A Biocatalytic Platform for the Synthesis of Enantiopure Propargylic Alcohols and Amines

Green and Enantioselective Synthesis via Cascade Biotransformations: From Simple Racemic Substrates to High-Value Chiral Chemicals

Asymmetric synthesis of chiral chemicals in high enantiomeric excess (ee) is pivotal to the pharmaceutical industry, but classic chemistry usually requires multi-step reactions, harsh conditions, and expensive chiral ligands, and sometimes suffers from unsatisfactory enantioselectivity. Enzymatic catalysis is a much greener and more enantioselective alternative, and cascade biotransformations with multi-step reactions can be performed in one pot to avoid costly intermediate isolation and minimise waste generation. One of the most attractive applications of enzymatic cascade transformations is to convert easily available simple racemic substrates into valuable functionalised chiral chemicals in high yields and ee. Here, we review the three general strategies to build up such cascade biotransformations, including enantioconvergent reaction, dynamic kinetic resolution, and destruction-and-reinstallation of chirality. Examples of cascade transformations using racemic substrates such as racemic epoxides, alcohols, hydroxy acids, etc. to produce the chiral amino alcohols, hydroxy acids, amines, and amino acids are given. The product concentration, ee, and yield, scalability, and substrate scope of these enzymatic cascades are critically reviewed. To further improve the efficiency and practical applicability of the cascades, enzyme engineering to enhance catalytic activities of the key enzymes using the latest microfluidics-based ultrahigh-throughput screening and artificial intelligence-guided directed evolution could be a useful approach.

Enantioselectivity in Vanadium-Dependent Haloperoxidases of Different Marine Sources for Sulfide Oxidation to Sulfoxides

This study explores the reasons behind the variations in the enantioselectivity of the sulfoxidation of methyl phenyl sulfide by marine-derived vanadium-dependent haloperoxidases (VHPOs). Twelve new VHPOs of marine organisms were overexpressed, purified, and tested for their ability to oxidize sulfide. Most of these marine enzymes exhibited nonenantioselective behavior, underscoring the uniqueness of AnVBPO from the brown seaweed Ascophyllum nodosum and CpVBPO from the red seaweed Corallina pilulifera, which produce (R)- and (S)-sulfoxides, respectively. The enantioselective sulfoxidation pathway is likely due to direct oxygen transfer within the VHPO active site. This was demonstrated through molecular docking and molecular dynamics simulations, which revealed differences in the positioning of sulfide within AnVBPO and CpVBPO, thus explaining their distinct enantioselectivities. Nonenantioselective VHPOs probably follow a different oxidation pathway, initiating with sulfide oxidation to form a positively charged radical. Further insights were gained from studying the catalytic effect of VO43- on H2O2-driven sulfoxidation. This research improves the understanding of VHPO-mediated sulfoxidation and aids in developing biocatalysts for sulfoxide synthesis.

Regioselective and enantioselective propargylic hydroxylations catalyzed by P450tol monooxygenases

Regioselective and enantioselective hydroxylation of propargylic C-H bonds are useful reactions but often lack appropriate catalysts. Here a green and efficient asymmetric hydroxylation of primary and secondary C-H bonds at propargylic positions has been established. A series of optically active propargylic alcohols were prepared with high regio- and enantioselectivity (up to 99% ee) under mild reaction conditions by using P450tol, while the C≡C bonds in the molecule remained unreacted. This protocol provides a green and practical method for constructing enantiomerically chiral propargylic alcohols. In addition, we also demonstrated that the biohydroxylation strategy was able to scaled up to 2.25 mmol scale with the production of chiral propargyl alcohol 2a at a yield of 196 mg with 96% ee, which's an important synthetic intermediate of antifungal drug Ravuconazole.

A Simple Access to γ- and ε-Keto Arenes via Enzymatic Divergent C─H Bond Oxyfunctionalization

Performing divergent C─H bond functionalization on molecules with multiple reaction sites is a significant challenge in organic chemistry. Biocatalytic oxyfunctionalization reactions of these compounds to the corresponding ketones/aldehydes are typically hindered by selectivity issues. To address these challenges, the catalytic performance of oxidoreductases is explored. The results show that combining the peroxygenase-catalyzed propargylic C─H bond oxidation with the Old Yellow Enzyme-catalyzed reduction of conjugated C─C triple bonds in one-pot enables the regio- and chemoselective oxyfunctionalization of sp3 C─H bonds that are distant from benzylic sites. This enzymatic approach yielded a variety of γ-keto arenes with diverse structural and electronic properties in yields of up to 99% and regioselectivity of 100%, which are difficult to achieve using other chemocatalysis and enzymes. By adjusting the C─C triple bond, the carbonyl group's position can be further tuned to yield ε-keto arenes. This enzymatic approach can be combined with other biocatalysts to establish new synthetic pathways for accessing various challenging divergent C─H bond functionalization reactions.

Lactones from Unspecific Peroxygenase-Catalyzed In-Chain Hydroxylation of Saturated Fatty Acids

γ- and δ-lactones are valuable flavor and fragrance compounds. Their synthesis depends on the availability of suitable hydroxy fatty acid precursors. Three short unspecific peroxygenases were identified that selectively hydroxylate the C4 and C5 positions of C8-C12 fatty acids to yield after lactonization the corresponding γ- and δ-lactones. A preference for C4 over C5 hydroxylation gave γ-lactones as the major products. Overoxidation of the hydroxy fatty acids was addressed via the reduction of the resulting oxo acids using an alcohol dehydrogenase in a bienzymatic cascade reaction.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as hyperispirone A from Hypericum beanii.