Phytochemistry of the mopane, Colophospermum mopane.

The polyphenolic pool of the heartwood of the mopane, Colophospermum mopane Kirk ex J. Leonard, exhibits extreme diversity and complexity. It comprises a variety of monomeric flavonoids, e.g. flavan-3-ols, flavan-3,4-diols including the mopanols and peltogynols, flavonols, dimeric proanthocyanidins, e.g. proguibourtinidins, profisetinidins, promopanidins, propeltogynidins, and a variety of profisetinidin-type triflavanoids. The di- and tri-meric proanthocyanidins are accompanied by several functionalized tetrahydropyrano- and hexahydrodipyrano-chromenes (phlobatannins) that originate from the bi- and tri-flavanoids, respectively, via rearrangement of the pyran heterocycle(s). Owing to the predominance of the 5-deoxy (A-ring) flavan-3-ols, the chain terminating moieties in the biosynthesis of oligo- and poly-meric proanthocyanidins, the di- and tri-meric analogs also exhibit diversity as far as interflavanyl bonding positions are concerned. Such heterogeneity results from the reduced nucleophilicity of the A-rings of 5-deoxy flavan-3-ols, compared to the A-rings of the 5-oxy analogs (catechins), hence permitting alternative centers to participate in proanthocyanidin formation. Biomimetic-type syntheses were extensively utilized to unequivocally establish constitution and absolute stereochemistry of both the conventional and pyran ring rearranged-type di- and tri-meric compounds. Comprehension of the intricate mechanistic and stereochemical course of the pyran ring rearrangement reactions also contributed significantly to unambiguous structure elucidations. The aerial parts of the mopane are rich in essential oils that comprise mainly alpha-pinene and limonene, which are presumably responsible for the strong turpentine odor of the pods. The leaves also contain significant concentrations of beta-sitosterol and stigmasterol which are apparently the source of sterols in various organs of the mopane moth, Gonimbrasia belina. Three diterpenes, dihydrogrindelic acid, labd-13E-en-15-oate and dihydrogrindelaldehyde are present in the bark and seeds, the latter compound exhibiting significant cytotoxicity against a human breast cancer cell line.