Mechanism of Brønsted acid-catalyzed conversion of carbohydrates

► Regioselectivity of the initial protonation of sugars determines selectivity of their conversion. ► Favorable protonation of fructose opens path to levulinic acid via HMF intermediate. ► Favorable protonation of glucose leads to reversion products and humin precursors. ► Novel direct mechanism of glucose transformation to levulinic acid is reported.