Thioesters nucleophilic attack

As shown in Figure 16.10, this reaction mechanism involves nucleophilic attack by —SH on the substrate glyceraldehyde-3-P to form a covalent acylcysteine (or hemithioaeetal) intermediate. Hydride transfer to NAD generates a thioester intermediate. Nucleophilic attack by phosphate yields the desired mixed carboxylic-phosphoric anhydride product, 1,3-bisphosphoglycerate. Several examples of covalent catalysis will be discussed in detail in later chapters. 

Allenyl bromide accepts the nucleophilic attack of amines or potassium thio-acetate leading to propargylic amines or thioesters via the alkenylidenecarbene 132 or zwitterion intermediate 133 [66],... 

Scheme 34 Overview of native chemical ligation (NCL). Two unprotected segments react in a reversible thiol/thioester reaction only the thioester product between the C-terminal thioester and the N-terminal cysteine can react further to form the desired amide bond via nucleophilic attack of the cysteine amine group.

The P-oxidation sequence involves three reactions, dehydrogenation, hydration, then oxidation of a secondary alcohol to a ketone, thus generating a P-ketothioester from a thioester. We shall study these reactions in more detail later (see Section 15.4.1). The P-ketothioester then suffers a reverse Claisen reaction, initiated by nucleophilic attack of the thiol coenzyme A (see Box 10.8). 

Nucleophilic attack by the amino group of the neighbouring aminoacyl thioester is catalysed by the C domain, and this results in amide (peptide) bond formation. Enzyme-controlled biosynthesis in this manner is a feature of many microbial peptides, especially those containing unusual amino acids not encoded by DNA and where post-translational modification (see Section 13.1) is unlikely. 

Since the thioester linkage is susceptible to nucleophilic attack but stable to TEA treatment during solid-phase peptide synthesis (using standard Boc protection), Weigel and colleagues " have envisioned that hydroxylamine derivatives could directly cleave resin-bound peptide thioesters 201 or 202 to form the corresponding peptide hydroxamates 203 (Scheme 88). 

Goethite (surface are 50 m g" ) has been reported to catalyse the hydrolysis of carboxylate and phosphorothioate esters (Torrents and Stone, 1994). The authors suggest that the sulphur of the thioester binds to the surface Fe of the goethite and thereby reduces the electron density at the P atom which in turn facilitates the nucleophilic attack by OH and promotes hydrolysis. 

The hydrolysis of peptides by these proteases represents classic nucleophilic catalysis. The relatively inert peptide is converted to the far more reactive ester or thioester acylenzyme, which is rapidly hydrolyzed. The use of the serine hydroxyl rather than the direct attack of a water molecule on the substrate is favored in several ways alcohols are often better nucleophiles than the water molecule in both general-base-catalyzed and direct nucleophilic attack the serine... 

Cross-linking is achieved by the activation of the Glu thioester group by silver ions, followed by nucleophilic attack of the N-terminal amino group from the other peptide. The process is repeated using the Glu a-thioester peptide to create a trimeric structure (Scheme... 

Two mechanisms (i.e. direct hydrolysis and alternatively a path via an unstable acyl phosphate intermediate) are involved in the hydrolysis in phosphate buffer of N-arylsulfonyl / -lactams such as (130).107 The acyl phosphate intermediate can be trapped with hydrazine. The alkaline hydrolysis of some torsionally distorted lactams, i.e. the bridged benz[rfe]isoquinolin-l-ones (131), in 70% (v/v) DMSO-water has been compared under the same conditions with the hydrolysis of AvA-dimethyl-1 -naphthamide (132). The relative rates of reaction and activation parameters indicate the effect of torsional distortion.108 The reaction of the tricyclic azetidinones (133) with trifluoroacetic acid gives the bicyclic thioesters (135). The mechanism may involve acid-catalysed elimination of methanethiol to give an azetinone intermediate (134) which, after nucleophilic attack of the thiol, is converted into (135).109... 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

However, most nucleophiles attack 5-oxazolones at the carbonyl group and the products are derivatives of a-amino acids formed by acyl-oxygen fission. Thus the action of alcohols, thiols, ammonia and amines leads, respectively, to esters, thioesters and amides orthophosphate anion gives acyl phosphates (Scheme 18). The use of a-amino acids in this reaction results in the establishment of a peptide link. Cysteine is acylated at the nitrogen atom in preference to the sulfur atom. Enzymes, e.g. a-chymotrypsin and papain, also readily combine with both saturated and unsaturated azlactones. A useful reagent for the introduction of an a-methylalanine residue is compound (202). Both the trifluoroacetamido and ester groups in the product are hydrolyzed by alkali to give a dipeptide. The alkaline hydrolyzate may be converted into the benzyloxycarbonyl derivative, which forms a new oxazolone on dehydration. Reaction with an ester of an amino acid then yields a protected tripeptide (equation 45). 

Another example of the activation of a hydroxy acid was described by Rastetter and Phillion [60] First the 0-protected hydroxyacid 68 reacts with a thiol group containing crown ether 67. Then the resulting thioester 69 reacts with potassium mrt-butoxide to give the alkoxide. At the same time a complexation of the potassium ion by the [18]crown-6 part of the molecule occurs. Thus, the alkoxide ion comes close to the carbonyl group of the molecule, so that nucleophilic attack leading to ring formation is facilitated (cooperation of dilution principle, template effect, and ion pair interaction). 

Fig. 2. Scheme of protein splieing. Cleavage pathway proposed for intein that possesses a cysteine residue in eaeh spliee junetion. In the initial step a linear thioester intermediate is formed by an N-S acyl rearrangement at Cysi (N-terminal amino acid of the intein). Next, traw -thioesterification that involves nucleophilic attack of the side-... 

Peptide bond cleavage at either the N- or C-terminal splice junction can occur independently. Replacement of Cysj to Alai allows only the C-terminal intein splicing and leads to C-extein that includes the cysteine at its N-terminus. Alternatively, the cleavage at N-terminus of the intein takes place by the nucleophilic attack at the thioester intermediate formed when the C-terminal Asn of the intein is substituted by Ala. 

The reaction of thiol esters with lithium ynolates (equation 67) takes place by a route different than the one shown in equation 65 for alcohol esters. Thiol esters (162) undergo a two-carbon homologation to S-keto thiol esters 165 in good yield. Intermediates 163 undergo a two-step rearrangement to a S-keto thiol ester enolate (165), via elimination of lithium thiolate to yield a ketene (164), followed by the nucleophilic attack of the thiolate on 164. Finally, the homologated S-keto thioester (165 ) is obtained on acidification of the reaction mixture . ... 

An alternative elegant native ligation approach has recently been proposed where the Staudinger reaction is exploited to couple the carboxy component as a thioester via trans-thioesterification to a suitable phosphine scaffold that reacts with azidoacyl peptides to produce the imino-X, -phosphine. This chemical ligation is followed by nucleophilic attack by the imino-X -phosphine nitrogen on the thioester to form the amide bondt °°l (Section 2.2.1.10). 

Dithioesters like (62) which can exist in the enol form react with sulfur ylides like dimethylsulfoxonium methylide (63) to give ketenethiols (64). The mechanism of the reaction involves nucleophilic attack by the ylide on the enol form of (62) (Scheme 34). Similar compounds (65) may be synthesised by treatment of a suitable thioester (62) with an alkyl halide in the presence of a base (Scheme 35). 

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