Regiochemical aspects of Diels-Alder reactions of cyclopentadienone

Abstract

The regiochemistry of the Diels-Alder reaction between cyclopentadienone and 6,6-dimethylfulvene (isopropylidenecyclopentadiene) has been studied. Two adducts (in addition to cyclopentadienone dimer) were isolated and their structures were shown to be endo-10-isopropylidenetricyclo[5,2,1,0^2,6]-deca-4,8- dien-3-one (6a) and endo-3-isopropylidenetricyclo[5,2,1,0^2,6]deca-4,8- dien-10-one (7a). Kinetic parameters for the Cope rearrangement (6a) ↔ (7a) have been determined. The adducts (6a) and (7a) were formed in the ratio 45 : 55 respectively. Thus, in contrast to FMO predictions, and to its reaction with cyclopentadiene, cyclopentadienone tends to behave as a diene towards the fulvene. A tentative explanation of these results, based on angle strain in the transition states for the formation of Diels-Alder adducts, is proposed.