Sn(OTf)2 as an Effective Lewis Acid in Reactions of Cyclopropyl Ketones with Acetic Anhydride: Application in the Synthesis of a 19-Nor-B-homo Steroid

Attempts to convert 3β‐acetoxy‐5,19‐cyclo‐pregna‐6,20‐dione (8) into a cyclocitrinol‐related steroid with a bicyclo[4.4.1]‐undecane AB‐ring substructure through a Lewis acid‐assisted fragmentation failed. Instead, treating 8 with an excess of Ac2O and BF3·Et2O afforded B‐homo steroid 9 in 90 % yield. This unexpected rearrangement is assumed to proceed via an intermediate bicyclobutonium cation. Remarkably, tricyclo[4.4.1.0]undecane‐1‐one (rac‐13), prepared as a model substrate, did not react under the same conditions. However, by screening various Lewis acids, stannous triflate was identified as a particularly efficient reagent for this and related Lewis acid‐assisted ring‐opening reactions of cyclopropyl ketones in the presence of Ac2O, and even catalytic amounts of Sn(OTf)2 (5 mol‐%) proved to be effective. The Lewis acid‐mediated ring‐opening rearrangement of bicyclo[4.4.1.0]‐undecane‐1‐one and related cyclopropyl ketones proceeds smoothly using a combination of stannous triflate [Sn(OTf)2] and Ac2O.