Isolation and Structure Elucidation by LC-DAD-MS and LC-DAD-SPENMR of Cyclopeptide Alkaloids from the Roots of Ziziphus oxyphylla and Evaluation of Their Antiplasmodial Activity

Nine cyclopeptide alkaloids (1-9), of which five (compounds 2, 3, 5, 8, and 9) are described herein for the first time, were isolated from roots of Ziziphus oxyphylla by means of conventional separation methods as well as semipreparative HPLC with DAD and ESIMS detection and LC-DAD-SPE-NMR. Structure elucidation was done by spectroscopic means. Nummularine-R (1), a previously known constituent from this species, was isolated along with its new derivatives O-desmethylnummularine-R (2) and O-desmethylnummularine-R N-oxide (3). In addition, the known compounds hemsine-A (4) and ramosine-A (6), as well as hemsine-A N-oxide (5), were isolated. Moreover, oxyphylline-C (7), a known constituent of Z. oxyphylla stems, was obtained, and two new compounds were identified, oxyphyllines-E (8) and-F (9). Just like oxyphylline-C, oxyphyllines-E and-F belong to the relatively rare class of neutral cyclopeptide alkaloids. The antiplasmodial activity and cytotoxicity of compounds 1, 2, 4, 6, and 9 were evaluated, and the most promising activity was found for O-desmethylnummularine-R (2), which exhibited an IC50 value of 3.2 +/- 2.6 mu M against Plasmodium falciparum Kl, whereas an IC50 value of >64.0 mu M was evident far its cytotoxicity against MRC-S cells.