NOVEL BENZYLIC ETHER REDUCTIONS IN LIGNAN BIOGENESIS IN FORSYTHIA-INTERMEDIA

Forsythia intermedia cell-free preparations catalyse the stepwise conversion of pinoresinol into secoisolariciresinol via lariciresinol; to our knowledge, this represents the first documented example of the reduction of benzylic ethers (or their quinone methide analogues) in plants. In this study, the specificity of the reductive steps was established: with (+)- and (-)-pinoresinols as substrates, it was found that their overall conversion into lariciresinol was highly enantioselective [i.e.> 99:1 in favour of the (+)-antipode], whereas (-)-secoisolariciresinol formation was more strictly enantioselective with only reduction of (+)-lariciresinol being observed. Under the conditions examined to date, it was not possible to demonstrate the NADP-dependent back reaction using (+/-)-lariciresinols as substrates. Interestingly, neither (+)- nor (-)-epipinoresinols [i.e. the diastereomers of (+/-)-pinoresinols] underwent similar reduction to afford lariciresinol, epilariciresinol or secoisolariciresinol.