Acetylation phenolic hydroxyl groups

The phenolic hydroxyl group of tyrosine, the imidazole moiety of histidine, and the amide groups of asparagine and glutamine are often not protected in peptide synthesis, since it is usually unnecessary. The protection of the hydroxyl group in serine and threonine (O-acetylation or O-benzylation) is not needed in the azide condensation procedure but may become important when other activation methods are used. 

The best known aryl ester is O acetylsalicylic acid better known as aspirin It is pre pared by acetylation of the phenolic hydroxyl group of salicylic acid... 

Acetic anhydride is very often used to introduce the acetyl group into an alcoholic or phenolic hydroxyl group or into a derivative of ammonia HNRR The reaction is accelerated to an extraordinary extent by the presence of a drop of concentrated sulphuric acid. 

Salicylic acid is manufactured on a large scale. In the dye industry it serves for the production of valuable azo-dyes which exhibit great fastness. To some extent these dyes are applied to mordanted fibres. In addition, the acid and its derivatives are widely used in pharmacy. Being a phenolcarboxylic acid it has a powerful disinfecting action (preservative). It has further proved itself an important antirheumatic and an analgetic. The derivative in which the phenolic hydroxyl group is acetylated (aspirin) has become especially popular. The first medicament of the series was the phenyl ester of salicylic acid, salol, which is produced as a by-product in the technical process. The preparation of salicylaldehyde has been described above (p. 235). 

That vomicine contains a phenolic hydroxyl group is not immediately obvious, for the alkaloid is insoluble in alcoholic alkali, does not give a ferric chloride color, and is not easily methylated or acetylated on the phenolic oxygen (187). Later work led to the isolation of what may be O-methylvomicine in small yield (191) and to O-acetylvomicine by the action of acetic anhydride sodium acetate (192). The presence of a phenolic grouping is evident, however, in vomicinic acid (CCXXXVIII ... 

Chemical methods for analysis of phenolic hydroxyl groups include determination of the increase in methoxyl content resulting from diazomethane methylation (Bjorkman and Person 1957), the increase in phenolic acetyl group content after acetylation (Lenz 1968, Mansson 1983), and low-molecular weight compounds derived from the degradation of phenolic structures (Chap. 5.2). The phenolic acetyl group of acetylated lignin may also be determined by an NMR spectroscopic technique (Lenz 1968, Robert et al. 1986) or by a selective deacetylation in pyrrolidine (aminolysis) (Mansson 1983). 

Aminolysis is a laborious procedure, e.g., for wood samples, a 6-7-h reaction period may be required for the aminolysis step alone. The accuracy of this method is critically dependent upon both a quantitative acetylation of phenolic hydroxyl groups and a selective deacetylation of phenolic acetyl groups. Although these requirements may not represent a serious concern in the analysis of soluble or reagent-accessible lignin preparations, they could present a problem in the case of lignocellulosic materials. 

In addition, the reducing end groups of carbohydrates behave like phenolic hydroxyl groups in the aminolysis method. Thus, a quantitative reduction of wood or pulp samples by borohydride prior to acetylation is essential. 

Note 4. See Chapter 7.1.2.1 for alternative methods. O-Acetylation modifies both aliphatic and phenolic hydroxyl groups. In the event subsequent O-methylation of carboxylic acid groups by ethereal diazomethane is required, it is essential to carry out the O-acetylation of phenolic hydroxyl groups as completely as possible as unacetylated phenolic hydroxyl groups also undergo O-methylation with diazomethane. 

The first clue to the mode of connection of the two cinnamic acid residues with the spermine moiety was given by analyzing the MS of 175 resulting from acetylation of a partial hydrolysis product of 173. Because of the formation of the peak triades mentioned at the beginning of Section III, the cinnamic acid residue that carries a free phenolic hydroxyl group (after... 

Acetylation of phenols and anilines. These substrates can be acetylated rapidly and in generally high yield by reaction with acetyl chloride using powdered NaOH and tetrabutylammonium hydrogen sulfate in an organic solvent (CH2CI2, dioxane, THE). Even 2,6-di-f-butyI-p-cresol can be acetylated in this way in 72% yield. Selective acylation of the phenolic hydroxyl group of estradiol is possible. ... 

On treatment of 11 with palladium, an interesting disproportionation reaction takes place which yields the colorless indole derivative 14 (27,28). Reaction of betanidin with acetyl chloride in trifluoroacetic acid afforded di-C>-acetyl-betanidin (13), which could be hydrolyzed back to 1 with concentrated hydrochloric acid (5). This demonstrated the presence of two phenolic hydroxyl groups in betanidin, and from consideration of the H-NMR data formula 1 was assigned to betanidin. 

G-Acetyldihydromorphimethine can be hydrogenated to 6-acetyl-totrahydromorphimethine [lxxi, R = Ac], which is insoluble in alkali despite having a free phenolic hydroxyl group, [lxxi, R = Ac] can also bo prepared by tho domothylation of a-totrahydrooodeimethine. It can... 

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