Stereochemistry and biosynthesis of (+)-lyoniresinol, a syringyl tetrahydronaphthalene lignan in Lyonia ovalifolia var. elliptica I:: isolation and stereochemistry of syringyl lignans and predicted precursors to (+)-lyoniresinol from wood

Steps leading to the biosynthesis of syringyl lignans and tetrahydronaphthalene and naphthalene lignans, especially the formation of the C2-C7' linkage, have not been elucidated. Lyoniresinol is a typical syringyl lignan, as well as a tetrahydronaphthalene lignan found in Lyonia ovalifolia var. elliptica. To demonstrate the biosynthetic pathway for (+)-lyoniresinol, three putative biosynthetic intermediates of lyoniresinol, syringaresinol, 5,5'-dimethoxylariciresinol, and 5,5'-dimethoxysecoisolariciresinol, were isolated from wood. The identity of the putative intermediates was confirmed by spectroscopic analyses, as well as by comparison of spectral and chromatographic data with those of authentic samples previously synthesized. The stereochemistry (enantiomeric composition and absolute configuration) of the isolated lignans were determined as (+/-)-syringaresinol, (8S,8'S)-(-)-5,5'-dimethoxylariciresinol [46% enantiomeric excess (e.e.)], (8S,8'S)-(+)-5,5'-dimethoxysecoisolariciresinol (91% e.e.), and (8R,8'R)-(+)-lyoniresinol (42% e.e.). The absolute configurations of (+)-and (-)-5,5'-dimethoxylariciresinols, and (+)-and (-)-5,5'-dimethoxysecoisolariciresinols were determined by their synthesis (catalytic reduction) from (8R,8'R)-(+)-and (8S,8'S)-(-)-syringaresinols and by subsequent chiral high-performance liquid chromatography analysis.