Prenylated Isoflavanones with Antimicrobial Potential from the Root Bark of Dalbergia melanoxylon .

Guill. & Perr (Fabaceae) is widely utilized in the traditional medicine of East Africa, showing effects against a variety of ailments including microbial infections. Phytochemical investigation of the root bark led to the isolation of six previously undescribed prenylated isoflavanones together with eight known secondary metabolites comprising isoflavanoids, neoflavones and an alkyl hydroxylcinnamate. Structures were elucidated based on HR-ESI-MS, 1- and 2-D NMR and ECD spectra. The crude extract and the isolated compounds of were tested for their antibacterial, antifungal, anthelmintic and cytotoxic properties, applying established model organisms non-pathogenic to humans. The crude extract exhibited significant antibacterial activity against Gram-positive (97% inhibition at 50 μg/mL) and antifungal activity against the phytopathogens , and (96, 89 and 73% at 125 μg/mL, respectively). Among the pure compounds tested, kenusanone H and (3)-tomentosanol B exhibited, in a panel of partially human pathogenic bacteria and fungi, promising antibacterial activity against Gram-positive bacteria including methicillin-resistant (MRSA) and showing MIC values between 0.8 and 6.2 μg/mL. The observed biological effects support the traditional use of and warrant detailed investigations of its prenylated isoflavanones as antibacterial lead compounds.