Macrocyclic pyrrolizidine alkaloids from Senecio anonymus. Separation of a complex alkaloid extract using droplet counter-current chromatography

Ten 12-membered macrocyclic pyrrolizidine alkaloids, all of them esters of the necines, retronecine or otonecine, have been isolated from Senecio anonymus. The separation, carried out by droplet counter-current chromatography, afforded senecionine 1], integerrimine 2], retrorsine 3], senkirkine 5], neosenkirkine 6], otosenine 10], hydroxysenkirkine 7], and a new alkaloid given the trivial name anonamine 9]. Traces of usaramine 4] and another new alkaloid, hydroxyneosenkirkine 8], were detected by 1H nmr. In addition, the previously unreported 3a beta-hydroxy-4-ethoxy-2,6-perhydroindoledione 11] was isolated. X-ray structures were obtained for neosenkirkine 6], hydroxysenkirkine 7], anonamine 9], and 11]. 1H-13C heteronuclear shift correlated nmr (HETCOR) provided unambiguous chemical shift assignments for 13C-nmr data. Antitumor activity was assayed using the A204-rhabdomyosarcoma cell line in soft agarose.