Total Synthesis of (+)-12-epi-Fischerindole U Isonitrile and Its Activity as a Substrate of the Halogenase WelO5

Mononuclear nonheme Fe(II) and 2-oxoglutarate-dependent enzymes (Fe(II)/2OG) constitute an enzyme superfamily that oxidizes substrates by activating molecular oxygen (O₂) via the highly oxidized iron(IV)-oxo (ferryl) intermediate. Despite the high similarity of their active sites, the enzymes of this superfamily perform a diverse array of reactions such as hydroxylation, halogenation, epoxidation, and desaturation. WelO5, a member of this superfamily, regio- and stereospecifically monochlorinates a free-standing substrate, 12-epi-fischerindole U isonitrile (FIUI), at its aliphatic C(13) to give 12-epi-fischerindole G (FIG). Researchers have proposed that a dynamic reconfiguration of the WelO5 active site governs the selective halogenation of FIUI. Unfortunately, the requirements of spectroscopic and structural methods for large amounts of fully assembled samples of protein-substrate complexes impede the investigation of such reconfiguration. In this work, we describe a synthesis of (+)-FIUI that makes use of the Baran group's strategy for coupling of indoles with carbonyl compounds. We apply EPR and LC-MS methods to demonstrate the ability of WelO5 to bind the synthetic (+)-FIUI and subsequently convert it to FIG.