Enzymatic Intermolecular Hetero-Diels–Alder Reaction in the Biosynthesis of Tropolonic Sesquiterpenes

Abstract

Diels-Alder reactions are among the most powerful synthetic transformations to construct complex natural products. Despite that increasing of enzymatic intramolecular Diels-Alder reactions have been discovered, natural intermolecular Diels-Alderases are rarely described. Here, we report an intermolecular hetero-Diels-Alder reaction in the biosynthesis of tropolonic sesquiterpenes and functionally characterize EupfF as the first fungal intermolecular hetero-Diels-Alderase. We demonstrate that EupfF catalyzed the dehydration of a hydroxymethyl-containing tropolone (<b>5</b>) to generate a reactive tropolone <i>o</i>-quinone methide (<b>6</b>) and might further stereoselectively control the subsequent intermolecular hetero-Diels-Alder reaction with (1<i>E</i>,4<i>E</i>,8<i>Z</i>)-humulenol (<b>8</b>) to produce enantiomerically pure neosetophomone B (<b>1</b>). Our results reveal the biosynthetic pathway of <b>1</b> and expand the repertoire of activities of Diels-Alder cyclases.