Peptide esters, preparation

N-Silylated peptide esters are acylated by the acid chloride of N-Cbo-glycine to N-acylated peptide bonds [11]. Likewise, acid chlorides, prepared by treatment of carboxylic acids with oxalyl chloride, react with HMDS 2 at 24°C in CH2CI2 to give Me3SiCl 14 and primary amides in 50-92% yield [12]. Free amino acids such as L-phenylalanine or /5-alanine are silylated by Me2SiCl2 48 in pyridine to 0,N-protected and activated cyclic intermediates, which are not isolated but reacted in situ with three equivalents of benzylamine to give, after 16 h and subsequent chro-... 

The first peptide amides prepared by solid-phase synthesis were obtained by ammonolysis of resin-bound benzyl esters of peptides in solvents containing methanol (Figure 5.16, A). The method was occasionally employed but was not popular because it was inefficient, producing some ester in addition to the amide. A new variant employing gaseous ammonia will likely rekindle this approach (see Section 8.3). During the early developments of solid-phase synthesis, it was known that the... 

WF DeGrado, ET Kaiser. Polymer-bound oxime esters as supports for solid-phase peptide synthesis. Preparation of protected fragments. J Org Chem 45, 1295, 1980. 

Amides and peptides. The preparation of these substances with I as the coupling reagent (9, 20-21) has been simplified by use of phase-transfer conditions (Bu4NHS04, KOH, H20 CHjCIj). The potassium salt of the acid can be prepared in situ either from the acid or the ethyl ester. In either case yields of amides are 70-95%.1... 

As mentioned above, thiazolidine-4-carboxylic acid is characterized by an anomalously low basicity and thus difficult acylation in peptide synthesis. 189 Therefore, the incorporation of this amino acid residue into a growing peptide chain is preferentially preformed via dipeptide derivatives. 139 Suitably N-protected amino acids are coupled directly to the thiazolidine-4-carboxylic acid by the acid fluoride 139 or iV-carboxyan hydride 1392111 methods. The resulting dipeptides are used as building blocks without risk of racemization 139 and standard coupling procedures are applied as pentachlorophenyl esters prepared by the mixed anhydride procedure 121 or PyBOP. 171 ... 

Preparation of the thioacids and their esters via SPPS is mainly restricted to the Boc methodology. The popular Fmoc approach is limited by aminolysis of the thioester bond during removal of the Fmoc group by piperidine. However, a modified Fmoc-deprotecting mixture (1-methyl pyrrolidine/hexamethyleneimine/HOBt/DMSO/NMP 25 2 2 35.5 35.5) gave the final desired peptide ester with 24% yield J24 ... 

Amino acid and peptide aldehydes with one to three residues have been prepared successfully using diisobutylaluminum hydride. Z-Protected amino aldehydes such as Z-Leu-H, Z-Phe-H, Z-Cys(Bzl)-H, Z-Pro-H have been synthesized with little or no racemization (Table l). 5 The diisobutylaluminum hydride reduction can be used with both peptide esters and Z amino acid esters. However, the Boc protecting group is less stable when refluxed with diisobutylaluminum hydride, thus resulting in its loss while reducing Boc-Ala-OMe or Boc-Ser(OBzl)-OMe. 13 ... 

Carboxy terminal amino acid or peptide thiols are prepared from various p-amino alcohols by conversion into a thioacetate (R2NHCHR1CH2SAc) via a tosylate followed by saponification.Several methods have been used to prepare N-terminal peptide thiols, the most common procedure is the coupling of (acetylsulfanyl)- or (benzoylsulfanyl)alkanoic acids or add chlorides with a-amino esters or peptide esters, followed by deprotection of the sulfanyl and carboxy groups. 8 16 Other synthetic methods include deprotection of (trit-ylsulfanyl)alkanoyl peptides, 1718 alkaline treatment of the thiolactones from protected a-sulfanyl acids, 19 and preparation of P-sulfanylamides (HSCH2CHR1NHCOR2, retro-thior-phan derivatives) from N-protected amino acids by reaction of P-amine disulfides with carboxylic acid derivatives, followed by reduction. 20,21 In many cases, the amino acid or peptide thiols are synthesized as the disulfides and reduced to the corresponding thiols by the addition of dithiothreitol prior to use. 

Sulfanylalkanoyl amino acids and peptides are prepared by reaction of the (acetyl-sulfanyl)- or (benzoylsulfanyl)alkanoic acids or acid chlorides with a-amino esters or peptide esters, followed by deprotection of the sulfanyl and carboxy groups. 8 101114 16 27 29 For example, the 3-(acetylsulfanyl)alkanoic acids 7 are prepared from the condensation of ethyl (diethoxyphosphoryl) acetate 5 with various aldehydes according to the Horner-Emmons reaction, providing the a, 3-unsaturated ethyl esters 6 (a mixture of Z- and E-isomers, 50 50), followed by saponification of the ethyl esters and Michael addition of thiolacetic acid. The 3-(acetylsulfanyl)alkanoic acids 7 can be coupled with a-amino esters or peptide esters and subsequent hydrolysis of the 3-(acetylsulfanyl) derivatives provides the desired products 8 (Scheme 2). 14 ... 

The p-nitrophcnyl ester prepared in part (e) is an active ester. The p-nitrophenyl group is a good leaving group and can be displaced by the amino nitrogen of valine ethyl ester to form a new peptide bond. 

Aminolysis of peptide esters occurs uneventfully but this approach to peptide bond formation is not used routinely because access to enantiomerically pure active esters of peptides is straightforward only when the activated residue is glycyl or prolyl (Section 3.2.2). Succinimido esters of small peptides with glycine or proline at the carboxy terminus have been used extensively for the preparation of larger segments in solution. Aminolysis by dipeptide ester 85 of dipeptide succinimido ester 84, obtained through the mixed anhydride (Scheme 19), gave enantiomerically pure (<0.5% l-d-l-l isomer) protected tetrapeptide 86 in 88% yield (Scheme 25). 

Protected peptides are cleaved from the resin with N-hydroxypiperidine (HOPip), followed by treatment with zinc in AcOH to afford the free acidJ Resin-bound peptides can be cleaved from the oxime resin using the tetrabutylammonium salts of side-chain protected amino acids to directly provide the a-amino-protected (Boc), side-chain-protected pep-tide.[ 3o- 32] Peptide esters can be obtained from peptidyl oxime resin by treating with amino acid estersJ Peptide hydrazides are derived from peptidyl oxime resins on treatment with anhydrous hydrazine.b Protected peptide amides can be also derived from peptidyl oxime resins on treatment with ammoniaJ l Cyclic peptide acids as carboxy components were successfully prepared by this procedure through the Pac ester. 

Carboxylic acid azides are prepared from carboxylic acid halides or from amino acid/peptide esters via hydrazides, followed by nitrosylation with nitrous acid in water or with alkyl nitrites in organic solvents.Also, the aforementioned diphenylphosphorazidate cf. Section 2.3.1.1.2.2.ii.e) is often used in this preparation method starting from carboxylic acids. 

In analogy to the active esters, thioesters are prepared by condensation of thiols with carboxylic acids/DC(3 or carboxylic acid chlorides. " Aryl thioesters react readily with nucleophiles like amines to give amides. Peptides are prepared without racemization. In the case of r-butyl thioesters activation is accomplished by treatment with Ag02CCp3. 

A conceptual alternative way to the activation of the carboxylic acid function is the reaction of carboxylic acids with amino groups activated as isocyanates - and isothiocyanates (equation 16). Preparation of these derivatives is racemization free. The reaction proceeds via mixed acid anhydrides in aromatic hydrocarbon solvents at elevated temperatures, and decarboxylation leads to the V-substituted amide. Pyridine as solvent enhances the conversion rate but increases also the amount of the urea side product via disproportionation. Application to peptide chemistry is limited, because peptide ester fragments tend to form hydantoins. ... 

Sufficient volatility for GLC analysis is found for A-acylated esters of amino acids and peptides. Their preparation requires a two-step derivatisation protocol and therefore introduces a potential source of error. There is also anxiety about the impurities that may be introduced in this way. However, this applies to any derivatisation protocol and experienced users of the GLC technique can obtain impressive reproducibility of results, sufficient to match the reliability of the classical Moore and Stein procedure. Flexibility because of the additional range of detectors available for GLC can be useful, e.g. highly sensitive electron-capture detectors for halogenated analytes or amino acids and peptides derivatised with halogen-containing groups. 

Synthesis of himastatin involved the preparation of the pyrroloindoline moiety A, its conversion to the bisindolyl unit A 2, synthesis of the peptidal ester moiety B, the subsequent joining of these units (A 2 and two B units), and cyclization leading to himastatin. The following brief account focuses on the protective group aspects of the synthesis. 

Phenol-type resins 13-15 have also been used to prepare LH-RH, enkephalin and bradykinin peptide analogs. Peptides were successfully cleaved by ammonoly-sis (NH3 in MeOH-DMF or DMF, 5-18h) or aminolysis with protected amino acid or peptide esters bearing the free a-amino group [24, 25, 35]. Marshall s mercaptophenol resin (16) has been used to prepare libraries of piperazine-2-carboxamides (consisting of 22 different amines) and 6-carboxybenzopyran-4-ones. Cleavage was performed using an excess of appropriate amines in pyridine for 24 to 48 h [36, 37]. Tetrahydro-P-carboline-3-carboxamides have also been similarly synthesized and released [38]. 

The first reported solid-phase synthesis of head-to-tail cyclic peptides was based on the intramolecular aminolysis of resin-bound o-nitrophenyl esters. The cyclization proceeds concurrently to cleave the peptide from the resin, after deprotection and neutralization of the AT-terminal residue (Scheme 2A). Accordingly, Fridkin et al. [3] reported the preparation of several simple, unhindered cyclopeptides, such as cyc/o(Ala-Gly-Ala-Ala). Similarly, Flanigan and Marshall [4] obtained activation of the resin-bound peptide ester, after elongation of the peptide chain, by oxidation of the 4-(methyl-thio)phenyl (MTP) linker to a sulfonyl ester. Subsquent deblocking of the A-terminal residue and intramolecular condensation yielded the desired cyclic peptide. However, this method was found not to be suitable for the synthesis of longer and more hindered cyclic peptides [5]. 

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