Stereo and chemical course of acid-catalyzed double bond migration of cholesta-5,7-dien-3β-ol to 5α-cholesta-8,14-dien-3β-ol

Abstract

Acid-catalyzed double bond migration of steroid 5,7-dienes to the 5α-8,14-dienes, a well-known reaction in steroid chemistry, was reinvestigated by using cholesta-5,7-dien-3β-ol 1, and the stereo and chemical course of this reaction was detailed. Treatment of 1 with 36% hydrochloric acid in refluxing ethanol for 3 h afforded a 6∶11∶65∶16∶2 mixture of dienols (93%): 5α- and 5β-cholesta-6,8(14)-dien-3β-ols 7a,b, 5α- and 5β-cholesta-8,14-dien-3β-ols 2a,b, and 5α-cholesta-14,16-dien-3β-ol 10a, along with a mixture of enones (1.7%): 5β-cholest-8(14)-, 5β-cholest-14- and 14-epi-5β-cholest-8-en-3-ones 11–13. The experiments using 7a,b and 2a,b suggested the reaction sequence: 1→7a,b⇆7,14-dienols 8a,b⇆2a,b⇆8(14),15-dienols 9a,b⇆10a,b, in which 8a,b⇆9a,b should be also implicated. The initial step, 1 to 7a,b proceeded irreversibly with the stereoselectivity, ca. 7∶3 of 5α-H to 5β-H. Dienols 7a, 8a, 2a and 10a with 5α-H were identified, which were equilibrated at 6∶0∶92∶2. Among dienols with 5β-H, only 7b and 2b were identified, which were equilibrated at 2∶98, and this interconversion proceeded in competition with an intramolecular hydride shift from the C-3α to C-6α of 7b, leading to the formation of a mixture of enones 11–13. The considerable difference in activation energies between 1→7a,b and 7a,b→8a,b/11–13 realized the predominant formation of 7a,b: by treatment at 30 °C for 44 h, 1 gave a 53∶27∶5∶15 mixture (94%) of 7a, 7b, 8a and 2a.