Ynamide-Mediated Thionoester and Dithioester Syntheses

Methyl chlorothioformate as a convenient reagent for thionoester synthesis

A promising reagent for introducing the methyl thionoester function into organic molecules through a straightforward synthetic sequence based on common magnesium organics is proposed. A scalable procedure for its production is elaborated. The potential of the reaction, along with its advantages/simplicity, convenience, and effectiveness, is demonstrated and discussed.

Facile Access to Piezoelectric Polyamides by Polyamidation of Carboxylic Acids and Ynamides for Potent Tumor Immunotherapy

Polyamides are a fascinating group of piezoelectric materials that are largely synthesized by the polycondesation of dicarboxylic acids with diamines or the ring-opening polymerization of special lactams. However, developing a green polymerization approach for the facile synthesis of piezoelectric polyamides is highly desirable but also challenging. Here, a simple polyamidation of carboxylic acids and ynamides is successfully established to synthesize versatile piezoelectric polyamides. This polymerization also possesses the merits of 100% atom economy, no waste generation, and additive-free as well as catalyst-free system. A range of polyamides are synthesized in moderate to good yields with satisfactory molecular weights. Importantly, piezoelectric polyamides containing tetraphenylethene (P1e/2b) can generate a bundant reactive oxygen species under ultrasound (US) exposure, thereby eliciting robust immunogenic cell death induction for augmented piezoelectric immunotherapy. Furthermore, P1e/2b exhibits distinctive aggregation-induced emission property that allows fluorescence imaging. In vivo evaluation using a bilateral tumor-bearing mouse model manifested that P1e/2b administration reinforced systemic immunity and triggered immune memory under US exposure, thereby leading to primary tumor eradication and distant tumor suppression. Therefore, this work represents a transition-metal-free and waste-free polymerization paradigm for the construction of piezoelectric polyamides, providing a distinct strategy to designing new piezoelectric polymers for effective tumor immunotherapy.

Synthesis of Dithioester Derivatives by Base-Mediated Fragmentation of 1,3-Dithiolanes

Dithioesters are important agents for chain transfer in polymer chemistry and precursors in the synthesis of heterocycles. Straightforward approaches to their synthesis are therefore in demand. Outlined herein is a method to access such compounds in one-pot. 2-Aryl-1,3-dithiolanes undergo ring fragmentation with LiHMDS in CPME to generate aryl-dithiocarboxylates in 5 min at 100 °C. These anions are subsequently captured in the second step by addition of various alkyl halides and diaryliodonium salts to furnish a large library of dithioesters in good yields. The method can be also employed in a one-pot, one-step manner for alkyl bromides and allows the synthesis of dithioesters in gram scale.

Synthesis of Bis-Thioacid Derivatives of Diarylethene and Their Photochromic Properties

Diarylethenes (DAEs) are an important class of photoswitchable compounds that typically undergo reversible photochemical conversions between the open and closed cyclized forms upon treatment with UV light or visible light. In this study, we introduced thioacid functional groups to several photochromic dithienylethene (DTE) derivatives and established a method that can be used to prepare these photoswitchable thioacids. Four thioacid-functionalized diarylethene derivatives were synthesized through the activation of carboxylic acids with N-hydroxysuccinimide, followed by reactions with sodium hydrosulfide with yields over 90%. These derivatives exhibited reversible photoswitching and photochromic properties upon treatment with ultraviolet (UV) and visible lights. The thioacid groups on these compounds can act as reaction sites for attaching other desirable functionalities. The photochromic properties of these new derivatives were characterized by using ultraviolet-visible (UV-vis) spectroscopy. The photocyclizations of one of the derivatives and its potassium salt were also characterized by using nuclear magnetic resonance (NMR) spectroscopy. The anions of the thioacid formed water-soluble photochromic systems, and their applications as colorimetric sensors in agarose hydrogels were demonstrated.

Recent advances in asymmetric synthesis of chiral amides and peptides: racemization-free coupling reagents

Over past decades, chiral amides and peptides have emerged as powerful and versatile compounds due to their various biological activities and interesting molecular architectures. Although some chiral condensation reagents have been applied successfully for their synthesis, the introduction of racemization-free methods of amino acid activation have shown lots of advantages in terms of their low cost and low toxicity. In this review, advancements in amide and peptide synthesis using racemization-free coupling reagents over the last 10 years are summarized. Various racemization-free coupling reagents have been applied in the synthesis of enantioselective amides and peptides, including ynamides, allenones, HSi[OCH(CF3)2]3, Ta(OMe)5, Nb(OEt)5, Ta(OEt)5, TCFH-NMI, water-removable ynamides, DBAA, DATB, o-NosylOXY, TCBOXY, Boc-Oxyma, NDTP, 9-silafluorenyl dichlorides, the Mukaiyama reagent, EDC and T3P. The racemization-free reagents described in this review provide an alternative greener option for the asymmetric synthesis of chiral amides and peptides. We hope that this review will inspire further studies and developments in this field.

Ynamide Coupling Reagents: Origin and Advances

Since the pioneering work of Curtius and Fischer, chemical peptide synthesis has witnessed a century's development and evolved into a routine technology. However, it is far from perfect. In particular, it is challenged by sustainable development because the state-of-the-art of peptide synthesis heavily relies on legacy reagents and technologies developed before the establishment of green chemistry. Over the past three decades, a broad range of efforts have been made for greening peptide synthesis, among which peptide synthesis using unprotected amino acid represents an ideal and promising strategy because it does not require protection and deprotection steps. Unfortunately, C → N peptide synthesis employing unprotected amino acids has been plagued by undesired polymerization, while N → C inverse peptide synthesis with unprotected amino acids is retarded by severe racemization/epimerization owing to the iterative activation and aminolysis of high racemization/epimerization susceptible peptidyl acids. Consequently, there is an urgent need to develop innovative coupling reagents and strategies with novel mechanisms that can address the long-standing notorious racemization/epimerization issue of peptide synthesis.This Account will describe our efforts in discovery of ynamide coupling reagents and their application in greening peptide synthesis. Over an eight-year journey, ynamide coupling reagents have evolved into a class of general coupling reagents for both amide and ester bond formation. In particular, the superiority of ynamide coupling reagents in suppressing racemization/epimerization enabled them to be effective for peptide fragment condensation, and head-to-tail cyclization, as well as precise incorporation of thioamide substitutions into peptide backbones. The first practical inverse peptide synthesis using unprotected amino acids was successfully accomplished by harnessing such features and taking advantage of a transient protection strategy. Ynamide coupling reagent-mediated ester bond formation enabled efficient intermolecular esterification and macrolactonization with preservation of α-chirality and the configuration of the conjugated α,β-C-C double bond. To make ynamide coupling reagents readily available with reasonable cost and convenience, we have developed a scalable one-step synthetic method from cheap starting materials. Furthermore, a water-removable ynamide coupling reagent was developed, offering a column-free purification of the target coupling product. In addition, the recycle of ynamide coupling reagent was accomplished, thereby paving the way for their sustainable industrial application.As such, this Account presents the whole story of the origin, mechanistic insights, preparation, synthetic applications, and recycle of ynamide coupling reagents with a perspective that highlights their future impact on peptide synthesis.

Copper-catalyzed room-temperature cross-dehydrogenative coupling of secondary amides with terminal alkynes: a chemoselective synthesis of ynamides

A copper-catalyzed aerobic oxidative cross-dehydrogenative coupling reaction between secondary amides and terminal alkynes has been developed. With the aid of ligands and 3 Å molecular sieves, ynamides can be efficiently synthesized at room temperature and conveniently scaled up. A legitimate mechanism involving nitrogen-centred radicals and copper trivalent intermediates has been proposed.

Synthesis and Functionalization of Thiophosphonium Salts: A Divergent Approach to Access Thioether, Thioester, and Dithioester Derivatives

Herein, we report a straightforward practical method for efficiently obtaining a diverse range of thiophosphonium salts. This method involves the direct coupling of commercially available thiols and aldehydes with Ph3P and TfOH. The setup is simple and carried out in a metal-free manner. The synthetic utility of these salts is demonstrated through various examples of C-P bond functionalizations, enabling the synthesis of thioether, deuterated thioether, thioester, and dithioester derivatives. These products, which serve as valuable building blocks, are obtained in high yields.

Ynamide-Mediated Thioamide and Primary Thioamide Syntheses

Environmentally friendly ynamide-mediated thioamidation of monothiocarboxylic acids with amines or ammonium hydroxide for the syntheses of thioamides and primary thioamides is described. Simple and mild reaction conditions enable the reaction to tolerate a wide variety of functional groups such as hydroxyl group, ester, tertiary amine, ketone, and amide moieties. Readily available NaSH served as the sulfur source, avoiding the use of toxic, expensive, and malodorous organic sulfur reagents and making this strategy environmentally friendly and practical. Importantly, the stereochemical integrity of α-chiral monothiocarboxylic acids was maintained during the activation step and subsequent aminolysis process, thus offering a racemization-free strategy for peptide C-terminal modification. Furthermore, a number of thioamide-modified drugs were prepared in good yields by using this protocol and the synthesized primary thioamides were transformed into backbone thiazolyl modified peptides.

Ni-Catalyzed Synthesis of Thiocarboxylic Acid Derivatives

A Ni-catalyzed cross-coupling of readily accessible O-alkyl xanthate esters or thiocarbonyl imidazolides and organozinc reagents for the synthesis of thiocarboxylic acid derivatives has been developed. This method benefits from a fast reaction time, mild reaction conditions, and ease of starting material synthesis. The use of transition-metal catalysis to access a diverse range of thiocarbonyl-containing compounds provides a useful complementary approach when compared with previously established methodologies.

Metal-Free Stereoselective Addition of Propiolic acids to Ynamides: A Concise Synthetic Route to Highly Substituted Ene-Diyne/Dienyne-(E)-N,O-Acetals

A straight forward and sustainable approach for 1,2-addition of propiolic acids to ynamide has led to bench stable sp2 (E)-enol-enamides of enediynes & dienynes. The reaction is chemo, regio, as...

An Efficient and Straightforward Approach for Accessing Thioesters via Palladium-Catalyzed C-N Cleavage of Thioamides

We first report the coupling of activated thioamides with alcohols to efficiently form thioesters via palladium-catalyzed C-N cleavage strategy. The new approach employs the thioamides as thioacylating reagent to give...

Electrophilic Sulfonium-Promoted Peptide and Protein Amidation in Aqueous Media

A novel amidation strategy using electrophilic sulfonium, which is soluble and stable in aqueous conditions, was developed. The sulfoniums could activate thioacid and carboxyl acid to efficiently react with amines to afford amides. This method enables applications in amidation in both aqueous media and solid-phase peptide synthesis, peptide/protein modifications, and reactive lysines of a proteome at pH 10 with activity-based protein profiling. A peptide ligand-directed labeling of the USP7-UBL2 domain was also performed using this method.

Photo-induced synthesis of β-sulfonyl imides from carboxylic acids

A photo-induced imidation process of carboxylic acids is described. Numerous carboxylic acids could convert to β-sulfonyl imides in the presence of N-sulfonyl ynamides under visible light irradiation. Control experiments and mechanistic studies demonstrate that this imidation process involves a hydroacyloxylation/radical rearrangement cascade. This protocol represents a direct imidation method from carboxylic acids under mild conditions, with broad scope and high atom-economy.

Single-Pot Preparation of 4-Amino-2-(het)aryl-5-Substituted Thiazoles Employing Functionalized Dithioesters as Thiocarbonyl Precursors

An effective, diversity oriented, one-pot reaction of 4-amino-2-(het)aryl/alkyl-5-functionalized thiazoles has been disclosed, utilizing aryl/heteroaryl/alkyl dithioesters as thiocarbonyl coupling partners in a modified Thorpe-Ziegler type cyclization. The reaction proceeds at room temperature, under mild conditions, in excellent yields, displaying broad functional group compatibility at 2 and 5 positions of thiazoles. This synthetic strategy has been further expanded for the one-pot construction of two highly potent tubulin polymerization inhibitors, i.e., 2-(het)aryl-4-amino-5-(3,4,5-trimethoxyaroyl) thiazoles, in high yields.

Photoinduced oxidative cyclopropanation of ene-ynamides: synthesis of 3-aza[n.1.0]bicycles via vinyl radicals

The first photoinduced synthesis of polyfunctionalized 3-aza[n.1.0]bicycles from readily available ene-ynamides and 2,6-lutidine N-oxide using an organic acridinium photocatalyst is reported. Applying a photocatalytic strategy to the reactive distonic cation vinyl radical intermediate from ynamide, a series of bio-valuable 3-azabicycles, including diverse 3-azabicyclio[4.1.0]heptanes and 3-azabicyclo[5.1.0]octanes that are challenging to accomplish using traditional methods, have been successfully synthesized in good to high yields under mild and metal-free conditions. Mechanistic studies are consistent with the photocatalyzed single-electron oxidation of ene-ynamide and the intermediacy of a putative cationic vinyl radical in this transformation. Importantly, this strategy provides new access to the development of photocatalytic vinyl radical cascades for the synthesis of structurally sophisticated substrates.