Ensuring that "slow-moving amines" don't fall behind: TCFH/ Oxyma and transient imine protection strategies

Background information:

The construction of amide bonds is widespread in the synthesis of pharmaceutical active molecules, natural products, and biomacromolecules. With the continued boom in the pharmaceutical market and the increasing complexity and diversity of molecular structures, developing synthetic strategies that combine high efficiency and high selectivity is crucial in organic synthesis.

However, in practice, when multiple reaction sites exist within a molecule, such as when there are both highly reactive primary amines/alcohols and less reactive hindered amines/anilines, how to ensure that the latter is precisely acylated while the former remains "unresponsive" usually requires cumbersome protection and deprotection steps, which is not only inefficient but also has poor atom economy.

Recently, a research team at Merck developed a novel, universal synthetic strategy for the selective acylation of low- reactivity amines using the TCFH- Oxyma combination without protecting groups. On one hand, the TCFH- Oxyma combination enables selective N-acylation, meaning that low- reactivity amines can be acylated with high selectivity in the presence of alcohols . On the other hand, a transient imine protection strategy is employed, introducing an electron-deficient aromatic aldehyde to form an imine in situ with a reactive primary amine, followed by acylation of the low- reactivity amine using the TCFH- Oxyma combination . These two approaches systematically solve the challenge of selective coupling of low-reactivity amines .

I. Low-activity amines vs. alcohols – TCFH/ Oxyma demonstrates its power

Studies have found that TCFH exhibits distinctly different N-selectivity and O-selectivity when combined with different nucleophilic additives (such as NMI, DMAP, and Oxyma ) (see Figure 1). The combination of TCFH and a catalytic amount of Oxyma demonstrates superior N-selectivity in the competition between amines and alcohols, enabling efficient and highly selective amide coupling of low-activity amines and carboxylic acids in the presence of alcohols . For example, for electron-deficient aniline, the product ratio reaches as high as 13:1; for sterically hindered α -methylproline derivatives, the selectivity exceeds 50:1, achieving almost specific formation of the target amide.

Figure 1. Selectivity of different condensing agents and combinations

This unique chemical selectivity stems from a specific reaction mechanism: Oxyma , as a nucleophilic catalyst , forms a highly active acyl- Oxyma intermediate with TCFH-activated carboxylic acids, which exhibits extremely high selectivity for amines .

explored the substrate range of low-reactivity amines using 1-propanol as a competitive nucleophile (Figure 2). The results showed that the selectivity of the aniline series compounds increased with increasing electron-richness of aniline, and various sterically hindered 2,6-substituted anilines reacted well. In addition, sterically hindered primary amines, secondary amines (including amino acids with secondary amines), and hindered α -tertiary amines also exhibited excellent chemoselectivity.

Figure 2. TCFH- Oxyma efficiently mediates N-selective coupling reaction.

II. Low-reactivity amines vs. active primary amines – Transient imine protection (in-situ imine masking)

When both reactive and inactive amino groups are present in the reactants, the research team employed a transient imine protection strategy (Figure 3): an electron-deficient aromatic aldehyde (such as 2-bromo-4-chlorobenzaldehyde) was added to the reaction system. Because the primary amine is more nucleophilic, it preferentially and rapidly binds to the aldehyde to form an imine, thus being temporarily shielded and unable to participate in subsequent coupling. Meanwhile, the inactive amino group can successfully couple with the carboxylic acid . After the reaction, a simple acidic post-treatment is all that is needed for the imine to hydrolyze, releasing the primary amine back into the reaction mixture .

This strategy is widely applicable , and can be used to obtain the target amide with excellent yield and selectivity in the presence of competitive nucleophiles , whether it is aniline with different electronic properties (from electron-rich to electron-deficient), or α -tertiary amines with large steric hindrance and amino acid-derived secondary amines, while keeping the chiral center intact.

Figure 3. Scope of application of transient imine protection strategy

III. Examples – Complex Substrate Coupling

This method can be directly applied to the late-stage modification of complex drug molecules and bioactive molecules. For example, in the case of derivatives of fingolimod, a drug for treating multiple sclerosis, this method successfully achieved single amidation without protecting groups, with a selectivity as high as 23:1, avoiding the hassle of multi- step protection operations. For peptides containing multiple amino groups (such as the dipeptide Pro-Lys), this method can also precisely couple only to the less reactive proline residues, exhibiting excellent selectivity.

Figure 4. Selective amide coupling of complex substrates

Overall conclusion:

The above work developed a general strategy that systematically solves two selectivity challenges in amide coupling without protecting groups. First, the TCFH- Oxyma combination can mediate the coupling reaction of amino groups with high selectivity while the hydroxyl group remains unaffected. Second, the imine transient protection strategy achieves precise coupling of a single, low-activity amine through reversible imine formation. This synthetic strategy avoids additional protecting - deprotecting steps, simplifying the synthetic route.

In addition, the reagent used (TCFH- Oxyma ) is low in cost, highly safe, and compatible with complex molecules, and can maintain the integrity of the chiral center, which has important practical value in drug development and complex molecule synthesis.

Company Introduction:

Suzhou Haofan Biotech Co., Ltd. (Stock Code: 301393.SZ), founded in 2003 and headquartered in Suzhou High-tech Zone, is a national high-tech enterprise providing specialty raw materials to pharmaceutical R&D and manufacturing companies worldwide. Its products are mainly used in the synthesis of peptides, nucleotides, and pharmaceuticals, covering a wide range of products including condensing agents for specialty amide bonds, protective agents, linking agents, protein cross-linking agents for antibody-drug conjugates, molecular building blocks, liposomes, and phosphorus reagents. To date, it has cumulatively developed and produced over 1,500 different products.

After more than two decades of unremitting efforts and accumulation, Haofan Biotech has continuously cultivated its expertise in the global peptide synthesis reagent field. It has now developed into a leading enterprise with extensive customized product coverage and significant advantages in large-scale production, capable of meeting the specific needs of various customers. We sincerely invite customers interested in this product to contact us to learn more about product details and explore cooperation opportunities .

参考文献：

[1] General chemoselective hindered amide coupling enabled by TCFH-catalytic Oxyma and transient imine protection.

DOI：10.1039/d4cc05313c