Ionic Liquid-Promoted Wagner–Meerwein Rearrangement of 16α,17α-Epoxyandrostanes and 16α,17α-Epoxyestranes

Ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]+[PF6]−) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]+[BF4]−) 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-Δ13-18-norsteroids, the results of the usual rearrangement, were obtained as minor components of the reaction mixtures. The ionic liquid [bmim]+[PF6]− was shown to induce C-ring aromatization of 16α,17α-epoxyestranes due to the formation of HF, the hydrolysis product of [PF6]−. Increasing amounts of HF and [PO2F2]− were detected by 19F and 31P NMR when the ionic liquid was reused. The structures of the steroidal products, 16-oxo-18-nor-13α-steroid derivatives, 16α-hydroxy-Δ13-18-norsteroids, and C-aromatic compounds were determined by two-dimensional NMR techniques and high-resolution mass spectrometry (HRMS). The ionic liquids were recirculated efficiently.