Thioester synthesis comprises transformations that form compounds of general structure R–C(=O)–S–R′, where sulfur replaces the alkoxy oxygen of an ordinary ester. The defining bond-forming event is acyl–sulfur bond formation, most directly achieved by reaction of a thiol or thiolate with an activated carboxylic-acid derivative. Carboxylic acids may first be converted into acyl chlorides, anhydrides, activated esters, or coupling-reagent intermediates; alternative catalytic routes include dehydrogenative coupling of alcohols with thiols and carbonylative processes ^[1]. Thioesters retain an acyl carbonyl and undergo nucleophilic acyl substitution because sulfur provides weaker resonance donation than oxygen. They are distinct from sulfides, which lack a carbonyl adjacent to sulfur, and from thiocarboxylic acids, R–C(=O)–SH. In chemoselective ligation, a peptide thioester undergoes transthioesterification and S-to-N acyl transfer, illustrating that the thioester is an activated acyl donor rather than simply a sulfur-substituted ether ^[2].