Enzyme-catalysed [6+4] cycloadditions in the biosynthesis of natural products

Pericyclic reactions are powerful transformations for the construction of carbon-carbon and carbon-heteroatom bonds in organic synthesis. Their role in biosynthesis is increasingly apparent, and mechanisms by which pericyclases can catalyse reactions are of major interest.sup.1. [4+2] cycloadditions (Diels-Alder reactions) have been widely used in organic synthesis.sup.2 for the formation of six-membered rings and are now well-established in biosynthesis.sup.3-6. [6+4] and other 'higher-order' cycloadditions were predicted.sup.7 in 1965, and are now increasingly common in the laboratory despite challenges arising from the generation of a highly strained ten-membered ring system.sup.8,9. However, although enzyme-catalysed [6+4] cycloadditions have been proposed.sup.10-12, they have not been proven to occur. Here we demonstrate a group of enzymes that catalyse a pericyclic [6+4] cycloaddition, which is a crucial step in the biosynthesis of streptoseomycin-type natural products. This type of pericyclase catalyses [6+4] and [4+2] cycloadditions through a single ambimodal transition state, which is consistent with previous proposals.sup.11,12. The [6+4] product is transformed to a less stable [4+2] adduct via a facile Cope rearrangement, and the [4+2] adduct is converted into the natural product enzymatically. Crystal structures of three pericyclases, computational simulations of potential energies and molecular dynamics, and site-directed mutagenesis establish the mechanism of this transformation. This work shows how enzymes are able to catalyse concerted pericyclic reactions involving ambimodal transition states. Enzymatic catalysis of pericyclic [6+4] cycloaddition reactions to form ten-membered rings is observed during biosynthesis of the macrocyclic antibiotic streptoseomycin, and the mechanism of these transformations is established.
Author(s): Bo Zhang [sup.1] , Kai Biao Wang [sup.1] , Wen Wang [sup.1] , Xin Wang [sup.2] , Fang Liu [sup.2] , Jiapeng Zhu [sup.3] , Jing Shi [sup.1] , [...]