Phenol precursors

Oxidative addition of a silyl-protected 4-(bromomethyl)phenol precursor to (tme-da)Pd(II)Me2 (tmeda = tetramethylethylenediamine), followed by ethane reductive elimination, resulted in formation of the benzylic complex 16 (Scheme 3.10). Exchange of tmeda for a diphosphine ligand (which is better suited for stabilizing the ultimate Pd(0) QM complex), followed by removal of the protecting silyl group with fluoride anion, resulted in the expected p-QM Pd(0) complex, 17, via intermediacy of the zwitterionic Pd(II) benzyl complex. In this way a stable complex of p-BHT-QM, 17b, the very important metabolite of the widely used food antioxidant BHT20 (BHT = butylated hydroxytoluene) was prepared. Similarly, a Pd(0) complex of the elusive, simplest /)-QM, 17a, was obtained (Scheme 3.10). 

C=0) is linked to both an alkyl group (or hydrogen atom) and a hydroxyl group. The medicinal activity of salicylic acid suggests that it is this phenolic precursor (phenol, Fig. 13.4.3), not acetylsalicylic acid (Fig. 13.4.4), that is the prime pain... 

Scheme 3)10. Indeed, independent photolysis of 2,4-cyclohexadien-l-ones 12 and 13 afforded the macrolides 15. These reactions likely proceed via a common intermediate, in this case dienylketene 14, which is trapped intramolecularly by the pendant hydroxyl group. Adjustment of the oxidation level and functional group interconversion then led efficiently to the desired macrolide 17. The sulfonyl group was used for two reasons first, to easily transform lactones 15 into dienyl lactones 16 needed for 17, and secondly, to control the regiochemistry of the Wessely oxidation of phenolic precursor needed to produce the photolysis substrates 12 and 13. 

Loubinoux, B., Gerardin, P., Kilbertus, G. and Miazimbakana, J. (1992). Reaction of wood and cellulose with quinone methides prepared in situ from phenolic precursors. Holzforschung, 46(2), 175-176. 

The application of radioactive phenolic precursors—quinic acid and shikimic acid (52), phenylalanine (30,53), tyrosine (53), and cinnamic acid (30,31,53)—to infected wheat leaves led to a solvent- and alkali-resistant incorporation of radioactivity into hypersensitively reacting host cells suggesting lignin formation had occurred. 

Lille, U., Heinmaa, I., Muurisepp, M. Pehk, T. 2002. Investigation of kukersite structure using NMR and oxidative cleavage On the nature of phenolic precursors in the kerogen of Estonian kukersite. Oil Shale, 19, 101-116. 

The sulfation of phenol and the glucuronidation of its hydroquinone metabolite were measured in human liver cytosols and microsomes, respectively. The rate of phenol sulfation varied between 0.31 and 0.92 nmol/mg protein/min this is slightly higher than the rate for mice (0.46) and lower than that for rats (1.20). The rate of hydroquinone glucuronidation was between 0.10 and 0.28 mnol/mg protein/min, slightly higher than that for rats (0.08) and lower than that for mice (0.22). These enzyme-kinetic data were subsequently used to simulate phenol metabolism in mice, rats and humans in vivo, using a com-partmental pharmacokinetic model with benzene as phenol precursor (Seaton et al., 1995). 

Benzoic acid is an important chemical intermediate which can also be used as a phenol precursor by decarbonylation in the presence of copper catalysts (Lummus process). It is produced industrially by oxidation of toluene by air in the presence of cobalt catalysts (Dow and Amoco processes equation 240). The reaction can be carried out without solvent, or in an acetic acid solvent. The oxidation of toluene without solvent uses a cobalt octoate catalyst and operates at higher temperature (180-200 CC). Yields of benzoic acid are about 80% for ca. 50% toluene conversion.361 In an acetic acid solution and in the presence of cobalt acetate, the reaction occurs at lower temperature conditions (110-120 °C) and gives higher yields in benzoic acid (90%).83,84... 

Several general synthetic methods have been developed that utilize the novel reactivity of quinone methides generated by oxidation of phenol precursors. Angle and Ranier have generated 2,6-dialkyl or 2,6-dialkoxy-substituted / -quinone methides 51 by oxidation of the corresponding phenols with Ag20. 

The efficiency of HRP for the coupling of iodotyrosine derivatives has also been studied by Eickhoff) who extended the chemistry to include the formation of biaryl linkages between small peptides in low yields (5-6 %) [30]. Biaryl-containing peptide libraries were also prepared by HRP oxidative coupling of appropriate phenolic precursors. 

In parallel studies to the ruthenium-based work, Ward and co-workers have also shown that PIFA (Scheme 44) [122, 128] and 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) (Scheme 45) [122, 129, 130] are effective reagents for coupling the phenolic precursors 185a,b. Yields are generally lower than in the fully methylated case (spirodienone products 188 can be formed when a hydroxyl group is present at the para position of the 2-benzyl group of 185a) and unbiased mixtures of atropisomers 186/187 and 189/190 are obtained. 

Scheme 51. Di-tert-butyl peroxide-mediated oxidative dimerization of a chiral phenol precursor to the magistophorenes.

Many natural products display structural motifs biosynthetically derived from ortho-quinol precursors, and some even feature ortho-quinol moieties in their final structural arrangement [1, 6]. Asatone (7) and related neolignans can be put forward as classic examples of complex natural products derived from cyclodimerization of oxidatively activated simple phenol precursors (Figure 5) biomimetic syntheses of 7 have accordingly been accomplished by anodic oxidation (Section 15.2.1) and by Pelter oxidation (Section 15.2.2) of the naturally occurring phenol 9 [34, 36]. 

Studies carried out by Bernal et al. (1995) on the ability of hot pepper (C. annuum) peroxidase and the phenolic precursors of capsaicin biosynthesis showed that hot pepper peroxidase, and especially hot pepper peroxidase iso enzyme B6 (Prx B6), was capable... 

Bernal, M.A., Calderon, A.A., Ferrer, M.A., Merino de Caceres, F. and Barcelo, R.A. (1 995) Oxidation of capsaicin and capsaicin phenolic precursors by the basic peroxidase isoenzyme B6 from hot pepper. Journal of Agricultural and Food Chemistry 43(2), 352-355. 

Plant phenolics represent a very large group of defensive compounds defined here as having a phenol (hydroxybenzene) moiety. In some instances substances having a phenolic precursor (e.g. methoxybenzene derivatives) have conveniently also been included in this category. Phenolics derive biosynthetically from hydroxycinnamoyl coenzyme A (yielding a phenyl-propanoid moiety). 

It was not long before the first activated carbon fibres (ACFs) were developed. In the work of Economy and Lin (1971, 1976) highly porous carbon fibres were prepared from Kynol, a fibrous phenolic precursor. Carbonization was carried out in nitrogen at 800°C and activation occurred in steam at 750-1000°C. The products appeared to be predominantly microporous and were found to be effective for the removal of low levels of certain pollutants (e.g. phenol and pesticides) from air or aqueous solutions. 

The thermolytic deprotection reaction is extremely clean. The infrared spectrum of the deprotected polymer is identical to that of the phenolic precursor as is the ultraviolet spectrum. The molecular weight of the phenolic copolymer precursor is unchanged by the t-BOC protection reaction/thermolysis cycle based on GPC data. 

Technical chlorophenol formulations are the major sources of PCDEs in the environment [4,40,45,46]. Hydroxy chlorodiphenyl ethers (OH-PCDE), also called polychlorinated phenoxyphenols (PCPP), are the main impurities of chlorophenols [39,47,48]. 2-OH-PCDEs are called predioxins, since they are phenolic precursors to polychlorinated dibenzo-p-dioxins (PCDD) [47]. The levels of 2-hydroxy-nonaCDEs have varied between 0.6 mg kg-1 and 1100 mg kg-1 in commercial pentachlorophenol preparations (sodium salts) [47]. 3,4,5,6-tetrachloro-2-(2,3,4,5,6-pentachlorophenoxy)phenol) is the main impurity of commercial pentachlorophenols [48]. 

Representatives of the 215 system were obtained from phenolic precursors and Fremy s salt or CAN (79ZN(B)624 83ZN(B)392). They undergo a... 

Compounds belonging to systems 224, 225, 226, and 227 are known. Compound 224 was prepared from the phenolic precursor with nitric acid (27LA172) dichromate oxidation of the dihydroxy precursor yielded 225 (37LA38). Quinone 226 was obtained from the 4-chloro- or -bromo-5-hydroxy... 

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