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Polyolefin allylsilane cyclisations

University of British Columbia

A series of Lewis acid-catalysed polyolefin cyclisations were performed, and the effect of the trimethylsilyl moiety and an ester group on the cyclisations were examined. It was found that an allylsilane not only directs formation of the resulting alkene to the less stable exocyclic position, but also activates an αβ-unsaturated ester in the cyclisation reactions. The allylsilanes 138 and 172 were synthesised and were treated with a variety of Lewis acids to give the monocyclic gem-dimethylcyclohexane 140 and the drimane 173b, respectively. The allylsilyl acid 123 was also synthesised and converted to structure 125. Elaboration of 140 and 125 into the monocyclic terpenoid, trixagol (153), and of 173b into several bicyclic terpenoids, albicanol (179b), albicanyl acetate (180), and isodrimenin (183), was achieved. The allylsilyl epoxides 191 and 226 were successfully synthesised and cyclised to the structures 189 and 229b, respectively. These are some of the first examples of an efficient cyclisation of an olefinic epoxide, and once again the trimethylsilyl group was found to be exceedingly useful in directing formation of the olefin to the exocyclic position. Karahana ether (193) was obtained in two steps from the cyclised product 189, and elaboration of 229b resulted in the first reported synthesis of 3-hydroxylabda-8(20),13-dien-15-oic acid (238). [diagrams not included]

Text

2010-04-26

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http://hdl.handle.net/2429/24197

Chemistry

University of British Columbia

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