Esterification methods

A great variety of cellulose organic esters other than those of present commercial importance have been prepared for laboratory study. Methods of preparation different from those in commercial use must be employed since anhydrides of acids above butyric do not readily esterify cellulose. The reaction of acid chlorides in the presence of pyridine has been widely used, and is usually considered to be a reliable preparative method. In most cases elevated temperatures are required, however, and the danger of side reactions which result in combined nitrogen and 

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The only other esterification method which rivals the present procedure in convenience, mildness of conditions, selectivity, and yield js the preparation of methyl esters wdth diazomethane [Methane, diazo-] 10 Esterification with trialkyloxonium salts, however, allows... 

Otera, J. (2003) Esterification Methods, Reactions and Applications, Wiley-VCH Verlag GmbH, Weinheim. 

S Kim, JL Lee, YC Kim. A simple and mild esterification method for carboxylic acids using the mixed carboxylic-carbonic anhydrides. J Org Chem 50, 560, 1985. 

Although a carboxylate anion is only a relatively modest nucleophile (see Section 6.1.2), it is possible to exploit an Sn2 reaction to prepare esters from carboxylic acids as an alternative to the usual esterification methods (see Section 7.9). Such methods might be useful, depending upon the nature and availability of starting materials. 

Especially for large-scale work, esters, may be more safely and efficiently prepared by reaction of carboxylate salts with alkyl halides or tosylates. Carboxylate anions are not very reactive nucleophiles so the best results are obtained in polar aprotic solvents54 or with crown ether catalysts.55 The reactivity for the salts is Na+ < K+ < Rb+ < Cs+. Cesium carboxylates are especially useful in polar aprotic solvents. The enhanced reactivity of the cesium salts is due both to high solubility and to the absence of ion pairing with the anion.56 Acetone has been found to be a good solvent for reaction of carboxylate anions with alkyl iodides.57 Cesium fluoride in DMF is another useful combination.58 Carboxylate alkylation procedures have been particularly advantageous for preparation of hindered esters that can be relatively difficult to prepare by the acid-catalyzed esterification method (Fischer esterification) which will be discussed in Section... 

While in CHEC(1984) <1984CHEC(2)1> there is an emphasis on decarboxylation reactions of pyridazinecarboxylic acids and the choice of esterification methods, in CHEC-II(1996) <1996CHEG-II(6)1> more examples of reactions with pyridazinecarboxylic esters appeared. More recently, the aminolysis of 1,2-diazinecarboxylic esters such as... 

Methyl cinnamate is a colorless crystalline solid mp 36.5 °C) with a fruity, sweet-balsamic odor. In addition to the common esterification methods, it can be prepared by Claisen condensation of benzaldehyde and methyl acetate in the presence of sodium. Methyl cinnamate is used in soap perfumes, as well as in blossom and oriental perfumes, and is sometimes added to aromas. 

The Huls esterification method later described is based on the following factors no acid catalysts are used for esterification instead, substances or substance mixtures which react in a neutral or alkaline way in the presence of water are used. Since strong inorganic acids with a dehydrating and oxidating effect are not a part of the esterification components, it is easier to obtain colorless esters with this method than with that usually employed. 

Standard solid-phase peptide synthesis requires the first (C-terminal) amino acid to be esterified with a polymeric alcohol. Partial racemization can occur during the esterification of N-protected amino acids with Wang resin or hydroxymethyl polystyrene [200,201]. /V-Fmoc amino acids are particularly problematic because the bases required to catalyze the acylation of alcohols can also lead to deprotection. A comparative study of various esterification methods for the attachment of Fmoc amino acids to Wang resin [202] showed that the highest loadings with minimal racemization can be achieved under Mitsunobu conditions or by activation with 2,6-dichloroben-zoyl chloride (Experimental Procedure 13.5). iV-Fmoc amino acid fluorides in the presence of DMAP also proved suitable for the racemization-free esterification of Wang resin (Entry 1, Table 13.13). The most extensive racemization was observed when DMF or THF was used as solvent, whereas little or no racemization occurred in toluene or DCM [203]. 

Olera, J. Esterification Methods. Reactions, and Applications. John Wiley Sons. Inc.. New York. NY. 2003... 

This ester was prepared from the optically active (S)-2-fluorohexanoic acid isolated above, by the esterification method described in this procedure. 

Examples of the use of these methods are given in Expts 5.148 and 5.149. The use of an acyl chloride or acid anhydride is the method of choice for the synthesis of phenyl esters (e.g. phenyl cinnamate see also Section 9.6.6), which cannot be prepared by the direct esterification methods described above. 

Aromatic esters may be prepared by direct esterification methods similar to those already described for aliphatic esters (Section 5.12.3, p. 695). A large range of examples of simple alkyl esters of aromatic carboxylic acids is included in Expt 6.163. Corresponding esterification of a simple aliphatic acid (e.g. acetic acid) with benzyl alcohol is illustrated in Expt 5.142. 

Chemical esterification methods use an alcohol and a carboxylic acid in the presence of a mineral acid as catalyst. Sulfuric acid, which is commonly used, leads to the formation of undesirable byproducts, requiring a difficult separation step (3). Moreover, in this case, the starting material is a high-value component (fatty acid). Consequently, researchers are interested in the alcoholysis reaction using a vegetable oil with low cost and largely produced in Brazil as a raw material for ester synthesis. 

GC RESULTS FOR FATTY ACIDS METHYL ESTERS FOLLOWING VARIOUS ESTERIFICATION METHODS [7]... 

The total contents of saponifiable and free acids in small biological samples were determined by MacGee and Allen [137]. A sample of tissue (3—5 mg) or serum (5—100 /xl) was hydrolysed with a lye, acidified and extracted with /7-hexane. Free acids were extracted from the /7-hexane into a small volume (5—10 ql) of trimethyl-(a,a,a-trifluoro-/ 7-tolyl)ammonium hydroxide. The quaternary salt was injected together with methyl propionate, and methyl esters produced by the pyrolysis were subjected to GC analysis (injection port temperature 240°C, column temperature 180°C, 10% EGSS-X). Comparison of this method with other esterification methods in Table 5.9 shows that it offers at least equally good results. 

Benzyl and p-nitrobenzyl esters are usually made by classical methods such as reaction of benzyl alcohol with an acid chloride in the presence of pyridine or with a carboxylic acid in the presence of a carbodiimide (see general esterification methods). We have already shown the conversion of the cesium salt of a carboxylic acid to its benzyl ester by reaction with benzyl bromide [Scheme 6.40] ... 

Esterification of cephalosporanic acids Usual esterification methods of these acids lead to mixtures of and A -esters. This undesired isomerization is prevented by conversion of the acid into the 3-nitro-2-pyridinethiol ester (2) by reaction with 1 and triphenylphosphine. This stable intermediate (obtainable in about 90% yield) reacts with various alcohols to form the desired esters in 60-80% yield (equation I). 

Racemization occurring during esterification of protected amino acids to hydroxymethyl polymers is a well-recognized problem.PP l This reaction is promoted by the use of DMAP as catalyst,P l and racemization is particularly pronounced in the case of cysteine.P By the use of optimized esterification methods racemization can be reduced, but not totally sup-pressed.P l When starting with halomethyl-functionalized resins, such as 2-chlorotrityl chloride resin,P l bromo-SASRIN resin,P l or 4-(bromomethyl)phenoxyacetamide res-in,P l racemization levels are significantly lowered. Alternatively, racemization is largely prevented by using the TDO esters without DMAP,f P°l or PyAOP in the presence of DIPEA at low temperature.P l... 

Many oil products are available that cannot be esterified directly using alkali catalysts as they have sufficient fatty acid content to neutralize an alkali catalyst, yet have high ester contents. An acid esterification step may be employed in advance of base catalyzed esterification. These pre-esterification methods need only provide sufficient acid and alcohol to esterify the fatty acids present (3, 68, 71), although... 

Much helpful information on starch esterification methods and on the probable properties of starch esters may be obtained by examining similar reactions and esters in the cellulose field. This is because both starch and cellulose consist of polymerized glucose units capable of esterification to the triester stage, and because the superior ability of the cellulose esters to form plastics, films, fibers, and lacquers has promoted an accumulation of information on cellulose esters which far exceeds the present knowledge of starch esters. 

In applying cellulose esterification methods to starch, it must be remembered that starch is much more easily hydrolyzed than cellulose. Many esterification procedures for cellulose are too drastic to be applied to starch unless milder reaction conditions are provided. 

One of the most convenient esterification methods developed earlier was based on the decarboxylation of unstable mixed carboxylic-carbonic anhydrides prepared by reaction of chloroformates with carboxylic acids (Ref. 38) according to scheme 43. 

A very mild esterification method involves reaction of mixed carboxylic-carbonic anhydrides derived from carboxylic acids and alkyl chloroformates in the presence of a catalytic amount of DMAP. ... 

The neutral phosphonate esters, D(EB)[(EB)P], D(4-MPe-2)[BP], D(4-MPe-2)[(iB)P] and D(4-MPe-2)[PP] were prepared by the Michaelis-Arbuzov Reaction in which alkyl halides were reacted with previously prepared trialkyl phosphites. The neutral phosphate, T(4-MPe-2)P, was prepared by a conventional esterification method in which 4-methyl-2-pentanol was reacted with POCI3 in the presence of pyridine. The temperature during the reaction was kept below 15°C to prevent disproportionation of the alkyl group. The neutral phosphinate ester, B[DBP], was prepared by esterification of dibutyl phosphorus oxychloride, (C Hg)2P0C1, in the presence of pyridine. 

Fischer-Speier esterification method. Esterification of acids by refluxing with excess alcohol in the presence of hydrochloric acid or other acid catalysts. 

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