Reduction of steroidal ketones

The reduction of an asymmetric cyclohexanone (e.g. a steroidal ketone) can lead to two epimeric alcohols. Usually one of these products predominates. The experimental results for the reduction of steroidal ketones with metal hydrides have been well summarized by Barton and are discussed in some detail in a later section (page 76) unhindered ketones are reduced by hydrides to give mainly equatorial alcohols hindered ketones (more accurately ketones for which axial approach of the reagent is hindered " ) are reduced to give mainly axial alcohols. 

The data available on the stereochemistry of reduction of steroidal ketones have been obtained largely in the course of synthetic work, rather than in studies devoted specifically to stereochemical problems. As discussed in an earlier section, the proportion of epimers depends on the steric environment of the ketone, the reagent, the solvent and the temperature. These factors will be discussed below. 

Reduction of Steroidal Ketones with Lithium Aluminum Deuteride... 

Deuteration with metal deuteride complexes reduction of steroidal ketones with lithium aluminum deuteride, 164 Dimethyl sulfoxide dicyclohexylcarbo-... 

The axial equatorial isomer ratio for cyclohexanols obtained by reduction of cyclohexanones depends upon the solution pH (see Table 10.6). In alkaline or initially neutral but unbuffered solutions [59, 60], this ratio approaches the value at thermodynamic equilibrium The equatorial-aicohol, for exanq>le 9, is generated [61] from reduction of steroid ketones. In acid buffers, the thermodynamically less... 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

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