Fig. 1.

Schematic representation of reaction network in which ketohexoses can isomerize to aldohexoses via 1,2-HS (r₁) and to 2-C-(hydroxymethyl)-aldopentoses via 1,2-CS (r₁₁) reactions. Retro-aldol reactions of hexose species (r₂, r₃, and r₁₂) lead to the formation of C₂, C₃, and C₄ carbohydrate fragments. Lewis acids can then catalyze the formation of α-hydroxy carboxylic acids from these smaller fragments (e.g., r₇, r₈, and r₉ in the formation of alkyl lactate from trioses). Side reactions, involving dehydration reactions of fructose to 5-HMF (r₅), redox and fragmentation reactions of unstable intermediates, and various humin-forming condensation reactions, lead to loss of yield of desired products.