Amino thioacids

A more versatile route to synthesize peptide thioesters is by the preparation of a peptide thioacid via the solid-phase method and then reaction with a halo derivative to form the thioester. This route can be achieved via the preparation of thioester linkers that can be applied in stepwise solid-phase peptide synthesis. The general structure of the N-protected amino thioacid attached to the linker is shown in Scheme 14. t65-80 ... 

Scheme 14 Structure of a Boc-Protected Amino Thioacid Attached to a Linker...

This method was modified by the preparation of the new linking reagent 37 which eliminated the necessity of synthesizing the amino thioacid needed for attachment to 36. This improved method was described by Canne et al. 81 and is outlined in Scheme 16. 

Dipeptide Amino thioester DCC Peptide thioacid 93 20060L823... 

Domling et al. made react (3-amino butyric thioacid, 89, the isobutyraldehyde 64, and 86 into the product 90, which simultaneously contains a 13-lactam group and a thiazole system. 

A report involving the solid-phase preparation of Fmoc-protected t t[CH2—S] pseudodipeptides revealed that the expected stereochemical patterns did not hold.147 It is known that the conversion of an a-bromo acid into the a-thioacid does not always occur with quantitative inversion of chirality, since it is side-chain dependent.146] With side chains such as benzyl (Phe) or butyl (Leu, lie), exceptions are not expected. But when the Fmoc-protected amino thiol 26 was condensed to a support-bound bromo acid 27, as shown in Scheme 8, the (S,R)-product 28 was obtained instead of the expected (S,S)-isomer. 

Preparation of the C-terminal thioacids and thioesters of peptides can be also achieved by solid-phase synthesis. A key step of the solid-phase synthesis is the preparation of the thioester-modified N-protected amino acid in the linkable form 32 so it can be immobilized on the resin as shown in Scheme 13J67 ... 

The attachment of the first amino acid to the linker 36 was accomplished by the conversion of the amino acid into the thioacid by H2S treatment of the A-hydroxysuccinimide ester followed by formation of the triethyl or A,A -dicyclohexyl ammonium salt and then reaction with the linkerJ65-80 ... 

The linker 37 with the first amino acid attached, compound 38, can be applied to stepwise solid-phase peptide synthesis. At the end of the synthesis, when the desired peptide sequence is completed, the thioacid-modified peptide fragment is cleaved from the solid support by HF and further S-alkylated with a N-bromoacetylated peptide to form an endothioester bond. The cleaved thioacid can also be reacted with an alkyl bromide to form the corresponding thioester. 

Turning finally to redox-to-condensation energy coupling, the simplest case is shown in reaction (5). COS in turn will be the substrate for various forms of condensation-to-condensation energy couplings. For example, C-C-bond formation by car-boxylation with COS will generate thioacids (3). Similarly, carboxylations of a-amino acids by COS generate thiocarbox-amides en route to aminoacyl N-carboxy-anhydrides ... 

Peptide carbothioic acids were synthesized using a thioacid linker 4 developed by Blake (Scheme 4).P - l A generalized synthesis of this Boc-Xaa-thioacid-linker has been described by Canne et The thioadd-resin was synthesized by dissolving Boc-Xaa-thioacid-linker DCH A salt (1.1 equiv) and HBTU (1.1 equiv) in a minimum volume of DMF and activated for 2 min. This mixture was added to amino-methyl-polystyrene-resin (1.0 equiv, -l.Ommol-g loading of amine) and coupled for Ih or until the reaction proceeded to >9 5% as monitored by the quantitative ninhydrin test. 

The oc-mercaptobenzhydryl linker 27 [71] was developed for the synthesis of thioacids by Boc synthesis. Acylation of the thiol group with Boc-amino acids leads to a resin-bound thioester. Subsequent peptide chain extension and then treatment with HF cleaves the ester-linker bond, releasing the C-terminal peptide thioacid. Unfortunately, owing to the nucleophile lability of thioesters, this approach cannot be applied to Fmoc-based methods. 

Examples of efficient ligation methods for creating artifically linked proteins or peptides include formation of thioesters by reaction of a thioacid and a bromoacetamide [119], formation of pseudoproline residues from an aldehyde and Cys or Thr [122,124], formation of an oxime from an amino-oxy function and an aldehyde or ketone [120,121], and formation of a hydrazone from a hydrazide and an aldehyde [121,125,126]. Several of the preceding methods may be used in series because of different reactivities. 

All native chemical ligations described to date use C-terminal thioesters or thioacids for reaction with an //-terminal nucleophile or electrophile, respectively. In the original method, an A-terminal Cys reacts with a C-terminal thioester to form a thioester intermediate that rearranges spontaneously to form a natural peptide bond with a Cys incorporated (Fig. 8) [123]. This method has been extended further to allow native bond formation without the exclusive use of //-terminal Cys. To accomplish this, an oxy-ethanethiol was attached to the amino function of the iV-terminal residue. 

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