Chromium trioxide Chromones

Conversion of an aldehyde into a carboxyl group is effected with chromium trioxide-sulfuric acid in moderate yield. This is a synthetically valuable conversion because of the availability of chromone-3-carboxaldehydes (477). Photooxidation of the aldehyde (477 R = H, Me, Cl, OMe or OAc) in the presence of NBS gives high yields of 3-carboxylic acids (80SC889). 

Chromium trioxide oxidizes 2-aminochroman-4-ols, available in high yield from salicyl-aldehyde and enamines, to chromones (74IJC26). 

Oxidation of a methyl side chain is not an attractive route to chromone-2-carboxylic acids because of the ease with which the pyrone ring is attacked by oxidising agents to form salicyclic acid or its derivatives [112]. When 2,3-dimethylchromone is oxidised [113] with selenium dioxide, the main product (57 per cent yield) is the 2-aldehyde (25) a small amount of the 2-carboxylic acid (26) is formed simultaneously. The aldehyde (25) may be oxidised to the carboxylic acid by chromium trioxide in cold acetic acid. 

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See also in sourсe #XX -- [ ]

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