Ionic Liquid-Promoted Wagner–Meerwein Rearrangement of 16α,17α-Epoxyandrostanes and 16α,17α-Epoxyestranes
作者:Anita Horváth、Ágota Szájli、Róbert Kiss、János Kóti、Sándor Mahó、Rita Skoda-Földes
DOI:10.1021/jo2006285
日期:2011.8.5
Ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([bmitn](+)[PF(6)](-)) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim](-)(+)[BF(4)](-)) were found to promote an unusual Wagner-Meerwein rearrangement of steroidal 16 alpha,17 alpha-epolddes leading to unnatural 13-epi-18-nor-16-one derivatives as the main products. These compounds were isolated in good to excellent yields. 16 alpha-Hydroxy-Delta(13)-18-norsteroid5, 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 16 alpha,17 alpha-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-13 alpha-steroid derivatives, 16 alpha-hydroxy-Delta(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.