Synthesis of Oleanolic Acid Analogues and Their Cytotoxic Effects on 3T3 Cell Line

Oleanolic acid (OA) is a known natural compound with many important biological activities. Thirteen oleanolic acid derivatives linked at C-3 and C-28 were synthesized and their structures were confirmed by 1H- and 13C NMR and mass spectral analyses. Among them, compounds 4, 6, 8-10, 12, 13 were synthesized for the first time. They were evaluated for their cytotoxic activity. They showed proliferative effect at low concentrations while cytotoxic effect was observed at high concentrations in a dose dependent manner.

We have first synthesized compounds 1 and 2 from the reaction of methyl iodide and OA. Compound 1 was reduced with LiAlH4 to give compound 3, and compound 2 gave compound 9 with MOMBr as a new compound. The compound 10 was then obtained from the reduction of compound 9 with LiAlH4 as a new oleanolic acid derivative. A diol derivative 11 was synthesized from OA and LiAlH4 at the room temperature. Compound 4 was obtained from the reaction of compound 3 with CBr4 as a new analogue of OA, and the reduction of compound 4 afforded compound 5 as a known product. In addition, we synthesized compounds 6-8 from compound 3 using MsCl, MeI and p-nitrobenzoyl chloride, respectively, in good yields. Compounds 6 and 8 are new analogues of OA. The new compounds 12 and 13 were also synthesized starting from OA using with MOMBr and TBDMSiCl as the reagents. The all synthesized compounds were purified by using column chromatography and/or crystallization.

In the present study, thirteen OA derivatives linked at C-28 and (or) C-3 were synthesized and evaluated for their cytotoxic activity on 3T3 cell lines which are the standard fibroblast cell lines, derived from Swiss albino mouse embryo tissue. 3T3 cell viability was observed at low concentrations of the tested triterpenoids while they displayed anti-proliferative effect at higher concentrations.

Oleanolic acid 28-methyl ester (2) showed fairly different behavior from all the other compounds tested and found to be the least cytotoxic compound. However, at 200 µM concentration, it exhibited the same cytotoxicity with compounds 3, 9 and 10 around 58-59%. Among the tested 13 compounds, 7 exhibited the most drastic decline for the viability from 12,5 µM to 25 µM concentration. Compound 6 displayed the most cytotoxic effect, almost in all concentrations, particularly at 6.25 and 25 µM concentrations while the highest cytotoxic effect at 50 µM was observed for compound 11 among all the tested triterpenoids. As a result, all the tested OA derivatives showed proliferative effect at 1,56 µM although no proliferative effect was observed for OA. Moreover, OA exhibited higher cytotoxic effect than its derivatives, particularly at higher concentrations (50, 100, 200 µM) with an exception for compound 11. Because, the latter showed highest proliferative effect at lowest concentration, and highest anti-proliferative effect at highest concentration which surpassed all the OA derivatives.