Pyrrolizidine Alkaloids from<i>Ageratum conyzoides</i>

Abstract

Pyrrolizidine alkaloids (PAs) are widely distributed in many plant families. They mainly occur in the genus Senecio (Asteraceae) such as Heliotropium (Boraginaceae) and Crotalaria (Fabaceae). PAs are of great pharmaceutical interest because of their toxic activities and their associations with Lepidoptera. Some of these insects Sequester the PAs from the plants and use them as defense agents and as precursors for the pheromones danaidone, danaidal, and hydroxydanaidal (l, 2, 3). This insect-PA relationship has been studied with a number of Lepidoptera species, e.g. Daninae, Ithomiinae, and Arctiidae (4, 5) leading to the conclusion that this fact can be used as a biological indicator for PA content. In this way obvations in Kenya became of great interest, which showed u.ctt, apart from Crotalaria and Heliotropium species, the native occurring Ageratum conyzoides (Asteraceae) is attractive for Lepidoptera, too (6). This association prompted us to undertake a phytochemical investigation of A. conyzoides. Two PAs have been isolated, which were found by MS to be isomeric forms. On account of 2D-NMR studies the absolute configuration of both PAs were deduced as follows: 0-retronecine-(-)viridiflorine ester [lycopsamine (1)] and an 0-heliotridine-(-)viridiflorine ester [echinatine (2)]. This is the first report of the occurrence of PAs in Ageratum species.