Isomeric Tropane Alkaloids from the Aerial Parts of Schizanthus tricolor

Abstract

Investigation of the aerial parts of Schizanthus tricolor yielded seven isomeric tropane alkaloids: 3 alpha-(1-methylitaconyl)-6 beta-senecioyloxytropane 3a-(1-methylitaconyl)-6 beta-angeloyloxytropane (2), 3 alpha-(1-methylmesaconyl)-6 beta-senecioyl-oxytropane (3), 3 alpha-(1-methylmesaconyl)-6 beta-angeloyloxytropane (4), 3 alpha-(1-methylmesaconyl)-6 beta-tigloyloxytropane (5), 3 alpha-(1-methylcitraconyl)-6 beta-senecioyloxytropane (6), and 3 alpha-(1-methylcitraconyl)-6 beta-angeloyloxytropane (7). Their structures were established by NMR including (1)H, (13)C NMR, HSQC, HMBC, COSY, and NOESY experiments, UV, IR, and mass spectrometry. Compounds 1, 6, and 7 are new to the literature. Alkaloids 1, 3, 4, and 5 and a mixture of 3, 4, and 5 were evaluated for in vitro antiplasmodial and cytotoxic activity. Compounds 1, 4, and 5 showed marginal inhibition of Plasmodium falciparum strain K1 with IC(50) values of 22.8, 24.8, and 36.0 mu M and displayed no cytotoxicity on MRC-65 cells (CC(50) > 64 mu M). Alkaloid 3 was inactive(IC(50) 63.5 mu M). The alkaloid mixture exhibited slightly higher activity (IC(50) 17.0 mu M) than the pure compounds, indicating some synergy between the different isomers.