AbstractNovel pyrimidine to pyrimidine transformation reactions are described. 1,3‐Dimethyl‐(or diethyl)‐uracil(1) are converted into isocytosine, 2‐thiouracil or uracil derivatives by treatment with guanidine, thiourea or urea, respectively. The latter two cases require base catalysis. The effects of some substituents at C‐5 and C‐6 of 1,3‐dialkylated uracils (1a → 1e) on this transformation were examined and a plausible mechanism is offered for their reaction. The utility of this reaction is exemplified by the facile two‐step conversion of pseudouridine into the anlileukemic agent, pseudoisocytidine, in good overall yield.