Thioesters, preparation

The inconvenience and risk of HF handling prompted the development of Fmoc-SPPS compatible protocols for thioester preparation. The principal obstacle is the instability of the thioester when exposed to 20% piperidine, the standard conditions for Fmoc-removal, and numerous alternative methods have been developed to circumvent this problem [79], The most straightforward involve using less nucleophilic conditions, such as a mixture of 25 vol.% 1-methylpyrrolidine, 2 vol.% hexamethyleneimine, and 2% HOBt (w/v) in NMP and DMSO (1 1, v/v) [80, 81] or a mixture of 1% HOBt (wA ) and 1 vol.% DBU in DMF [82]. These methods are effective when preparing short sequences. 

Scheme 4 Examples of peptide thioester preparation by C-terminal activation and thiolysis...

Clippingdale AB, Barrow CJ, Wade ID (2000) Peptide thioester preparation by Fmoc solid phase peptide synthesis for use in native chemical ligation. J Pept Sci 6 225-234... 

Various antioxidants behave differently in mixtures with carbon black. Most substituted phenols and certain amines (for example, diphenyl-p-phenylenediamines) become still less effective in the presence of carbon black (phenomenon of antagonism). However, thioesters prepared from these compounds are not only themselves stronger antioxidants, but also manifest synergism in mixtures with carbon black. 

Peptide thioester preparation via intramolecular O-to-S acyl transfer through Fmoc chemistry has been studied by computational chemistry and found to occur via an anionic stepwise mechanism in which the cleavage of the C-O bond is the rate-limiting... 

Fig. 3 Protein splicing mechanism of the Mxe GyrA intein. The asparagine dashed bo)(j is essential for cleavage, thus, replaced by an alanine residue in engineered intein suitable for protein a-thioester preparation...

Kawakami T, Sumida M, Nakamura K, Vorherr T, Aimoto S (2005) Peptide thioester preparation based on an N-S acyl shift reaction mediated by a thiol ligation auxiliary. Tetrahedron Lett 46 8805-8807... 

This chapter concerns the preparation and reactions of acyl chlorides acid anhydrides thioesters esters amides and nitriles These com pounds are generally classified as carboxylic acid derivatives and their nomenclature is based on that of carboxylic acids... 

Polyhydric alcohol mercaptoalkanoate esters are prepared by reaction of the appropriate alcohols and thioester using -toluenesulfonic acid catalyst under nitrogen and subsequent heating (16,17). Organotin mercapto esters are similarly produced by reaction of the esters with dibutyltin oxide (18). Pentaerythritol can be oxidized to 2,2-bis(hydroxymethyl)hydracryhc acid [2831-90-5] C H qO, ... 

Diaryl derivatives of the 1,3-oxathiolylium system (29) are prepared by acid-catalyzed cyclization of the /3-keto thioesters (28) which are readily prepared from thioacid salts (27)... 

Activity is also retained when oxygen at the 21 position is replaced by sulfur. Preparation of one of these compounds follows a route quite analogous to the foregoing thus, displacement of the mesylate group in the cortisone (56) derivative 57 with the anion from thiopivalic acid affords thioester 58. Reduction of the 11-ketone by means of borohydride affords tixocortol pivalate (59)[13j. 

Carboxylic acids, RC02H, occupy a central place among carbonyl compounds. Not only are they valuable in themselves, they also serve as starting materials for preparing numerous acyl derivatives such as acid chlorides, esters, amides, and thioesters. In addition, carboxylic acids are present in the majority of biological pathways. We ll look both at acids and at their close relatives, nitriles (RC=N), in this chapter and at acyl derivatives in the next chapter. 

The investigation of minor groove-binding polyamides was greatly accelerated by the implementation of solid-phase synthesis [48]. Originally demonstrated on Boc-y9-Ala-PAM resin with Boc-protected monomers, it was also shown that Fmoc chemistry could be employed with suitably protected monomers and Fmoc-y9-Ala-Wang resin (Fig. 3.8) [49]. Recently, Pessi and coworkers used a sulfonamide-based safety-catch resin to prepare derivatives of hairpin polyamides [50]. Upon activation of the linker, resin-bound polyamides were readily cleaved with stoichiometric quantities of nucleophile to provide thioesters or peptide conjugates. 

Carboxylic acids such as acetic acid react with alcohols such as methanol or with methoxytrimethylsilane 13 a in the presence of trimethylchlorosilane (TCS) 14 in THF or 2-methyl-THF to give esters such as methyl acetate in 97% yield and hex-amethyldisiloxane 7. Even methyl pivalate can be readily prepared in 91% yield [111]. Reaction of a variety of carboxylic acids, for example N-benzoylglycine 329, with two equivalents of yS-trimethylsilylethanol 330 and with 14 has been shown to afford esters such as 331 in 98% yield [112, 112 a]. Likewise, silylated carboxylic acids react with silylated alcohols or thiophenols in the presence of 4-trifluoro-methylbenzoic anhydride and TiCl4/AgCl04 to furnish esters or thioesters in high yields [113, 114] (Scheme 4.43). 

The palladium-catalyzed carbonylation of aryl halides in the presence of various nucleophiles is a convenient method for synthesizing various aromatic carbonyl compounds (e.g., acids, esters, amides, thioesters, aldehydes, and ketones). Aromatic acids bearing different aromatic fragments and having various substituents on the benzene ring have been prepared from aryl iodides at room temperature under 1 atm... 

The N-linked pentasaccharide core Man3(GlcNAc)2 glycopeptide bearing the extracellular MMP inducer sequence 517 (emmprin 34—58) has been successfully linked to G(l) PAMAM aminodendrimer by thioester activation (Fig. 63). The resulting octameric 30 kDa construct was obtained in low yield, but was purified by preparative electrophoresis and fully characterized by MALDI-TOF mass spectrometry. The multivalent architecture was built to evaluate the requirement of emmprin multimerization for inducing MMP expression.384... 

Peptides typically are prepared for this ligation process using a-alkyl thioesters, because they are simple to make at the time of peptide synthesis. However, due to the relatively slow reaction kinetics of alkyl thioesters, most native chemical ligation processes have been catalyzed through the use of thiol compound additives, such as benzyl mercaptan or thiophenol (Dawson et al., 1997). These compounds react with the initial a-alkyl thioester to form another intermediate, an aryl thioester, which is more reactive toward the N-terminal cysteine on the other peptide to be coupled. A study... 

Native chemical ligation also can be extended to the conjugation of peptides or proteins to other molecules or surfaces. For instance, Reulen et al. (2007) prepared liposomes that contained cysteine-PEG-phospholipid derivatives and then coupled thioester-modified peptides or proteins to form a protein-liposome conjugate. Using this procedure, approximately 100 molecules of a collagen binding protein could be coupled to the cysteine-containing liposomes. 

Despite the many simple methods for preparation of carboxylic esters and thioesters, in some instances, use of 1-acylbenzotriazoles 915 as O and S acylating agents may be advantageous. For example, easy to prepare salicylic acid derivative 941 reacts with cyclopentanol under microwave irradiation to give 92% yield of cyclopentyl salicylate in 10 min <2006JOC3364>. In another example, L-phenylalanine derivative 942 reacts with benzyl mercaptan... 

The preparation of thiadiazoles 180 from 1-cyanoformamide 177 (2-nitrilo-acetimidic acid), or its alkyl esters 178, using disulfur dichloride under mild conditions was previously reviewed in CHEC(1984) and CHEC-II(1996). The synthesis of thiadiazoles from the thioesters 179, formed by the addition of alkylthiols to cyanogens, was more recently investigated (Equation 36) <1998JME379>. In several cases, the esters 178 or thioesters 179 prepared from cyanogen were not isolated but added directly to disulfur dichloride <1996W038431, 1998JME379>. 

Activated esters (see Section 2.9) Activated esters of peptides are rarely used because there is no general method available for converting an (V -protected peptide into the ester with a guarantee that it will be a single isomer. Attempts have been made to overcome this obstacle (see Section 7.8). However, solid phase synthesis allows the preparation of thioesters of segments (see Section 7.10). Once the ester is in hand, it can be aminolyzed without generation of a second isomer if suitable conditions are employed. 

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