Gold-Catalyzed Oxidative Transformation of Free Sugars into Biobased Platform Molecules

Due to their easy conversion into high-added value products,sugarlactones and their derivatives are very attractive biobased platformmolecules. Yet, conventional transformation of free sugars into suchactivated compounds is not so handy: a multistep procedure requiringprotection/oxidation/lactonization-esterification/deprotection isoften necessary. We report herein a procedure allowing one to rapidlyand efficiently form lactones/esters directly from free sugars undermild conditions, catalyzed with a small amount (0.36 mol %) of recyclablegold nanocatalyst under oxygen atmosphere. The conditions were optimizedusing galactose as a model, quantitatively and selectively affording1,4-galactonolactone in 2 h at room temperature. The procedure wasthen successfully applied to a variety of hexoses and pentoses leadingto excellent conversion (>86%). Due to the equilibrium betweenlactoneregioisomers and ester forms, a mixture of 1,4-lactone, 1,5-lactoneand methyl ester can be generally obtained depending on the sugarseries. A subsequent reaction of the crudes with benzylamine leadsto a total conversion of lactones/esters into corresponding amides,confirming the efficiency of the procedure and paving the way to aone-pot transformation of free sugars into high added value sugar-basedderivatives. Based on NMR and ESR analyses, a mechanism of the reactioninvolving CH3O & BULL; radical species seemsto be taking shape. Rapid, efficientand selective catalytic transformationof free sugars into activated platform molecules was performed usingO(2) under mild conditions.