Phenol , methoxy substituted

The low specificity of electron-donating substrates is remarkable for laccases. These enzymes have high redox potential, making them able to oxidize a broad range of aromatic compounds (e.g. phenols, polyphenols, methoxy-substituted phenols, aromatic amines, benzenethiols) through the use of oxygen as electron acceptor. Other enzymatic reactions they catalyze include decarboxylations and demethylations [66]. 

Laccase is one of the main oxidizing enzymes responsible for polyphenol degradation. It is a copper-containing polyphenoloxidase (p-diphenoloxidase, EC 1.10.3.2) that catalyzes the oxidation of several compounds such as polyphenols, methoxy-substituted phenols, diamines, and other compounds, but that does not oxidize tyrosine (Thurston, 1994). In a classical laccase reaction, a phenol undergoes a one-electron oxidation to form a free radical. In this typical reaction the active oxygen species can be transformed in a second oxidation step into a quinone that, as the free radical product, can undergo polymerization. 

Photo-induced electron-transfer decarboxylation reactions have been reviewed. A variety of methyl- and methoxy-substituted phenol radical cations have been generated by either photo-induced electron transfer or photo-ionization in dry solvents such as acetonitrile. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only transient species observed. The 2-methoxyphenol radical cation was found to be more reactive than the 4-methoxy radical cation. 

On the other hand, PIFA-induced oxidation of p-methoxy-substituted phenols (15) in the presence of electron-rich styrene derivatives (16) resulted in new carbon-carbon bond formation via an intermolecular 1,3-cycloaddition to afford frazzs-dihydrobenzofurans (17) stereoselectively [35,36] [Eq. (4)]. A formal synthesis of neolignans such as kadsurenone (18) and denudatin B (19) was achieved by this methodology. 

The Photo-oxidation of Methoxy Substituted Phenols. As we are now aware of the fact that there are several routes to form phenols in the chemical reactions of lignin, we have looked into the nature of free radical induced photooxidation of phenols (IS). There is no doubt that photolysis of methoxy phenols in solutions or in solid state produces yellow products. Molecular oxygen has often been shown to be a necessary reactant in the oxidation, although the radical nature of the mechanism is not understood (16). 

Surprisingly, the bulkiness of the 4-substituent on the phenol ring did not have a significant influence on the cis/trans ratio of the formed alkylcyclohexylamines at the same reaction conditions, whereas a bulky substituent in 2-position made the reduction of the intermediate cyclohexanone more difficult. After one hour of reaction time, 2-tert. butylcyclohexanone was the only product formed. Prolonged reaction led to the slow formation of 2-alkylcyclo-hexanols. In the case of the methoxy-substituted phenol, hydrogenolysis of the ether group took place. 

Peddinti also reported a simple and efficient one-pot synthesis of benzoxazolic bicyclo[2.2.2]octenones 29-31 by subjecting 2-methoxy-substituted phenolic aldimines of type 26 to a similar treatment with DIB in the presence of the same kind of dienophiles 24, or furans, in MeOH [47], This clever domino reaction starts with a DIB-mediated oxidative cychzation of the phenolic aldimines into the phenolic benzoxazoles 27, which are then converted, with a second equivalent of DIB, into the MOBs 28 that are finally trapped with excess of dienophile to furnish selectively the expected [4+2] cycloadducts 29-31 (Fig. 11) [47]. 

---