[en] Monomode microwave-assisted coupling of D-glucuronic acid with alcohols, in the presence of various impregnated acid catalysts, was successfully performed, affording in almost quantitative yields the corresponding monosubstituted b-D-glucofuranosidurono-6,3-lactones in less than 10 min at 85°C. This study evidences the synergy of microwaves and impregnated acid catalysts as a fast and clean strategy in the field of carbohydrate chemistry.
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Catalysts preparation: Both silica-based supports, namely Silica Gel 60 (with BET-specific surface area of 500 ± 50 m2/g and average pore diameter of 60 ± 5 Å) and Si-MCM-41 (with BET-specific surface area of 1000 m2/g and pore size of 23-27 Å), were predried prior to use at 200 °C overnight. Silica-supported sulfuric acid catalysts were prepared according to Kogelbauer by aqueous incipient wetness impregnation of Silica Gel 60 with appropriated aqueous solutions of H2SO4 yielding catalysts with respective loadings of 35% (i.e., 35 g pure H2SO4/100 g final catalyst), 50% and 70%. The same procedure was used for the preparation of 35% H2SO4/MCM-41, H3PO4/SiG60 and silica-supported Keggin type heteropolyacid. After preparation, each catalytic system was dried for 24 h in static air at 120 °C and stored in a desiccator under vacuum.
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For sake of comparison, the weight loss associated to the desorption of H2SO4 reached 33.4% for the catalyst recovered after a blank reaction, where H2SO4/SiG60 was stirred together with methanol, under microwave heating at 85 °C for 10 min.
General procedure for the microwave-assisted synthesis of compounds 1-7: d-GlcA (200 mg, 1.03 mmol), alcohol (5-25 equiv) and the acid catalyst were introduced together in a specific 10 mL microwave vial. The vial was next placed in the microwave cavity and irradiated under stirring. Control of the temperature during the experiment was assigned by an infrared detector. For classical thermally driven reactions, the reactions were performed under agitation in a thermostated oil bath. After heating, the crude mixture was filtrated to eliminate solid acid catalyst and then chromatographed on silica gel. Spectral and physico-chemical data for 1, 3 and 4-6 (α,β) are proposed in Supplementary Data S4. i-Propyl d-glucofuranosidurono-6,3-lactone 7: White solid (recrystallised in CHCl3); mp 110-112 °C (decomp.). IR (KBr) ν 3440 cm-1, 2976 cm-1, 1755 cm-1. Compound 7β: TLC (EtOAc/n-hexane 1/1 v/v): Rf 0.11. 1H NMR (600 MHz, D2O) δ 5.17 (s, 1H, H-1), 4.88 (m, 2H, H-3, H-4), 4.59 (d, 1H, J4,5 6.6 Hz, H-5), 4.21 (s, 1H, H-2), 3.87 (m, 1H, OCH(CH3)2), 1.17 (d, 6H, OCH(CH3)2). 13C NMR (D2O) δ 177.23 (C{double bond, long}O), 107.66 (C-1), 83.79 (C-3), 76.79 (C-4), 76.69 (C-2), 69.03 (C-5), 64.25 (OCH(CH3)2), 23.59 (OCH(CH3)2).