Antimalarial activity of azafluorenone alkaloids from the Australian tree Mitrephora diversifolia.

Mass-directed isolation of the CH2Cl2/MeOH extract from the roots of the Australian tree Mitrephora diversifolia resulted in the purification of the new azafluorenone alkaloid 5,8-dihydroxy-6-methoxyonychine (1) together with the known natural product 5-hydroxy-6-methoxyonychine (2). The structures of 1 and 2 were determined by extensive 1D and 2D NMR and MS data analyses. Both compounds were isolated during a drug discovery program aimed at the identification of new antimalarial leads from a prefractionated natural product library. When tested against two different strains of the parasite Plasmodium falciparum (3D7 and Dd2), 2 displayed IC(50) values of 9.9 and 11.4 microM, respectively, while 1 showed minimal activity.