Effective Synthesis of a Novel Betulinic Acid Conjugate with Mitochondria-Targeting Cation F16.

Currently, mitochondria are considered as an attractive universal target in the development of new anticancer drugs. These organelles are essential in energy production, regulation of cell death pathways, generation of reactive oxygen species, as well as maintenance of calcium homeostasis. Various approaches are being developed to deliver biologically active compounds into the mitochondria of tumour cells, including conjugation of cytotoxic substances with mitochondria-targeted lipophilic cations. Among the currently known low molecular weight lipophilic cationic molecules, (E)-4(1H-indol-3-ylvinyl)-N-methylpyridinium iodide (F16) is of great interest. This mitochondria-toxic cationic compound with luminescent properties is selectively accumulated in mitochondria and can selectively trigger apoptosis and necrosis of tumour cells, making it an attractive targeted agent for theranostic use. Meanwhile, betulinic acid, an available natural pentacyclic triterpenoid, has been considered as a promising scaffold for development of new anticancer agents in recent years. The antitumour effect of this natural product arises from affecting the mitochondria of tumour cells through formation of reactive oxygen species. The present article details an efficient synthesis of a novel multifunctional hybrid agent in which a cytotoxic triterpenoid, betulinic acid, is carbon-carbon bonded to the cationic F16 fragment at the C-2 position of ring A through a phenylethynyl spacer. The starting substrates in the synthesis were C-2 propynyl derivative of betulinic acid and N-aryl-substituted 4-(1H-indol-3-ylvinyl)-pyridine. The derivative of betulinic acid with a terminal acetylenic group was prepared by the reaction of C-alkylation with propargyl bromide of potassium enoxytriethylborate generated from betulonic acid. To obtain the N-aryl-substituted analogue of F16, CuI-catalyzed Ullmann-Goldberg reaction was applied. The synthesis of the target conjugate was successfully completed by the cross-coupling of the terpene and heterocyclic components according to Sonogashira in the presence of CuI/Pd(PPh₃)₂ catalyst.