Stereoselective synthesis of ( E )-halomethylidene[1,3]thiazolo[3,2-a ]thieno[3,2-e ] pyrimidinium and analogous [1,3]oxazolo[3,2-a ] thieno[3,2-e ]pyrimidinium halides starting from 3-N -substituted 2-propargylthio(oxy)thieno[2,3-d ] pyrimidin-4-ones Abstract: A convenient procedure for the stereoselective synthesis of [1,3]thiazolo[3,2-a ]thieno[3,2-e ]pyrimidinium halides and analogous [1,3]oxazolo[3,2-a ] thieno[3,2-e ]pyrimidinium halides 5 and 6 is reported. 2-Propargylthio-3-R -thieno[2,3-d ]pyrimidin-4-ones and 2-propargyloxy-3-R -thieno[2,3-d ]pyrimidin-4-ones were treated with bromine or iodine in AcOH to afford ( E )-halomethylidene[1,3]thiazolo[3,2-a ]thieno[3,2-e ]pyrimidinium and ( E )-halomethylidene-[1,3]oxazolo[3,2-a ]thieno[3,2-e ] pyrimidinium halides, respectively.