| The ultraviolet photoinactivation of cellular electron transport has been previously shown by Snyder and Raporport to result from photooxidative cleavage of the side chain (beta),(gamma)-double bond of the menaquinones (Vitamins K). The mechanisms of this process and the fate of the naphthoquinone moiety were not determined. This photooxidation was reinvestigated using an argon ion laser and a wavelength dependence of the photodegradation was observed. Irradiation of menaquinone-1 with 457.9 nm blue light under oxygen afforded the known 3'-hydroperoxide and a new intramolecular 1,2,4-trioxane resulting from the photoaddition of the quinone carbonyl and molecular oxygen to the side chain (beta),(gamma)-double bond. Subsequent irradiation of this trioxane with ultraviolet light was shown to fragment the trioxane ring into three carbonyl groups resulting in the overall oxidative cleavage of the side chain olefin. The ultraviolet fragmentation provided a high yield of acetone and a low yield of an extremely labile quinone-aldehyde. The acetone was isolated as a derivative and the structure of the aldehyde was confirmed by an alternate synthesis. In a parallel series of experiments aimed at the preparation of peroxides similar to the 1,2,4-trioxanes observed in the laser photolysis, the dye-sensitized photooxidation of the chromenol isomers of the menaquinones was examined. Sensitized photooxidation of the chromenol isomer of menaquinone-1 resulted in a high yield of a 10b-hydroperoxide analogous to the product of tocopherol oxidation reported by Foote. The chromenol acetates derived from menaquinone-1 and O-methyllapachol underwent a novel photooxidative rearrangement in quantitative yield to provide epoxyketones bearing a 10b-acetoxy group. Confirmation of the unusual structure assigned to the products and determination of the relative stereochemistry of the epoxy and acetoxy groups was achieved by X-ray crystallography. The epoxy and acetoxy groups exhibit trans stereochemistry, therefore a mechanism attended by a 1,4-acetoxy shift is presented. The photooxidation products are extremely sensitive due to the facile solvolysis of the acetoxy group. Addition of water or alcohols led to epimeric mixtures of hemiketals or ketals respectively. The menaquinone-1 chromanol acetate undergoes a similar photooxidation, however hydrolysis of this epoxyquinone ketal leads to an unstabile hemiketal which undergoes pyran ring opening to form the epoxyquinone. Vitamin K derived peroxides and certain laser generated 1,2,4-trioxanes in which the peroxide group is part of a cycle hydroperoxy hemiketal system underwent a base promoted decomposition which evolved singlet oxygen. The decompositions were monitored by addition of N,N-dimethyl-biacridylidene which chemiluminesces when oxidized by singlet oxygen. The construction of a sensitive photomultiplier electrometer used for the detection of weak chemiluminescence is reported. |
