Methylene blue, oxygen sensitizer

Fig. 3. a) First order plot of oxygen uptake in the Methylene-blue (MB)-sensitized photooxidation of GA 8.4 pM and 1.3 mM histidine (control) in phosphate buffer pH 7. b) Percentage radical scavenging activity for the control molecule Trolox and GA at pH 7.4 in phosphate buffer 10 mM (hydroxyl radical) and pH 10 in sodium carbonate buffer 50 mM (anion superoxide radical). 

Similar oxidative ring closures by singlet oxygen (methylene blue as sensitizer, - 78°C) have also been reported by Dutch workers [87JCS(P1)-2481]. For instance, (42) gave a mixture of the two diastereomers (43) and (44). 

The dioxetane (29) has been obtained quantitatively by photo-oxygenation of 9-(2-adamantylidene)-iV-methylacridan in CH2CI2 at -72 C using Methylene Blue as sensitizer. In toluene at 25 °C, (29) is catalytically decomposed by lanthanide chelates with energy transfer. Base-induced oxygenation and chemiluminescence of 10-methylacridinium and 1-methylquinolinium salts have been observed in DMSO. The chemiluminescence has its origin in... 

Photosensitized oxygenation of a chaicone. Photooxygenation of the chalcone (1) with Methylene Blue as sensitizer gives the flavanol (2). Rose Bengal is a less efficient sensitizer. The reaction may be a model for the biological oxidation of chalcones to flavanoids. 

An irradiated soln. of di- er -butyl disulfide in methanol containing methylene blue as sensitizer treated with oxygen until uptake stops after 3.5 hrs. tert-butyl er -butanethiolsulfinate. Y 15%. F. e. s. R.W. Murray and S. L. Jindal, J. Org. Chem. 37, 3516 (1972) review of thiolsulfinic acid esters s. N. Isenberg and M. Grdinic, Intern. J. Sulfur Chem. 8, 307 (1973). 

Startg. oxazole irradiated 12-14 hrs. with a 275 w. sunlamp in methylene chloride containing methylene blue as sensitizer while dry oxygen is bubbled through the stirred soln. co-cyanocarboxylic acid. Y 80-90%. F. e., also with 9,10-diphenylanthracene peroxide in refluxing toluene, s. H. H. Wasserman and E. Druckrey, Am. Soc. 90, 2440 (1968). 

Methyl-4,5-diphenyloxazole in methanol containing methylene blue as sensitizer irradiated 6 hrs. with a 150 w. floodlamp N-acetyldibenzamide. Y 86%. Also from the isomeric 4-methyl-2,5-diphenyloxazole and f. e. s. H. H. Wasser-man and M. B. Floyd, Tetrahedron SuppL 7, 441 (1966) with 9,10-diphenyl-anthracene peroxide, singlet oxygen reactions, s. Am. Soc. 89, 3073 (1967). 

With short periods of irradiation (with high-pressure mercury lamps) under oxygen in chloroform containing methylene blue as a sensitizer, variously substituted 2-arylthiazoles are converted in the corresponding 2-aryloxazoles (823). 

The Methylene Blue sensitized photoaddition of singlet oxygen to ethyl l//-azepine-l-car-boxylate was reported originally to yield only a C2-C5 adduct 269 however, a reinvestigation with the methyl ester, and using tetraphenylporphine as a sensitizer, revealed that in addition to adduct 46 the [6 + 2] cycloadduct 47 is also produced.270... 

Af.fV-Dimethylhydrazones are photolyzed in the presence of oxygen, under conditions that are assumed to generate singlet oxygen (medium-pressure mercury lamp with Methylene Blue as a sensitizer) in 48-88% yields (see Scheme 23). 

In 1999, Clennan and Sram reported a study of the photo-oxidations of a series of tetrasubstituted alkenes (Fig. 5) in methylene blue-doped zeolite Y [11], The ene regiochemistries are very sensitive to the size of the allylic substituent, R, in solution. The A/B ratio increases from 0.49 to 2.4 as the substituent, R, is changed from methyl to ferr-butyl. This phenomenon has been attributed [12] to a sterically induced lengthening of the carbon-2 oxygen bond in the perepoxide intermediate I and subsequent preferred opening of this long bond (Fig. 5). 

Similarly, oxygenation of 1,2-digermetrene gave the digermin 91b in 80% yield. Comparable results were also obtained when methylene blue (MB) was employed as the sensitizer On the other hand, the reaction does not involve singlet oxygen, since 90a is stable under photooxygenation in the presence of the photosensitizer TPP. 

Competitive photooxygenation of the substrate pair 1-methyl-cyclopentene (AJ/l-methylcyclohexene (A2) was studied by Kopecky and Reich.123 Using rose bengal, erythrosin, and eosin, as well as methylene blue and hematoporphyrin as sensitizers, the ratio kg1/kg2 was found to be independent of the nature of the sensitizer used, in agreement with the assumption that singlet oxygen is the oxygenating intermediate. 

It has recently been shown by Spikes150 that uracil and a number of substituted uracils are subject to dye-sensitized photooxidation under certain conditions. Methylene blue and Eosin Y were active photosensitizers in the pH range 8-11.5, but were inactive below pH 8. FMN was very active over the pH range of 2.4-11.5. Photooxidation was measured with a rotating platinum oxygen electrode. 

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