Norrish–Prins reaction as a key step in the synthesis of 14β-hydroxy-5α (or 5β or Δ 5,6)-pregnane derivatives

Abstract

Numerous bioactive glycosteroids are characterized by aglycones bearing a 14β-hydroxy pregnane skeleton like boucerin and isoramanone. In general, the syntheses of the latter are achieved by acidic hydrolysis of the corresponding glycosteroids. These aglycones were also obtained by a combined Norrish type I–Prins reaction starting from the corresponding 12-keto-pregnane derivatives. However, for the Norrish–Prins reaction, no reports describe the influence of the A/B ring junction (cis or trans or Δ 5,6 double bond) or the influence of the substitution pattern at position 20. Herein, we describe the use of Norrish type I–Prins reactions to synthesize isoramanone and boucerin derivatives and their A/B cis and trans analogs. The influence of the parameters mentioned above is also presented. These studies showed that the A/B ring junction has little influence on the Norrish type I–Prins reaction but that the substitution pattern at position 20 is important. The presence of a dioxolane group induced not only the formation of the desired 14β-hydroxy pregnane derivatives in the highest yields but also the formation of new spiro derivatives.