Ynamide Coupling Reagents: From Low-Epimerization Activation Pathways to More Practically Usable N→C Inverse Peptide Synthesis

Introduction

In peptide synthesis, what is often truly difficult to control is not whether an amide bond can be formed, but rather how to minimize epimerization at the α-stereogenic center of the activated carboxylic acid component during activation and the subsequent bond-forming process. This issue is particularly prominent in peptide fragment coupling, head-to-tail cyclization, and any route in which preservation of the stereochemical integrity at the peptide acid terminus is a major concern.

Ynamide coupling reagents have attracted attention in recent years not simply because they increase the intrinsic strength of carboxylic acid activation, but because they organize carboxylic acid activation along a pathway distinct from conventional uronium, phosphonium, or carbodiimide systems. In this pathway, a relatively stable and manageable α-acyloxyenamide activated intermediate is first formed, followed by subsequent aminolysis or alcoholysis; in sulfur-containing transformations, the corresponding sulfur-containing activated intermediate is generated before the subsequent bond-forming step takes place. It is precisely this strategy of “first managing the activated intermediate clearly, and then optimizing the downstream bond-forming step” that has enabled ynamide systems to show good epimerization control in reported models and applications, while also pushing this approach further toward a more practically usable direction for N→C inverse peptide synthesis.

1. In Peptide Synthesis, What Is Truly Difficult to Control Is the Retention of Stereochemistry After Carboxylic Acid Activation

Conventional forward peptide synthesis is already highly mature, but stereochemical problems have not disappeared. The mainstream routes currently still begin with amino-protected amino acids, activate the carboxyl group first, and then form a bond with the free amino group on the peptide chain. Protecting groups, additives, and condition optimization can reduce side reactions, but they cannot eliminate the risk of racemization or epimerization of the activated carboxylic acid component during activation and the subsequent bond-forming process. For amino acids and peptide acids containing α-stereogenic centers, this remains a key factor affecting yield, purity, and scalability.

This problem is especially sensitive in N→C inverse peptide synthesis, although the high-risk step is not identical across all routes. The activated α-aminoester route reported in 2014 was not based on repeated activation of peptide acids; rather, it served as a proof of concept for inverse peptide synthesis by activating the amino-terminal component. The peptide thiocarboxylic acid route reported in 2023, by contrast, advanced N→C chain extension through C-terminal peptide thiocarboxylic acid/active ester transformations. In 2024, the combination of ynamide chemistry with a transient protection strategy pushed the incorporation of formally unprotected amino acids to a more practically usable level. The central challenge shared by all three routes is how to continuously suppress epimerization of the residue corresponding to the C terminus of the growing chain during N→C extension.

1.1 Significance of Low-Epimerization Activation Pathways in Different Peptide-Bond-Forming Tasks

Research scenario	Immediate experimental objective	Key difficulty	Why simply increasing activation strength is not enough
Forward peptide synthesis	Form a single amide bond	The α-stereogenic center of the activated carboxylic acid component can still be compromised during activation and subsequent aminolysis	Faster activation does not necessarily mean better stereochemical integrity; overly strong activation may also amplify side reactions
Peptide fragment coupling	Connect longer and more complex peptide segments	Peptide acid substrates are more complex, making it more difficult to suppress side reactions and epimerization simultaneously	Simply increasing reactivity often makes it difficult to balance selectivity with intermediate controllability
Head-to-tail cyclization	Construct macrocycles or conformationally constrained scaffolds	Dilution effects, conformational preorganization, and competing side reactions all coexist	The goal is not merely to close the ring, but also to preserve the original stereochemical information as much as possible
Inverse peptide synthesis	Achieve chain extension with as few protection and deprotection steps as possible	Repeated activation of peptide acid termini that are highly sensitive to epimerization is required	This is a route-level problem that requires simultaneous control over activated-intermediate stability and the conditions for the subsequent bond-forming step

2. Key Features of Ynamide Coupling Reagents and the Extension of Their Methodology

A 2016 study showed that ynamides can serve as a class of low-racemization, low-epimerization coupling reagents for amide and peptide synthesis. Subsequent mechanistic studies further demonstrated that the key feature of this system lies not in simply increasing the strength of carboxylic acid activation, but in first converting the carboxylic acid into a stable α-acyloxyenamide active ester, followed by subsequent aminolysis, alcoholysis, or sulfur-based transformation. For carboxylic acid substrates bearing α-stereogenic centers, the value of this activation mode lies in the fact that it not only helps control stereochemical loss during the activation stage, but also improves selectivity and stability during the subsequent bond-forming stage.

The later development of this approach suggests that ynamide coupling reagents are best viewed as an extendable carboxylic acid activation pathway. By 2022, this chemistry had already been extended to peptide fragment coupling, head-to-tail cyclization, and solid-phase peptide synthesis. A 2024 review further summarized its applications in esterification, thioamide incorporation, and inverse peptide synthesis.

2.1 Representative Application Scenarios of Ynamide Coupling Reagents and Their Methodological Significance

Experimental task	Major challenge	Main value of the ynamide system
Conventional amidation or dipeptide coupling	Need to balance yield with stereochemical integrity	Formation of a relatively stable α-acyloxyenamide active ester prior to the subsequent bond-forming step helps reduce the risk of stereochemical loss during activation and subsequent aminolysis
Peptide fragment coupling	More complex substrates, with side reactions and epimerization carrying a higher cost	Better suited to balancing coupling efficiency with retention of the α-stereogenic center in complex peptide acid substrates
Head-to-tail cyclization	Macrocycle formation and stereochemical retention often need to be solved simultaneously	Suitable for evaluating ring-closure efficiency and stereochemical retention in parallel, thereby more clearly revealing the value of this system in complex bond-forming tasks
Thioamide or thiolated peptide incorporation	Requires precise sulfur-containing modification at defined sites within the peptide backbone	Relevant sulfur-containing activated intermediates can be generated from monothiocarboxylic acid donors and ynamides for construction of thiopeptide bonds/thioamide sites, showing that the application of this system is not limited to ordinary amide-bond formation
Solid-phase peptide synthesis	Diffusion, swelling, and reaction rate within the resin environment can limit conversion	After mechanistic and condition optimization, the method can be extended to solid-phase peptide synthesis and is suitable for use under conditions where water promotes subsequent aminolysis and swellable resins are employed

3. From Amidation to Esterification and Macrolactonization: Extension of the Ynamide-Based Carboxylic Acid Activation Pathway

Ynamide coupling reagents were subsequently applied to intermolecular esterification and macrolactonization. A 2020 study reported a one-pot, two-step intermolecular esterification mediated by ynamides, showing that the α-acyloxyenamide intermediate formed from a carboxylic acid and an ynamide can continue to react with alcohols and phenols under appropriate conditions. Work on macrolactonization published in the same year further demonstrated that this pathway can also be extended to more complex ring-closing esterification tasks. Based on these developments, the 2024 review summarized ynamide coupling reagents as a class of general coupling reagents that have expanded from amide-bond formation to ester-bond formation.

These two types of extension are not significant in exactly the same way. Intermolecular esterification shows that the activated intermediate generated in the ynamide system is not limited to amine nucleophiles. Macrolactonization further shows that this pathway can do more than construct linear ester bonds; it can also enter macrocyclic tasks that depend more strongly on conformational control, side-reaction management, and functional-group compatibility.

Extended task	Key information	What this work demonstrates
Intermolecular esterification	The α-acyloxyenamide intermediate can continue to react with alcohols and phenols	This carboxylic acid activation pathway can be used not only for amide-bond formation, but also for ester-bond formation
Macrolactonization	The same activation logic can enter acid-catalyzed ring-closing processes	The ynamide system is capable not only of linear bond formation, but also of handling more complex macrocycle construction tasks

4. Inverse Peptide Synthesis: The Ynamide System Begins to Enter Peptide Chain Extension Routes

A 2024 study reported a practically operable inverse peptide synthesis method that combines a transient protection strategy. Formally unprotected amino acids are used as starting building blocks, and transient protection, activation, aminolysis, and in situ deprotection are carried out within the same reaction system. The core problem addressed by this work is the long-standing issue of severe racemization or epimerization in N→C peptide chain extension.

Placing this advance back into the broader methodological context makes its position easier to understand. The activated α-aminoester route reported in 2014 demonstrated that advancing inverse peptide synthesis by activating the amino-terminal component is chemically feasible. The peptide thiocarboxylic acid route reported in 2023 further showed that N→C chain extension can move toward fewer protecting-group operations and higher atom economy. By 2024, the combination of ynamide chemistry with a transient protection strategy had directly pushed low-epimerization activation into a more practically usable inverse peptide synthesis design.

4.1 Three Representative Milestones in N→C Inverse Peptide Synthesis

Time and representative work	Core concept	What it advanced
2014, activated α-aminoester route	Approached inverse peptide synthesis by activating the amino-terminal component	Demonstrated that inverse peptide synthesis is chemically feasible
2023, peptide thiocarboxylic acid route	Advanced N→C chain extension with fewer protecting-group operations	Showed that inverse peptide synthesis can move toward cleaner and more atom-economical directions
2024, ynamide + transient protection route	Achieved a practically operable inverse peptide synthesis approach based on low-epimerization activation	Pushed inverse peptide synthesis toward a more practically useful route design

5. Three Issues Worth Continued Attention in the Future Development of Ynamide Coupling Reagents

Focus	Progress to date	Questions worth watching going forward
Workup and process friendliness	Water-wash-removable ynamide coupling reagents have already appeared; byproducts can be removed by acidic aqueous washing, and some routes can avoid column chromatography	Whether these advantages can be maintained reliably on larger scales and with more complex substrates
Compatibility with complex peptide tasks	Already extended to peptide fragment coupling, head-to-tail cyclization, and solid-phase peptide synthesis	Whether the scope can continue to expand to longer peptide segments, a broader range of side-chain types, and more complex sequences
Generality of inverse peptide synthesis	The first practically operable ynamide-mediated inverse peptide synthesis route has already appeared	Whether it can develop from a representative method into a more general peptide chain extension strategy

6. Product Navigation Table for Research on Ynamide Coupling Reagents (Choose Table 1–Table 3 by Research or Experimental Goal)

Current research or experimental goal	Recommended table to consult first	Why this table should be consulted first	Suggested table(s) to consult next	Navigation note
Want to first clarify which core reagents constitute the ynamide coupling reagent system itself, and identify which types of key chemicals are the real research focus	Table 1	Table 1 concentrates on core terminal ynamide coupling reagents such as MYTsA and MYMsA, as well as upstream precursors such as N-methyl-p-toluenesulfonamide and N-methylmethanesulfonamide, making it the most suitable starting point for understanding the system itself	Then see Table 2	First clarify “which compounds are the core reagents, which are the precursors, and which structural types are the true research objects”; after that, it becomes easier to build a solid experimental route by moving on to condition tuning and comparison with control systems
Want to build the basic experimental workflow for ynamide coupling reagents from scratch and evaluate how bases, acids, additives, and active esters affect the reaction outcome	Table 2	Table 2 brings together condition-tuning and active-ester reference components such as DIPEA, cesium carbonate, p-toluenesulfonic acid monohydrate, HOAt, NHS, Oxyma, and HOBt, making it the most suitable table for reaction screening and mechanism-related condition optimization	Then see Table 1	First establish the condition window, reagent addition logic, and reference activation modes; then return to the core ynamide reagents in Table 1, which makes it easier to judge the performance differences of different reagents under specific conditions
Want to compare ynamide coupling reagents with classical coupling systems and determine whether they are truly worth using in place of conventional coupling methods	Table 3	Table 3 focuses on classical coupling reagents such as DCC, DIC, EDC·HCl, CDI, HATU, HBTU, PyBOP, COMU, TOTU, and TBTU, making it best suited for parallel control experiments and for assessing the strengths and weaknesses of different routes	Then see Table 1 and Table 2	First clarify the activation types and common experimental practices of the classical control systems; then return to Tables 1 and 2 to examine the reagent and condition setup of the ynamide system, which makes it easier to see where its real advantages come from
Want to carry out low-epimerization amidation or peptide coupling, with emphasis on how different activation pathways affect stereochemical retention	Table 1 and Table 3	Table 1 provides the core ynamide coupling reagents, while Table 3 provides the most common carbodiimide, uronium, and phosphonium control systems; these two tables are well suited for direct side-by-side comparison of activation pathways and stereochemical retention	Then see Table 2	After completing the primary comparison between the core reagents and classical coupling reagents, further refine the conditions with components such as HOAt, Oxyma, HOBt, and NHS in Table 2, which is more suitable for epimerization-control studies
Want to study the differences between α-acyloxyenamide active esters and intermediates such as NHS esters or acyl imidazoles	Table 2 and Table 3	NHS, HOAt, Oxyma, and HOBt in Table 2 help establish active-ester and additive references; CDI, DCC, and EDC·HCl in Table 3 correspond to different classical activation pathways and are suitable for intermediate-level comparisons	Then see Table 1	First clarify the experimental significance of the different reference intermediates and activation modes; then return to Table 1 to examine the activated intermediates formed by ynamide reagents, which is more helpful for mechanistic analysis and route evaluation
Want to extend the ynamide system to fragment coupling, head-to-tail cyclization, or more complex peptide tasks	Table 1 and Table 2	Table 1 provides the core ynamide reagents responsible for activation, while Table 2 provides key condition components such as DIPEA, HOAt, and p-toluenesulfonic acid monohydrate that affect subsequent aminolysis, cyclization, or acid-catalyzed transformations	Then see Table 3	First establish a combination of “core reagent + condition tuning,” then compare it with the conventional high-activity coupling reagents in Table 3, which makes it easier to judge whether ynamide reagents or classical systems are more suitable for complex tasks
Want to perform thioamide formation, sulfur-containing backbone modification, or related downstream transformations	Table 2	Components such as cesium carbonate and p-toluenesulfonic acid monohydrate in Table 2 are more closely aligned with the actual needs of sulfur-containing transformations or acid-catalyzed follow-up reactions after ynamide-mediated activation	Then see Table 1 and Table 3	First screen the downstream transformation conditions and media, then separately combine them with the ynamide reagents in Table 1 and the classical coupling reagents in Table 3 for comparison, which makes it easier to determine which type of system is more suitable for sulfur-containing backbone modification
Want to start from routinely available laboratory reagents and quickly establish a beginner-level comparison experiment of “ynamide system vs. classical system”	Table 3	Table 3 covers the most common and easiest-to-use classical coupling reagents, making it suitable for first establishing a conventional benchmark	Then see Table 1 and Table 2	First get the conventional system working smoothly and obtain comparable baseline data; then add the ynamide reagents in Table 1 and the condition-tuning components in Table 2, which is most suitable for building an interpretable comparative experimental design
Want to start from the preparation, recovery, and route optimization of ynamide reagents rather than directly carrying out coupling applications	Table 1	Table 1 contains both the core terminal ynamide reagents and the key upstream precursors, making it the table most directly connected to reagent preparation and structural-source studies	Then see Table 2	First clarify the origins, precursors, and structural differences of the reagents; then introduce the condition-tuning components in Table 2 to examine how these differences affect actual coupling and downstream transformations
Want to systematically determine whether a specific substrate is better suited to ynamide coupling reagents or to conventional systems such as HATU, COMU, or DIC/Oxyma	Table 3	Table 3 is best suited for first establishing a comparative framework among different classical activation families	Then see Table 1 and Table 2	First use Table 3 to rapidly locate the classical candidate systems; then include the ynamide reagents in Table 1 in parallel screening and fine-tune the base, acid, and additive conditions with Table 2, which is more consistent with real-world selection workflows

Table 1 | Core Ynamide Coupling Reagents and Upstream Precursor Components

Classification	CAS No.	Aladdin Cat. No.	Name	Grade or Purity	Product Features and Applications
Upstream haloalkene starting material for ynamide reagents	75-35-4	D433595	1,1-Dichloroethylene (VDC)	≥99%, contains 200 ppm MEHQ as inhibitor	Can be used for the upstream preparation of terminal ynamide coupling reagents such as MYTsA and MYMsA, and is suitable for reagent synthesis, route optimization, and comparison of precursor sources.
Upstream precursor for p-toluenesulfonyl-type ynamide reagents	640-61-9	M130028	N-Methyl-p-toluenesulfonamide	≥98% (HPLC)	An important nitrogen-containing precursor for p-toluenesulfonyl-type terminal ynamide coupling reagents; suitable for one-step preparation, recovery studies, and route optimization of MYTsA-type reagents.
Upstream precursor for methanesulfonyl-type ynamide reagents	1184-85-6	M121825	N-Methylmethanesulfonamide	≥98%	An important nitrogen-containing precursor for methanesulfonyl-type terminal ynamide coupling reagents; suitable for the preparation, recovery and reuse studies, and screening of different synthetic routes for MYMsA-type reagents.
p-Toluenesulfonyl-type terminal ynamide coupling reagent	1005500-75-3	N397619	N-Ethynyl-N,4-dimethylbenzenesulfonamide	≥98%	A representative terminal ynamide coupling reagent that can be used to construct α-acyloxyenamide active esters; suitable for studies on low-epimerization amidation, peptide coupling, fragment coupling, and head-to-tail cyclization.
Methanesulfonyl-type terminal ynamide coupling reagent	1675790-91-6	N397622	N-Ethynyl-N-methylmethanesulfonamide	≥98%	A representative methanesulfonyl-type terminal ynamide coupling reagent; suitable for parallel comparison with p-toluenesulfonyl-type reagents in terms of activation efficiency, substrate compatibility, workup convenience, and peptide-coupling performance.

Table 2 | Condition-Tuning Components and Active Ester/Additive Reference Components for Ynamide Systems

Classification	CAS No.	Aladdin Cat. No.	Name	Grade or Purity	Product Features and Applications
Non-nucleophilic organic base	7087-68-5	D109322	N,N-Diisopropylethylamine	Distilled grade, ≥99.5%	A commonly used non-nucleophilic base; suitable for subsequent aminolysis of ynamide active esters, solid-phase coupling, and optimization of the basic reaction window, facilitating comparison of how base strength and reagent addition order affect reaction rate and stereochemical retention.
Inorganic basic promoter for downstream transformations	534-17-8	C432848	Cesium carbonate	purum p.a., ≥98% (T)	Suitable for studies of downstream transformations of ynamide systems under relatively strong basic conditions, especially for condition screening in thioamide, thioester, and related sulfur-containing derivatization steps.
Component for acid-catalyzed downstream transformations	6192-52-5	T104290	p-Toluenesulfonic acid monohydrate	AR, ≥98.5%	Commonly used in acid-catalyzed downstream transformations; suitable for condition optimization in macrolactonization, alcoholysis, or ring-closing steps involving ynamide active esters.
Low-epimerization coupling additive	39968-33-7	H109328	1-Hydroxy-7-azabenzotriazole	≥99%	Can be used together with DIPEA to accelerate aminolysis and coupling on resin; suitable for comparing how additives affect the reaction rate, completeness of conversion, and epimerization control of ynamide active esters.
Classical NHS active ester construction component	6066-82-6	H109330	N-Hydroxysuccinimide (NHS)	≥98%	A classical component for constructing NHS active esters; suitable for comparison with the α-acyloxyenamide active esters formed by ynamides, in order to evaluate the stability and downstream coupling performance of different pre-activated intermediates.
Oxyma-type low-epimerization additive	3849-21-6	E138773	Ethyl (hydroxyimino)cyanoacetate	≥98%	A precursor to Oxyma-type additives, commonly used in low-epimerization coupling systems; suitable for parallel comparison with the ynamide activation pathway in terms of side-reaction control, substrate compatibility, and coupling efficiency.
Classical benzotriazole additive	123333-53-9	H106176	1-Hydroxybenzotriazole Monohydrate	≥97%	A classical coupling additive; suitable for comparing activation efficiency, reaction selectivity, and protection of sensitive chiral substrates among HOAt, Oxyma, and ynamide systems.

Table 3 | Classical Reference Coupling Reagents and Activation-System Components

Classification	CAS No.	Aladdin Cat. No.	Name	Grade or Purity	Product Features and Applications
Classical carbodiimide coupling reagent	538-75-0	D106074	N,N′-Dicyclohexylcarbodiimide	≥99%	A classical carboxylic acid activating reagent; suitable for comparing coupling efficiency, handling of precipitated byproducts, and applicability to epimerization-sensitive substrates with ynamide coupling reagents.
Imidazole-type activating reagent	530-62-1	C109315	N,N'-Carbonyldiimidazole (CDI)	≥99%	Can channel carboxylic acids into the acyl imidazole activation pathway; suitable for comparing intermediate types, reaction mildness, and inverse-peptide-synthesis-related activation strategies with the ynamide active ester pathway.
HOAt-type uronium coupling reagent	148893-10-1	H109327	HATU	≥99%	A highly active HOAt-type uronium coupling reagent; suitable for comparing performance in difficult couplings, fragment couplings, and substrates requiring high stereochemical fidelity with ynamide reagents.
HOBt-type uronium coupling reagent	94790-37-1	H106174	HBTU	≥99%	A commonly used peptide coupling reagent; suitable for comparing activation efficiency, ease of reagent addition, and side-reaction control in conventional amidation and solid-phase peptide synthesis with the ynamide platform.
Classical carbodiimide coupling reagent	693-13-0	N420184	N,N'-Diisopropylcarbodiimide	≥98.5%	Commonly used in peptide and solid-phase coupling; can be combined with Oxyma, HOBt, HOAt, and others; suitable for comparing low-epimerization strategies and process operability with ynamide systems.
Water-soluble carbodiimide coupling reagent	25952-53-8	E106172	N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride	≥98%	A water-soluble carboxylic acid activating reagent; suitable for amidation control experiments in aqueous media or where simpler workup is desired, and for comparison of coupling performance in different media with ynamide systems.
Phosphonium-type coupling reagent	128625-52-5	P109336	1H-Benzotriazol-1-yloxytripyrrolidinophosphonium Hexafluorophosphate	≥98%	A classical phosphonium-type coupling reagent; suitable as a high-activity reference in peptide coupling and fragment coupling, for comparison with ynamide systems in reaction rate, workup, and stereochemical retention.
Oxyma-type uronium coupling reagent	1075198-30-9	C340003	COMU	≥98%	A representative Oxyma-type uronium coupling reagent; suitable for comparing overall performance in low-epimerization coupling, compatibility with sensitive substrates, and peptide synthesis with ynamide reagents.
Oxyma-type tetrafluoroborate uronium coupling reagent	136849-72-4	E102847	TOTU	≥98%	An Oxyma-derived uronium coupling reagent; suitable for parallel evaluation with COMU and ynamide reagents in terms of coupling activity, epimerization control, and difficulty of byproduct handling.
HOBt-type tetrafluoroborate uronium coupling reagent	125700-67-6	T109338	TBTU	≥98%	A classical tetrafluoroborate uronium coupling reagent; suitable as a benchmark system for routine amidation and peptide coupling, and for comparing reactivity, condition compatibility, and coupling performance under different activation pathways with ynamide reagents.

Note: The above are representative Aladdin products. For more product specifications, please refer to the product list at the end of the article or search the Aladdin website using the product name/CAS/catalog number.

References

[1] Hu L, Xu S, Zhao Z, et al. Ynamides as Racemization-Free Coupling Reagents for Amide and Peptide Synthesis. Journal of the American Chemical Society. 2016;138(40):13135-13138. doi:10.1021/jacs.6b07230.

[2] Xu S, Jiang D, Peng Z, et al. Ynamide-Mediated Peptide Bond Formation: Mechanistic Study and Synthetic Applications. Angewandte Chemie International Edition. 2022;61(46):e202212247. doi:10.1002/anie.202212247.

[3] Yang J, Wang C, Xu S, Zhao J. Ynamide-Mediated Thiopeptide Synthesis. Angewandte Chemie International Edition. 2019;58(5):1382-1386. doi:10.1002/anie.201811586.

[4] Hu L, Zhao J. Ynamide Coupling Reagents: Origin and Advances. Accounts of Chemical Research. 2024;57(6):855-869. doi:10.1021/acs.accounts.3c00743.

[5] Liu T, Peng Z, Lai M, et al. Inverse Peptide Synthesis Using Transient Protected Amino Acids. Journal of the American Chemical Society. 2024;146(6):4270-4280. doi:10.1021/jacs.4c00314.

[6] Suppo JS, Subra G, Bergès M, et al. Inverse Peptide Synthesis via Activated α-Aminoesters. Angewandte Chemie International Edition. 2014;53(21):5389-5393. doi:10.1002/anie.201402147.

[7] Tatsumi T, Sasamoto K, Matsumoto T, et al. Practical N-to-C Peptide Synthesis with Minimal Protecting Groups. Communications Chemistry. 2023;6:231. doi:10.1038/s42004-023-01030-0.

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