Phenolic hydroxyl group Esterification

The esterification of the phenolic hydroxyl group in salicylic acid with acetic acid results not only in an... 

Properties. Vanillin is a colorless crystalline solid mp 82-83 °C) with a typical vanilla odor. Because it possesses aldehyde and hydroxyl substituents, it undergoes many reactions. Additional reactions are possible due to the reactivity of the aromatic nucleus. Vanillyl alcohol and 2-methoxy-4-methylphenol are obtained by catalytic hydrogenation vanillic acid derivatives are formed after oxidation and protection of the phenolic hydroxyl group. Since vanillin is a phenol aldehyde, it is stable to autoxidation and does not undergo the Cannizzarro reaction. Numerous derivatives can be prepared by etherification or esterification of the hydroxyl group and by aldol condensation at the aldehyde group. Several of these derivatives are intermediates, for example, in the synthesis of pharmaceuticals. 

The reactions of the phenolic hydroxyl group which have been included in this chapter comprise esterification with carboxylic acids and related reactants, with inorganic materials, with replacements producing nitrogen, sulphur and halogen-containing compounds, procedures for removal of the OH group and miscellaneous reactions. 

Diazomethane is not ideal for esterification of phenolic acids because the phenolic hydroxyl groups are also methylated, albeit at a slower rate, which may lead to mixtures of partially methylated products. However, esterification can usually be achieved without O-methyl ether formation by cooling the reaction mixture to below 0°C [54,55]. 

Salicylates are derivatives of 2-hydroxybenzoic acid (salicylic acid). They were discovered in 1838, following the extraction of salicylic acid from willow bark. Salicylic acid was used medicinally as the sodium salt but replaced therapeutically in the late 1800s by the acetylated derivative, acetylsalicylic acid (ASA), or aspirin (Figure 13.2). Therapeutie utility is enhanced by esterification of the phenolic (hydroxyl) group as in aspirin, and by substitution of a hydrophobic/lipophilic group at C-5 as in diflimisal. 

How the aliphatic monomers are incorporated into the suberin polymer is not known. Presumably, activated co-hydroxy acids and dicarboxylic acids are ester-ified to the hydroxyl groups as found in cutin biosynthesis. The long chain fatty alcohols might be incorporated into suberin via esterification with phenylpro-panoic acids such as ferulic acid, followed by peroxidase-catalyzed polymerization of the phenolic derivative. This suggestion is based on the finding that ferulic acid esters of very long chain fatty alcohols are frequently found in sub-erin-associated waxes. The recently cloned hydroxycinnamoyl-CoA tyramine N-(hydroxycinnamoyl) transferase [77] may produce a tyramide derivative of the phenolic compound that may then be incorporated into the polymer by a peroxidase. The glycerol triester composed of a fatty acid, caffeic acid and a>-hydroxy acid found in the suberin associated wax [40] may also be incorporated into the polymer by a peroxidase. 

Sequential Methylation Technique. A sequential methylation-extraction technique has been devised primarily to separate phenolics from aromatic and aliphatic acids. The procedure, outlined below, allows quantitative conversion of phenolic hydroxyls to methyl ethers in one step and esterification of carboxyl groups in another. 

An additional method of ACEC synthesis was based on hydroxybenzoic acids as the starting materials [32]. The synthesis starts from esterification of the acid with 3-cyclohexenyl-1-methyl alcohol. The following hydroxybenzoic acids were used 2-hydroxybenzoic (salicylic), 2,4-di-hydroxybenzoic ( 3-resorcylic) and 3,4,5-trihydroxybenzoic (gallic) acid. The phenolic hydroxyls in the cy-clohexenylmethyl esters were transformed in glycidyl ethers in the usual way (reaction with excess ECH followed by dehydrochlorination). Eventually, epoxidation with PAA was carried out (Scheme 51). The epoxide groups content in the ACECs thus prepared was above 85% of the calculated value. 

Organosolv lignin obtained with a mixed formic and acetic acids pulping technique [9] is bound to present a number of formylated and acetylated phenolic and aliphatic hydroxyl groups. The results in Table 1 confirm this by the evidence of esterification by formyl and acetyl groups mainly of the aliphatic but also of the phenolic... 

Most derivatizations for gas chromatography are esterifications or etherifications. For example, analytes containing carboxyl groups are often converted into methyl [156], [161], pentafluorobenzyl [255]-[258], or triethyl esters [259]-[261], Analytes containing acidic hydroxyl groups (phenols, chlorophenols, glycol ethers) or amino groups (e.g.. aniline) can be derivatized with such per-fluorinated compounds as heptafluorobutyric anhydride, pentafluoropropionic anhydride [169], or pentafluorobenzoyl chloride [262]. 

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