Light carboxylic acid protection

Ester formation is the main and most efficient means of protecting carboxylic acids. The protection of a carboxylic acid as an amide is infrequent, as its removal normally requires drastic conditions. Most of the work concerned with the use of light-sensitive protecting-groups for carboxylic acids is in peptide synthesis. Carboxylic acids are protected as photosensitive esters, including ester linkages to polymer supports or as (difficult to prepare) photosensitive amides. Many of these techniques may be readily applied to sugar acids. 

In order to develop a material with these properties, the well-documented thermal characteristics of the aromatic polyamides were combined with the sensitivity of a photolabile protecting group. In 1973, Amit and Patchornik reported that N-substituted-ortho-nitroanilides are light-sensitive and undergo a photoinduced rearrangement to produce the corresponding carboxylic acid in excellent yield. (4)... 

The HTPs were then obtained by neutralization of the polymers in solution with highly reactive metal alkoxides or alkyl metals under strictly anhydrous conditions, following a procedure described in detail elsewhere (6,7). When the very rapid reaction is completed, the polymer is dried under vacuum to constant weight. Quantitative neutralization of the carboxylic acid groups can be achieved as evidenced by infrared spectroscopy. Once in the bulk state, all samples were kept under atmospheric conditions and protected from light. 

Diastereoselective [2+2] photocycloaddition of a polymer-supported cyclic chiral enone with ethylene has been reported (Scheme 12.33) [43]. The auxiliary was derived from (-)-8-(p-methoxyphenyl) menthol (87). Protection of the secondary alcohol and demethylation were carried out to give (-)-8-(p-hydroxyphenyl)menthyl acetate (88). An alkyl linker was introduced and finally loaded to poly (ethylene glycol) grafted Wang resin. Deprotection of the alcohol functionality was followed by esterification with cyclohexen-3-one-l-carboxylic acid to provide the chiral enone 89. The photochemical reaction with ethylene was performed by irradiating with light (k > 280 nm). Trifluoroacetic acid (TFA) or aqueous hydrolysis with... 

Amino carboxylic acids or their hydrochlorides, when treated with PC15 in acetyl chloride1089 or, better, in CC14 (which dissolves PC15)1090 give the amino acid chloride hydrochlorides. Since these salts are very sensitive to moisture, the reaction must be carried out in a closed apparatus and the product must be collected on a protected frit and finally washed with a suitable solvent (light petroleum or CC14) and sucked dry. 

Reagent for characterising and protecting carboxylic acids. Light-yellow cryst. Mp 75-76°. 

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