The Diels-Alder Reaction: an Update

In this update on the Diels-Alder reaction we would like to present an overview on how this very important reaction has progressed in the last decade, since the last spate of monographs and reviews appeared in the literature, and which deal with the reaction in the overall sense. Initially we present the Diels-Alder reaction in a generalised form, much as discovered over seventy years ago, so that its present relevance as probably the single most important reaction in the synthetic organic chemist’s tool-box is clear. Thus, the original version of the Diels-Alder reaction (Scheme 1) joins together a wide variety of conjugated dienes and alkenes with electron withdrawing groups (the dienophiles), to produce a cyclohexene ring in which practically all six carbon atoms can be substituted as desired. The reaction may be executed under relatively simple reaction conditions by heating together the two components, diene and dienophile, in non-polar solvents, followed by evaporation which leads usually to high yields of the product(s). The reaction is disciplined by the WoodwardHoffmann rules 4-6 as a [ 4 s + 2 s ] cycloaddition occurring in a concerted but probably not symmetrically synchronous fashion, thus leading to highly predictable product structures in which two new carbon-carbon sigma bonds are formed in a stereospecific manner with the creation of up to four new stereogenic centres. The classical empirical rules have now found strong theoretical basis in the WoodwardHoffmann rules, with regards to regiochemistry (“ortho” and “para” orientations) and stereochemistry (endo transition state kinetically favoured over the exo transition state in most of the reactions). The practising synthetic organic chemist will certainly be well aware of the kinds of dienes and