Clinical trials have evidenced that several natural compounds, belonging to the phytochemical classes of alkaloids, terpenes, phenols and flavonoids, are effective for the management of various types of cancer. Latest research has proven that natural products and their semisynthetic variants may serve as a starting point for new drug candidates with a diversity of biological and pharmacological activities, designed to improve bioavailability, overcome cellular resistance, and enhance therapeutic efficacy. This review was designed to bring an update regarding the anticancer potential of betulonic acid and its semisynthetic derivatives. Chemical derivative structures of betulonic acid including amide, thiol, and piperidine groups, exert an amplification of the in vitro anticancer potential of betulonic acid. With the need for more mechanistic and in vivo data, some derivatives of betulonic acids may represent promising anticancer agents.
Novel lupane‐type triterpenoid‐isoxazole conjugates were designed by direct placing of isoxazole linker at C(17) of triterpenoid. The suggested synthetic sequence demonstrates successful combination of electro‐organic synthesis and conventional approaches. TEMPO‐mediated electrooxidation of betulin to betulinal was developed and optimized at boron‐doped diamond anodes with potassium acetate as inexpensive supporting electrolyte. Betulinal‐derived oxime was further selectively electro‐oxidized at a graphite anode to nitrile oxide, which proved to be stable and isolable species. The same reaction sequence was performed with 3β‐lupane‐3,28‐diol. Nitrile oxides were characterized by 15N NMR and X‐ray crystallography. The isolable nitrile oxides allowed creation of isoxazole library by 1,3‐dipolar cycloaddition reactions with various alkynes. Some of the title conjugates exhibit cytostatic properties against breast cancer cell line MCF7, glioblastoma multiform cell line U‐87 MG and lung carcinoma cell line A549 with growth inhibition (GI50) concentrations up to 11 μm, while being harmless to immortalized human fibroblasts hTERT (GI50 >100 μm).
The reactions of halides or pseudohalides of group I and II metals with carbamate-protected aziridines and azetidines in liquid sulfur dioxide as solvent resulted in the efficient ring-opening of these heterocycles. Sulfur dioxide, as a highly polar solvent, solubilizes the inorganic salts and acts as a mild Lewis acid to accelerate the ring-opening. For this reason, carb-
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