Reactions Steglich esterification

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. 

The Steglich Esterification is a mild reaction, which allows the conversion of sterically demanding and acid labile substrates. It s one of the convenient methods for the formation of terf-butyl esters because r-BuOH tends to form carbocations and isobutene after a subsequent elimination under the conditions employed in the Fischer Esterification. 

Steglich esterification of a carboxylic acid with 2,2,2-trichloroethanol using DCC-DMAP affords a mild and convenient method for preparing TCE esters [Scheme 6,88],203 More traditional methods involving the reaction of an acid chloride with trichloroethanol in the presence of pyridine have also been used. 

Fig. 7 Macrolactonizations mediated by (a) the Mitsunobu reaction (b) the Steglich esterification run in the presence of DMPA, which gives only an urea derivative and (c) high yields of lactones, which are obtained in the presence of a proton source (DMPA, HCl).

The second example is based on Steglich esterification, for which it was shown that the yield of esterification reaction using dicyclohexylcarbodiimide is greatly increased by the addition of 4-dimethylarninopyridine (DMPA). However, in the case of w-hydroxyacids under these reaction conditions, the N-acylurea derivative is formed almost exclusively (Fig. 7b), with almost no lactone formation. The reason for this failure appears to lie in the proton-transfer steps. By operating in the presence of a proton source (DMPA-HCl), the lactoniza-tion occurred in excellent yield (Fig. 7c). 

Organic Syntheses Based on Name Reactions STEGLICH-HASSNER Direct Esterification... 

Schreiner et al. also reported the use of catalyst 76 in conjunction with various carboxyhc acids for the enantioselective Steglich esterification of trows-cycloalkane-1,2-diols [73]. The reaction, which proceeds through the in situ formation of an anhydride, affords high levels of selectivity (s ranging from 6 to >50) and prevents the use of unstable or difficult to access anhydrides (Scheme 41.25). 

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