DIELS-ALDER CYCLOADDITIONS OF ELECTRON-RICH, ELECTRON-DEFICIENT, AND PUSH-PULL DIENES WITH CYCLIC DIENOPHILES - HIGH-PRESSURE-INDUCED REACTIONS AND THEORETICAL CALCULATIONS

High-pressure-promoted (4 + 2) cycloadditions of cyclic dienophiles (mainly beta-angelica lactone and cyclopentenone) with electron-rich and push-pull hetero-substituted dienes have been investigated. Only one regio- and stereoisomer was obtained in all cases. While these dienophiles reacted smoothly with alkoxy- and (silyloxy)butadiene derivatives, replacement of oxygen by sulfur resulted in a dramatic decrease in the reactivity of the dienes. On the other hand, push-pull dienes exhibited reactivity similar to that of the electron-rich dienes, in that the dienophilicity was not changed substantially by the influence of an electron-withdrawing group. Theoretical calculations of activation enthalpies have been carried out by the semiempirical AM1 method. These values and frontier orbital considerations have permitted us to interpret the experimental results and to establish a relative order of reactivity of the dienes and dienophiles herein studied.