Toward Understanding Base-Catalyzed Isomerization of Saccharides

This Perspective addresses recent advances in isomerization of aldoses into ketoses catalyzed by bases. In recent years, catalytic activity of a number of bases was reported, but the structure-performance correlations are typically missing or remain poorly understood. In this Perspective, the reaction mechanism and kinetics of the base-catalyzed isomerization are discussed. An overview of products obtained via base-catalyzed transformations of D-glucose (Glc), D-lactose (Lac), D-galactose (Gal), and aldopentoses D-ribose (Rib), D- arabinose (Ara), D-xylose (Xyl), and D-lyxose (Lyx) is provided. Side reactions that occur under base conditions are summarized. Catalysis by soluble metal hydroxides and general bases is considered. Moreover, structure-selectivity relationships observed for soluble bases as well as methods for stabilization of ketoses via complexation are discussed. Influences of organic (co-)solvents on reaction kinetics and acceleration of the reaction rate in aqueous phase in the presence of soluble salts-also referred to as salt effect-are addressed. Next, solid bases applied as catalysts for isomerization are considered with a focus on contributions of homogeneous and heterogeneous catalysis into the isomerization processes. The applicability of a filtration test and a contact test as tools to estimate impact of leached species onto catalysis is critically discussed.