Organic compounds mostly consist of just ten to a dozen non-metallic elements including
C, H, N, P, O, S, and halogens. This may be one of the main reasons why chemists, until
relatively recently, tended to rely heavily on those reactions involving only non-metallic
elements. Many of them including the Diels-Alder reaction, the Claisen and Cope rearrangements
continue to be important. Even so, their combined synthetic scope has been
rather limited.