Cyclization of natural allene oxide fatty acids. The anchimeric assistance of beta, gamma-double bond beside the oxirane and the reaction mechanism.

Formation of cyclopentenones was followed from linoleic, alpha-linolenic and gamma-linolenic acid hydroperoxides (HPOD, HPOT(alpha) and HPOT(gamma), respectively) via allene oxides in the presence of flax seed allene oxide synthase. Although 13-HPOT(alpha) and 9-HPOT(gamma) were effective cyclopentenone precursors, 13-HPOD, 9-HPOD(gamma) and 9-HPOT(alpha) were not. These results suggest that the presence of a double bond in beta, gamma-position toward the hydroperoxide function causes the strong effect of anchimeric assistance, increasing the cyclization rate by 2-3 orders of magnitude. The minor 15(E) isomer was formed from 13-HPOT along with usual 12-oxo-10,15(Z)-phytodienoic acid (12-oxo-PDA). The remarkable (about 2-fold) suppression of 12-oxo-PDA formation was observed under acidic (pH 5.5) conditions in comparison to the alkaline (pH 7.8) ones. The mechanism of double bond-assisted allene oxide cyclization, comprising dipolar pericyclic ring closure in zwitterionic intermediate, is proposed.