Ring-Locking Enables Selective Anhydrosugar Synthesis from Carbohydrate Pyrolysis

Anhydrosugars are common but low-yield products of carbohydrate pyrolysis. Their fixed conformation, protected stereoselectivity, and retention of most of their chemical functionality make these sugar derivatives highly desirable precursors for biologically active compounds, biodegradable surfactants, and polymers. Anhydrosugar production is very inefficient though, due to the nonselective nature of carbohydrate pyrolysis. Here we demonstrate how they form with excellent purity by ring-locking sugars prior to thermal treatment. We found that the selectivity to 1,6-anhydro-β-D-glucopyranose (levoglucosan, LGA) increased from 2% to greater than 90%, after alkoxy or phenoxy substituents were placed at the anomeric carbon of glucose, followed by fast pyrolysis of the resulting sugar at 600 °C. LGA formation becomes the dominant reaction pathway when the substituent group inhibits the pyranose ring from opening and fragmenting into non-anhydrosugar products. These findings introduce the ring-locking concept to sugar pyrolysis chemistry and offer a chemical-thermal treatment approach for upgrading simple and complex carbohydrates.