Aromatics from pyrones: esters of terephthalic acid and isophthalic acid from methyl coumalate

Abstract

The Diels–Alder reaction of methyl coumalate with alkenes bearing electron-withdrawing groups provides terephthalates or isophthalates in good yields, with the regioselectivity depending on the electron-withdrawing group. The reaction of methyl coumalate with the salt of acrylic acid gave only the monoester of isophthalic acid. Density functional theory (B3LYP/6-31 + G(d,p)) computations of the energies of the competing transition states of the para-selective Diels–Alder reactions are in good agreement with experiment. The surprising regioselectivity of methyl coumalate with activated alkenes is attributed to a secondary orbital interaction between the pyrone oxygen and the dienophile LUMO, which switches the regiochemistry expected from simple frontier molecular orbital theory arguments.