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Abstract
Scheme 1-Reaction of amino-ester 1 with ethoxycarbonyl isothiocyanate led to the formation of thiourea derivative 12. Compound 12 is cyclized to 2-ethoxycarbonylaminopyridazino[5`,4`:3,4]thieno[3,2-d]-1,3-thiazin-4-one derivative 13 by action of sulfuric acid, and 3-ethoxycarbonyl-4-oxopyrimido[4`,5`:4,5]thieno[2,3-c]pyridazine-2(1H)-thione 14, by 2 N methanolic sodium hydroxide. Similarly the reaction of compound 1 with benzoyl isothiocyanate in boiling acetone led to the formation of benzoylthiourea derivative 15. An ensuing alkaline cyclization of 15 afforded 1,2-dihydro-2-thioxopyrimido[4`,5`:4,5]thieno[2,3-c]pyridazin-(3H)-one 16. The cyclic amide 16 furthermore was defined using as a comparison its cyclic thioether isomer 17 obtained by ring closure in sulphuric acid. Treatment of 2-amino derivative 17 with benzoyl isothiocyanate afforded 2-(3-benzothioureido) derivative 18, which underwent ring cyclization upon treated with conc. sulfuric acid at room temperature in presence of NBS gave tetracyclic derivative 19.