Studies in the synthesis and interconversion of isomeric triazolothienopyrimidines

4-Hydrazinothieno2,3-d]pyrimidines were cyclized with triethyl orthoformate and formic acid to give 1,2,4-triazolo4,3-c]thieno3,2-e]pyrimidines and 1,2,4-triazolo2,3-c]thieno3,2-e]pyrimidines depending on the reaction conditions employed.