Derivatization by Alcoholysis of the Intermediate Mixed Anhydrides

Haloalkyl anhydrides, especially the perfluorinated ones of acetic, propionic and butyric acids, are able to promote condensation reactions with an alcohol (see Table 1 for the reagents and abbreviations), when in a molar excess over the alcohol. TCE, TFE or HFIP in combination with TFA or HFBA have often been used to attain higher ECD response. Heating is usually employed without the catalytic presence of an organic base. This approach will be mentioned again in the next section.

At the beginning of the 1990s, chloroformates with the simplest alkyl groups, i.e. methyl and ethyl chloroformate (MCF and ECF), were shown to act as exceptionally rapid esterification reagents. The catalytic presence of pyridine has been found to be a prerequisite for ester formation; water can be, or for some applications should be, a constituent of the medium, together with an alcohol and commonly acetonitrile also. This promising treatment has a broader utility and will be discussed further. The reaction mechanism is based, as in the first case, on alcoholysis of a mixed anhydride intermediate. The advantageous use of this approach in comparison with the former can be seen from Table 2.

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