Chiral Br?nsted acid-catalyzed enantioselective ionic [2+4] cycloadditions
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- 作者：Alina Borovika ; Pavel Nagorny<sup> ; sup><sup>nagorny@umich.edusup>
- 关键词：Oxocarbenium ion ; Ionic Diels&ndash ; Alder ; Cycloaddition ; Chiral counterion ; N-Triflyl phosphoramide
- 刊名：Tetrahedron
- 出版年：2013
- 出版时间：8 July, 2013
- 年：2013
- 卷：69
- 期：27-28
- 页码：5719-5725
- 全文大小：1517 K
文摘
The first asymmetric chiral N-triflylphosphoramide-catalyzed ionic [2+4] cycloaddition reaction of unsaturated acetals is described. This reaction proceeds through the intermediacy of a vinyl oxocarbenium/chiral anion pair, and the chiral N-triflylphosphoramide anion controls the stereoselectivity of the cycloaddition step. Moderate enantioselectivities (up to 80:20 e.r.) have been obtained when ¦Á,¦Â-unsaturated dioxolanes were employed as the dienophiles. These reactions demonstrate strong dependence on the counterion coordinating properties and solvent polarity, a behavior characteristic of oxocarbenium ions.