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
✍ Scribed by Darius Paul Kranz; Aike Meier zu Greffen; Sherif El Sheikh; Jörg Martin Neudörfl; Hans-Günther Schmalz
- Publisher
- John Wiley and Sons
- Year
- 2011
- Tongue
- English
- Weight
- 449 KB
- Volume
- 2011
- Category
- Article
- ISSN
- 1434-193X
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✦ Synopsis
Abstract
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.