Facile Entries for Regioselective Synthesis of [1,2,4]Triazolo-[4,3-a]pyrimidin-5(1H)-ones from 2-Thiouracil

Three new alternative synthetic strategies based on reactions of hydrazonoyl halides 3 with 2-methylthiouracil 2 and treatment of either 2-pyrimidinyl thiohydrazonates 8 or the diazonium coupling products of active (pyrimidin-2- ylthio)methylene compounds 12 with sodium ethoxide in ethanol are described for the title compounds. The mechanisms and regiochemistry of the studied reactions are discussed