Mechanism of linoleic acid hydroperoxide reaction with alkali

Treatment of (13 S ,9 Z ,11 E )‐13‐hydroperoxy‐9,11‐octadecadienoic acid (13 S ‐HPODE) with strong alkali resulted in the formation of about 75% of the corresponding hydroxy acid, (13 S ,9 Z ,11 E )‐13‐hydroxyl‐9,11‐octadecadienoic acid (13 S ‐HPODE), and the remaining 25% of products was a mixture of several oxidized fatty acids, the majority of which was formed from (9 Z ,11 R,S ,12 S,R )‐13‐oxo‐11, 12‐epoxy‐9‐octadecenoic acid by Favorskii rearrangement (Gardner, H.W., et al. (1993) Lipids 28 , 487–495). In the present work, isotope experiments were completed in order to get further information about the initial steps of the alkali‐promoted decomposition of 13 S ‐HPODE. 1 . Reaction of [hydroperoxy‐ 18 O 2 ]13 S ‐HPODE with 5 M KOH resulted in the formation of [hydroxy‐ 18 O]13 S ‐HPODE and [epoxy‐ 18 O](9 Z ,11 R,S ,12 S,R )‐13‐oxo‐11, 12‐epoxy‐9‐octadecenoic acid; 2 . treatment of a mixture of [U‐ 14 C]13 S ‐HPODE and [hydroperoxy‐ 18 O 2 ]13 S ‐HPODE with KOH and analysis of the reaction product by radio‐TLC showed that 13 S ‐HPODE was stable under the reaction conditions and did not serve as precursor of other products; 3. reaction of a mixture of [U‐ 14 C]13‐oxo‐9,11‐octadecadienoic acid (13‐OODE) and [hydroperoxy‐ 18 O 2 ]13 S ‐HPODE with KOH resulted in the formation of [U‐ 14 C‐epoxy‐ 18 O](9 Z ,11 R,S ,12 S,R )‐13‐oxo‐11,12‐epoxy‐9‐octadecenoic acid; 4. treatment of a mixture of [hydroperoxy‐ 18 O 2 ] 13 S ‐HPODE and [carboxyl‐ 18 O 1 ]13 S ‐HPODE with KOH afforded (9 Z ,11 R,S ,12 S,R )‐13‐oxo‐11,12‐epoxy‐9‐octadecenoic acid having an 18 O‐labeling pattern which was in agreement with its formation by intermolecular epoxidation. It was concluded that (9 Z ,11 R,S ,12 S,R )‐13‐oxo‐11, 12‐epoxy‐9‐octadecenoic acid is formed from 13 S ‐HPODE by a sequence involving initial dehydration into the α,β‐unsaturated ketone, 13‐OODE, followed by epoxidation of the Δ 11 double bond of this compound by the peroxyl anion of a second molecule of 13 S ‐HPODE. Rapid conversion of hydroperoxides by alkali appreared to require the presence of an α,β‐unsaturated ketone intermediate as an oxygen acceptor. This was supported by experiments with a saturated hydroperoxide, methyl 12‐hydroperoxyoctadecanoate, which was found to be much more resistant to alkali‐promoted conversion than 13 S ‐HPODE.