Structural Studies on α-Pyrone Cycloadducts. Manifestation of the Early Stages of CO2 Extrusion by retro Hetero-Diels - Alder Reaction

Abstract

Structures of the α-pyrone (pyran-2-one) cycloadducts 4–8 show deviations of some bond distances from their normal values, consistent with manifestation of the early stages of the retro hetero-Diels–Alder decarboxylation reaction in the ground state structures. Thus the bridgehead C–O(CO) and C–CO(O) bonds are lengthened and the bridging C–O bond is shortened. The degree of lengthening of the C–O(CO) and C–CO(O) bonds is similar, whereas in the calculated transition state structure there is significant asymmetry in the extent of C–CO(O) and C–O(CO) bond breaking.