A highly regioselective synthesis of delta-lactones from meadowfoam fatty acids

Meadowfoam fatty acids, when treated with mineral acid catalysts in the presence of polar nonparticipating solvents, undergo a facile ring closure to form delta-lactones. Perchloric and sulfuric acids catalyze the cyclization at concentrations of 0.6-13 mole equivalents, both neat and in the presence of solvent. Under constant acid concentrations, methylene chloride was found to increase the rate of reaction, the regioselectivity for the formation of delta-lactone, and the overall yield. In the absence of solvent, increased acid concentration improved the yield of lactone but reduced regioselectivity for the delta-isomer. Solvent polarity plays a significant role in the regioselectivity of the cyclization for delta-lactone, with solvents of higher dielectric strength providing larger delta/gamma ratios (38:1) and higher yields up to 92%.