Esterification carboxylic esters

When the related saccharin derived sultam (R)-29 is converted into the (Z)-boron enolate and subsequently treated with aldehydes,. vy -diastereomers 30 result almost exclusively. Thus, the diasteromeric ratios, defined as the ratio of the major product to the sum of all other stereoisomers, surpass 99 1. Hydroperoxide assisted saponification followed by esterification provides carboxylic esters 31 with recovery of sultam 32106a. 

This chapter covers not only nuclear and extranuclear quinoxahnecarboxylic acids (and anhydrides) but also the carboxylic esters, acyl halides, carboxamides, carbohydrazides, carbonitriles, carbaldehydes, and (ketonic) acyl derivatives of quinoxaline a few related speceis are also included. To avoid repetition, the interconversions of these quinoxaline derivatives are discussed only at the first opportunity thus the esterification of quinoxalinecarboxylic acids in covered as a reaction of carboxylic acids rather than as a preparative route to carboxylic esters, simply because the section on carboxylic acids precedes that on carboxylic esters. To minimize any confusion, appropriate cross-references have been inserted. 

Carboxylic Esters and of Esterification of Carboxylic Acids. Acid Hydrolysis of an Ester with Heavy Oxygen as Isotopic Indicator. J. chem. Soc. [London] 1939, 838. 

In 2006, Zeitler demonstrated the use of alkynyl aldehydes in redox esterification [114]. As in previous examples, the author proposes the formation of an activated carboxylate that acts as an acylating agent Eq. 19. A variety of a,P-unsaturated carboxylic esters 199 are formed in moderate yields with E-selectivity up to >95 5. 

Yield ketones after oxidation with Mn02 Yield methyl esters after esterification carboxylic acids... 

EtjN,663 and N,N -carbonyldiimidazole (100).664 In the latter case easily alcoholyzed im-idazolides (101) are intermediates. BF3 promotes the esterification by converting the acid to RCO+ BF3OH, so the reaction proceeds by an AacI type of mechanism. The use of BF etherate is simple and gives high yields.665 Carboxylic esters can also be prepared by treating carboxylic acids with /-butyl ethers and acid catalysts.666... 

From Boron Halides. Using boron halides is not economically desirable because boron halides are made from boric acid. However, this method does provide a convenient laboratory synthesis of boric acid esters. The esterification of boron halides with alcohol is analogous to the classical conversion of carboxylic acid halides to carboxylic esters. Simple mixing of the reactants at room temperature or below in a solvent such as methylene chloride, chloroform, pentane, etc, yields hydrogen halide and the borate in high yield. 

ESTERIFICATION. This article describes methods for the production ol carboxylic esters ... 

Alternative reactions employ coupling reagents such as DCC (Steglich Esterification), preformed esters (transesterification), carboxylic acid chlorides or anhydrides. These reactions avoid the production of water. Another pathway for the production of esters is the formation of a carboxylate anion, which then reacts as a nucleophile with an electrophile (similar reactions can be found here). Esters may also be produced by oxidations, namely by the Baeyer-Villiger oxidation and oxidative esterifications. 

Primary, secondary, and tertiary carboxylic amides, carboxylic esters, and carboxylic acids are protonated by mineral acids or sulfonic acids at the carboxyl oxygen to a small extent (Figure 6.9). This corresponds to the activation discussed in Section 6.2.3. This activation is used in acid hydrolyses of amides and esters, in esterifications of carboxylic acids and in Friedel-Crafts acylations of aromatic compounds with carboxylic acids. 

Fig. 6. 22. Aa[2 mechanism of the acid-catalyzed hydrolysis of carboxylic esters (read from left to right) Aa[2 mechanism of the Fischer esterification of carboxylic acids (read from right to left). H means migration of a proton.

The Aac2 mechanism (Figure 6.19) of ester hydrolysis represents an SN reaction at the carboxyl carbon, which accurately follows the general mechanism of Figure 6.5. Acid-catalyzed hydrolyses of carboxylic esters that are derived from primary or from secondary alcohols take place according to the Aac2 mechanism. The reverse reactions of these hydrolyses follow the same mechanism, namely, the acid-catalyzed esterifications of carboxylic acids with methanol, with primary or with secondary alcohols. In the esterifications, the same intermediates as during hydrolysis are formed but in the opposite order. 

To an organic chemist, the term ester normally means an ester of a carboxylic acid, unless some other kind of ester is specified. Replacing the —OH group of a carboxylic acid with the —OR group of an alcohol gives a carboxylic ester. The following condensation, called the Fischer esterification, shows the relationship between the alcohol and the acid on the left and the ester and water on the right. 

Trans) esterification Alcohol/carboxylic acid/carboxylic ester... 

Polyesters can be prepared by the direct esterification of carboxyl groups with alcoholic hydroxyl groups in the same or different compounds. Similarly, polyesters can be prepared from the reaction of carboxylic esters and alcoholic hydroxyl groups in the same or different... 

Unlike carboxylic esters, open-chained S-alkyl thiocarboxylates cannot be obtained by direct proton-catalyzed esterification. Whereas thiocarboxylic acids react with alcohols to form esters as expected, thiol esters are formed on reaction of carboxylic acids with thiols, but the equilibrium is shifted towards the educts and the reaction is not useful for synthetic purposes. Therefore, activation is necessary and several methods have been developed to meet this requirement. 

Starting from ethyl 4, 4, 4-trifluorocrotonate, racemic aziridine carboxylic acid 96 was prepared as shown in Scheme 9.22 [46] and was then subjected to lipase-catalyzed esterification. Methyl ester 97 was obtained in 35% yield with excellent enantiomeric purity. Acid-catalyzed ring opening of aziridine 97 proceeded regio- and stereoselectively, affording 2-substituted (2R,3R) or (2/ ,3S)-3-amino-4,4,4-trifluorobutanoates 98 in high yields [47]. 

Esterification. Carboxylic acids react with alkyl (-butyl ethers in the presence of catalytic amounts of sulfuric acid or p-toluenesulfonic acid to form esters.1 Yields are generally good to excellent. The reaction is carried out by heating the reactants either under reflux or on a steam bath for several minutes until the evolution of isobutylene ceases. 

The colored acid chloride was developed by Butenandt s group for use in ah investigation of the sex attractant principle secreted by the female silkworm moth, bombykol, which indeed was isolated as the 4 -nitroazobenzene-4-carboxylic ester (2). Twelve milligrams of pure ester, m.p. 95-96°, was isolated from 500,000 pairs of scent glands. Oxidation of 1 mg. of this ester with permanganate in acetone, separation of the colored fragment, and esterification with diazomethane gave an... 

Esterification. Carboxylic acids are converted into their ethyl esters when heated with an excess of the reagent. Even hindered acids such as 2,4,6-trimethylbenzoic acid are esterified. An acid catalyst usually is not necessary and may be deleterious however, esterification of nicotinic acid and of hippuric acid was accomplished with p-toluenesulfonic acid in dimethylformamide. 

In addition to hydrolyzing carboxylic ester bonds, lipases can catalyze a variety of esterification reactions in non-aqueous media. Industrial applications of lipases have focused on their enantioselectivity, regioselectively and substrate... 

Fischer esterification works well to prepare simple carboxylic esters. The diazomethane method would also react with the phenol, making the phenyl ether. 

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