AbstractThe reaction of 3‐amino‐4,6‐dimethylthieno[2,3‐b]pyridine‐2‐carboxamide(1a)or its N‐aryl derivatives1b‐dwith carbon disulphide gave the pyridothienopyrimidines2a‐d, whilst when the same reaction was carried out usingN1‐arylidene‐3‐amino‐4,6‐dimethylthieno[2,3‐b]pyridine‐2‐carbohydrazides(1e‐h), pyridothienothiazine3was obtained. Also, refluxing of1b‐dwith acetic anhydride afforded oxazinone derivative4. Compounds2aand2b‐dwere also obtained by the treatment of thiazine3with ammonium acetate or aromatic amines, respectively. When compound2awas allowed to react with arylidene malononitriles or ethyl α‐cyanocinnamate, novel pyrido[3″,2″:4′,5′]thieno[3′,2′:4,5]pyrimido[2,1‐b][1,3]thiazines5a‐cwere obtained. Treatment of2b‐dwith bromine in acetic acid furnished the disulphide derivatives6a‐c. U.V. irradiation of2b‐dresulted in the formation of pyrido[3″,2″:4′,5′]thieno[3′,2′:4,5]pyrimido[2,1‐b]benzthiazoles7a‐c. The reaction of2a‐dwith some halocarbonyl compounds afforded the correspondingS‐substituted thiopyrido thienopyrimidines8a‐j. Compound8bwas readily cyclized into the corresponding thiazolo[3″,2″‐a]‐pyrido[3′,2′:4,5]thieno[3,2‐d]pyrimidine9upon treatment with conc. sulphuric acid. Heating of2a,bwith hydrazine hydrate in pyridine afforded the hydrazino derivatives11a,b. Reaction of ester8cwith hydrazine hydrate in ethanol gave acethydrazide10. Compounds10and11a,bwere used as versatile synthons