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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
Author(s) -
Kranz Darius Paul,
Meier zu Greffen Aike,
El Sheikh Sherif,
Neudörfl Jörg Martin,
Schmalz HansGünther
Publication year - 2011
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201100020
Subject(s) - chemistry , lewis acids and bases , acetic anhydride , acetic acid , organic chemistry , medicinal chemistry , stereochemistry , catalysis
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.