Reactions of Hydro(pero)xy Derivatives of Polyunsaturated Fatty Acids/Esters with Nitrite Ions under Acidic Conditions. Unusual Nitrosative Breakdown of Methyl 13-Hydro(pero)xyoctadeca-9,11-dienoate to a Novel 4-Nitro-2-oximinoalk-3-enal Product

13(S)-Hydroperoxy- and 13(S)-hydroxyoctadeca-9,11-dienoic acids (1a/b), 15(S)-hydroperoxy- and 15(S)-hydroxyeicosa-5,8,11,13-tetraenoic acids (2a/b), and their methyl esters reacted smoothly with NO2- in phosphate buffer at pH 3-5.5 and at 37 degreesC to afford mixtures of products. 1b methyl ester gave mainly the 9-nitro derivative 3b methyl ester (11% yield) and a peculiar breakdown product identified as the novel 4-nitro-2-oximinoalk-3-enal derivative 4 methyl ester (15% yield). By GCMS hexanal was also detected among the products. Structures 3b and 4 methyl esters were secured by N-15 NMR analysis of the products prepared from 1b methyl ester upon reaction with (NaNO2)-N-15. 4 methyl ester (14% yield) was also obtained from la methyl ester along with the nitrated hydroperoxy derivative 3a methyl ester (10% yield). Under the same conditions, 2a/b methyl esters gave mainly the corresponding nitrated derivatives 5a/b, with no detectable breakdown products, whereas the model compound (EE)-2,4-hexadienol (6) afforded two main nitrated derivatives identified as 7 and 8. A reaction pathway for 1a/b methyl esters was proposed involving conversion of nitronitrosooxyhydro(pero)xy intermediates which would partition between two competing routes, viz., loss of HNO2, to give 3a/b methyl esters, and a remarkably facile fission leading to 4 methyl ester and hexanal.