Thioesters are compounds with the functional group C-S-CO-C. They are the product of esterification between a carboxylic acid and a thiol. Thioesters are widespread in biochemistry, the best-known derivative being acetyl-CoA.
Contents |
Thioesters have been prepared in many ways,[1] but the main route involves condensation of thiols and carboxylic acids in the presence of dehydrating agents:[2]
A typical dehydration agent is DCC. Acid anhydrides and some lactones also react with thiols in the presence of base.
They also arise via carbonylation of alkynes and alkenes in the presence of thiols.[3]
The carbonyl center in thioesters is reactive toward nucleophiles, the reactivity being reminiscent of but milder than acid chlorides. Thus, thioesters and amines combine to give amides:
The C-H groups adjacent to the carbonyl in thioesters are mildly acidic and undergo aldol condensations. This kind of reaction occurs in the biosynthesis of fatty acids.
Thioesters are common intermediates in many biosynthetic reactions, including the formation and degradation of fatty acids and mevalonate, precursor to steroids. The biosynthesis of lignin, which comprises a large fraction of biomass, proceeds via the thioester derivative of caffeic acid.[4] Examples include malonyl-CoA, acetoacetyl-CoA, propionyl-CoA, cinnamoyl-CoA. Acetogenesis proceeds via the formation of acetyl-CoA. These thioesters arise analogously to those prepared synthetically, the difference being that the dehydration agent is ATP. In addition, thioesters play an important role in the tagging of proteins with ubiquitin, which tags the protein for degradation.
As posited in a "Thioester World," thioesters are possible precursors to life.[5] As de Duve explains:
Thionoesters are isomeric with thioesters. In a thionoester, sulfur replaces the carbonyl oxygen in an ester. Methyl thionobenzoate is C6H5C(S)OCH3. Such compounds are typically prepared by the reaction of the thioacyl chloride with an alcohol, but they can also be made by the reaction of Lawesson's reagent with esters.[6]
|