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

Due to their easy conversion into high-added value products, sugar lactones and their derivatives are very attractive biobased platform molecules. Yet, conventional transformation of free sugars into such activated compounds is not so handy: a multistep procedure requiring protection/oxidation/lactonization-esterification/deprotection is often necessary. We report herein a procedure allowing one to rapidly and efficiently form lactones/esters directly from free sugars under mild conditions, catalyzed with a small amount (0.36 mol %) of recyclable gold nanocatalyst under oxygen atmosphere. The conditions were optimized using galactose as a model, quantitatively and selectively affording 1,4-galactonolactone in 2 h at room temperature. The procedure was then successfully applied to a variety of hexoses and pentoses leading to excellent conversion (>86%). Due to the equilibrium between lactone regioisomers and ester forms, a mixture of 1,4-lactone, 1,5-lactone and methyl ester can be generally obtained depending on the sugar series. A subsequent reaction of the crudes with benzylamine leads to a total conversion of lactones/esters into corresponding amides, confirming the efficiency of the procedure and paving the way to a one-pot transformation of free sugars into high added value sugar-based derivatives. Based on NMR and ESR analyses, a mechanism of the reaction involving CH3O• radical species seems to be taking shape.