Esters thiol

IS the key carbon-carbon bond forming step Acetyl coenzyme A (Section 26 1) A thiol ester abbreviated as... 

Thiol esters are obtained in 91% yield by the addition of aioyl chloride to a slurry of an alkaline salt xanthate in acetone without cooling (54) ... 

Scheme 68 shows the conversion of the phenoxymethylpenicillin-derived disulfide (see Scheme 10) to penem derivative (91) (78JA8214). Of particular interest in this sequence is the reductive acylation step to afford (89) and the Wittig ring closure to give (90). The rate of the latter reaction was found to be greatly infiuenced by the steric and electronic character of both the thiol ester and the carboxyl blocking group. 

These neutral conditions can be used to prepare thiol esters of acid- or base-sensitive compounds including penicillins. ... 

Thiol esters, which are more reactive to nucleophiles than are the corresponding oxygen esters, have been prepared to activate carboxyl groups for both lactoniza-tion and peptide bond formation. For lactonization S-f-butyl and S-2-pyridyP esters are widely used. Some methods used to prepare thiol esters are shown below. The S-r-butyl ester is included in Reactivity Chart 6. 

The procedure described is essentially that of Shioiri and Yamada. Diphenyl phosphorazidate is a useful and versatile reagent in organic synthesis. It has been used for racemlzatlon-free peptide syntheses, thiol ester synthesis, a modified Curtius reaction, an esterification of a-substituted carboxylic acld, formation of diketoplperazines, alkyl azide synthesis, phosphorylation of alcohols and amines,and polymerization of amino acids and peptides. - Furthermore, diphenyl phosphorazidate acts as a nitrene source and as a 1,3-dipole.An example in the ring contraction of cyclic ketones to form cycloalkanecarboxylic acids is presented in the next procedure, this volume. 

Migration of the perfluoromethylthio group occurs in perfluoroalkyl acetylenic thiol esters and substituted Mmethylpyrroles [78, 79] (equation 18)... 

The reaction of hydroxyethyl-TPP with the oxidized form of lipoic acid yields the energy-rich thiol ester of reduced lipoic acid and results in oxidation of the hydroxyl-carbon of the two-carbon substrate unit (c). This is followed by nucleophilic attack by coenzyme A on the carbonyl-carbon (a characteristic feature of CoA chemistry). The result is transfer of the acetyl group from lipoic acid to CoA. The subsequent oxidation of lipoic acid is catalyzed by the FAD-dependent dihydrolipoyl dehydrogenase and NAD is reduced. 

Several additional points should be made. First, although oxygen esters usually have lower group-transfer potentials than thiol esters, the O—acyl bonds in acylcarnitines have high group-transfer potentials, and the transesterification reactions mediated by the acyl transferases have equilibrium constants close to 1. Second, note that eukaryotic cells maintain separate pools of CoA in the mitochondria and in the cytosol. The cytosolic pool is utilized principally in fatty acid biosynthesis (Chapter 25), and the mitochondrial pool is important in the oxidation of fatty acids and pyruvate, as well as some amino acids. 

Dewar and Turchi described the Comforth rearrangement of 5-alkoxyoxazole-4-thiocarboxylates as a potentially general method for the synthesis of 5-thiooxazole-4-carboxylic esters. Specifically, they found that thiol ester 13 underwent thermal isomerization to the corresponding 5-thiooxazole 14 in 94% yield. 

Ben2yloxyindolyl-3)-Q -acetvlamino-Q -methylthiopropionic acid methane-thiol ester Hydrogen Sulfuric acid... 

S-Alkylation of a thiocarboxylic acid with an a-halogenated carbonyl compound gives a thiol ester in which the two carbons to be connected... 

The HSAB principle predicts that the equilibrium should lie to the right, because the hard acid CH3CO should have a greater affinity for the hard base RO than for the soft base RS . Indeed, thiol esters are easily cleaved by OR or hydrolyzed by... 

Thiol acids and thiol esters can be prepared in this manner, which is analogous to 10-8 and 10-24. Anhydrides and aryl esters (RCOOAr) are also used as substrates, but the reagents in these cases are usually SH and SR. Thiol esters can... 

Alcohols, when treated with a thiol acid and zinc iodide, give thiol esters (R COSR). ... 

Acid derivatives that can be converted to amides include thiol acids (RCOSH), thiol esters (RCOSR), ° acyloxyboranes [RCOB(OR )2]. silicic esters [(RCOO)4Si], 1,1,1-trihalo ketones (RCOCXa), a-keto nitriles, acyl azides, and non-enolizable ketones (see the Haller-Bauer reaction 12-31). A polymer-bound acyl derivative was converted to an amide using tributylvinyl tin, trifluoroacetic acid, AsPh3, and a palladium catalyst. The source of amine in this reaction was the polymer itself, which was an amide resin. 

See Wallace, O.B. Springer, D.M. Tetrahedron Lett., 1998, 39, 2693 for cleavage of thiol esters to thiols with NaSMe in methanol and Choi, J. Yoon, N.M. Synth. Commun., 1995, 25, 2655 for borohydride exchange resin-Pd(OAc)2 reductive cleavage of thiol esters to thiols. 

Carboxylic esters, thiol esters, and amides can be made, respectively, by acid-... 

As with alcohols, ketenes add thiols to give thiol esters (R2C=C=0- - RSH-R2CHCOSR). 

In either the acid catalysis or the niekel carbonyl (or other metallic catalyst) method, if alcohols, thiols, amines, etc. are used instead of water, the product is the corresponding ester, thiol ester, or amide, instead of the carboxylic acid. 

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