Acridine-Isoxazole and Acridine-Azirine Hybrids: Synthesis, Photochemical Transformations in the UV/Visible Radiation Boundary Region, and Anticancer Activity.

Easy-to-handle -hydroxyacridinecarbimidoyl chloride hydrochlorides were synthesized as convenient nitrile oxide precursors in the preparation of 3-(acridin-9/2-yl)isoxazole derivatives via 1,3-dipolar cycloaddition with terminal alkynes, 1,1-dichloroethene, and acrylonitrile. Azirines with an acridin-9/2-yl substituent attached directly or via the 1,2,3-triazole linker to the azirine C2 were also synthesized. The three-membered rings of the acridine-azirine hybrids were found to be resistant to irradiation in the UV/visible boundary region, despite their long-wave absorption at 320-420 nm, indicating that the acridine moiety cannot be used as an antenna to transfer light energy to generate nitrile ylides from azirines for photoclick cycloaddition. The acridine-isoxazole hybrids linked at the C9-C3 or C2-C3 atoms under blue light irradiation underwent the addition of such hydrogen donor solvents, such as, toluene, -xylene, mesitylene, 4-chlorotoluene, THF, 1,4-dioxane, or methyl -butyl ether (MTBE), to the acridine system to give the corresponding 9-substituted acridanes in good yields. The synthesized acridine-azirine, acridine-isoxazole, and acridane-isoxazole hybrids exhibited cytotoxicity toward both all tested cancer cell lines (HCT 116, MCF7, and A704) and normal cells (WI-26 VA4).