Aromatic 0-4 substructure model

Umezawa T, Fliguchi T (1987) Mechanism of aromatic ring cleavage of h-O-4 lignin substructure models by lignin proxidase. FEBS Lett 218 255-260... 

Aromatic Ring Cleavage of Nonphenolic 0-0-4 Lignin Substructure Model Compounds and Veratryl Alcohol by Lignin Peroxidase. 

Aromatic Ring Cleavage of Phenolic 0-0-4 Substructure Model Compounds by Laccase. When vanillyl alcohol was used as a substrate, only biphenyl formation (C5-C5 linked) occurred and no evidence for the formation of any ring-opened products was obtained (26). Hence, we also examined the effect of laccase on the sterically hindered 4,6-di-<-butylguaiacol substrate 50, as it would be unlikely to undergo such free-radical coupling reactions... 

The purpose of the present paper is to describe the aromatic ring cleavage of lignin substructure model compounds by white-rot basidiomycetes and by lignin peroxidase of P. chrysosporium. The aromatic ring cleavage of synthetic lignin (DHP) by the enzyme will also be described. 

Figure 1. /3-0-4 lignin substructure model dimers 1-4 and their degradation by white-rot fungi, Phanerochaete chrysosporium, Coriolus versicolor, and Coriolus hirsutus. The ether bond between the C/ and the B-aromatic nucleus is referred to as /3-0-4 bond in lignin chemistry. 

Aromatic Ring Cleavage of a (0-0-4)-(0-0-4) Lignin Substructure Model Trimer by Lignin Peroxidase... 

Figure 5. Proposed mechanisms for aromatic rin, substructure model dimers by lignin peroxidases.

Current photoresists cannot be used for 157 nm technology, mainly because their transmittance at 157 nm is too low. Although materials with aromatic substructures are quite useful for the 248-nm process, only purely aliphatic polymers are employed in the current 193 nm technology. For an illuminating wavelength of 157 nm, even the absorptivity of most aliphatic compounds is too high. Therefore, only partially fluorinated polymers with absorption characteristics carefully optimized by experiment [10] and molecular modeling [11] can be used. The solubility switch after illumination is usually achieved by addition of a photo-activatable super-acid (e.g. a diaryl iodonium hexafluoroantimonate) [12], which typically cleaves an add-labile tert-butyl ester in the polymer (Scheme 4.9). 

For lignin substructures containing a carbon substituent in the aromatic C-5 position, or for stilbenes originating from phenylcoumaran structures, the oxidative degradation leads to the formation of isohemipinic acid methyl ester (4) (Fig. 6.3.2, R=CH3). However, Gellerstedt and co-workers have shown that model compounds of the biphenyl type also give rise to isohemipinic acid in addition to the expected bis-vanillic acid structure (7). The reason seems to be incomplete alkylation (pH 11, 24 h) with the result that the substrate is alkylated at only one of the available positions. To achieve a more complete alkylation, it is necessary to increase the pH to 13 or alternatively, to extend the alkylation time to 84 h. In both cases, the yield of the bis-vanillic acid structure is substantially increased as shown in Table 6.3.1 nevertheless, the concomitant formation of isohemipinic acid cannot be completely avoided. 

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