Site-selective chlorination of pyrrolic heterocycles by flavin dependent enzyme PrnC

Halogenation of pyrrole requires strong electrophilic reagents and often leads to undesired polyhalogenated products. Biocatalytic halogenation is a highly attractive approach given its chemoselectivity and benign reaction conditions. While there are several reports of enzymatic phenol and indole halogenation in organic synthesis, corresponding reports on enzymatic pyrrole halogenation have been lacking. Here we describe the in vitro functional and structural characterization of PrnC, a flavin-dependent halogenase that can act on free-standing pyrroles. Computational modeling and site mutagenesis studies identified three key residues in the catalytic pocket. A moderate resolution map using single-particle cryogenic electron microscopy reveals PrnC to be a dimer. This native PrnC can halogenate a library of structurally diverse pyrrolic heterocycles in a site-selective manner and be applied in the chemoenzymatic synthesis of a chlorinated analog of the agrochemical fungicide Fludioxonil. Pyrroles are an important scaffold in medicinal chemistry with various bioactivities; however, the selective chemical halogenation of pyrroles remains challenging. Here, the authors develop an enzymatic site-selective chlorination of pyrrolic heterocycles by a flavin-dependent halogenase PrnC and apply it to the chemoenzymatic synthesis of a chlorinated analogue of the fungicide Fludioxonil.