Synthesis and in vitro antitumor activity of isoazamitosene and isoiminoazamitosene derivatives

Seven isoazamitosene derivatives, mitomycin analogues, were synthesized and tested for cytotoxicities against leukemia and gastric cancer cell lines. Preparation of a pyrrolo[1,2-a]benzimidazole (3) (azamitosene ring system) was completed by utilizing the Lewis acid-catalized cyclization, with o-chloronitrotoluene as the starting material. Nitration of 3 produced a mixture of two isomers (5-nitro isomer (4) and 7-nitro isomer (5)) in product ratio of 36 : 52. 4 was directly converted into quinone (7) by reduction and Fremy oxidaton. Finally, quinone derivatives (8, 9, 10, and 11) were synthesized by 1,4-addition of 7 with cyclic secondary amines. From above-mentioned 5, 8-nitro compound (15) was prepared in 4 steps. At pH 3, Fremy oxidation of 15 produced quinone (16), whereas iminoquinone derivatives (17a and 17b) at pH 7. Isoazamitosene derivatives (8, 9, 10, and 11), containing cyclic amino groups at the 7-position, showed potent cytotoxicity on P388, SNU-1, and KHH tumor cell lines. Among them, 8 had stronger cytotoxicity against SNU-1 cell line than mitomycin and adriamycin. Considering these results, isoazamitosene derivatives may had unique cytotoxicity profiles. However, isoiminoazamitosene derivatives (17a and 17b) revealed very weak cytotoxicity.