Cyclization and Ring Transformation via Spirocyclic Arenium Ions

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  • Nagumo Shinji
    Department of Chemistry and Life Science, School of Advanced Engineering, Kogakuin University
  • Mizukami Megumi
    School of Pharmacy, Hokkaido Pharmaceutical University

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  • スピロ型アレーニウムイオンを経由する環化反応および骨格変換反応
  • スピロガタ アレーニウムイオン オ ケイユ スル カンカ ハンノウ オヨビ コッカク ヘンカン ハンノウ

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Abstract

Our cyclizations and ring transformations via two types of spirocyclic arenium ions are described in this article. A spiro [5.2] octadienyl cation, which is well known as a Cram’s arenium ion, has attracted considerable attention in the aspect from physical organic chemistry. Our effort to utilize the intermediate as a synthetic tool resulted in the development of a lactonization of β-aryltosylate with an internal ester, a lactone ring transformation, and an ether ring transformation via the Cram’s arenium ion. We also developed a 7-endo selective Friedel-Crafts cyclization of various epoxy unsaturated esters promoted by Lewis acids. Detailed experimental data showed that a spiro [5.5] undecadienyl cation is a key intermediate of this reaction. The reaction mechanism seems to involve ipso-cyclization and subsequent skeletal rearrangement of the resulting spirocyclic arenium ion. The Friedel-Crafts cyclization was also applied to the formation of tetrahydro-2-benzazepine.

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