1311. Pyrimidines. Part XIV. Synthesis of 2,5-dihydroxypyrimidine

Abstract

2,5-Dihydroxypyrimidine has been synthesised from 5-benzyloxy-4-hydroxy-2-mercaptopyrimidine. Its properties are compared with those of a-and p-isouracil which Tafel and Houseman had tentatively considered t o be tautomeric forms of 2,5-dihydroxypyriniidine.THERE are four possible dihydroxypyrimidines, of which three have been well-characterised, namely, 2,4-dihydroxy-(uracil), 4,5-dihydro~y-,~ and 4,6-dihydro~y-pyrimidine.~ The fourth isomer, 2,5-dihydroxypyrimidine, may have been prepared by Tafel and Houseman but in view of doubts concerning the identity of their material the present synthetic work was undertaken. In 1901 Tafel reduceduric acid electrolytically and obtained a substance, C5H8N,02 which he called " purone." When this was treated with acid or alkali it was irreversibly isomerised to " isopurone."oxidised the latter with bromine water and obtained two compounds, C,H,N202, which were isomeric with uracil. These they called a-and p-isouracil. a-Isouracil crystallised as needles, which carbonised without melting at about 350", and with ferric chloride it gave a brown-violet colour which soon faded. It was more soluble in dilute acid or alkali than in water alone, and it was gradually decomposed by cold dilute acid or by hot water with the formation of a greygreen, amorphous, sparingly soluble precipitate. It did not react with phenylhydrazine in acetic acid, but on prolonged warming with an excess of this reagent in ethanol it gave an amorphous, osazone-like precipitate whose composition was not established. Tafel and Houseman tentatively suggested formulz (I) or (11) for a-isouracil. The p-isomer was found to be very stable; it dissolved in alkali and could be recovered on acidification. Reaction with phenylhydrazine in acetic acid gave a crystalline product, but this was not fully investigated. Because of its apparent ketonic nature, p-isouracil was given the provisional structure (111). Attempts to interconvert the a-and p-isomers were unsuccessful.