Thioesters, 0-amino synthesis

Arthur L. Weber (1998), now working at the Seti Institute of the Ames Research Center at Moffett Field, reports the successful synthesis of amino acid thioesters from formose substrates (formaldehyde and glycolaldehyde) and ammonia synthesis of alanine and homoserine was possible when thiol catalysts were added to the reaction mixture. On the basis of his experimental results, Weber (1998) suggests the process shown in Fig. 7.10 to be a general prebiotic route to amino acid thioesters. 

Chemoselective ligation of peptides using the free amino terminal as nucleophile and a thiobenzyl thioester C-terminal amino acid as electrophile provides an efficient approach to the synthesis of proteins with several hundred residues [30]. From this perspective the introduction of non-natural amino acids into proteins becomes a possibility rather than a problem and chemoselective ligation is thus a prospect for the future for the incorporation of new functionality. 

Chemical ligation methods for peptide synthesis using thioester chemistry in solution have been previously documented (see Vol. E 22a, Section 4.1.5). Generalized procedures for solid-phase ligation have been developed that simplify the overall procedure. One method uses a safety-catch acid labile linker at the C-terminus and was used for the synthesis of a 71-amino acid chemokine, vMIP I (Section 5.3.2.1). Another procedure uses a selectively cleavable glycolate ester linkage (Section 5.3.2.2). 

Among the electrophilic handles proposed for head-to-tail and side-chain-to-tail cyclization of peptides on solid support by intrachain aminolysis with concurrent detachment of the product from the resin in the protected form (see Section 6.8.3.1.3), generally the oxime resin (also called Kaiser resin)1364 365 and a thioester resin[363l are recommended (see Scheme 14). In addition to the classical head-to-tail cyclization,[3431 the oxime resin is used for side-chain cyclizations as well as for the synthesis of multicyclic peptides vide infra). Due to its dual functions, the oxime resin can be employed only with Boc/Bzl chemistry it is not compatible with Fmoc/tBu chemistry where the basic N -deprotection leads to free amino groups and thus to premature cyclization reactions. To avoid this premature cleavage of the... 

A closely related E. coli protein is a 79-kDa multifunctional enzyme that catalyzes four different reactions of fatty acid oxidation (Chapter 17). The amino-terminal region contains the enoyl hydratase activity.32 A quite different enzyme catalyzes dehydration of thioesters of (3-hydroxyacids such as 3-hydroxydecanoyl-acyl carrier protein (see Eq. 21-2) to both form and isomerize enoyl-ACP derivatives during synthesis of unsaturated fatty acids by E. coli. Again, a glutamate side chain is the catalytic base but an imidazole group of histidine has also been implicated.33 This enzyme is inhibited irreversibly by the N-acetylcysteamine thioester of 3-decynoic acids (Eq. 13-8). This was one of the first enzyme-activated inhibitors to be studied.34... 

Scheme 30 Synthesis of 1-Oxoethylene Peptides using an a-Amino Acid Thioester and a Grignard Reagent1651...

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 ... 

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 ... 

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. 

In the synthesis of a-amino acids [290] through addition of the carbanion of MMTS to nitriles the overall process involves three other steps frequently encountered in sulfur-mediated chemistry a Pummerer-type rearrangement, with a less common migration of a methylthio group, and a Raney nickel desulfurization following transesterification of the thioester function. 

Numerous researchers have employed thiols as weak bases in the thioalkylation reaction to ligate unprotected peptides with a haloacetyl group to form thioethers at pH 7 8.5[90 91 131-133 or thioesters at acidic to basic conditions. 108"110 Of these two reactions, thioether formation is often the choice because thioesters suffer from instability in aqueous basic conditions. Haloacetyl derivatives, either as carboxylic acids or active esters, can be attached to either the N-terminal or side-chain amines during the stepwise solid-phase synthesis of either the peptide or the core and are stable to either HF or TFA cleavage conditions. Capping an amino group with a chloroacetyl group is compatible with Fmoc chemistry when used at a terminal step. 

Peptide thioester 84 was prepared from 81 (0.3 g of resin). All amino acids were protected with N-terminal Boc groups. The side-chain protections were as follows Arg(Tos), Asp(OCy), Cys[Bzl(4-Me)], Glu(OCy), His(Tos), Lys[Z(2-Cl)], Ser(Bzl), and Thr(Bzl). Each synthesis cycle consisted of (1) a 25-min deprotection with 55% TFA/CH2C12 and (2) coupling with Boc amino acid (4 equiv) and BOP or HBTU (4 equiv) in the presence of DIPEA (6 equiv) in DMF for 1 h. All couplings were monitored by the... 

In the synthesis of F extended thioester substrates, two methods are used to prepare the thiols. One method involves coupling of L-a-sulfanylcarboxylic acid (HSCHR5C02H) with N-terminal unprotected peptides)3,21 the other method is to convert the a-amino group of an... 

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