Various types of condensing agents for esterification are known, and the amidinium type is one of these. For example, chloro-N,N,N',N'-tetramethylformamidinium hexafluorophosphate (1) reacts with 2-mercaptopyridine (2) to give the thiouronium salt 3. The compound 3 will easily react with carboxylic acids to give thioesters, like 4, in high yield.