On the mechanism of the hydrogen chloride catalysed isomerization of 5α-cholest-8(14)-en-3β-ol to 5α,17β-cholest-14-en-3β-ol: fate of the 17α-hydrogen atom

Abstract
(4R)-[3H,2-14C]Mevalonic acid was incubated with a rat liver homogenate, in the presence of the inhibitor AY–9944, to yield [3H3,14C5]cholesta-5,7-dien-3β-ol, which was reduced (Li–NH3) to [17α,20,24R-3H3,14C5]-5α-cholest-7-en-3β-ol. [3H3,14C5]-5α-Cholest-7-en-3β-ol was isomerized (platinum–acetic acid–hydrogen) and the resulting [3H3,14C5]-5α-cholest-8(14)-en-3β-ol was acetylated. The acetate was treated with hydrogen chloride in chloroform at –78° followed by aqueous NaHCO3 to give [20,24R-3H2,14C5]-5α,17β-cholest-14-en-3β-yl acetate. The isomerization, proceeded with the loss of ca. 0.9 atom of tritium. The mechanism of the hydrogen chloride catalysed isomerization is discussed.

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