Probing the Role of Individual OH Sites in Carbohydrate Conversion Suggests Strategies for Increasing Product Selectivity and Avoiding Humins

Carbohydrates will likely play a central role in the future production of organic chemicals, but the understanding of pathways that are accessible in carbohydrate upgrading has remained unsatisfactory. The polyfunctionality of carbohydrates and their complex appearance in linear and cyclic forms with different stereochemistries can lead to diverse and interconverting pools of isomers along the entire reaction pathway. Here, we identify avoidable pathways toward byproducts and humins using an experimental high-resolution investigation into the routes of carbohydrate conversion with an emphasis on the conversion of C5 carbohydrates by Lewis acidic SnCl4 in dimethyl sulfoxide (DMSO)/water. Reaction tracking with C5 carbohydrates and deoxy variants was used to evaluate the effects of loss of individual functional groups on the pathways that are accessible in carbohydrate conversion. Especially, the removal of the hydroxy group at C5 enhances (and accelerates) the selectivity of carbohydrate conversion through an acyclic pathway, not only relative to the formation of furanic compounds but also relative to the formation of humins. Blocking the primary alcohol in xylose may hence benefit future biomass conversion strategies. Even in the absence of intermolecular reactions toward humins, the conversion of pentose follows second-order kinetics.