In this report, we describe the semisynthesis of two series of ursolic and betulinic acid derivatives through designed by modifications at the C-3 and C-28 positions and demonstrate their antimalarial activity against chloroquine-resistant P. falciparum (W2 strain). Structural modifications at C-3 were more advantageous to antimalarial activity than simultaneous modifications at C-3 and C-28 positions. The ester derivative, 3β-butanoyl betulinic acid (7b), was the most active compound (IC₅₀ = 3.4 µM) and it did not exhibit cytotoxicity against VERO nor HepG2 cells (CC₅₀ > 400 µM), showing selectivity towards parasites (selectivity index > 117.47). In combination with artemisinin, compound 7b showed an additive effect (CI = 1.14). While docking analysis showed a possible interaction of 7b with the Plasmodium protease PfSUB1, with an optimum binding affinity of −7.02 kcal/mol, the rather low inhibition displayed on a Bacillus licheniformis subtilisin A protease activity assay (IC₅₀ = 93 µM) and the observed accumulation of ring forms together with a delay of appearance of trophozoites in vitro suggests that the main target of 3β-butanoyl betulinic acid on Plasmodium may be related to other molecules and processes pertaining to the ring stage. Therefore, compound 7b is the most promising compound for further studies on antimalarial chemotherapy. The results obtained in this study provide suitable information about scaffolds to develop novel antimalarials from natural sources.

在这份报告中，我们描述了通过在C-3和C-28位置进行修饰设计而设计的两个系列的熊草酸和桦木酸衍生物的半合成，并证明了它们对耐氯喹的恶性疟原虫（W2株）具有抗疟活性。与在C-3和C-28位置同时进行修饰相比，在C-3处进行结构修饰更有利于抗疟疾活性。酯衍生物3β-丁酰桦木酸（7b）是活性最高的化合物（IC ₅₀  = 3.4 µM），对VERO和HepG2细胞均无细胞毒性（CC ₅₀  > 400 µM），显示出对寄生虫的选择性（选择性）索引> 117.47）。与青蒿素合用，化合物7b表现出加和效应（CI = 1.14）。对接分析显示7b与疟原虫蛋白酶PfSUB1可能发生相互作用，最佳结合亲和力为-7.02 kcal / mol，地衣芽孢杆菌枯草杆菌蛋白酶A蛋白酶活性测定（IC ₅₀  = 93 µM）和观察到的环形式积累以及滋养体在体外的出现延迟表明，疟原虫上3β-丁酰基桦木酸的主要靶标可能与其他与环阶段有关的分子和过程有关。因此，化合物7b是抗疟化学进一步研究的最有希望的化合物。在这项研究中获得的结果提供了有关支架的适当信息，以从自然资源开发新型抗疟药。