Photooxygenation of Pregnanes

AbstractThe course of the singlet‐oxygen reaction with pregn‐17(20)‐enes and pregn‐5,17(20)‐dienes was studied to compare the reactivity of the two alkene moieties present in some steroid families. Thus, from commercially available (3β,5α)‐hydroxy‐androstan‐17‐one and (3β)‐3‐hydroxyandrost‐5‐en‐17‐one, the following 3‐{[(tert‐butyl)dimethylsilyl]oxy}‐substituted, 17(20)‐unsaturated pregnanes were prepared (see Fig. 1): (3β,5α)‐21‐norpregn‐17(20)‐ene 1; (3β,5α,17Z)‐pregn‐17(20)‐ene 2, (3β,5α,16α,17E)‐pregn‐17(20)‐en‐16‐ol 3, (16β,5α,17E)‐pregn‐17(20)‐en‐16‐ol 4, (3β,5α,16β,17E)‐pregn‐17(20)‐en‐16‐ol acetate 5, (3β,16α)‐21‐norpregna‐5,17(20)‐dien‐16‐ol 6, (3β,16α,17E)‐pregna‐5,17(20)‐dien‐16‐ol 7, (3β,17Z)‐pregna‐5,17(20)‐diene 8, (3β,17E)‐pregna‐5,17(20)‐dien‐21‐ol 9 and (3β,17E)‐5,17(20)‐dien‐21‐ol acetate 10. The oxygenated products (see Fig. 2) obtained from 1–10 and 1O2, generated by irradiation of Rose Bengal in 3O2‐saturated pyridine solution, were characterized by 1H‐, 13C‐NMR, and MS (EI, FAB, HR‐EI, ESI‐ and UV‐MALDI‐TOF) data. Major products were those formed by the ene reaction involving as intermediates the corresponding hydroperoxides and the cyclic tautomers of the allylic hydroperoxides, i.e., the corresponding oxiranium oxide‐like intermediate (Scheme 5).