Structural Engineering of S -Adenosyl- l -methionine for Biocatalytic Fluoromethylation via Nucleophilic and Radical Pathways

Fluorine-containing compounds are ubiquitous in pharmacology, diagnostics, agrochemistry, and materials science. Fluoromethylation is a reliable method for introducing fluorine into the parent structure. Recently, fluorinated S-adenosyl-l-methionine (F-SAM) and its stabilized analogues have been utilized by methyltransferases to selectively fluoromethylate bioactive molecules. However, the inherent instability of F-SAM and the limited enzyme recognition of the stable analogues restrict their broader application. Therefore, next-generation fluoromethylation reagents for biocatalysis are highly desirable. Here, we engineered the carboxyl and base moieties of F-SAM with bioisosteric substitution and developed three F-SAM analogues. Among them, 7-deazaadenine-tetrazole-substituted F-SAM (F-7dz-tSAM) is highly stable and has kinetic properties comparable to those of SAM with several O-, S-, and C-methyltransferases and fluoromethylates natural products regio- and stereoselectively. Impressive turnover numbers and high conversions were achieved when halide methyltransferase was coupled for the regeneration of F-7dz-tSAM. With F-7dz-tSAM replacing SAM in biosynthesis pathways, we efficiently prepared fluorinated derivatives of two clinically used drugs, diosmin and physostigmine. More importantly, F-7dz-tSAM is utilized by the B12-dependent radical SAM methyltransferase CysS for radical fluoromethylation in better yield than the labile F-SAM.