Isomeric Tropane Alkaloids from the Aerial Parts of Schizanthus tricolor

Investigation of the aerial parts of Schizanthus tricolor yielded seven isomeric tropane alkaloids: 3α-(1-methylitaconyl)-6β-senecioyloxytropane (1), 3α-(1-methylitaconyl)-6β-angeloyloxytropane (2), 3α-(1-methylmesaconyl)-6β-senecioyloxytropane (3), 3α-(1-methylmesaconyl)-6β-angeloyloxytropane (4), 3α-(1-methylmesaconyl)-6β-tigloyloxytropane (5), 3α-(1-methylcitraconyl)-6β-senecioyloxytropane (6), and 3α-(1-methylcitraconyl)-6β-angeloyloxytropane (7). Their structures were established by NMR including ¹H, ¹³C 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 IC₅₀ values of 22.8, 24.8, and 36.0 μM and displayed no cytotoxicity on MRC-65 cells (CC₅₀ > 64 μM). Alkaloid 3 was inactive (IC₅₀ 63.5 μM). The alkaloid mixture exhibited slightly higher activity (IC₅₀ 17.0 μM) than the pure compounds, indicating some synergy between the different isomers.