Acid-catalysed transformation of oleic acid, and 12-hydroxystearic acid, into γ-octadecanolactone. Evidence for the occurrence of a novel side reaction

γ-Octadecanolactone is produced in the reaction between oleic acid (cis-9-octadecenoic acid) and concentrated sulfuric acid at 80 °C. To account for its formation, two types of mechanism have been considered. Both begin with protonation at C-9/C-10 to give a carbocation, following which the positive charge is transmitted to C-4, where nucleophilic attack by the carboxyl group is facilitated. One of them invokes a "zipperlike" succession of 1,2-shifts of intervening methylenic protons, whereas the other amounts to a stepwise migration of the alkene function along the carbon chain. Results obtained through the use of deuterated sulfuric acid to catalyse formation of the lactone allow for the existence of both types. However, each is characteristic of a different region of the C18 structure. Of the total isotope incorporated (individual molecules contained from zero to nine atoms of deuterium), >85% had been distributed relatively uniformly over the chain segment from C-7 to C-17, according to 2H nuclear mag...