Lignans and sesquiterpenoids from Lactuca sibirica

The genus Lactuca (Asteraceae) comprises approximately 100 species. One of these, L. virosa is an old medicinal plant with analgesic, antitussive and sedative properties attributed to the guaiane-type sesquiterpene lactone lactucin and its derivatives present in its dry latex, known as lactucarium. We have recently confirmed the reputed analgesic and sedative activities of the compounds in mice [1]. Moreover, biological studies have shown that some lactucin-type guaianolides inhibit acetylcholinesterase activity, cyclooxygenase-2 protein expression and DNA binding of the central transcription factor NFκB [2]. Until now, L. sibirica (L.) Benth. ex Maxim., a species distributed in continental Europe and Asia, has not been a subject of phytochemical analysis.

An ethanolic extract from aerial parts of the plant was prepared and chromatographed on a silica gel column, followed by preparative TLC and semi-preparative HPLC to give the previously known lactucin-type guaianolides 8-deoxylactucin, jacquinelin, 11β,13-dihydrolactucin and crepidiaside B, along with the known furofuran lignan lactucaside (8α-hydroxypinoresinol-4α-O-β-glucopyranoside). In addition, two new furofuran lignans were isolated and characterized as 8α-hydroxypinoresinol-4α-O-β-(6-p-methoxyphenylacetyl)-glucopyranoside and 8α-hydroxypinoresinol-4α-O-β-(6-p-hydroxyphenylacetyl)-glucopyranoside on the basis of spectroscopic data. This is the first time, to our knowledge, that furofuran lignans containing acylated sugar moieties have been found in plants. Furofuran lignans are rare secondary metabolites in Lactuca species. Prior to this work, lactucaside has been reported from L. indica, a folk medicine in Asia [3]. It is interesting to note that the compound showed antidiabetic activity in rats [3].

References: 1. Wesolowska, A. et al. (2006)J. Ethnopharmacol. 107:254.

2. Rollinger, J.M. et al. (2005) Curr. Drug Discov. Technol. 2:185.

3. Hou, C.C. et al. (2003)J. Nat. Prod. 66:625.