Charting the Evolution of Chemoenzymatic Strategies in the Syntheses of Complex Natural Products

Bolstered by recent advances in bioinformatics, genetics, and enzyme engineering, the field of chemoenzymatic synthesis has enjoyed a rapid increase in popularity and utility. This Perspective explores the integration of enzymes into multistep chemical syntheses, highlighting the unique potential of biocatalytic transformations to streamline the synthesis of complex natural products. In particular, we identify four primary conceptual approaches to chemoenzymatic synthesis and illustrate each with a number of landmark case studies. Future opportunities and challenges are also discussed.

Regiodivergent addition of phenols to allylic oxides

The regiodivergent addition of substituted phenols to allylic oxides has been demonstrated using C2-symmetric palladium complexes. Complex phenol donors tyrosine, estradiol, and griseofulvin follow the predictive model.

Palladium-catalyzed carbonylation of halo arene-cis-dihydrodiols to the corresponding carboxylates. Access to compounds unavailable by toluene dioxygenase-mediated dihydroxylation of the corresponding benzoate esters

A series of arene-cis-dihydrodiol carboxylates was prepared by palladium-catalyzed carbonylation of (1S, 2S-cis)-3-iodo-3,5-cyclohexadiene-1,2-diol, which is obtained in high titers by enzymatic dihydroxylation of iodobenzene. Both the free diol and the corresponding acetonide were subjected to this protocol to produce various arene-cis-dihydrodiol carboxylates that are unavailable by fermentation of the corresponding benzoates or are produced in low yields. The comparison of yields obtained from fermentation versus carbonylation was made for all compounds investigated. Experimental and spectral data are provided for all new compounds.

Benzoate dioxygenase fromRalstonia eutrophaB9 – unusual regiochemistry of dihydroxylation permits rapid access to novel chirons

Oxidation of benzoic acid by a microorganism expressing benzoate dioxygenase leads to the formation of an unusualipso,orthoarenecis-diol in sufficient quantities to be useful for synthesis.

Toluene dioxygenase mediated oxidation of halogen-substituted benzoate esters

A series of ortho-, meta-, and para- halogen-substituted methyl benzoate esters was subjected to enzymatic dihydroxylation via the whole-cell fermentation with E. coli JM109 (pDTG601A). Only ortho-substituted benzoates were metabolized. Methyl 2-fluorobenzoate yielded one diol regioselectively whereas methyl 2-chloro-, methyl 2-bromo- and methyl 2-iodobenzoates each yielded a mixture of regioisomers. Absolute stereochemistry was determined for all new metabolites. Computational analysis of these results and a possible rationale for the regioselectivity of the enzymatic dihydroxylation is advanced.

Several generations of chemoenzymatic synthesis of oseltamivir (Tamiflu): evolution of strategy, quest for a process-quality synthesis, and evaluation of efficiency metrics

Four generations of chemoenzymatic approaches to oseltamivir are presented. The first two generations relied on the use of cyclohexadiene-cis-diol derived enzymatically from bromobenzene. The third and fourth generation used the corresponding diol obtained from ethyl benzoate by fermentation with E. coli JM109(pDTG601a). Oseltamivir was obtained from ethyl benzoate by intersecting intermediate 39 (third-generation synthesis) and intermediate 45 (fourth-generation synthesis). Both of these advanced approaches benefited from symmetry considerations and translocation of the acrylate double bond with concomitant elimination of the C-1 hydroxyl. The syntheses are evaluated for overall efficiency by the use of efficiency metrics and compared with other syntheses of oseltamivir (both academic and industrial).

Chemoenzymatic synthesis of carbasugars (+)-pericosines A-C from diverse aromatic cis-dihydrodiol precursors

cis-Dihydrocatechols, derived from biological cis-dihydroxylation of methyl benzoate, iodobenzene and benzonitrile, using the microorganism Pseudomonas putida UV4, were converted into pericosines A, C, and B, respectively. This approach constitutes the shortest syntheses, to date, of these important natural products with densely packed functionalities.