2-Cyclopropylbenzoic acids in the synthesis of phthalides and 3,4-dihydroisocoumarins

Abstract

2-Cyclopropylbenzoic acids, under the action of strong protic acids (FSO3H, H2SO4), are converted to 3-ethylphthalidium ions. In solutions in these inorganic acids, the 3-ethylphthalidium ions are isomerized to 3-methyl-3,4-dihydroisocoumarinium ions. As a result, a thermodynamic equilibrium is established, with the concentrations of both types of cyclic ions depending on the nature of the substituent in the aromatic part of the substrate. Neutralization of the original solutions of 3-ethylphthalidium ions or a mixture of these with 3-methyl-3,4-dihydroisocoumarinium ions, gave either 3-ethylphthalides or their mixtures with 3-methyl-3,4-dihydroisocoumarins. The neutral 3-ethylphthalides and 3-methyl-3,4-dihydroisocoumarins, when subjected to the action of inorganic acids, are also isomerized to form in each case a mixture of ions with concentrations matching the concentrations of ions formed from the corresponding 2-cyclopropylbenzoic acids.