Electrophilic heterocyclization of 6-alken(yn)ylsulfanyl-pyrazolo[3,4-d]pyrimidin-4(5H)-ones

6-Allylsulfanyl-1-arylpyrazolo[3,4- d ]pyrimidin-4(5 H )-ones react with iodine and sulfuric acid to give angular pyrazolothiazolopyrimidine derivatives. The reaction of 6-(prop-2-yn-1-ylsulfanyl)-1-(4-tolyl)-pyrazolo[3,4- d ]pyrimidin-4(5 H )-one with sulfuric acid gives angularly fused pyrazolo[4,3- e ][1,3]thiazolo-[3,2- a ]pyrimidin-4-one, whereas in the reaction with sodium methoxide linearly fused pyrazolo[3,4- d ][1,3]-thiazolo[3,2- a ]pyrimidin-4-one was formed. Linearly fused pyrazolo[3′,4′:4,5]pyrimido[2,1- b ][1,3]thiazole derivatives were also obtained by reaction of 1-aryl-6-(3-phenylprop-2-en-1-ylsulfanyl)pyrazolo[3,4- d ]pyrimidin-4(5 H )-ones with sulfuric acid.