4- pyridinium chlorosulfite chloride

Carboxyl Activation Synthesis of Esters and Amides from Carboxylic Acids. 4-(Dimethylamino)pyridinium chlorosulfite chloride is more reactive than either thionyl chloride or thionyl chloride/pyridine for carboxyl activation. Aliphatic and aromatic, as well as amino acids (in racemic form), undergo activation (via the acyl halide) and subsequent esterification by reaction with an alcohol at — 20 °C (eq 1). The esterification step requires the addition of a second equivalent of DMAP and this method has been applied to a range of functionalized carboxylic acids. 

Eligh yields of esters are obtained from both acid-sensitive (cyanoacetic acid) and base-sensitive carboxylic acids (3-phenyl-propionic acid and trichloroacetic acid) and, in these latter cases, use of 4-(dimethylamino)pyridinium chlorosulfite chloride is much more effective than use of thionyl chloride alone. The esterification process has been claimed to be independent of the steric environment of the carboxyl function, though this reagent may be of more limited value with heavily substituted benzoic acids. Carboxyl activation, in the presence of a primary amine, leads to the corresponding amide in excellent yield (eq 2). In both the esterification and amidation processes and the oxime dehydration reaction discussed below, recovery of DMAP is straightforward. 

Dehydration of Aldoximes. A range of alkyl, aryl, and heteroaryl aldoximes undergo smooth dehydration with4-(dhnethyl-amino)pyridinium chlorosulfite chloride at —10 to +10 °C to give the corresponding nitriles in 70-100% yield (eq 3). ... 

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