The Diels-Alder reaction is one of the most powerful and widely used methods in synthetic chemistry for the stereospecific construction of carbon-carbon bonds. Despite the importance of Diels-Alder reactions in the biosynthesis of numerous secondary metabolites, no naturally occurring stand-alone Diels-Alderase has been demonstrated to catalyse intermolecular Diels-Alder transformations. Here we report a flavin adenine dinucleotide-dependent enzyme, Morus alba Diels-Alderase (MaDA), from Morus cell cultures, that catalyses an intermolecular [4+2] cycloaddition to produce the natural isoprenylated flavonoid chalcomoracin with a high efficiency and enantioselectivity. Density functional theory calculations and preliminary measurements of the kinetic isotope effects establish a concerted but asynchronous pericyclic pathway. Structure-guided mutagenesis and docking studies demonstrate the interactions of MaDA with the diene and dienophile to catalyse the [4+2] cycloaddition. MaDA exhibits a substrate promiscuity towards both dienes and dienophiles, which enables the expedient syntheses of structurally diverse natural products. We also report a biosynthetic intermediate probe (BIP)-based target identification strategy used to discover MaDA.

FAD-dependent enzyme-catalysed intermolecular [4+2] cycloaddition in natural product biosynthesis.

The interest in functional supramolecular systems for the design of innovative materials and technologies, able to fundamentally change the world, is growing at a high pace. The huge array of publications that appeared in recent years in the global literature calls for systematization of the structural trends inherent in the formation of these systems revealed at different molecular platforms and practically useful properties they exhibit. The attention is concentrated on the topics related to functional supramolecular systems that are actively explored in institutes and universities of Russia in the last 10–15 years, such as the chemistry of host–guest complexes, crystal engineering, self-assembly and self-organization in solutions and at interfaces, biomimetics and molecular machines and devices. The bibliography includes 1714 references.

Functional supramolecular systems: design and applications

Cytochrome P450 enzymes in fungal natural product biosynthesis

An ongoing challenge in chemical research is to design catalysts that select the outcomes of the reactions of complex molecules. Chemists rely on organocatalysts or transition metal catalysts to control stereoselectivity, regioselectivity and periselectivity (selectivity among possible pericyclic reactions). Nature achieves these types of selectivity with a variety of enzymes such as the recently discovered pericyclases-a family of enzymes that catalyse pericyclic reactions1. Most characterized enzymatic pericyclic reactions have been cycloadditions, and it has been difficult to rationalize how the observed selectivities are achieved2-13. Here we report the discovery of two homologous groups of pericyclases that catalyse distinct reactions: one group catalyses an Alder-ene reaction that was, to our knowledge, previously unknown in biology; the second catalyses a stereoselective hetero-Diels-Alder reaction. Guided by computational studies, we have rationalized the observed differences in reactivities and designed mutant enzymes that reverse periselectivities from Alder-ene to hetero-Diels-Alder and vice versa. A combination of in vitro biochemical characterizations, computational studies, enzyme co-crystal structures, and mutational studies illustrate how high regioselectivity and periselectivity are achieved in nearly identical active sites.

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An enzymatic Alder-ene reaction.

Enzymatic dimerization in the biosynthetic pathway of microbial natural products.

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 stereoselectively control the subsequent intermolecular hetero-Diels-Alder reaction with (1E,4E,8Z)-humulenol (8) to produce enantiomerically pure neosetophomone B (1). Our results reveal the biosynthetic pathway of 1 and expand the repertoire of activities of Diels-Alder cyclases.

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Yongsheng Che

Papers

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