A method was developed for the synthesis of new fused 3-alkyl-substituted derivatives of thieno[3,2- d ]pyrimidin-4(3 H )-ones based on 11-(2-furyl)-8,8-dimethyl-7,10-dihydro-8 H -pyrano[3′′,4′′:5′,6′]pyrido[3′,2′:4,5]-thieno[3,2- d ]pyrimidin-4(3 H )-one via nucleophilic substitution in the pyrimidine ring. A series of fused 4-amino-substituted 11-furylthieno[3,2- d ]pyrimidine derivatives was also synthesized from the corresponding 4-chloro derivative.