Background: The structural modification of natural products with the aim to improve the anticancer activity is a popular current research direction. The pentacyclic triterpenoid compounds oleanolic acid (OA) and glycyrrhetinic acid (GA) are distributed widely in nature.
Methods: In this study, various oleanolic acids and glycyrrhetinic acids were designed and synthesized by using the combination principle. The in vitro anticancer activities of new OA and GA derivatives were tested by the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) method with SGC-7901 (gastric cancer), MCF-7 (breast cancer), Eca-109 (esophageal cancer), HeLa (cervical cancer), Hep-G2 (hepatoma cancer) and HSF (normal human skin fibroblast) cells. 
Results and conclusion: The screening results showed that the compound 3m presented the highest inhibitory activities against SGC-7901, MCF-7 and Eca-109 cell lines with IC₅₀ values of 7.57±0.64 µM, 5.51±0.41 µM and 5.03±0.56 µM, respectively. In addition, this compound also showed effective inhibition of Hep-G2 cells with an IC₅₀ value of 4.11±0.73 µM. Moreover, compound 5b showed the strongest inhibitory activity against Hep-G2 cells with an IC₅₀ value of 3.74±0.18 µM and compound 3l showed strong selective inhibition of the HeLa cells with the lowest IC₅₀ value of 4.32±0.89 µM. A series of pharmacology experiments indicated that compound 5b could induce Hep-G2 cells autophagy and apoptosis. These compounds will expand the structural diversity of anti-cancer targets and confirm the prospects for further research.
Keywords: oleanolic acid, glycyrrhetinic acid, cytotoxic properties, synthesis, apoptosis