A Dimroth rearrangement of pyrimidine nucleosides
N 4-Acylamino-2′-deoxycytidine derivatives, 4a–c, undergo acid-promoted cyclisation to give [1,2,4]triazolo[4,3-c]pyrimidin-5(6H)-ones, 5a–c, in the presence of pyridinium chloride. This reaction has been demonstrated for a series of analogues. Treatment of the cyclised products in basic media gives rise to a novel Dimroth-type rearrangement leading to [1,2,4]triazolo[1,5-c]pyrimidin-5(6H)-ones, 7a–c. The crystal structure of one such product, 15, was confirmed by X-ray analysis.