Trimerization Products of Trifluoroacetone: Critical Solvent Effect on Position and Kinetics of Anomeric Equilibria

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• 作者：Pierre-Lo&iuml ; c Saaidi ; Mathieu Guyonnet ; Erwann Jeanneau ; Paul Fleurat-Lessard ; Jens Hasserodt
• 刊名：Journal of Organic Chemistry
• 出版年：2008
• 出版时间：February 15, 2008
• 年：2008
• 卷：73
• 期：4
• 页码：1209 - 1216
• 全文大小：240K
• 年卷期：v.73,no.4(February 15, 2008)
• ISSN：1520-6904

文摘

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In the presence of bases, trifluoroacetone is known to trimerize leading to configurationally labile 6-methyl-2,4,6-tris(trifluoromethyl)tetrahydro-2H-pyran-2,4-diols 1a,b and 2a,b, structurally close to fluorinatedcarbohydrates. We report herein a complete study of their behavior in solution. The remarkable solventeffect on the two equilibria (1a 1b; 2a 2b) was rationalized using solvent basicity measures andpolarity scales. Solvents of weak donor number were found to favor the diastereoisomers 1a and 2a,which were subsequently isolated. According to their X-ray analyses, they both possess a concave structurewith 1,3-cis-diaxial hydroxyl groups. A complementary kinetic study illustrated that acidic conditionscan drastically reduce the equilibration rate, allowing the use of a wide range of solvents. Finally, areexamination of previously published trimerization conditions using sodium or magnesium amalgamrevealed that, contrary to the suggestion by two independent reports, 1,3,5-tris(trifluoromethyl)cyclohexane-1,3,5-triol 3/4 was neither formed as the principal product in place of 1a,b and 2a,b nor could it bedetected as a minor product.