Structural characteristics-guided investigation of Ailanthus altissima (Mill.) Swingle resulted in the isolation and identification of seven undescribed potential Michael reaction acceptors (1–7). Ailanlactone A (1) possesses an unusual 1,7-epoxy-11,12-seco quassinoid core. Ailanterpene B (6) was a rare guaianolide-type sesquiterpene with a 5/6/6/6-fused skeleton. Their structures were determined through extensive analysis of physiochemical and spectroscopic data, quantum chemical calculations, and single crystal X-ray crystallo-graphic technology using Cu Kα radiation. The cytotoxic activities against HepG2 and Hep3B cells of isolates were determined. En-couragingly, ailanaltiolide K (4) showed significant cytotoxicity against Hep3B cells with IC50 values of 1.41 ± 0.21 μM, whose covalent binding mode was uncovered in silico.