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 BF3·Et2O-promoted reaction of 3β-acetoxy-5,19-cyclo-pregnan-6β-ol-20-one with different nucleophiles was investigated. B-homo steroids (3β-acetoxy-B-homo-6a-β-alkoxy-pregna-5(10)-en-20-ones) were obtained with primary and secondary alcohols, while the reaction with common carboxylic acids selectively afforded the corresponding 3β-acetoxy-6β-(acyloxymethyl)-pregna-5(10)-en-20-ones. The transformations are supposed to proceed via the rearrangement of a cyclopropyl-methyl cation (bicyclobutonium) intermediate, which is regioselectively opened in dependence on the nucleophile employed. The method represents an efficient, diversity-oriented entry to new B-ring-modified steroids, which are of potential pharmaceutical relevance.

The first synthetic entry towards the cyclocitrinols, a new class of natural products possessing a steroid-analogue structure with a characteristic bicyclo[4.4.1]undecane AB ring system, was elaborated. In the key step, a cyclopropanated intermediate (prepared from dehydroepiandrosterone) was subjected to reductive fragmentation (using SmI 2 in THF).

Key indicators: single-crystal X-ray study; T = 100 K; mean (C-C) = 0.003 Å; R factor = 0.037; wR factor = 0.079; data-to-parameter ratio = 9.9.In the title compound, C 23 H 34 O 4 , the C/D and D/E rings are trans fused and the A/B ring possesses an anti fusion. The two cyclohexane rings adopt a chair conformation while the cyclohexene ring exhibits a half-chair conformation. The cyclopentane ring displays an envelope conformation with the C atom bearing the methyl group as the flap. In the crystal, the molecules are linked by O-HÁ Á ÁO hydrogen bonds, forming chains along the b axis.

Me3Si‐allyl as the nucleophile gives the highest yield of the rearranged alcohol (XIII) after hydrolysis.