Ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim] +[PF 6] -) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim] +[BF 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 [bmim] +[PF 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 2F 2] - 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- 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.
ASJC Scopus subject areas
- Organic Chemistry