Antimicrobial activities of new and known flavanoid metabolites of Dalea lanata var. terminalis (Fabaceae)

The phytochemical investigation of extracts of Dalea lanata var. terminalis root and aerial portions has led to the isolation and characterization of a suite of bioactive phenolic compounds. The structures of a previously undescribed flavone (1), a flavanone (3), and an isoflavan (9), were deduced by NMR spectroscopy, supported by HR-ESI mass spectrometry. Nine known compounds were also isolated and identified. Compounds 1, 3, and 9 were summarily names lanatans A-C. The absolute configurations of chiral centers were determined by circular dichroism (CD) spectroscopy or verified for known compounds by optical rotation. The in vitro antimicrobial activities were determined for all previously untested compounds against biomedically relevant pathogens. The strongest activities observed for the new compounds were for lanatan A (1); MICs of 2.9 μM against methicillin resistant Staphylococcus aureus (MRSA) and 3.0 μM against vancomycin resistant Enterococcus faecalis (VRE). Lanatan B (3) was ten-fold less active (MIC 13.5 μM) against these pathogens, and lanatan C (9) was essentially inactive. The known compounds 2, 4, and 8 exhibited moderate activities, with MICs of 15.7, 9.6, and 3.3 μM, respectively, toward the fungal pathogen Cryptococcus neoformans. D. lanata var. terminalis expresses metabolites in common with Dalea spp. of the US Southwest and West studied previously in our laboratory, including D. albiflora, D. nana, D. searlsiae, and D. pogonathera, suggesting a common lineage. The known biosynthetic precursor of flavonoids, trans-cinnamic acid (12) was isolated for the first time, to our knowledge, from a Dalea species.