New lignans identified in softwoods

Research in our group has revealed that softwood knots, i.e. the branch bases inside tree stems, commonly contain 5-10% (w/w) of free lignans. A few lignans dominate in most industrially important softwood species, the most common lignans being 7-hydroxymatairesinol, secoisolariciresinol, lariciresinol, and nortrachelogenin. In addition, some minor lignans, of which some still are unidentified, are present in varying amounts in softwoods. We here report on the isolation and structural characterisation of two new 7-hydroxy divanillyl butyrolactol lignans, 1 and 2, which earlier, based solely on GC-MS analyses, incorrectly have been addressed as isomers of liovil in most softwoods. Some spruce species can contain up to 0.9% (w/w) of 1 and 2 in total. Additionally, we report on the isolation and structural characterisation of two previously unknown butyrolactone lignans, 3 and 4, which are present in amounts up to 0.5% (w/w) in, e.g. spruce species. The lignans were first extracted from softwood knots and then fractionated using appropriate techniques of normal and reversed-phase HPLC, reversed-phased TLC, and crystallisation from alcohol. The purified lignans were analysed using GC-FID, GC-EI-MS, LC-ESI-MS, and ¹H and ¹³C NMR techniques. We conclude that knots in many softwood species constitute the richest known source of easily accessible lignans in nature. Our research has brought us close to a complete characterisation of all significant lignans in softwood knots. Nevertheless, some minor lignans still remain unidentified and should be characterised in the near future.