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 (5) to generate a reactive tropolone o-quinone methide (6) and might further *

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 × 10 5 fold in the biosynthesis of fungal natural product ilicicolin H. We demonstrate IccD is highly periselective towards 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.

Ambimodal reactions involve a single transition state leading to multiple products. In such reactions, transition state theory gives no information about the ratio of products that are formed, and molecular dynamics must be performed to predict this ratio. Understanding the relationship between the transition structure and the product ratio is a long-standing problem in molecular dynamics. We have studied 15 ambimodal pericyclic reactions and investigated the relationship between the TS bond lengths in the saddle points and the product ratios from trajectory simulations. A linear correlation, ln(B:A) = -9.4(Bond 3 - Bond 2), is found with R = 0.92, where A and B refer to the products formed upon formation of bonds 2 and 3, respectively. The correlation shows that the ratio of products formed after the bifurcation is related to the partial bond lengths, and corresponding bond orders, in the transition state.

The pericyclases are Nature's route to spirotetronates, statins, myceliothermophins, leporins and more.

We report an ambimodal trispericyclic transition state leading to [6+4]-, [4+6]-, and [8+2]cycloadducts in the reactions of 8,8-disubstituted heptafulvenes with 6,6-dimethylfulvene. The potential energy surfaces for these reactions were explored with ωB97X-D density functional theory. Quasi-classical direct molecular dynamics simulations gave information on the ratios of products expected in these reactions.Communication pubs.acs.org/JACS