From biomass to chiral chemicals via chemocatalysis: advances and prospects

The use of renewable lignocellulosic biomass to produce optically pure chiral chemicals are important, due to the versatile inherent stereostructures and multiple functional groups. Typically, as the two main units of hemicellulose and cellulose components in lignocellulosic biomass, D-xylose and D-glucose bear multiple chiral centers (e.g., 2R-3S-4R for D-xylose and 2R-3S-4R-5R for D-glucose). Lignin biosynthesis produces β-O-4 units, and the α and β carbon atoms in side chain exhibit (R,R/S,S) or (R,S/S,R) stereostructures. Currently, chiral chemicals derived from biomass are predominantly produced through microbial fermentation. However, this method faces several disadvantages, including low productivity, poor substrate compatibility, high costs, and complicated separation and purification processes. Recently, increasing efforts have been devoted to upgrading biomass to produce chiral chemicals via chemocatalysis. Although most platform chemicals obtained from biomass are achiral compounds or racemic mixtures, some progresses have been made in the synthesis of chiral chemicals. This review discusses typical chiral chemicals derived from biomass through chemocatalysis, including lactones (e.g., R/S-valerolactone), carboxylic acids (e.g., D/L-glyceric acid, D/L-lactic acid), polyols (e.g., tetrose), furans, oligosaccharides, and others. Two strategies are generally employed. One approach involves first producing achiral platform chemicals from biomass, followed by the introduction of asymmetric catalysts to reconstruct stereocenters. The second relates to selectively preserve one or more inherent stereocenters in the natural biomass structure during complex cascade reactions in which biomass feedstock acts as a "chiral pool", thus eliminating the establishment of stereocenter. The feedstock, methods employed, and enantioselectivity and applications of the target chiral chemicals are discussed. Despite these advances, the synthesis of optically pure chemicals from biomass is still in its infancy. The coming decade presents both extraordinary challenges and opportunities in biomass-derived chiral chemistry. Future research should focus on: (1) The integration of well-established asymmetric catalysis techniques with biomass' inherent chiral pools which presents an unprecedented opportunity to expand the chemical space of sustainable chiral compounds, (2) Mastering polyfunctional complexity of chiral chemicals through holistic utilization of biomass' multichiral centers, (3) Unlocking lignin's stereochemical treasury that represents the next frontier in biomass valorization.