Phenolic compounds in species of the Blechnaceae

Rosmarinic acid and other caffeic and 4-coumaric acid derivatives are widespread in land plants. These phenolic compounds contribute to the medicinal uses of plants. Ferns (Polypodiopsida), however, are still under-investigated in terms of phenolic natural products and their biosyntheses. We analysed 32 fern species in the Blechnaceae, encompassing all three subfamilies, for content of caffeoyl-5-O-quinic acid, rosmarinic acid and related compounds using LC/ESI-MS/MS and HPLC. A compound previously unknown from ferns, but known in two species of Lamiaceae, was purified, its structure elucidated by NMR, and named blechnic acid B. While caffeoyl-5-O-quinic acid was present in extracts from all investigated species, rosmarinic acid was only found in four species (Doodia maxima, Lomariocycas tabularis, Neoblechnum brasiliense, Oceaniopteris ciliata) belonging to superclade B of the Blechnoideae subfamily. Blechnic acid B, a novel compound in ferns, is composed of two moieties of isorinic acid (caffeoyl-4'-hydroxyphenyllactic acid). This lignan was only detected in Neoblechnum brasiliense, together with three related lignans having one or two 3,4-dihydroxyphenyllactic acid moieties. Our study suggests that genes for biosynthesis of caffeoyl-5-O-quinic acid were already established in ancestor species of the Blechnaceae. Blechnic acid B and its three derivatives might have formed through a selective coupling of the rosmarinic/isorinic acid monomers with the help of a dirigent protein.