Literature Sharing: Rapid Construction of a Tyr C6–Trp C′ Linkage: Application in the Total Synthesis of Micitide 982, a Noncanonical Cyclic Peptide

          Today, we are sharing a research article led by Professor Hugh Nakamura from the Hong Kong University of Science and Technology (HKUST), published in Angewandte Chemie International Edition. This study developed an efficient synthetic strategy that integrates electrochemically assisted nickel-catalyzed cross-electrophile coupling with regioselective Larock macrocyclization, successfully constructing the highly strained tyrosine C6–tryptophan C5′ (Tyr C6–Trp C5′) biaryl linkage and accomplishing the total synthesis of micitide 982, a ribosomally synthesized and post-translationally modified peptide (RiPP). This work provides a modular and generalizable approach for synthesizing analogous complex cyclic peptides.

01 Research Background

         Ribosomally synthesized and post-translationally modified peptides (RiPPs) have attracted significant attention in natural product research due to their structural diversity and remarkable biological activities. Many RiPPs contain noncanonical biaryl linkages, which often exhibit atropisomerism and present considerable synthetic challenges. Micitide 982, a RiPP discovered in 2023, features a core structure comprising a highly strained 13-membered ring formed by three amino acids (L-tyrosine, L-proline, and L-tryptophan) and cross-linked via a rare Tyr C6–Trp C5′ biaryl bond (Figure 1a). Due to its extreme scarcity (only 5 mg was isolated from 10 L of fermentation broth) and unknown bioactivity, chemical synthesis became essential for functional exploration. However, conventional macrocyclization strategies via amide bond formation proved ineffective—yields were below 5%—due to the excessive ring strain imposed by the rigidity and linearity of the biaryl bridge. Thus, a novel synthetic strategy was urgently needed.

02 Innovative Highlights

• Development of a tandem electrochemical coupling–macrocyclization strategy: Overcoming the limitations of traditional amide-based macrocyclization for highly strained peptides, this work pioneers the integration of Ni-electrocatalytic cross-coupling and Pd-catalyzed Larock indole annulation, enabling efficient and mild incorporation of biaryl motifs into complex peptide scaffolds.
• Achievement of the first total synthesis of micitide 982: This strategy successfully addresses the challenge of constructing the constrained Tyr C6–Trp C5′ linkage, confirming the proposed structure of the natural product.
• Demonstration of high modularity and broad applicability: The use of a peptide precursor bearing terminal halogen and alkyne groups allows late-stage, modular incorporation of diverse biaryl units, greatly facilitating the construction of structural analogues (Figure 1d, Figure 3).

Figure1.a) Representative noncanonical cyclic peptides. b) Proposed biosynthetic pathway. c) Biosynthesis. d) This work.

03 Results and Discussion

• Failure of conventional macrocyclization and strategic insight

The authors first attempted classical amide bond macrocyclization routes. Precursors 2 and 3 were synthesized and subjected to various coupling reagents (EDCI, DCC, HATU, etc.), but all attempts failed, affording the desired cyclic product 4 in less than 5% yield (Figure 2a). These results underscore the incompatibility of traditional methods with the high ring strain induced by the rigid biaryl unit and highlight the need for an alternative cyclization logic.

• Design of a new convergent strategy and retrosynthesis

A new convergent route was designed (Figure 2b). The key insight was to first install the biaryl motif onto a tripeptide linker bearing bromide and alkyne terminals via Ni-catalyzed cross-coupling, followed by Larock cyclization to simultaneously form the biaryl bond and macrocycle. Retrosynthetically, the target molecule was disconnected into biaryl iodide 22 and peptide linker 19, which would be coupled in two steps.

Figure 2. a) High level summary of documented failures. b) A synthetic approach to micitide 982 (1).

• Crucial optimization of electrochemical Ni-catalyzed coupling

The coupling between peptide linker 19 and biaryl unit 22 was a critical first step. Initial attempts using stoichiometric Zn/Mn as reductants in Ni-catalyzed systems performed poorly (~11% yield). The team introduced electrochemical synthesis, using electrical current instead of metal reductants. Ultimately, under conditions employing NiBr₂/dtbbpy (L12) as the catalytic system and a constant current of 4 mA at room temperature, the desired coupled product 23 was obtained in 47% yield (Scheme 1). This step proceeded under mild conditions, demonstrating excellent compatibility with the complex peptide substrate.

• Larock macrocyclization constructs the core scaffold

With linear precursor 23 in hand, intramolecular Larock reaction catalyzed by Pd(OAc)₂/tBu₃P successfully forged the highly strained 13-membered ring via Tyr C6–Trp C5′ bond formation, yielding the core cyclic peptide 24 in 45% yield (Scheme 1). NMR spectroscopy confirmed its structure matched that of the natural product.

Scheme 1. Concise construction of the Tyr C6-to-Trp C5 linkage and the total synthesis of micitide 982

• Theoretical study on conformational stability

Why did Larock cyclization succeed where amide cyclization failed? DFT calculations revealed that the cyclized product exists as two atropisomers, 26 and 27. The rotational energy barrier between them was high (32.6 kJ/mol), and isomer 26 was thermodynamically more stable by 11.4 kJ/mol, existing as a single, isolable conformer in a >99:1 ratio at room temperature (Scheme 2). Calculations also indicated that cyclization proceeded through a more stable transition state leading to 26, confirming it as the kinetically favored product.

 Scheme 2. Thermodynamic stability of conformers 26 and 27.

• Demonstration of modular applicability

To showcase the generality of this platform, four diverse biaryl building blocks (28–31) were synthesized. All were successfully incorporated into peptide linker 19 via the “electrochemical coupling – Larock cyclization” two-step sequence, affording structurally diverse cyclic peptide analogues 32–35 in good overall yields (Figure 3). This powerfully demonstrates the strategy’s potential for building libraries of biaryl-linked cyclic peptides.

Figure 3. Substrate scope for the modular biarylation

04 Conclusion

This study developed a convergent synthetic strategy that combines Ni-electrocatalytic cross-electrophile coupling with Pd-catalyzed Larock macrocyclization, overcoming the challenge of constructing the highly strained Tyr C6–Trp C5′ biaryl linkage and achieving the first total synthesis of micitide 982 on a gram scale. The key advantage of this approach is its high modularity and convergence: by designing a universal peptide precursor bearing halogen and alkyne terminals, diverse biaryl units can be flexibly introduced at a late stage. As demonstrated, this method is not only applicable to the synthesis of complex natural products but also enables the rapid construction of analogue libraries, providing a powerful tool for cyclic peptide-based drug discovery. This work transcends the limitations of traditional enzymatic approaches, showcasing the power of synthetic chemistry to create complex biomolecules and opening new avenues for the synthesis and study of RiPPs and other biaryl-based bioactive molecules.

Original Article:

Ogawa H, Nagata Y, Chan T K, et al. Rapid Construction of a Tyr C6–Trp C5′ Linkage: Application in the Total Synthesis of Micitide 982, a Noncanonical Cyclic Peptide[J]. Angewandte Chemie, 2025: e202516053.

 https://doi.org/10.1002/anie.202516053