Stereochemistry of Hydrogen Removal During Oxygenation of Linoleic Acid by Singlet Oxygen and Synthesis of 11(S)-Deuterium-Labeled Linoleic Acid

Exposure of unsaturated fatty acids to singlet oxygen results in the formation of hydroperoxides. In this process, each double bond in the acyl chain produces two regioisomeric hydroperoxides having an (E)-configured double bond. Although such compounds are racemic, the hydrogen removal associated with the oxygenation may, a priori, take place antarafacially, suprafacially or stereorandomly. The present study describes the preparation of [11(S)-H-2]linoleic acid by two independent methods and the use of this stereospecifically labeled fatty acid to reveal the hidden stereospecificity in singlet oxygenations of polyunsaturated fatty acids. It was found that linoleic acid 9(R)- and 13(S)-hydroperoxides formed from [11(S)-H-2]linoleic acid both retained the deuterium label whereas the 9(S)- and 13(R)-hydroperoxides were essentially devoid of deuterium. It is concluded that polyunsaturated fatty acid hydroperoxides produced in the presence of singlet oxygen in e.g., plant leaves are formed by a reaction involving addition of oxygen and removal of hydrogen taking place with suprafacial stereochemistry. This result confirms and extends previous mechanistic studies of singlet oxygen-dependent oxygenations.