Benzylic hydroxyl groups

Condensation of normeperidine (81) with 3-chloropropan-l-ol affords the compound possessing the alcohol side chain (88). The hydroxyl is then converted to chlorine by means of thionyl chloride (89) displacement of the halogen by aniline yields pimino-dine (90). ° Condensation of the secondary amine, 81, with styrene oxide affords the alcohol, 91 removal of the benzyllic hydroxyl group by hydrogenolysis leads to pheneridlne (92). ... 

The treacheries inherent in naive attempts at pattern recognition are illustrated by the finding that ester known cetiedil, is said to be a peripheral vasodilator. Clemmen-sen reduction of Grignard product removes the superfluous benzylic hydroxyl group and esterification of the sodium salt of the resulting acid ) with 2- l-cycloheptylamino)ethyl chloride produces cetiedil (28). ... 

The anti stereospecificity of epoxidation by the peracid is interpreted as due to association of the reagent with the allylic hydroxyl group which directs the entering oxygen atom to the same face of the molecule. The stereospecificity of bromohydrin formation is explicable in terms of steric approach control involving initial attack of the bulky bromine atom on the face opposite to the benzylic hydroxyl group (7). 

The designations 1 and 2 for the diol epoxides indicate that the benzylic hydroxyl group and the epoxide oxygen are cis (syn) or trans (anti), respectively. 

In studies on the oxidation of lignin that had alternately been methylated at the p-hy-droxyl and benzylic hydroxyl groups, Leopold (1952) concluded that methylation caused the low yield of vanillin obtained in the oxidation. As mentioned in Section 6.6.3, the replacement of the OH group by OMe seriously impedes C-C bond cleavage in the water reaction medium. 

Generating tin hydride in situ by using a catalytic amount of tributyltin chloride and sodium cyanoborohydride [24] did not solve the problem. Although this method inherently produces the desired low concentration of n-BujSnH to promote cyclization over direct reduction of the starting material and reduces drastically the amount of tin residue, a displacement reaction between solvent and the benzylic hydroxyl group takes place. Eventually, the best method was the use of high dilution conditions at approximately 70 °C in benzene followed by the KF workup. 

It was felt that the reactivity of the benzylic hydroxyl group needed to be increased under the reduction conditions. Benzylic hydrogenolysis using a variety of transition metal catalysts has revealed this to be a... 

Manganese dioxide is a reagent that oxidizes only allylic or benzylic hydroxyl groups to ketones,... 

Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide structure. Several representative substitution reactions are illustrated in Fig. 1.5. At moderate temperatures ( 100°C) and under mildly alkaline conditions, benzylic hydroxyl groups in phenolic units are converted to thiols by reaction with bisulfide (Q, Fig. 1.5). At higher temperatures and alkalinities, e.g., under kraft pulping conditions, the mercaptide group undergoes a series of transformations in which the sulfur is ultimately eliminated. 

The homolytic oxidative reagent, nitrobenzene, either abstracts an electron from a hydroxide ion as reported by Ashby and coworkers (21,22) (Scheme 1 of Figure 3) or from the benzylic hydroxyl group (Scheme 2 of Figure 3). 

The mechanism proposed in Figure 3 can easily be applied to the oxidative depolymerization of the lignin macromolecule by nitrobenzene or copper(II) in aqueous alkali. A recent model of softwood lignin (24) contains 27% free benzylic hydroxyl groups and 12% aromatic ring-conjugated double bonds (stilbene... 

Alkaline hydrolysis of lignin increases the number of reactive benzylic hydroxyl groups and may also be important in further depolymerizing the lignin once the oxidative-cleavage reaction has occurred. The formation of a p-electron-withdrawing -CHO substituent on aryl lignin units should increase the rate of hydrolysis of the ether bonds (26). Hydrolysis also forms p-phenylate ions, which then protect the benzaldehyde from further reaction via the Cannizzaro reaction, as mentioned earlier. 

Alkylation. Figure 15 illustrates that individual hydroxyl groups of a phenolic p-O-4 dimer may be selectively methylated with proper agents. Methyla-tion with diazomethane is largely limited to the phenolic hydroxyl group plus nonenolizable carbonyl units [332,358]. The benzylic hydroxyl group can be specifically methylated with methanolic hydrochloric acid [359]. All the phenolic and aliphatic hydroxyl groups can be methylated with dimethyl sulfate in alkali [332]. 

Treatments of spruce MWL in methanol-dioxane solution containing p-toluenesulfonic acid (at 30°C) resulted in hydrolysis of the a-aryl ether units and methylation of the benzylic hydroxyl groups [360,361]. A complete... 

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