Phenols formation pathways

The importance of phenol formation by the proposed pathway was probed by irradiating l,3-diphenoxy-2-methyl-2-propanol (5) under the same conditions. Compared to 3, the rate of phenol formation was approximately 2 times slower. Since the 11-transfer step in Scheme II is not available to 5, the results provide support for the scheme as an important, but not sole, pathway for phenol formation. Irradiation of and 5 with an air purge resulted in faster rates of phenol formation (ca. 5-fold) relative to N2. These findings parallel the accelerated fluorescence intensity loss from polymer 1 films in air as compared to the results in vacuo (see Table I). 

HRP catalyzes the oxidative dehydrogenation of a wide range of electron-rich aromatic compounds. The result of this radical formation pathway is dimerization and subsequent oligomerization of the substrates [76-78]. Peroxidases have been used to catalyze polymerizations of phenols (e.g. p-cresol and guaiacol) and aromatic amines (e.g. aniline, and o-phenyldiamine) [79, 80]. N- and O-dealkylations are also useful electron transfer reactions catalyzed by peroxidases. These reactions are used in industrial wastewater treatment and may have synthetic applications [81]. 

Group 3 (Tressl et al., Berlin) identified mainly furans, pyrroles, phenols and sulfur-containing compounds, emphasizing particularly their formation pathways. 

A scheme for the formation of guaiacols from ferulic acid has also been proposed by Manley et al. (1974). The biosynthesis of various phenolic acids from p-coumaric acid (H.84) was studied by Friedrich (1976). Formation pathways for simple phenols in food flavors have been reviewed (Maga, 1978a). The two primary pathways could be the decarboxylation of phenolic carboxylic acids and the thermal degradation of lignin. Secondary pathways include bacterial, fungal, yeast enzymic and glycosidic reactions. 

The production of toluene 1,2-epoxide (C) and 2-methy oxepin (D) by the pathway (b) was proposed by Klotz et al. (2000). Their reaction rates with OH was found to be fast experimentally and theoretically, and they are thought to be one of the formation pathways to the open-ring compounds described below (Cartas-Rosado and Castro 2007). In the case of benzene, it has been reported that phenol is formed from the photolysis of benzene oxide and oxepin (Klotz et al. 1997), but the cresols are not formed from toluene 1.2-epoxide or 2-methyl oxepin (Klotz et al. 2000). 

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

Lim et al. also investigated HMTA-phenolic reactions with somewhat larger model compounds (e.g., two- and four-ring compounds) and established that similar reaction pathways to those described previously occurred.50 For these model compounds (as opposed to one-ring model compounds), which are more representative of typical oligomeric systems, increased molecular weight favored die formation of hydroxybenzylamines but not benzoxazines. This was suggested to be a steric effect. 

The metabolism of phenols under anaerobic conditions has been examined under denitrifying, sulfate-reducing, Fe (lll)-reducing, and anaerobic nonmethanogenic conditions. It is plausible to suggest a common pathway that has been elucidated for denitrifying bacteria. This comprises (a) activation of phenol by the formation of phenylphosphate, (b) carboxylation at a position para to... 

Nitrophenols are phytotoxic, and dinoseb (6-iec-butyl-2,4-dinitrophenol) has been used as a herbicide, while nitrophenols have been detected in rainwater and plausible mechanisms for their abiotic formation have been proposed (Kohler and Heeb 2003 Vione et al. 2005). The pathway for the degradation of phenols with a single nitro group depends on the position of the substituents, while... 

On the basis of the previous data, possible reaction pathways may be proposed (Fig. 39.6). To confirm this hypothesis, the key intermediate 2-(ethoxymethoxy)phenol (2-EMP) was synthesized and fed, alone or together with water (Table 39.7). As expected, MDB was formed up to 47.0%, together with a smaller amount of 3-MC, confirming that 2-EMP is intermediate for both the above prodncts. Worthy of note is the formation of significant amounts of PYC, showing that the formation of 2-EMP is at equilibrinm with starting reagents. Lastly, the presence of water not only increased the amount of PYC, but also aided the formation of 2-MP. 

Chain cleavage with subsequent formation of phenolic products, rather than the photo-Fries rearrangement to form salicylates and dihydroxybenxophenones, has been identified as the major initial degradation pathway of PC exposed to natural weathering conditions. 

Irradiation of at longer wavelengths (>280 nm) provided phenyl formate (6) as a major volatile product, together with minor amounts of phenol and phenoxyacetone (4), as well as other products. A possible pathway for formation of phenyl formate by oxidation and subsequent cleavage is provided in Scheme III. Phenoxyacetic acid (7) was also identified as a minor product by mass-gc analysis. Photolysis of phenoxyacetone ( ) and phenoxyacetic acid (7)12 yields phenol together with photo-Fries products (also shown in Scheme III). 

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