The synthesis of alkenes via epi-phosphonium species: 1. An anti-Wittig elimination

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• 作者：Lawrence ; Nicholas J. ; Muhammad ; Faiz
• 关键词：alkenes ; elimination reactions ; phosphines ; Wittig reaction
• 刊名：Tetrahedron
• 出版年：1998
• 出版时间：December 10, 1998
• 年：1998
• 卷：54
• 期：50
• 页码：15345-15360
• 全文大小：1016 K

文摘

Anti-1,2-phosphinyl alcohols 11 and their corresponding syn-isomers upon treatment with phosphorus trichloride and triethylamine give E and Z alkenes respectively, by an anti elimination. This is in marked contrast to the syn Horner-Wittig elimination of the corresponding 1,2-phosphinoyl alcohols. The 1,2-phosphinyl alcohols 11 were prepared by the reduction of 1,2-phosphinoyl alcohols with cerium(III) chloride/lithium aluminium hydride. The anti elimination is explained by the formation of a transient epi-phosphonium species. An unexpected E-selective Horner-Wittig elimination during the cerium(III) chloride/lithium aluminium hydride reduction of a 1,2-phosphinoyl alcohol in which the diphenylphosphinoyl group is adjacent to an aryl group is described. This led to the synthesis of the antimitotic agent E-combretastatin A-4. An alternative synthesis of the 1,2-phosphinyl alcohols from the corresponding phosphine-borane complex is also described.