6-Thioxanthylic acid

Studies on the mechanism of action of 6-mercaptopurine are complicated by the fact that its anabolic product, thioinosinic acid, is further metabolized by oxidation to 6-thioxanthylic acid [219] and by methylation to 6-(methylthio)purine ribonucleotide [206, 296]. the effects of which could be even more important than those of thioinosinic acid itself, since the methylthio compound is about 20 times as potent as a feedback inhibitor [289]. 

To this newly formed 9-thioxanthyl sodium add dropwise, with stirring and cooling, 13.1 g of N-methyl-3-chloromethyl-piperidine in 30 to 40 ml of absolute benzene, then continue stirring at about 25°C for VA hours, and heat subsequently to 40°C for 1 hour. Decompose the resulting mixture by adding carefully a small amount of water, and then extract the newly formed base from the benzene solution by means of dilute hydrochloric acid. 

Protonation of the olefin, or protonation and subsequent dehydration of the parent alcohol, gives cations which are then subjected to laser excitation or steady-state irradiation. Cations generated in this way were identified by their characteristic absorption spectra, which also indicated cation stability over the time scale of the individual experiments by the lack of change in their absorption spectra. Among the numerous cations generated in acidified solution for photochemical studies are the xanthyl and thioxanthyl [7-15], dibenzosuberenyl [10], triphenylmethyl [10,15], a,co-diphenylpolyenyl [16], and 1,1-diarylethyl [17] cations. Media included acetonitrile acidified with trifluoroacetic acid (TFA-ACN) or aqueous sulfuric acid [7-9,11,14,15], TEA in 2,2,2-trifluoro-ethanol (TFA-TFE) [10,12,13], n-heptane acidified with TFA [9], and BFj-etherate in methylene chloride [16]. The absorption spectral data for several cations have been previously reviewed [6]. Characterization of the cation excited states will be discussed in Section III. 

Stable organic salts have been used to eliminate the requirement for acidic media and to avoid the problems associated with competitive radical generation that occur via homolytic bond cleavage of carbon-heteroatom bonds in laser flash photolytic experiments [28-30]. Treatment of several 9-arylxanthenols and thioxanthenols with fluoroboric acid in propionic anhydride gave crystalline xanthyl and thioxanthyl cation tetrafluoroborate salts [Eq. (5)]. 

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