Tetraphenylporphyrin-Catalyzed Tandem Photooxygenation of Polyenes and 1,4-Dienes: Multiple and Diverse Oxyfunctionalizations

Tandem singlet oxygen reactions photocatalyzed by meso-tetraphenylporphyrin were performed with substrates designed for primary alpha-gem-selectivity or high vinylogous gem-selectivity. The 1,3,5-triene ester 1 was designed as a model compound for the natural compound crocin and resulted in a highly regioselective singlet oxygen ene reaction with e-hydrogen activation (93:7 ene vs. [4+2] cycloaddition). The initially formed hydroperoxide 2a adds another singlet oxygen molecule in a slower reaction step to furnish the endoperoxide 3a. From the hydroperoxide stage, oxygen transfer gives the epoxide 4, reduction leads to the allylic alcohol 2b and subsequent photooxygenation (to 3b) followed by reduction results in the furan derivative 5. The allylic alcohol could also be reacted with other dienophiles as shown for the PTAD adduct 6. From the 1,4-diene 7, a gem-regioselective singlet oxygen ene reaction generates a mixture of the hydroperoxides 8a and 9a. Product 8a adds another singlet oxygen molecule to give the endoperoxide 10a. Catalyst activity was demonstrated by analysis of the changes in the electronic absorption properties

Interactions of plasmalogens and their diacyl analogs with singlet oxygen in selected model systems

Plasmalogens (Plg) are phospholipids containing vinyl ether linkage at the sn-1 position of the glycerophospholipid backbone. In spite of being quite abundant in humans, the biological role of plasmalogens remains speculative. It has been postulated that plasmalogens are physiological antioxidants with the vinyl ether functionality serving as sacrificial trap for free radicals and singlet oxygen. However, no quantitative data on the efficiency of plasmalogens to scavenge these reactive species are available. In this study, rate constants of quenching of singlet oxygen, generated by photosensitized energy transfer, by several plasmalogens and, for comparison, by their diacyl analogs, were determined by time-resolved detection of phosphorescence at 1270 nm. Relative rates of the interaction of singlet oxygen, with plasmalogens and other lipids in solution and liposomal membranes were measured by electron paramagnetic resonance oximetry and product analysis, employing HPLC-EC detection of cholesterol hydroperoxides and iodometric assay of lipid hydroperoxides. Results show that singlet oxygen interacts with plasmalogens significantly faster than with the other lipids, with he corresponding rate constants being by one-two orders of magnitude greater. The quenching of singlet oxygen by plasmalogens is mostly reactive in nature and results from its preferential interaction with the vinyl ether bond. The data suggest that plasmalogens could protect unsaturated membrane lipids against oxidation induced by singlet oxygen, providing that the oxidation products are not excessively cytotoxic