Ionic liquid-promoted Wagner-Meerwein rearrangement of 16α,17α-epoxyandrostanes and 16α,17α-epoxyestranes.

Ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6)) and 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)) were found to promote an unusual Wagner-Meerwein rearrangement of steroidal 16α,17α-epoxides leading to unnatural 13-epi-18-nor-16-one derivatives as the main products. These compounds were isolated in good to excellent yields. 16α-Hydroxy-Δ(13)-18-norsteroids, the results of the usual rearrangement, were obtained as minor components of the reaction mixtures. The ionic liquid bmimPF(6) was shown to induce C-ring aromatization of 16α,17α-epoxyestranes due to the formation of HF, the hydrolysis product of PF(6). Increasing amounts of HF and PO(2)F(2) were detected by (19)F and (31)P NMR when the ionic liquid was reused. The structures of the steroidal products, 16-oxo-18-nor-13α-steroid derivatives, 16α-hydroxy-Δ(13)-18-norsteroids, and C-aromatic compounds were determined by two-dimensional NMR techniques and high-resolution mass spectrometry (HRMS). The ionic liquids were recirculated efficiently.