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.