Mechanistic insights on the conversion of sugars into 5-hydroxymethylfurfural

A sustainable bio-economy requires the efficient utilization of all of the components of biomass. After glucose, mannose and galactose are the most abundant 6-carbon sugars in plant hemicellulose; lactose is a common dairy byproduct. Here, we demonstrate the conversion of these sugars into 5-hydroxymethylfurfural (HMF), a key intermediate for renewable fuels and chemicals. Chromium salts enable the efficient conversion of mannose into HMF but are less effective for galactose, lactose, and the related hexulose, tagatose. Isotopic labeling studies and the reactivity of psicose and sorbose indicate that the key aldose-to-ketose conversion occurs via a 1,2-hydride shift, which is followed by dehydration of the furanose tautomer. These data valorize mannose, galatactose, and lactose as sources of HMF, and inform the design of new catalysts for the conversion of aldoses to ketoses.