N-alkyl-4-boronopyridinium halides versus boric acid as catalysts for the esterification of alpha-hydroxycarboxylic acids.

[reaction: see text] Boric acid is a highly effective catalyst for the dehydrative esterification reaction between equimolar mixtures of alpha-hydroxycarboxylic acids and alcohols. In contrast, N-methyl-4-boronopyridinium iodide (2a) is a more effective catalyst than boric acid for the similar esterification in excess alcohol. A heterogeneous catalyst, such as N-polystyrene-bound 4-boronopyridinium chloride, is also an effective catalyst and can be recovered by filtration.