Chemotaxonomy of Plantago. Iridoid glucosides and caffeoyl phenylethanoid glycosides

Abstract

Data for 34 species of Plantago (Plantaginaceae), including subgen. Littorella (= Littorella uniflora), have been collected with regard to their content of iridoid glucosides and caffeoyl phenylethanoid glycosides (CPGs). In the present work, 21 species were investigated for the first time and many known compounds were found together with three new iridoid glucosides. Of these, arborescoside and arborescosidic acid, both of the uncommon type with an 8,9-double bond, were present in several species, while 6-deoxymelittoside was found only in P. subulata. The known compounds deoxyloganic acid, caryoptoside and rehmannioside D were isolated from the genus for the first time. The earlier reported occurrence of sorbitol in the family was confirmed, and this compound was shown by NMR spectroscopy to be the main sugar in the three species investigated for this. The combined data show that CPGs are present in all species investigated. With regard to the iridoids, the distribution patterns showed a good correlation with the classification of Rahn. Thus, aucubin is typical for the whole genus, while bartsioside and catalpol as well as 5-substituted iridoids are each characteristic for a subgenus in the family. Finally, the close relationship between Plantago and Veronica suggested by chloroplast DNA sequence analysis, could be corroborated by the common occurrence of the rare 8,9-unsaturated iridoids in these two genera.