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-di-one (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.