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Biocatalysis has the potential to address the need for more sustainable organic synthesis routes. Protein engineering can tune enzymes to perform in cascade reactions and for efficient synthesis of enantiomerically enriched compounds, using both natural and new-to-nature reaction pathways. This review highlights recent achievements in biocatalysis, especially the development of novel enzymatic syntheses to access versatile small molecule intermediates and complex biomolecules. Biocatalytic strategies for the degradation of persistent pollutants and approaches for biomass valorization are also discussed. The transition of chemical synthesis to a greener future will be accelerated by implementing enzymes and engineering them for high performance and new activities. 

Lactones are cyclic esters with extensive applications in materials science, medicinal chemistry, and the food and perfume industries. Nature’s strategy for the synthesis of many lactones found in natural products always relies on a single type of retrosynthetic strategy, a C−O bond disconnection. Here, we describe a set of laboratory-engineered enzymes that use a new-tonature C−C bond-forming strategy to assemble diverse lactone structures. These engineered “carbene transferases” catalyze intramolecular carbene insertions into benzylic or allylic C−H bonds, which allow for the synthesis of lactones with different ring sizes and ring scaffolds from simple starting materials. Starting from a serine-ligated cytochrome P450 variant previously engineered for other carbene-transfer activities, directed evolution generated a variant P411-LAS-5247, which exhibits a high activity for constructing a five-membered ε-lactone, lactam, and cyclic ketone products (up to 5600 total turnovers (TTN) and >99% enantiomeric excess (ee)). Further engineering led to variants P411-LAS-5249 and P411-LAS-5264, which deliver six-membered δ-lactones and seven-membered ε-lactones, respectively, overcoming the thermodynamically unfavorable ring strain associated with these products compared to the γ-lactones. This new carbene-transfer activity was further extended to the synthesis of complex lactone scaffolds based on fused, bridged, and spiro rings. The enzymatic platform developed here complements natural biosynthetic strategies for lactone assembly and expands the structural diversity of lactones accessible through C−H functionalization. 

The selective conversion of mixtures of Z/E alkenes into chiral products is a synthetic challenge. Biocatalytic strategies can transform isomeric alkenes into stereopure compounds, but enzymes typically convert only one alkene isomer, thereby limiting the overall yield. Additional strategies have been used to interconvert alkene isomers, often at the cost of increasing energy consumption and chemical waste. Here, we present engineered hemoproteins derived from a bacterial cytochrome P450 that efficiently catalyze α-carbonyl alkylation of isomeric silyl enol ethers, producing stereopure products. Through screening and directed evolution, we generated P450BM3 variant P411-SCA-5188, which catalyzes stereoconvergent carbene transfer in Escherichia coli with high efficiency and stereoselectivity to various Z/E mixtures of silyl enol ethers. In contrast to established stereospecific transformations that leave one isomer unreacted, P411-SCA-5188 converts both isomers to a stereopure product. This biocatalytic approach simplifies the synthesis of chiral α-branched ketones by eliminating the need for stoichiometric chiral auxiliaries, strongly basic alkali-metal enolates, and harsh conditions, delivering products with high efficiency and excellent chemo- and stereoselectivities.