Contemporary Applications of Thioamides and Methods for Their Synthesis

Thioamides are naturally occurring isosteres of amide bonds in which the chalcogen atom of the carbonyl is changed from oxygen to sulfur. This substitution gives rise to altered nucleophilicity and hydrogen bonding properties with importance for both chemical reactivity and non-covalent interactions. As such, thioamides have been introduced into biologically active compounds to achieve improved target affinity and/or stability towards hydrolytic enzymes but have also been applied as probes of protein and peptide folding and dynamics. Recently, a series of new methods have been developed for the synthesis of thioamides as well as their utilization in peptide chemistry. Further, novel strategies for the incorporation of thioamides into proteins have been developed, enabling both structural and functional studies to be performed. In this Review, we highlight the recent developments in the preparation of thioamides and their applications for peptide modification and study of protein function.

Practical N-to-C peptide synthesis with minimal protecting groups

Accessible drug modalities have continued to increase in number in recent years. Peptides play a central role as pharmaceuticals and biomaterials in these new drug modalities. Although traditional peptide synthesis using chain-elongation from C- to N-terminus is reliable, it produces large quantities of chemical waste derived from protecting groups and condensation reagents, which place a heavy burden on the environment. Here we report an alternative N-to-C elongation strategy utilizing catalytic peptide thioacid formation and oxidative peptide bond formation with main chain-unprotected amino acids under aerobic conditions. This method is applicable to both iterative peptide couplings and convergent fragment couplings without requiring elaborate condensation reagents and protecting group manipulations. A recyclable N-hydroxy pyridone additive effectively suppresses epimerization at the elongating chain. We demonstrate the practicality of this method by showcasing a straightforward synthesis of the nonapeptide DSIP. This method further opens the door to clean and atom-efficient peptide synthesis.

Oxidative Imidation of Benzylic and Cycloalkane C(sp3)-H Bond Donors Using N-Aroyloxyquinuclidinium Salts and Nitriles under Photoredox Catalysis

A series of N-aroyloxyquinuclidinium salts were prepared and used as reagents to perform efficient three-component Ritter-Mumm-type oxidative C-H imidation of donors of 1° and 2° benzylic C-H bonds used as limiting reagents with nitriles as a source of imide nitrogen under photocatalytic conditions; these reagents also exhibit somewhat lower reactivity toward cycloalkanes.

Amine Activation: "Inverse" Dipeptide Synthesis and Amide Function Formation through Activated Amino Compounds

A copper(II)/HOBt-catalyzed procedure for the synthesis of dipeptides and "general" amides has been developed using microwave irradiation to considerably hasten the reaction. As an alternative to using traditional carboxylic acid activation, the method relies on the use of N-acyl imidazoles as activated amino partners. By doing so, a nonconventional way to reach dipeptides and amides has been proposed through the challenging and less studied N → C direction synthesis. A series of dipeptides and "general" amides have been successfully synthesized, and the applicability of the method has been illustrated in gram-scale syntheses. The mild reaction conditions proposed are completely adequate for couplings in the presence of sensitive amino acids, affording the products without detectable racemization. Furthermore, experimental observations prompted us to propose a plausible reaction pathway for the couplings.

Chemo- and Site-Selective Replacement of N-Terminal Carbamates in Peptides

In peptide synthesis, it is important to distinguish the terminal amino group and carry out the selective transformation of only the N-terminal protecting group. We describe herein a reaction for the chemo- and site-selective replacement of carbamates with various other carbamates only at the N-terminus of peptides. We demonstrate the scope of carbamates and peptides and the introduction of fluorine into a peptide. This strategy is applicable to the late stage of peptide synthesis.

Ring Expansion of Thiolactams via Imide Intermediates: An Amino Acid Insertion Strategy

The Ag^I -promoted reaction of thiolactams with N-Boc amino acids yields an N-(α-aminoacyl) lactam that can rearrange through an acyl transfer process. Boc-deprotection results in convergence to the ring-expanded adduct, thereby facilitating an overall insertion of an amino acid into the thioamide bond to generate medium-sized heterocycles. Application to the site-specific insertion of amino acids into cyclic peptides is demonstrated.

Depsipeptide synthesis using a late-stage Ag(i)-promoted macrolactonisation of peptide thioamides

Ag(i)-Triggered activation of the thioamide and attack by the C-terminal carboxylate generates an isoimide intermediate that undergoes an intramolecular acyl transfer to furnish the cyclic depsipeptide.

Diboronic Acid Anhydride-Catalyzed Direct Peptide Bond Formation Enabled by Hydroxy-Directed Dehydrative Condensation

We report the catalytic direct peptide bond formations via dehydrative condensation of β-hydroxy-α-amino acids, affording the serine, threonine, or β-hydroxyvaline-derived peptides in high to excellent yields with high functional group tolerance, minimum epimerization, and excellent chemoselectivity. The key to the success of these atom-economical transformations is the use of diboronic acid anhydride catalyst for the hydroxy-directed reactions.

Convenient synthesis of thioamidated peptides and proteins

The unique physicochemical properties of a thioamide bond, which is an ideal isostere of an amide bond, have not been fully exploited because of the tedious synthesis of thionated amino acid building blocks. Here, we report a purification-free and highly efficient synthesis of thiobenzotriazolides of Fmoc-protected and orthogonally protected 20 naturally occurring amino acids including asparagine, glutamine, and histidine. The near-quantitative conversion to the respective thioamidated peptides on solid support demonstrates the robustness of the synthetic route. Furthermore, the unaltered incorporation efficiency of thiobenzotriazolides from their stock solution till 48 h suggests their compatibility toward automated peptide synthesis. Finally, utilizing an optimized cocktail of 2% DBU + 5% piperazine for fast Fmoc-deprotection, we report the synthesis of a thioamidated Pin1 WW domain and thioamidated GB1 directly on solid support.

Synthesis of N-acetoxy-N-arylamides via diacetoxyiodobenzene promoted double acylation reaction of hydroxylamines with aldehydes

A facile and efficient synthesis of N-acetoxy-N-arylamides through double acylations of hydroxylamines with aldehydes and diacetoxyiodobenzene is reported. The yields of the products are good to excellent.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Synthesis of Peptides by Silver-Promoted Coupling of Carboxylates and Thioamides: Mechanistic Insight from Computational Studies

The mechanism of the recently described N→C direction peptide synthesis through silver-promoted coupling of N-protected amino acids with thioacetylated amino esters was explored by using density functional theory. Calculation of the potential energy surfaces for various pathways revealed that the reaction proceeds through silver-assisted addition of the carboxylate to the thioamide, which is followed by deprotonation and silver-mediated extrusion of sulfur as Ag2 S. The resulting isoimide is the key intermediate, which subsequently rearranges to an imide through a concerted pericyclic [1,3]-acyl shift (O-sp(2) N 1,3-acyl migration). The proposed mechanism clearly emphasises the requirement of two equivalents of Ag(I) and basic reaction conditions, which is in full agreement with the experimental findings. Alternative rearrangement pathways involving only one equivalent of Ag(I) or through O-sp(3) N 1,3-acyl migration can be excluded. The computations further revealed that peptide couplings involving thioformamides require significant conformational changes in the intermediate isoformimide, which slow down the rearrangement process.

A facile one pot route for the synthesis of imide tethered peptidomimetics

A simple and efficient method for the synthesis of N,N’-orthogonally protected imide tethered peptidomimetics is presented. The imide peptidomimetics were synthesized by coupling the in situ generated selenocarboxylate of N^α-protected amino acids with N^α-protected amino acid azides in good yields.

Fully Synthetic Granulocyte Colony-Stimulating Factor Enabled by Isonitrile-Mediated Coupling of Large, Side-Chain-Unprotected Peptides

Human granulocyte colony-stimulating factor (G-CSF) is an endogenous glycoprotein involved in hematopoiesis. Natively glycosylated and nonglycosylated recombinant forms, lenograstim and filgrastim, respectively, are used clinically to manage neutropenia in patients undergoing chemotherapeutic treatment. Despite their comparable therapeutic potential, the purpose of O-linked glycosylation at Thr133 remains a subject of controversy. In light of this, we have developed a synthetic platform to prepare G-CSF aglycone with the goal of enabling access to native and designed glycoforms with site-selectivity and glycan homogeneity. To address the synthesis of a relatively large, aggregation-prone sequence, we advanced an isonitrile-mediated ligation method. The chemoselective activation and coupling of C-terminal peptidyl Gly thioacids with the N-terminus of an unprotected peptide provide ligated peptides directly in a manner complementary to that with conventional native chemical ligation-desulfurization strategies. Herein, we describe the details and application of this method as it enabled the convergent total synthesis of G-CSF aglycone.

Steric and Electronic Effects in the Synthesis and Regioselective Hydrolysis of Unsymmetrical Imides

The AgI-promoted coupling reaction of thioamides and carboxylic acids is shown to be a useful method for the generation of unsymmetrical imides. The reaction proceeds efficiently with unhindered and electron-rich or neutral coupling partners, but not with hindered thioamides (such as thiopivalamides) or electron deficient thioamides (such as trifluorothioacetamides). Intriguingly, thioformamides are also ineffective coupling partners, despite having minimal steric or electronic influence. Hindered carboxylic acid coupling partners (such as pivalic acid) are tolerated, but electron deficient acids, such as trifluoroacetic acid, are ineffective coupling partners. Furthermore, an interplay of both steric and electronic effects is observed in the subsequent hydrolysis of unsymmetrical imides. Imides with a dimethoxybenzoyl group give high regioselectivity upon hydrolysis, favouring cleavage of the distal acyl group. Imides with a p-nitrobenzoyl or pivaloyl group give reversed selectivity, favouring cleavage of the proximal acyl group.