The 2-oxo-2H-[ 1,2,4]oxadiazolo [2,3-a]pyrimidine-7-carbamate 3 belongs to a new class of potent peripheral vasodilators. This compound was shown to exert a strong and long-lasting antihypertensive effect in experimental animals as well as in humans [l]. Its synthesis was previously reported [2], the last step being a highly regioselective cyclization of the pyrimidine-N-oxide dicarbamate 2 (Scheme 1). Subsequently, we examined this reaction in more detail and found that thermal ring closure of 2 led to a 95 : 5 mixture of the regioisomers 3 and 4. The structure of the major isomer 3 has been unequivocally established before by an X-ray crystallographic analysis [2 b]. The minor component, isolated from the mother liquor by high pressure liquid chromatography, was identified as 4 on the basis of its spectroscopic properties.