Electrophilic heterocyclization of 6-alken(yn)ylsulfanyl-pyrazolo[3,4-<Emphasis Type="Italic">d</Emphasis>]pyrimidin-4(5<Emphasis Type="Italic">H</Emphasis>)-ones

6-Allylsulfanyl-1-arylpyrazolo3,4-d]pyrimidin-4(5H)-ones react with iodine and sulfuric acid to give angular pyrazolothiazolopyrimidine derivatives. The reaction of 6-(prop-2-yn-1-ylsulfanyl)-1-(4-tolyl)-pyrazolo3,4-d]pyrimidin-4(5H)-one with sulfuric acid gives angularly fused pyrazolo4,3-e]1,3]thiazolo-3,2-a]pyrimidin-4-one, whereas in the reaction with sodium methoxide linearly fused pyrazolo3,4-d]1,3]-thiazolo3,2-a]pyrimidin-4-one was formed. Linearly fused pyrazolo3′,4′:4,5]pyrimido2,1-b]1,3]thiazole derivatives were also obtained by reaction of 1-aryl-6-(3-phenylprop-2-en-1-ylsulfanyl)pyrazolo3,4-d]pyrimidin-4(5H)-ones with sulfuric acid.