Of thiolesters

A thiol-terminated PS was used as a sample in the experiment. It was based on a living polymerized carboxyl-terminated PS with M = 93,800 and Mu = 100,400. The polydispersity was Mp( /M = 1.07. The degree of polymerization was about 900 and, thus, its contour length was about 220 nm. The thiol groups were substimted for the carboxylic ends using 1,10-decanedithiol by means of thiolester bonding, anticipating the preferential interaction between... 

Synthesis of thiolesters from carboxylic acids R1C02H. 

In analogy to the formation of thiolesters, some selenolesters have also been synthesized from carboxylic acid, phenylselenol, and A -carbonyldi-1,2,4-triazole.tl80]... 

The DCLs were generated from a series of thiolesters and thiols, respectively. The reaction between five thiolesters (16-20) and four thiols (1,2,5, and 9) generated library DCL-C (Scheme 6.3). For every thiol added, five additional thiolesters are formed, making a total of 25 thiolesters, all constantly undergoing exchange with the five thiols during the whole process. 

This enzyme [EC 5.1.1.11], also known as phenylalanine racemase (ATP-hydrolyzing), catalyzes the reaction of ATP with L-phenylalanine to produce o-phenylalanine, AMP, and pyrophosphate. In this unusual racemase reaction, a thiol group of an enzyme-bound pantotheine forms a thiolester from an initial aminoacyl-AMP intermediate then, as is typical of acyl thioesters, the a-proton becomes labile, thereby permitting reversible inversion of configuration to produce an equilibrated mixture of thiolester-bound enantiomers. Hydrolysis of the thiolester yields the product. 

Electrophilic catalysis by Lewis acids is also observed here no ambiguity arises with general acid catalysis, as Lewis acids and proton acids are not the same. An interesting example is the strong catalysis of thiolester hydrolysis by mercuric and silver ions. These soft acids presumably coordinate with the sulfur and, by virtue of the consequent electron withdrawal, make the carbonyl group much more susceptible to attack in the addition mechanism, or, in favorable cases, promote unimolecular SN1 cleavage of the sulfur-carbon bond.122... 

As a model study of methyl cobalamine (methyl transfer) in living bodies, a methyl radical, generated by the reduction of the /s(dimethylglyoximato)(pyridine)Co3+ complex to its Co1+ complex, reacts on the sulfur atom of thiolester via SH2 to generate an acyl radical and methyl sulfide. The formed methyl radical can be trapped by TEMPO or activated olefins [8-13]. As a radical character of real vitamin B12, the addition of zinc to a mixture of alkyl bromide (5) and dimethyl fumarate in the presence of real vitamin B12 at room temperature provides a C-C bonded product (6), through the initial reduction of Co3+ to Co1+ by zinc, reaction of Co1+ with alkyl bromide to form R-Co bond, its homolytic bond cleavage to form an alkyl radical, and finally the addition of the alkyl radical to diethyl fumarate, as shown in eq. 11.4 [14]. 

An early example of DSR employed dynamic thiolester systems, generated from transthiolesterification reactions, combined with hydrolases in aqueous solution [2, 3]. The transthiolesterification reaction, of fundamental biological importance [4], is the reversible reaction between a thiolester and a thiol. This dynamic reaction is rapid and sufficiently stable under mild conditions in aqueous solution. Dynamic thiolester systems could thus efficiently be generated from a series of thiolester compounds and thiols. During equilibration, the hydrolase was applied as an external selection pressure to resolve the fittest constituents of the system, where the optimal thiolesters were continuously hydrolyzed to acid and thiol products. During... 

In order to evaluate the optimal substrate for cholinesterases, a variety of thiolesters and thiols were used to generate dynamic thiolester systems under mild reaction conditions, compatible with the target enzyme. The reversible thiolesterification reactions were performed in deuterated water at neutral pH, and the equilibration of the systems was followed by 1H-XMR spectroscopy. Based on the structure of... 

The geometry of enolborinates of thiolesters depends mainly upon the nature of the sulfur substituent. Masamune and coworkers [691] have shown that the reaction of thiophenyl propionate (MeCI COSPh) with 2.14 (R = OTf) leads to E-6.85, while triethylmethylthiol propionate (MeCH2COSCEt3) leads to Z-6.85 (Figure 6.75). A similar trend is observed in the reaction of MeCI COS-tert-Bu with 6.83 (X = Br) [1248], The reaction of Z-enolborinates Z-6.85 generated from 2.14 (R = OTf), 2.61 (R = OTf) or 6.83 (X = Br) with aldehydes is highly diaste-... 

Lithiation of Thiolesters, Thionesters, and Dithioesters with LDA (General Procedure)... 

Another reaction that goes better with thiol esters than with ordinary esters is enolization. This is an equilibrium reaction and the enol has lost the conjugation present in the ester. Again, a thiol ester has less to lose so is more enolized, and it is the enolization of thiolesters of coenzyme A that we are now going to discuss. 

A careful study of the formation of thiolesters R R CH CO SR , by the addition of a thiol to a keten, suggests that a radical path is followed. At least this accounts for the very irreprodudble rates. Photochemical reactions of 2-aminothiophenol (25) with carboxylic acids or cyclohexene provide a challenge from a mechanistic point of view the. S-methyl... 

Preparation.—In addition to a number of reactions of thiols, discussed in the preceding section, a variety of other routes have been explored for the preparation of sulphides. U.v. irradiation of thiolesters gives mixtures (CHa CO SPh gives PhSH, PhSMe, o- and p-AcPhSH, and disulphides as major products), while addition of an alkyl halide to a thiolester in the presence of strong base (LiH, PhgCLi, etc.) provides a one-step route. A novel /3-keto-sulphide synthesis from a 1,2-diketone is illustrated by the conversion of (29) into (30). ° Alternative routes to jS-keto-sulphides often... 

It has been observed that the characteristic absorption of thiolesters in the region of 232 mp disappears in the presence of thiols or disulfides and, possibly, in the presence of some of the products of further oxidation of the disulfides. On the other hand, in the one case where a thiolether was tested, no decrease in absorption was obtained. The behavior relative to the two reactants is given in Fig. 1. It may be seen that under the usual conditions for spectrophotometric observations, rather high ratios of thiol thiolester are required for complete suppression. 

Recent calculations which take into account the corrected value of 0.320 for the Ei value of the DPN-DPNH couplet show that the AFJ for the hydrolysis of the pyrophosphate bond in ATP is —8,640 calories (Anfinsen and Kielley, personal communication). If this value is correct, the AFo for the hydrolysis of thiolesters, acetyl-P and N-acetyl imidazole will be about —8,600, —11,200 and —14,000 calories respectively instead of the higher values which are reported in this paper. 

Thiol esters are better acylating agents than oxygen esters and such reactions are of some biological significance. Studies by Bruice and his coworkers have indicated that the pattern of thiolester... 

Conventional structure-rate studies for the hydrolysis of thiolesters have been reported. ... 

Our assay for the detection of active acetate was based on the enzymic reaction with sulfanilamide. In the presence of the enzyme obtained from pigeon liver, the acetylation of sulfanilamide increased with increasing addition of active acetate . In our purification work we used, however, another, much less time-consuming assay which was based on the delayed color development as given by thiolesters with sodium nitro-prusside. When ammonium hydroxide is added to the mixture of thiolesters and nitroprusside no color is seen in the first moment. It only gradually develops in the following minutes due to the hydrolysis of the thiolester in the alkaline medium. The... 

Taking advantage of Lynen s spectrophotometric methods and the stability characteristics of thiolesters, it was readily shown that the incubation of phosphotransacetylase with acetyl-P and substrate levels of CoA led to the accumulation of a thiolester which was isolated and identified as acetyl-CoA.< > This represented the first net synthesis of acetyl-CoA in vitro and established once and for all the mechanism of the phosphotransacetylase catalyzed reaction. From equilibrium measurements of reaction (25), it was calculated that the standard free energy of hydrolysis of acetyl-CoA is about the same as that of the pyrophosphate bond of ATP (i.e. —8 kcal). The energy-rich nature of acetyl CoA was thus firmly established. 

Trifluorothioacetic acid undergoes free-radical and nucleophilic addition to olefins and ketones respectively, paralleling in many ways the well-known reactions of thioacetic acid. Novel uses of thiolesters in synthesis... 

Intrinsic proteins may be anchored in the membrane by several different mechanisms. Attachments through interactions with lipids include fatty acid acylation of the aminoacyl terminus or the formation of thiolesters with cysteine residues. In addition, a number of proteins are covalently linked to phosphatidylinositol. 

This Zn-CHjCHBrj-TiCU-TMEDA reagent can also undergo the ethylidenation of thiolester and amide as shown in Scheme 8.21 [29]. 

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