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

Abstract

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 magnetic resonance spectra recorded in the presence of the shift reagent Yb(fod)3, and 13C nuclear magnetic resonance data. This evidence is consistent with the occurrence of a side reaction that creates many protonation sites over that specific region of the lactone's precursors. By contrast, as very little of the isotope had reached the proximity of the lactone ring, the latter is more likely to have been formed via the 1,2 shift type of mechanism. These events can also be initiated at other sites, as shown by the preparation of γ-octadecanolactone, and a comparably deuterated form of it, from 12-hydroxystearic acid. Reactions of these three stearic acid derivatives with HI, HBr, and HCl are described, by way of comparison.