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 1. [4+2] cycloadditions (Diels-Alder reactions) have been widely used in organic synthesis 2 for the formation of six-membered rings and are now well-established in biosynthesis 3-6. [6+4] and other 'higher-order' cycloadditions were predicted 7 in 1965, and are now increasingly common in the laboratory despite challenges arising from the generation of a highly strained ten-membered ring Reprints and permissions information is available at http://www.nature.com/reprints.