Diversity of secondary metabolites from some medicinal plants and cultivated lichen mycobionts

Abstract

Studies on the structural determination, biosynthesis, and biological activities of secondary metabolites from natural sources are significant in the field of natural products chemistry. This review focuses on diverse secondary metabolites isolated from medicinal plants and cultivated mycobionts of lichens in our laboratory. Monoterpene-tetrahydroiso- quinoline glycosides and alkaloids isolated from Cephaelis acuminata and Alangium lamarckii gave important information on the biosynthesis of ipecac alkaloids. A variety of glycosides linked with a secologanin unit and indole alkaloids were obtained from medicinal plants belonging to the families of Rubiaceae, Apocynaceae, and Loganiaceae. Plant species of the four genera Fraxinus, Syringa, Jasminum,and Ligustrum of the family Oleaceae were chemically investigated to provide several types of secoiridoid and iridoid glucosides. The biosynthetic pathway leading from protopine to ben- zophenanthridine alkaloids in suspension cell cultures of Eschscholtzia californica was elucidated. The structures and biological activities of the bisbenzylisoquinoline alkaloids of Stephania cepharantha and Nelumbo nucifera were also investigated. In addition, the mycobionts of lichens were cultivated to afford various types of metabolites that differ from the lichen substances of intact lichens but are structurally similar to fungal metabolites. The biosynthetic origins of some metabolites were also studied. These findings suggest that cultures of lichen mycobionts could be sources of new bioactive compounds and good systems for investigating secondary metabolism in lichens.