Xanthines European Pharmacopoeia

The different xanthines have various alkyl groups attached to the three nitrogen atoms not participating in the double bond. Examples are given below. 

The xanthines are alkaloids, but they do not react to the classical alkaloid reagents that are discussed in 3.3 Alkaloids. 

The test is based on a partly specific oxidation called the mmexide reaction. In the first step of the analysis a few milligrams (or the prescribed quantity) of the substance to be examined are added to 0.1 ml of strong hydrogen peroxide solution R and 0.3 ml of dilute hydrochloric acid R. The mixture is heated to dryness on a water-bath until a yellowish-red residue is obtained. 

The N-methyl groups of murexoin stems from the N-methyl groups of caffeine, and will therefore, in the murexide product of the other xanthines, be absent or replaced by the alkyl groups of the table above. But this will not affect the appearance of the product. 

Whereas the exact reaction mechanisms of the test are not easily elucidated in detail, some knowledge has been gathered about the structural prerequisites a compound must meet before it can give a positive murexide reaction. It appears that some rules can be set up about the substituents on the 6-member heterocyclic ring, including the nature of the substituents, which in the case of the xanthines includes a 5-membered heterocyclic ring. 

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