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 Ac 2 O and BF 3 · Et 2 O 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 Ac 2 O, and even catalytic amounts of Sn(OTf) 2 (5 mol‐%) proved to be effective.