Chemical synthesis of rare carbohydrates

Rare carbohydrates (e.g. D-tagatose, D-allulose, and D-xylulose) are regarded as ideal sugar replacers in a number of food formulations due to their relative sweetness and caloric value. However, their scarcity and high cost have limited their exploitation as potential sugar replacers. Selected rare carbohydrates are industrially produced through enzymatic methods. Unfortunately, enzymatic conversion has drawbacks, such as prolonged reaction time, limited reuse of enzymes, use of buffers to control pH, low stability of the enzymes, and the use of highly pure substrates. This review discusses recent progress in the chemical synthesis of rare carbohydrates, including homogenous catalysis (e.g. soluble alkalis and amines), heterogeneous catalysis (e.g. zeolite-based and molybdenum-based), combined methods (e.g. chemoenzymatic and electrosynthesis), and subcritical fluids. The scientific literature suggests that zeolite-based catalysis represents the most promising strategy for the synthesis of rare carbohydrates, and significant progress has been made in elucidating the reaction mechanisms of zeolite-based catalysts. The idea of synthesizing rare carbohydrates over the combination of solid catalysis and subcritical conditions from byproduct streams is also discussed here. Importantly, additional efforts and research strategies are needed to determine the thermodynamic properties and phase behavior of chemical systems to further develop this idea.