REGIOSELECTIVE AND DIASTEREOSELECTIVE SYNTHESIS OF DIMERIC LIGNANS USING OXIDATIVE COUPLING

The oxidative dimerization of monolignols such as (E)-isoeugenol (1), (E)-methyl ferulate (2) and (E)-coniferyl alcohol (3) has been performed using two catalytic systems: horseradish peroxidase (HRP)-H2O2 and tetraphenylporphyrinato-manganese(III) acetate or chloride-iodosylbenzene or H2O2. The kinetically controlled diastereoselective formation of trans beta-5 dimers was obtained in both cases. No diastereoselection between threo and erythro beta-O-4 dimers was observed in the HRP-catalyzed reaction. The trans stereochemistry of dehydrodiferulic acid methyl ester (5) was assigned by X-ray diffractometric analysis. The nature of the enzyme-substrate complex and the influence of pH and methanol content in the HRP-catalyzed reaction were also studied.