Oxidation reactions of 4H-chalcogenopyrans
We have studied the oxidation of 4H-chalcogenopyrans and chalcogenopyrylium salts by lead tetraacetate. We have established that the major reaction products on oxidation of 2,6-diphenyl-4H-thiopyran and selenopyran are 2,6-diphenyl-4H-thio(seleno)pyran-4-ones, while 2-benzoyl-5-phenyl(seleno)thiophenes are formed as impurities. The major product on oxidation of 2,6-diphenyl-4H-pyran is 2-benzoyl-5-phenylfuran. We have shown that 2,4,6-triphenyl-4H-pyran does not undergo oxidation by hydrogen peroxide in neutral medium, in contrast to its S and Se analogs. This supports the hypothesis that the heteroatom participates in the oxidation process.