Phytochemical and Antimicrobial Screening of Flavanones and Chalcones from Galenia africana and Dicerothamnus rhinocerotis

Abstract

This study focused on an 80% ethanol:water extract of Galenia africana and Dicerothamnus rhinocerotis in which a phytochemical study revealed the presence of flavonoids as the major secondary plant metabolites. Eleven pure flavonoids viz., (E)-2′,4′-dihydroxychalcone 1, (S)-7-hydroxyflavanone 2, (E)-2′,4′-dihydroxy-2,3-dihydrochalcone 3, (S)-5,7-dihydroxyflavanone 4, (S)-2′,5,7,-trihydroxyflavanone 5, (S)-5,7-dihydroxy-2′-methoxyflavanone 6, 5,7-dihydroxy-4H-chromen-4-one 7, (S)-5-hydroxy-7-methoxyflavanone 8 and (E)-2-hydroxy-3′,6′-dimethoxychalcone 9 were isolated from G. africana, while [sakuranetin] (S)-4′,5-dihydroxy-7-methoxyflavanone 10 and [eriodictyol-3′,7-dimethyl ether] (S)-4′,5-dihydroxy-3′,7-dimethoxyflavanone 11 were isolated from D. rhinocerotis. Compounds 6 and 9 are new while this is the first reported isolation of 1, 2, 3, 4, 5, 7, 8, 10 and 11 from these plants. All isolated compounds were tested for their antimycobacterial activity against the reference strain Mtb H37Rv. The most active compound, 9, demonstrated a MIC ₉₉ of 5 μM against Mtb H37Rv American Type Culture (ATCC) and (ATCC27294), which were also sensitive to Isoniazid (INH) and Rifampicin. The antibacterial activity of 9 might be ascribed to the presence of features such as the α,β-unsaturated ketone and the substitution patterns on the A and B rings.