Esterification with trichloroethanol

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. 

Boc-(5-HSer(Bzl)-OTce Typical Procedure for Esterification with 2,2,2-Trichloroethanol P l... 

Mierzwa and Witek (14) found that after esterification of 2,4-D and MCPA with 20% 2,2,2-trichloroethanol in trifluoroacetic anhydride in the presence of SO a lower limit of 0.096 ppb 2,4-D and 0.06 ppb MCPA in 1 litre samples of water was obtained. 

Chlorophenoxy acids are chromatographed after esterification. Methyl esters [498] and propyl esters [499] prepared by transesterification from methyl esters by heating for 5 min with n-propanol and sulphuric acid are often used. For a very sensitive analysis, Mierzwa and Witek [500] proposed the following procedure. They esterified acids with 20% of 2,2,2-trichloroethanol in TFA anhydride in the presence of sulphuric acid by heating at 100°C for 15 min (or several hours at room temperature) and analysed the derivatives on a column packed with 15% of QF-1 plus 10% of DC-200 (1 1) at 195°C. Trichloroethyl esters permit down to hundredths of 1 ppb of chlorophenoxy acids to be... 

The silica gel may adsorb the esters of the short-chain fatty acids. For the esterification of those acids, a chloroform solution of the Cj—C aliphatic acids (50/rl) is treated with an equal volume of a freshly prepared 1% solution of 2,2,2-trichloroethanol in chloroform, together with 150 /A of heptafluorobutyric anhydride (HFBA) in a closed tube at room temperature for 30 minutes. Excess trichloroethanol is removed by reaction with 50 1 of a 25% solution of palmitic acid in chloroform, added subsequently and left to react at room temperature for 15 minutes. (The trichloroethyl palmitate p>eak comes off the GC column long after the shorter-chain fatty acid ester peaks). Further purification is achieved by shaking with 100 /il of chloroform and 100 1 of 0.1 M HCl the aqueous layer is discarded, and the organic layer is washed with 100 fi of 0.1 M NaOH and evaporated to dryness. The residue is dissolved in 100 fil of diethyl ether for analysis by GC with electron capture detection [46]. 

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