New Dammarane and Malabaricane Triterpenes from Caloncoba echinata

Three novel triterpenes, (11R,20R)-11,20-dihydroxy-24-dammaren-3-one (1), (17S,20R,24R)-17,25-dihydroxy-20,24-epoxy-14(18)-malabaricen-3-one (2), and (17R,20S,24R)-17,25-dihydroxy-20,24-epoxy-14(18)-malabaricen-3-one (3), were isolated from leaves of Caloncoba echinata. The structures were established using mainly 800 MHz NOESY and HMBC connectivities. The absolute stereochemistry of C-11 in 1 and that of C-17 in 2 were established by the Mosher method. The stereochemistry of the side chains of the malabaricanes is compatible with their biosynthesis by a cascade opening of diepoxides. The isolated triterpenes inhibited growth of Plasmodium falciparum parasites in vitro apparently via incorporation into erythrocyte membrane, as suggested by transformation of erythrocytes into stomatocytes at a concentration level at which the growth inhibition was observed.