The Diels–Alder reaction is of very great importance in the synthesis of naturally occurring compounds, partly because of the large number of structures to which it is applicable and partly because of its stereospecificity. Although the Diels–Alder reaction between butadiene and ethylene is successful, it is generally unsatisfactory, or fails completely, with other simple alkenes or alkynes. The ene-reaction is a reaction of an allylic compound with an alkene in a manner which resembles both cycloaddition and a-sigmatropic shift of hydrogen. The occurrence and stereochemistry of sigmatropic rearrangements can be accounted for, like other pericyclic reactions, in terms of the symmetry of frontier orbitals. Although stereocentres can be created in Diels–Alder reaction, racemic mixtures are obtained unless one or both reactants is asymmetric. An approach being developed to induce enantioselectivity is the use of a chiral catalyst. Electrocyclic equilibria that occur at ambient temperature give rise to the phenomenon of valence tautomerism.