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Trimerization products of trifluoroacetone: critical solvent effect on position and kinetics of anomeric equilibria.

Saaidi PL, Guyonnet M, Jeanneau E, Fleurat-Lessard P, Hasserodt J

Laboratoire de Chimie, UMR CNRS 5182, Université de Lyon-ENS, 46 Allée d'Italie, 69364 Lyon, France, and Centre de Diffractométrie Henri Longchambon, Université de Lyon-UCB, 43 bd du 11 Novembre 1918, 69622 Villeurbanne, France.

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 fluorinated carbohydrates. We report herein a complete study of their behavior in solution. The remarkable solvent effect on the two equilibria (1a <--> 1b; 2a <--> 2b) was rationalized using solvent basicity measures and polarity 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 structure with 1,3-cis-diaxial hydroxyl groups. A complementary kinetic study illustrated that acidic conditions can drastically reduce the equilibration rate, allowing the use of a wide range of solvents. Finally, a reexamination of previously published trimerization conditions using sodium or magnesium amalgam revealed 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 be detected as a minor product.

Published 8 February 2008 in J Org Chem, 73(4): 1209-16.
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