Additive-Free Peptide Synthesis Using Pentafluorophenyl Esters as the Sequence Oligopeptide Synthesis via a Flow Reaction
Today, we share an important research article published in Organic Process Research & Developmentby the team of Tomohiro Hattori and Hisashi Yamamoto from the Peptide Science Center at Chubu University, Japan. Addressing long-standing challenges in traditional peptide synthesis such as excessive additive use, byproduct generation, and process complexity, this study developed a novel flow chemistry synthesis strategy based on pentafluorophenyl (Pfp) esters. The core of this method lies in leveraging the high reactivity and stability of Pfp esters to achieve efficient, rapid condensation with stoichiometric amino acid esters—including the highly challenging N-methyl amino acid esters—without requiring any additives. The research further established a continuous flow system integrated with a DBU polymer for online deprotection, successfully accomplishing the sequential assembly from dipeptides to pentapeptides (including bioactive sequences like Leu-enkephalin and thymopentin). The products were obtained with high purity, and the system enabled automated, long-duration synthesis on a gram scale. This work provides a brand-new solution for developing green, efficient, and scalable manufacturing processes for peptide pharmaceuticals.
01 Research Background
Peptide synthesis chemistry has evolved over a century, yet its industrial production still faces numerous challenges: traditional methods (including Solid-Phase Peptide Synthesis, SPPS, and Liquid-Phase Peptide Synthesis, LPPS) typically require excess coupling reagents and activators, leading to increased purification burden and byproduct generation (e.g., deletion sequences). Furthermore, the deprotection steps under strong acid/base conditions often have poor compatibility with subsequent condensation reactions, making efficient one-pot continuous synthesis difficult. Flow chemistry technology, known for its high reaction efficiency and safe, controllable processes, shows great potential in peptide synthesis. However, existing flow processes still cannot avoid additive use and intermediate purification. Pentafluorophenyl (Pfp) esters, as a class of highly reactive activated amino acid esters, have been used in peptide chemistry for some time, but their precise reaction characteristics under additive-free conditions and their potential in continuous flow assembly have not been fully explored. This study aims to systematically evaluate the unique advantages of Pfp esters in stoichiometric reactions and design a flow system integrated with online deprotection functionality to achieve truly additive-free, continuous oligopeptide synthesis.
02 Innovative Highlights
Chemical Innovation: Revealing the High Reactivity and Broad Applicability of Pfp Esters under Additive-Free Conditions
The study found that, compared to other aryl esters (e.g., nitrophenyl esters), Fmoc-amino acid-Pfp esters could quantitatively form dipeptides in just 5 minutes using only 1.1 equivalents of nucleophilic amino acid esters in THF solution without any catalysts. Crucially, this strategy is also effective for sterically hindered, low-reactivity N-methyl amino acid esters, solving the long-standing inefficiency problem in N-methyl peptide synthesis.
Process Innovation: Pioneering a Continuous Online Deprotection-Condensation Cycle Based on DBU Polymer
The research team designed a column reactor packed with DBU polymer for the efficient online removal of the Fmoc protecting group. This design enabled seamless integration of the deprotection step with subsequent condensation reactions within the flow system, avoiding interference from liquid additives on downstream reactions, which was the core to the successful operation of the entire continuous process.
System Innovation: Constructing a Flexible, Scalable Multi-Stage Flow Assembly Platform
By designing a flow system with multi-channel feeding and matched reaction line volumes, the successful sequential continuous synthesis from di- to tetra- and pentapeptides was achieved. The system ensured complete reaction at each step by adjusting line lengths and reactor combinations (e.g., connecting two DBU polymer columns in series for tetrapeptide synthesis), ultimately yielding target oligopeptides with high yields (mostly >80%) and high diastereoselectivity (dr >20:1).
Application Innovation: Demonstrating Feasibility for Gram-Scale Long-Term Operation
Using the synthesis of the tripeptide Fmoc-L-Ala-L-Ala-L-Ala-Ot-Bu as an example, the system could stably operate continuously for 150 hours. Furthermore, the synthesis of thymopentin achieved a yield of 159 grams over 72 hours, proving the significant potential of this flow process for scale-up from laboratory to production.
03 Results and Discussion
3.1 Systematic Evaluation and Optimization of Pfp Ester Reactivity
The team first compared the activity of different aryl esters (phenyl ester, p-nitrophenyl ester, pentafluorophenyl ester, etc.) in an alanine model reaction. Results showed that Fmoc-L-Ala-OPfp far outperformed other esters, completing the reaction almost quantitatively in just 5 minutes under additive-free conditions. This superiority stems from the strong electron-withdrawing effect of the Pfp group, significantly increasing the electrophilicity of the carbonyl and thus accelerating the nucleophilic attack by the amine.
Table 1. Effects of the Acid Aryl Esters on Peptide Bond Formation
3.2 Successful Breakthrough in N-Methyl Peptide Synthesis
This strategy was successfully applied to the condensation of various N-methyl amino acid esters. Excellent isolated yields and high stereoselectivity were obtained for corresponding N-methyl dipeptides, regardless of whether they involved highly reactive alanine and phenylalanine, or amino acids with bulky side chains like valine, leucine, and isoleucine. This demonstrates the generality of the Pfp ester strategy for challenging substrates, opening a new pathway for preparing complex N-methylated bioactive peptides.
Scheme 1. Peptide Bond Formation Using Fmoc-Amino Acid Pfp Esters and N-Methyl Amino Acid Esters
3.3 Construction of the Continuous Flow System and Dipeptide Synthesis
The team constructed a preliminary flow synthesis system. Two streams of THF solutions containing Fmoc-amino acid-Pfp ester (1) and amino acid tert-butyl ester (2), respectively, were pumped by syringe pumps at a flow rate of 1.0 mL/min into an empty mixer, then through a DBU polymer column heated to 50°C for deprotection. The N-deprotected dipeptide ester (4a-4g) was obtained directly at the outlet. The entire process took only minutes, achieving genuine integrated "reaction-deprotection" flow operation.
Scheme 2. Flow-Type Sequential Peptide Bond Formation and Fmoc-Deprotection
3.4 Flow Assembly of Tripeptides and Longer Sequences & Investigation of Substrate Scope
Building on the dipeptide synthesis flow (Route A), a third amino acid solution stream (Route B) was introduced, successfully assembling tripeptides. This system demonstrated an exceptionally broad substrate scope: amino acids with sulfur-containing side chains (cysteine), ethers (serine), esters (aspartic acid/glutamic acid derivatives), amino groups (lysine), and bulky alkyl side chains (valine, leucine, isoleucine) all participated smoothly in the reaction (6h-6y). Notably, phenylglycine, prone to racemization, also maintained excellent stereochemical integrity under these mild, rapid conditions (6f, 6g). Furthermore, the tripeptide synthesized using N-methylvaline (6z) was obtained in 79% yield, further highlighting the method's advantage for synthesizing difficult sequences.
Scheme 3. Substrate Scope of Tripeptide Synthesis
3.5 Synthesis of Tetrapeptides, Pentapeptides, and Preparation of Bioactive Peptides
To address potential incomplete deprotection due to increased flow rates in longer peptide chain synthesis, the team introduced a third amino acid solution stream (Route C) after Routes A+B and connected two DBU polymer columns in series, successfully synthesizing various tetrapeptides (8a-8g). Even more impressively, by using dipeptide esters as starting materials, pentapeptide sequences were successfully synthesized, including important bioactive peptides—Leu-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH) and thymopentin (Arg-Lys-Asp-Val-Tyr) (8h, 8i). These protected peptides could be quantitatively converted to the target products under simple final deprotection conditions (e.g., TFA treatment). Moreover, thymopentin was continuously prepared on a 159-gram scale, fully demonstrating the immense application prospects of this flow system for preparing practical peptide pharmaceuticals.
Scheme 4. Substrate Scope of Tetrapeptide Synthesis
04 Conclusion and Future Perspectives
This study successfully developed a novel additive-free peptide synthesis strategy based on Pfp esters and accordingly constructed an efficient, scalable continuous flow synthesis platform. The core value of this work lies in: ① Chemically, fully exploring and validating the unique reactivity and functional group tolerance of Pfp esters under additive-free, stoichiometric reaction conditions, particularly solving the challenge of N-methyl peptide synthesis; ② Process-wise, achieving online integration of deprotection and condensation steps through clever reactor design (DBU polymer column), avoiding intermediate purification and additive interference; ③ Practically, successfully realizing continuous flow assembly from di- to pentapeptides and long-term operation on a gram scale, providing a practical and feasible technological path for the green, economical, and scalable production of peptide pharmaceuticals. In the future, this platform could be further integrated with automated control systems to achieve fully automated rapid construction of oligopeptide sequences. Simultaneously, exploring the application of this strategy to the synthesis of more complex modified peptides (e.g., cyclic peptides, glycopeptides) would greatly expand its application boundaries. In summary, this research is not only a significant breakthrough for flow chemistry in peptide synthesis but also provides an inspiring example for addressing long-standing issues in organic synthesis, such as byproduct generation and multi-step operations.
Originial Article:
Hattori T, Matsunaga Y, Yamamoto H. Additive-Free Peptide Synthesis Using Pentafluorophenyl Esters as the Sequence Oligopeptide Synthesis via a Flow Reaction[J]. Organic Process Research & Development, 2025, 29(11): 2815-2822.