Enzyme-Catalyzed Inverse-Electron Demand Diels–Alder Reaction in the Biosynthesis of Antifungal Ilicicolin H

The pericyclases are a growing superfamily of enzymes that catalyze pericyclic reactions. We report a pericyclase IccD catalyzing an inverse-electron demand Diels–Alder (IEDDA) reaction with a rate acceleration of 3 × 105-fold in the biosynthesis of fungal natural product ilicicolin H. We demonstrate IccD is highly periselective toward the IEDDA cycloaddition over a competing normal electron demand Diels–Alder (NEDDA) reaction from an ambimodal transition state. A predicted flavoenzyme IccE was identified to epimerize the IEDDA product 8-epi-ilicicolin H to ilicicolin H, a step that is critical for the observed antifungal activity of ilicicolin H. Our results reveal the ilicicolin H biosynthetic pathway and add to the collection of pericyclic reactions that are catalyzed by pericyclases.