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Please login or sign up for a free trial to access the full content. Variation 1: By Brønsted Acids

DOI: 10.1055/sos-SD-020-00774

Zhang, M.; Hanson, P. R.Science of Synthesis, (200720927.

Brønsted acids are classical acid catalysts for transesterification and are still employed quite often (Scheme 82). Amongst the most popular acids are hydrogen chloride, sulfuric acid, and 4-toluenesulfonic acid. Other acids that have been utilized are hydrogen bromide, hydrogen iodide, hydrogen fluoride, acetic acid, trifluoromethanesulfonic acid, camphorsulfonic acid, tetrafluoroboric acid, and perchloric acid. Transesterification must be performed under anhydrous acidic conditions, because the ester is hydrolyzed otherwise. Therefore, in situ generation is required for acids that are not readily available in the anhydrous form. For instance, in situ generation of hydrogen chloride through the reaction between acetyl chloride and an alcohol is used for transesterification of propanol and methyl esters,[‌343‌] and hydrogen bromide is prepared from the reaction between triphenylphosphine and carbon tetrabromide for the transesterification of primary alcohols with ethyl acetate, e.g. to give 243 (Scheme 82).[‌344‌]

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