Chemotaxonomy of the acanthaceae. Iridoids and quaternary amines

Abstract

Fourteen of the 40 investigated species of Acanthaceae sensu lato contain iridoid glucosides. A total of 20 iridoids are now reported from the family; most have been isolated in the present work. Seven of the compounds are so far unique for the family, namely 6- O-acetylshanzhiside methyl ester, 6- O-acetylbarlerin, 8( S)-7,8-dihydroaucubin, eranthemoside, hygrophiloside, 6- epi-stilbericoside, and thunbergioside. A plausible biogenetic scheme interrelating the iridoid glucosides is presented. Notable amounts of betaine were isolated or detected by 1H NMR spectroscopy in crude extracts of virtually all (31/32) species belonging to subfamilies Acanthoideae and Ruellioideae. Trigonelline was similarly found in 14 species belonging to all five subfamilies. Other, unidentified quaternary methylammonium compounds were detected in the subfamilies Thunbergioideae, Mendoncioideae and Nelsonioideae. Quaternary methylammonium compounds are not usual in the order Scrophulariales and therefore the common occurrence of such compounds in the five subfamilies confirms their positioning within the Acanthaceae; this contrasts with the opinion of many taxonomists who prefer to raise one or more of these subfamilies to family rank. The common occurrence of the rare iridoid glucoside stilbericoside in Thunbergia, Stilbaceae and Retziaceae supports Dahlgren's placing of the two latter families in Scrophulariales.