Quinoid Intermediates

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

Quinone methides are the key intermediates in both resole resin syntheses and crosslinking reactions. They form by the dehydration of hydroxymethylphenols or dimethylether linkages (Fig. 7.24). Resonance forms for quinone methides include both quinoid and benzoid structures (Fig. 7.25). The oligomerization or crosslinking reaction proceeds by nucleophilic attack on the quinone methide carbon. 

When the ortho-para directing group is one with an unshared pair (this of course applies to most of them), there is another effect that increases the amount of para product at the expense of the ortho. A comparison of the intermediates involved (p. 683) shows that C is a canonical form with an ortho-quinoid structure, while D has a para-quinoid structure. Since we know that para-quinones are more stable than the ortho isomers, it seems reasonable to assume that D is more stable than C, and therefore contributes more to the hybrid and increases its stability compared to the ortho intermediate. 

While tocopherylacetic aicd (51), the lower Crhomologue of 3-(5-tocopheryl)-propionic acid (50) showed a changed redox behavior (see Section 6.5.1), compound 50 displayed the usual redox behavior of tocopherol derivatives, that is, formation of both ortho- and para-quinoid oxidation intermediates and products depending on the respective reaction conditions. Evidently, the electronic substituent effects that... 

Oxo-a-tocopherol (55) proved to be a very interesting compound with regard to forming various intermediate tautomeric and quinoid structures. It undergoes an intriguing rearrangement of its skeleton under involvement of different o-QM structures. The 4-oxo-compound was prepared from 3,4-dehydro-a-tocopheryl acetate via its bromohydrin, which was treated with ZnO to afford 4-oxo-a-tocopherol (55). 

Third, the pronounced difference in stability between the naphtho[2,3-sulfur extrusion processes. In contrast to the substantial instability of 116, the increased stability of 114 may be due to the energy required to convert two aromatic rings into the o-quinoid form in 114a. 

An interesting way to generate telluronium dications involves electron transfer through a 71-conjugated system to a spatially remote sulfoxide sulfur atom in a domino manner. Treatment of substrate 141 with triflic anhydride results in reduction of the terminal sulfoxide group with simultaneous oxidation of the tellurium atom in the para-position and formation of a trichalcogen dicationic moiety 144143 through the intermediate sulfonium salt 142 and quinoid structure 143 (Scheme 52). 

The ct-amino group of the substrate SAM replaces that of active site lysine as the Schiff base partner of the cofactor (external aldimine. Scheme 2(b)) and the C-a proton of SAM is next abstracted by the e-NH2 function of active site lysine to form a quinoid intermediate (Scheme 2(c)). 

Nevertheless, Li et al have proposed a model in which the Tyrl52 residue (see Section 5.04.2.2.5) mediates the formation of ACC from SAM via a catalytic mechanism involving a quinoid intermediate. 

After formation of the aldimine, numerous factors in the enzyme facilitate deprotonation of the a-carbon (Fig. 3, Step II). The lysine liberated by transimi-nation is utilized as a general base and is properly oriented for effective deprotonation [11]. Furthermore, the inductive effects of the ring system are tuned to increase the stabilization of the quinoid intermediate. For example, the aspartate group that interacts with the pyridyl nitrogen of the co enzyme promotes proto-nation to allow the ring to act as a more effective electron sink. In contrast, in alanine racemase, a less basic arginine residue in place of the aspartic acid is believed to favor racemization over transamination [12]. 

To explain the observed optical induction, a substrate was incorporated into the molecular model of the protein. A substrate such as a-ketoglutarate could be included in the protein model with a geometry that allowed stereoselective protonation of the quinoid intermediate by solvent, consistent with the enantiomeric excess (ee) of the 1-stereoisomer product. Moreover, the geometry consistent with production of the d-enantiomer appeared too sterically crowded for most substrates. However, pyruvic acid, which was the only substrate to favor the d-enantiomer product, was small enough to adopt the alternative geometry and also had the potential to interact with an arginine group. 

A possible reductive role for veratryl alcohol oxidase is proposed in Figure 5. Laccases from C. versicolor can produce both polymerization and depolymerization of lignin (29). In phenolic lignin model dimers, laccase can perform the same electron abstraction and subsequent bond cleavage as found for lignin peroxidase (30). The phenolic radical is however likely to polymerize unless the quinoid-type intermediates can be removed, for example by reduction back to the phenol. Veratryl alcohol oxidase, in... 

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