9- -2-amino-6-oxopurine

Guanine (2-amino-6 oxopurine) Urocil (2,4 -dioxopy rimidine) ... 

The interrelationship between the 2- and 8-positions is exemplified by 2-oxo-103 and 8-oxopurine, which are both converted by xanthine oxidase to 2,8-dioxopurine. Some unexpected results of oxidation can best be explained by assuming stereospecific factors. Thus, whereas purine, its 2-methyl, and its 2-amino derivatives give the 6-oxo derivatives, the 2-phenyl- and 2-oxopurines mentioned above oxidize at the 8-position.112... 

The first purine prepared from an imidazole was a 2-oxopurine, namely 7-methylxanthine (338), which was obtained from 4-amino-l-methylimidazole-5-carboxamide and diethyl carbonate in a sealed tube (Scheme 133) (24HCA713). 

Aminoimidazole esters have also been used as a source of 2-oxopurines, the additional nitrogen being derived from urea, cyanates and similar compounds. In this manner the amino ester (340) and potassium cyanate furnished a ureidoimidazole which smoothly cyclized to xanthine with dilute alkali (Scheme 142) (42JCS232). Methyl isocyanate similarly gave 1-methylxanthine and related derivatives have also been prepared (49JCS1071,50JCS1884). 

A major route to thioxopurines involves thiation of the corresponding oxopurine. Thus a good yield of 6-thioxo-l,6-dihydropurine is obtained from hypoxanthine and phosphorus pentasulfide (61JA4038). Thiation of an appropriate diaminopyrimidine and concomitant cyclization to a thioxopurine is sometimes possible as in the preparation of 2-amino-l-methyl-6-thioxo-l,6-dihydropurine from 2,6-diamino-5-formamido-3-methyl-4-oxo-3,4-dihydropyrimidine and phosphorus pentasulfide in refluxing pyridine over 30 hours... 

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