The murexide reaction of caffeine was investigated to clarify the pathway of the coloration. From the reaction mixture of caffeine with nitric acid, 1, 3-dimethylalloxan (II) and 1, 3, 7-trimethyl-2, 6-dioxo-8-nitro-1H, 3H, 7H-xanthine (IV) were isolated. Compound II was found to be the key intermediate, since it was converted to a purple-red-colored substance, murexoin (III), by treatment with conc. ammonia. It was found that 1-hydroxy-5, 7-dimethyl-2, 4, 6-trioxo-1H, 5H, 7H-oxazolo [4, 5-d] pyrimidine (I), previously obtained by the oxidation of caffeine with hydrogen peroxide and hydrochloric acid, was also transformed to III with conc. ammonia. Consequently, the murexide reaction of caffeine was shown to have two pathways of coloration depending on the oxidizing agent employed. From the spectral data, a symmetrical structure (III) was assigned to murexoin in solution. Amalic acid, which has been reported as an intermediate of the murexide reaction of caffeine, can be ruled out on the basis of our experimental results.
