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Concept
Thioester
Summary
In organic chemistry, thioesters are organosulfur compounds with the molecular structure . They are analogous to carboxylate esters () with the sulfur in the thioester replacing oxygen in the carboxylate ester, as implied by the thio- prefix. They are the product of esterification of a carboxylic acid () with a thiol (). In biochemistry, the best-known thioesters are derivatives of coenzyme A, e.g., acetyl-CoA. The R and R' represent organyl groups, or H in the case of R.
The most typical route to thioester involves the reaction of an acid chloride with an alkali metal salt of a thiol:
RSNa + R'COCl -> R'COSR + NaCl
Another common route entails the displacement of halides by the alkali metal salt of a thiocarboxylic acid. For example, thioacetate esters are commonly prepared by alkylation of potassium thioacetate:
CH3COSK + RX -> CH3COSR + KX
The analogous alkylation of an acetate salt is rarely practiced. The alkylation can be conducted using Mannich bases and the thiocarboxylic acid:
CH3COSH + R'_2NCH2OH -> CH3COSCH2NR'_2 + H2O
Thioesters can be prepared by condensation of thiols and carboxylic acids in the presence of dehydrating agents:
RSH + R'CO2H -> RSC(O)R' + H2O
A typical dehydration agent is DCC. Efforts to improve the sustainability of thioester synthesis have also been reported utilising safer coupling reagent T3P and greener solvent cyclopentanone. Acid anhydrides and some lactones also give thioesters upon treatment with thiols in the presence of a base.
Thioesters can be conveniently prepared from alcohols by the Mitsunobu reaction, using thioacetic acid.
They also arise via carbonylation of alkynes and alkenes in the presence of thiols.
Thioesters hydrolyze to thiols and the carboxylic acid:
RC(O)SR' + H2O → RCO2H + RSH
The carbonyl center in thioesters is more reactive toward amine nucleophiles to give amides:
In a related reaction, but using a soft-metal to capture the thiolate, thioesters are converted into esters.
Thioesters provide useful chemoselectivity in the synthesis of biomolecules.
Official source
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