FORMATION OF (-)-ARCTIGENIN IN FORSYTHIA-INTERMEDIA

Forsythia intermedia cell-free extracts were examined for their abilit y to catalyse the enantioselective and regiospecific O-methylation of matairesinol. In contrast to the enzymatic steps defined from (+)-pino resinol to (-)-matairesinol, the conversion of matairesinol into arcti genin was not highly enantioselective; both (+)- and (-)- antipodes of matairesinol served as substrates for methylation, with the naturally occurring (-)-enantiomer slightly preferred. But the cell-free extrac ts also catalysed the synthesis of (+)- and (-)-isoarctigenins, with ( -)-matairesinol again the preferred substrate. Thus the O-methylation of matairesinol, catalysed by F. intermedia cell-free extracts, is nei ther highly enantioselective nor regiospecific. No evidence that subse quent methylation of either arctigenin or isoarctigenin occurred to af ford dimethyl matairesinol was obtained, i.e. the O-methyltransferase( s) only catalysed monomethylation. Taken together, it is proposed that post-coupling methylation does not proceed via regiospecific methylat ion of matairesinol to give arctigenin directly. Instead, regiospecifi c glucosylation first occurs to afford matairesinoside; subsequent met hylation affords arctiin, which is then converted into arctigenin via action of a beta-glucosidase.