On the α-lithiation-rearrangement of N-toluensulfonyl aziridines: mechanistic and synthetic aspects |
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Authors: | Peter O'Brien Clare M RosserDarren Caine |
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Affiliation: | a Department of Chemistry, University of York, Heslington, York YO10 5DD, UK b GlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage SG1 2NY, UK |
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Abstract: | A detailed study of the rearrangement of five cycloalkene N-toluenesulfonyl (tosyl) aziridines using sec-butyllithium (with and without added ligands such as (−)-sparteine and TMEDA) has been carried out. Allylic sulfonamides were the main products from the cyclopentene and cyclohexene aziridines whereas bicyclic sulfonamides were obtained from the cycloheptene and cyclooctene aziridines. In most cases, p-toluenesulfonamide (TsNH2) was produced as a by-product and a mechanistic explanation for its formation is forwarded. These reactions are believed to involve α-lithiation to a lithiated aziridine which can then partition through two pathways: (i) rearrangement to allylic or bicyclic sulfonamides via C-H insertion reactions or (ii) reductive alkylation to alkenes via attack by sec-butyllithium and subsequent elimination of TsNH2. In the (−)-sparteine reactions, the products were generated with 38-66% ee and the sense of asymmetric induction involved lithiation of the S-aziridine stereocentre. This is opposite to that observed with epoxides. |
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Keywords: | aziridines sulfonamides rearrangement lithiation carbenes and carbenoids |
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