Synthesis,purification and characterization of 7-ketocholesterol and epimeric 7-hydroxycholesterols

Various methods were assessed for the synthesis of 7-ketocholesterol and epimeric 7-hydroxycholesterols. Upon the oxidation of cholesteryl acetate with t-butyl chromate, the resulting ketosterol acetate crystallized from methanol consisted of about 25% unoxidized cholesteryl acetate. After the sterol acetates were hydrolyzed in an aqueous K₂CO₃ medium, preparative TLC was used to fractionate the ketone from cholesterol. Of all the reducing agents employed, only LiAlH₄ reduced completely the purified 7-ketocholesterol to 7-hydroxycholesterols without side reaction products and ketone contamination. Yields of 21.3 mg of 7α-hydroxycholesterol and 72.6 mg of 7β-hydroxycholesterol were obtained by preparative TLC of a diol mixture prepared by the LiAlH₄-reduction of 100 mg of 7-ketocholesterol. To accomplish the preparative TLC separation of diol bands without overlapping, a double development of a chromatoplate with ethyl ether-cyclohexane (90∶10, v/v) and ethyl ether was essential. Data on the melting point, optical rotation and infrared spectra, as well as TLC and GLC characteristics, were obtained for purified 7-ketocholesterol and epimeric 7-hydroxycholesterols.