Pericyclic Reactions of 2-Pyrones with Nonconjugated Dienes. Conformational Analysis of the Double Diels-Alder Adducts by Molecular Mechanics Calculation

Pericyclic reactions of 5-methoxycarbonyl-2-pyrone (Ic) with nonconjugated dienes (IIa-j) were investigated. The diene (Ic) reacted readily with IIa-j to give [4+2]π cycloadducts (IIIa-j) that lost carbon dioxide to afford intramolecular double Diels-Alder (DDA) adducts (V). The structures of these adducts were determined from spectral evidence. The reactivity of the addends and the regiochemistry of the primary adducts are discussed on the basis of modified neglect of diatomic overlap (MNDO) calculation data. The relative stability of the twistene- and isotwistene- type compounds and the conformation of some isotwistene-type compounds are discussed on the basis of the molecular mechanics (MM2) calculation data.