces of the supports. In these cases reduction of the ketosteroid by alkoxyborohydrides is substantially suppressed and thus most of the reduction is brought about by BH4− itself, a relatively sterically undemanding reductant. The net result is that whilst reductions of 6-ketosteroids in solution by potassium borohydride typically gives the 6α- and 6β-alcohols in the ratio 15:85, with the supported steroid the ratios can be as high as 90:10. |
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