Design, Synthesis, and Anticancer Activity of Novel Fused Purine Analogues

6‐Mercaptopurine has been utilized for the synthesis of various fused purine analogues through different chemical reactions to yield [1,4]thiazino[4,3,2‐gh]purines 2, 3, 10a,b, 14, (8‐oxo‐[1,4]thiazino[4,3,2‐gh]purin‐7(8H)‐ylidene) acetate 4, [1,4]thiazepino[4,3,2‐gh]purine 6, thiazolo[3,4,5‐gh]purine 7, imidazo[1,5,4‐gh]purin‐5‐amine 8, 5‐methylimidazo[1,5,4‐gh]purine 9, [1,2,4]triazino[4,3‐i]purines 16, 18, 21, [1,2,4]triazino[4,5,6‐gh]purine 20, 5‐methyl‐2‐(7H‐purin‐6‐yl)‐1H‐pyrazol‐3(2H)‐one 22, [1,2,4]triazolo[4,3‐i]purine 23, [1,2,5]triazepino[5,4,3‐gh]purine 24, and ethyl 6‐(2‐(ethoxycarbonyl)hydrazinyl)‐9H‐purine‐9‐carboxylate 26. Seventeen of the newly synthesized compounds were selected by the NCI, Maryland, USA, and were tested for their anticancer activity in an initial single high dose in the full NCI 60 cell line panel among which [1,4]thiazino[4,3,2‐gh]purine‐7,8‐dione 2, 7‐benzyl‐[1,4]thiazino[4,3,2‐gh]purine 10b, and 3‐(2,4‐dimethoxyphenyl)‐7H‐[1,2,4]triazolo[4,3‐i]purine 23 were found to possess very potent anticancer activity against most of the cancer cell lines.