Toward Value-Added Chemicals from Carbohydrates via C–C Bond Cleavage and Coupling Transformations

Renewable carbohydrates are nearly inexhaustible libraries of chemical building blocks, and significant research efforts have been devoted to their valorization to valuable chemicals in the past decades. The commonly recognized main transformation routes include dehydration and C–C bond cleavage pathways, which lead to the production of conventional platform chemicals such as 5-hydroxymethylfurfural/furfural, lactic acid/lactates, and so on. With the huge availability of carbohydrates on earth, the production of other fine chemicals is very attractive but remains sparse. This Review therefore emphasizes the utilization strategies of carbohydrates based on in situ C–C bond cleavage to lower carbon fragments, such as glycolaldehyde and erythrose, and their subsequent transformations, e.g. hydrogenation, hydrogenolysis, oxidation, nucleophilic addition, and amination. The isolation of reactive intermediates is avoided, leading to the formation of a variety of "unconventional" useful scaffolds, such as ethylene glycol, ethanol, keto-alcohols, glycolic acid, formic acid, C4 skeleton α-hydroxy esters, N-containing compounds, etc. Inspired by the transformation of active intermediates, the direct conversion of monosugars with similar structures through C–C coupling to furan-based chemicals is also briefly reviewed. The primary focus of this Review is to show the spectacular range of fine chemicals that can be accessed from carbohydrates via C–C bond cleavage and coupling approaches. A summary of the reviewed works and some opportunities and challenges within this attractive field are underlined for future research in sugar chemistry.