Ergosterol selective reduction

Selective reduction of ergosterol and its derivatives has been carried out using electron transfer reduction. Product distribution depends on the hydroxy substituent as well as on the solvent and the metal used (Scheme 86). ... 

Carbon—carbon double bonds which are conjugated to other multiple bonds can also be reduced selectively by dissolving metals. For example, in the selective reduction of ergosterol (7) the distribution of the regioisomers depends on the hydroxy substituent, as well as on the solvent and the metal employed 1 °. Excellent yields of the (ran. -fused product 8 are obtained when sodium in /erf-butyl alcohol/tetrahydrofuran is used. Only a trace of the epimeric civ-product, formed as a result of / -face protonation, is found. The isolated double bond is not affected under the reaction conditions. 

Addition of deuterium to ergosterol acetate was shown to form the 5a,6a-dideuterio reduction product under mild conditions (Equation 15.4). This result illustrates a number of features of the reactivity of Wilkinson s catalyst. First, the conjugated diene is reduced in preference to the C21-C22 double bond. Second, the diene is reduced from the less-crowded a-face of the steroid. Neither the remaining trisubstituted double bond nor the trans disubstituted A double bond are affected. Finally, no isotopic exchange takes place during this reaction. This selectivity has been used to specifically label prostaglandins with tritium (Equation 15.5). The sensitive 3-keto alcohol is unperturbed. 

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