ABSTRACT
Cyclic reactions such as the Diels-Alder cycloaddition, the Cope and the Claisen rearrangements have substantial synthetic and biological significance. The biosynthesis of lanosterol derivatives and associated biomimetic cyclisations, many fragmentation reactions such as decarboxylation, and extrusion of nitrogen and sulphur dioxide constitute part of this large class of reactions. The eponymous selection rules originated from strong qualitative arguments based on symmetry and explain the stereochemical course of thermal cyclic reactions. Qualitative and quantitative molecular orbital theory has figured extensively in discussions of transition state structure in cyclic reactions and the original idea of concertedness appears to have arisen because of the high degree of stereospecificity invariably observed in these systems. The application of ab initio methods to cyclic reactions, even though they involve approximations, has centred on the prediction of transition structures which can then be utilised for calculating kinetic isotope effects for multiple isotopic substitution.
