Antimicrobial Abietane Diterpenoids from Clerodendrum eriophyllum
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Clerodendrum eriophyllum Gürke (Verbenaceae) is a shrub or small tree distributed in the dry bushlands of Eastern Kenya. The plant has no record of previous phytochemical analysis. A bioassay guided isolation of the methanol extract of C. eriophyllum afforded one new abietane diterpenoid, namely 12-hydroxy-8,12-abietadiene-3,11,14-trione (1) and nine known abietane analogs (2-10). The structures of the compounds 1-10 were determined by 1D and 2D NMR spectra, including COSY, HMQC and HMBC experiments, and ESI-HRMS. The isolated compounds 3, 6-8 exhibited in vitro antimicrobial activity against a panel of microorganisms, including potent activities against C. neoformans by 3 and 7 with IC50 0.58 and 0.96µg/mL respectively. Circular Dichroism (CD) spectra of isolated compounds 1, 3, 6-8 were registered to evaluate the stereochemistry.