Biocatalytic valorization of Furans: Opportunities for inherently unstable substrates.

Biogenic furans (Furfural and 5-hydroxymethylfurfural) are expected to become relevant building blocks based on their high functionalization and versatility. However, the inherent instability of furans poses considerable challenges for their synthetic modifications. Valorization routes of furans typically generate by-products, impurities, wastes, and a cumbersome downstream processing, compromising their ecological footprint. Biocatalysis may become an alternative, given the high selectivity of enzymes, together with the mild reaction conditions applied. This review critically discusses the options for enzymes in the upgrading of furans. Based on the reported literature, there is a variety of biocatalytic transformations applied to furans, with successful cases both in aqueous and in water-free media. Options comprise the biodetoxification of toxic furans in hydrolysates, selective syntheses based on oxidation-reduction processes, solvent-free esterifications, or carboligations to afford C12 derivatives. Reported strategies show in general promising but still modest productivities (range of 2-30 g product L-1 d-1, depending on the example). There are opportunities with high potential, deserving further development, scale-up and techno-economic assessment, to entirely validate them as realistic alternatives.