Catalytic and Enantioselective Route to Medium-Ring Heterocycles. Asymmetric Zirconium-Catalyzed Ethylmagnesation of Seven- and Eight-Membered Rings

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• 作者：Michael S. Visser ; Nicola M. Heron ; Mary T. Didiuk ; John F. Sagal ; and Amir H. Hoveyda
• 刊名：Journal of the American Chemical Society
• 出版年：1996
• 出版时间：May 8, 1996
• 年：1996
• 卷：118
• 期：18
• 页码：4291 - 4298
• 全文大小：360K
• 年卷期：v.118,no.18(May 8, 1996)
• ISSN：1520-5126

文摘

A variety of medium-ring heterocycles, preparedefficiently by the Ru-catalyzed diene metathesis method,undergo asymmetric catalytic ethylmagnesation to afford nonracemicunsaturated alcohols and amides in excellentenantiomeric purity (>98% ee). Noteworthy features of thesestudies are as follows: (i) Eight-membered unsaturatedtosyl amides are readily prepared by transition-metal-catalyzedmetathesis. (ii) With six-, seven-, andeight-memberedN-containing substrates the presence of an electron-withdrawing Ts unitis required for efficient carbomagnesation(corresponding alkylamines are inert). Chiral medium-ringheterocycles are resolved by the Zr-catalyzed C-C bond-forming reaction to afford recovered starting materials in up to >99%ee. The kinetic resolution data indicate thatsimple steric models can reliably predict the sense of the asymmetricinduction in the asymmetric carbomagnesationor kinetic resolution. However, experimental results presentedherein also illustrate that the observed levelsofenantioselectivity cannot be predicted on the basis of suchparadigms.