文摘
From the roots of various Stemona species four new dehydrotocopherols (chromenols) were isolated and their structures and stereochemistry elucidated by spectroscopic methods. The double bond between C-3 and C-4 proved to be a typical chemical character of the genus found in most of the species. Various C-methylations of the aromatic ring reflect differences in methyltransferase activities and agreed with the current species delimitations showing an exclusive accumulation of dehydro-??-tocopherol for the Stemona tuberosa group, whereas different provenances of Stemona curtisii were characterized by dehydro-??-tocopherol accompanied by small amounts of dehydro-??-tocopherol. From Stemona collinsae all four tocopherols were isolated with a clear preponderance of dehydro-??-tocopherol accompanied by smaller amounts of the rare dehydro-??-tocopherol. Stemona burkillii and a group of unidentified species showed a weak accumulation trend towards dehydro-??-tocopherol, whereas Stemona cochinchinensis and especially Stemona kerrii clearly differed by a preponderance of chromanol derivatives. In Stemona cf. pierrei no tocopherols could be detected at all. Based on TLC tests and microplate assays with the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) the antioxidant capacities of all chromenol derivatives were comparable with that of ??-tocopherol showing no significant differences among each other, except for a more rapid kinetic behaviour of the 5,7,8-methylated dehydro-??-tocopherol.