Laccases: blue enzymes for green chemistry

Laccases are oxidoreductases belonging to the multinuclear copper-containing oxidases. The overall outcome of their catalytic cycle is the reduction of one molecule of oxygen to two molecules of water and the concomitant oxidation of four substrate molecules to give four radicals.1 Typical substrates of laccases are phenols and aliphatic or aromatic amines, the reaction products being mixtures of dimers or oligomers derived by the coupling of the reactive radical intermediates. For instance, these biotransformations have been exploited to isolate new dimeric derivatives of natural phenolic derivatives (resveratrol and its analogues, ?-estradiol, totarol, sylibin and its analogues).2 In these studies a significant influence of the solvent on the reaction outcomes has also been observed.3 More recently, we have described the use of laccase-catalyzed reactions for the selective hydroxylation of ergot alkaloids and for the synthesis of the bisindole alkaloid anhydrovinblastine.4 Additionally, laccases oxidation of non-phenolic groups, particularly benzyl and - more generally - primary alcohols, is also possible thanks to the ancillary action of the so-called "mediators" (i.e., TEMPO, HBT, ABTS): the oxidation step is performed by the oxidized form of a suitable mediator, generated by its interaction with the laccase. Accordingly, a series of sugar derivatives and of natural glycosides have been oxidized.5