Enol acetates hydrogenolysis over

Desoxycholic acid (III) is commercially available, but its 7-hydroxy analog, cholic acid (I), is much more abundant in animal bile. One of the best conversions is due to Fieser and Rajagopalan (1949), who oxidized cholic acid (I) with N-bromosuccinimide to the 7-keto derivative (II), which without purification is reduced in 68% over-all yield by the Huang-Minlon modification of the Wolff-Kishner method to desoxycholic acid (III). An alternative reduction, devised by Hirschmann et al. (1951), involves the catalytic hydrogenolysis of the enol acetate of II, but the yield is inferior. 

Enol esters are much more labile to hydrogenolysis than enol ethers. Thus, 1-acetoxy-cyclohexene and ethyl 3-acetoxycrotonate are almost quantitatively hydrogenolyzed over Adams platinum in ethanol or acetic acid (eqs. 13.46 and 13.47).91 92... 

The phosphoric acid ester of an enol ether is even more readily hydrogenolyzed than an enol acylate. When the (Z)-enol phosphate 51 was subjected to hydrogenation in ethyl acetate at 0.3 MPa H2, the hydrogenolysis products were isolated in 95% yield not only over platinum oxide but also over 5% Pd-C and 5% Rh-Al203 (eq. 13.50).94 The hydrogenolysis of an enol phosphate has been utilized in conversion of one of two oxo groups to the methylene as shown in eq. 13.51 95... 

---