Synthesis and Biological Evaluation of Oxygen‐containing Heterocyclic Ring‐fused 23‐Hydroxybetulinic Acid Derivatives as Antitumor Agents

A collection of isoxazole and oxadiazole substituted 23-hydroxybetulinic acid (HBA) derivatives were designed, synthesized and evaluated for their antitumor activity. Most of the newly synthesized compounds exhibited more potent antiproliferative activity than patent compound 23-hydroxybetulinic acid, especially 13e and 14a were about four- to sevenfold more potent against all tested cancer cell lines than 23-hydroxybetulinic acid. Furthermore, the in vivo antitumor activity of 13e and 14a was validated in H22 liver cancer and B16 melanoma xenograft mouse models. The structure–activity relationships of these 23-hydroxybetulinic acid derivatives were also discussed based on the present investigation.