NMR studies of the tautomer distributions of D-fructose in lower alcohols/DMSO-d6

The tautomer distributions of D-fructose, a popular model compound for biorefinery, in various lower alcohols and DMSO-d(6) (ROHs/DMSO-d(6)) mixtures at 298 K were investigated by quantitative C-13 NMR. The tautomer distributions of D-fructose in ethanol/DMSO-d(6) and methanol/DMSO-d(6) with different volume ratios of alcohols to DMSO-d(6) were also examined. For these normal alcohols (methanol, ethanol, n-propanol and n-butanol)/DMSO-d(6), the tautomer distribution of D-fructose in methanol/DMSO-d(6) exhibited the shortest equilibration time (46 h) and the lowest concentration of beta-furanose (39.0%) at equilibrium. The fraction of fructofuranose (alpha-furanose and beta-furanose) at equilibrium and time needed to reach equilibrium in branched chain alcohols/DMSO-d(6) obey the following order: 2-propanol/DMSO-d(6) (62.2%, 320 h) > tert-butanol/DMSO-d(6) (61.6%, 307 h) > 2-butanol/DMSO-d(6) (60.6%, 216 h) > isobutanol/DMSO-d(6) (57.8%, 134 h). For the diols/DMSO-d(6), the time for the tautomer distribution of D-fructose to reach the equilibrium was found to be: 1,4-butanediol/DMSO-d(6) (113 h) > 1,2-propanediol/DMSO-d(6) (38 h) > ethylene glycol/DMSO-d(6) (36 h). For ethanol/DMSO-d(6) and methanol/DMSO-d(6) with respectively different volume ratios, the experimental results showed that the high concentration of ethanol or methanol increases the conversion rate of tautomers. (C) 2018 Elsevier B.V. All rights reserved.