Antibacterial Pterocarpans from Erythrina subumbrans

Seven pterocarpans, erybraedin B ( 1), erybraedin A ( 2), phaseollin ( 3), erythrabyssin II ( 4), erystagallin A ( 5), erythrabissin-1 ( 6) and erycristagallin ( 7), two flavanones, 5-hydroxysophoranone ( 8) and glabrol ( 9), and one isoflavone, erysubin F ( 10), were isolated from the stems of Erythrina subumbrans (Leguminosae). Their structures were identified by means of spectroscopy. This is the first report of the isolation of the non-alkaloidal compounds from Erythrina subumbrans and the observed dehydration of 6a-hydroxypterocarpans 5 and 6 in CDCl 3 to the corresponding pterocarpenes 11 and 12, respectively. Compounds 8 and 9 were isolated for the first time from the genus Erythrina. Compounds 2 and 4 exhibited the highest degree of activity against Streptococcus strains with an MIC range of 0.78–1.56 μg/ml, whereas compound 7 exhibited the highest degree of activity against Staphylococcus strains, including drug-resistant strains (MRSA and VRSA), with an MIC range of 0.39–1.56 μg/ml. Interestingly, compounds 2, 4, 5 and 7 were more active against several strains of Streptococcus and Staphylococcus than the standard antibiotics vancomycin and oxacillin. Compound 7 showed the highest level of activity against all VRSA strains tested, with an MIC range of 0.39–1.56 μg/ml, which were resistant to both antibiotics. These compounds may prove to be potent phytochemical agents for antibacterial activity, especially against the MRSA and VRSA strains.