Two 2,6-Dioxabicyclo[3.3.1]nonan-3-ones from Phragmanthera capitata (Spreng.) Balle (Loranthaceae)

Phytochemical investigation of the leaves of Phragmanthera capitata collected on Cassia spectabilis tree led to the isolation of two natural lactones, rel-(1R,5S,7S)-7-[2-(4-hydroxyphenyl)ethyl]-2,6-dioxabicyclo[3.3.1]nonan-3-one (1) and 4-{2-[rel-(1R,3R,5S)-7-oxo-2,6-dioxabicyclo[3.3.1]non-3-yl]ethyl}phenyl 3,4,5-trihydroxybenzoate (2) together with the known compounds betulinic acid (3), dodoneine (4), quercetin 3-O--L-rhamnopyranoside (5), quercetin 3-O--L-arabinofuranoside (6), quercetin (7), betulin (8), lupeol (9), and sitosterol (10). Their structures were established by means of modern spectroscopic techniques, and the relative configuration of compound 1 was confirmed by X-ray analysis. Compounds 1 and 2 were tested in vitro for their antiplasmodial activity against the Plasmodium falciparum chloroquine sensitive-strains NF54 and 3D7. Compound 2 exhibited good antiplasmodial activity against both strains with IC50 of 2.4 and 4.9M, respectively, while compound 1 was inactive.