Pyrimidines. 65 . Synthesis of 6‐substituted thieno[2,3‐ d ]pyrimidine‐2,4(1 H ,3 H )‐diones

Several 6‐substituted thieno[2,3‐ d ]pyrimidine‐2,4(1 H ,3 H )‐dione derivatives were synthesized. 6‐Ethoxycarbonyl derivatives 3 and 7 were prepared by treatment of 6‐chloro‐5‐formyluracil 1 and 6‐chloro‐5‐cyanouracil 6 with ethyl 2‐mercaptoacetate in the presence of a base. Electrophilic substitution reactions (Vilsmeier‐Haack reaction, bromination, and nitration) of 5,6‐unsubstituted thieno[2,3‐ d ]pyrimidine 9 , prepared by condensation of 6‐mercaptouracil 8 with chloroacetaldehyde, afforded the corresponding 6‐formyl‐, 6‐bromo‐, and 6‐nitrothieno[2,3‐ d ]pyrimidines 10, 15 and 16 , respectively.