Chemical Synthesis

COMU – Safer and More Efficient Peptide Coupling Reagent


Peptide synthesis relies heavily on efficient and reliable coupling reagents. A low tendency for racemization is a key requirement. This is especially true for solid phase peptide synthesis—quantitative yields with short reaction times are of utmost importance to make the synthesis of large peptides feasible.

A plethora of methods for the formation of the peptide bond have been reported. The most successful approaches known today involve active ester formation with uronium/guanidinium salts. The most popular members of this family are peptide synthesis reagents based on benzotriazole derivatives such as HOBt or HOAt, both of which are also commonly used as additives in carbodiimide mediated peptide coupling.

Scheme 1: The benzotriazole derivatives HOBt and HOAt are commonly used additives to suppress racemization in peptide coupling reactions. HBTU, TBTU, and HATU are the most successful coupling reagents today based on HOBt and HOAt.

HBTU, TBTU, and HATU are the most popular peptide coupling reagents based on HOBt and HOAt. How can these successful reagents be improved? Recent findings in the groups of Fernando Albericio in Spain and Ayman El-Faham in Egypt showed that the incorporation of a hydrogen bond acceptor in the iminium part of the coupling reagent resulted in a significant improvement in performance.1 Additionally, replacing one dimethylamino moiety with a more polar morpholino group proved to be an optimal enhancement.

Safety concerns also lead to a strong demand for improvements. HOBt derivatives have generally been regarded as potential explosives. Recent reclassifications have made economical shipping and storage increasingly difficult. In search for efficacious replacements for benzotriazoles, the Albericio and El-Faham groups demonstrated that ethyl (hydroxyimino)cyanoacetate (Oxyma) is a potent alternative to HOBt or HOAt.

Ethyl (hydroxyimino)cyanoacetate (Oxyma)

Scheme 2: Ethyl (hydroxyimino)cyanoacetate (Oxyma) Aldrich Prod. No. 233412

In direct comparison to HOBt and HOAt, ethyl (hydroxyimino)cyanoacetate (Oxyma) showed a remarkable capacity to inhibit racemization as an additive in carbodiimide mediated amide bond formation. Impressingly, its coupling efficiency in solid and liquid phase peptide coupling is superior to HOBt, and at least comparable to HOAt.2 DSC and ARC studies of Oxyma show only low thermal risks.

Advantages of Ethyl (hydroxyimino)cyanoacetate (Oxyma)

  • Non-explosive replacement for HOBt and HOAt with comparable performance to HOAt
  • Outperforms HOAt in sterically demanding peptide bond formation
  • Suitable for carbodiimide mediated peptide coupling
  • Less epimerization than HOBt in fragment condensation reactions


Scheme 3: COMU – the ultimate coupling reagent. Aldrich Prod. No. 712191

Combining the above findings - the advantageous influence of a morpholino group in the peptide coupling reagent, and the potency of ethyl (hydroxyimino)cyanoacetate to substitute the benzotriazole moiety as a leaving group – the groups of Albericio and El-Faham constructed COMU (712191) as a safer and more efficient coupling reagent.3

HBTU, HATU, and similar peptide coupling reagents based on benzotriazoles predominantly exist in the less reactive guanidinium or N-form, which is less reactive than the uronium or O-form.4

Scheme 4: Guanidinium and uronium form of HATU (also called N- and O-form)

Notably, COMU only exists as the more reactive uronium structure. Comparative studies proved that COMU exhibits a similar capacity as a peptide coupling reagent as the current gold standard, HATU.3 The following table shows an impressive evaluation of COMU’s performance in relation to other common coupling reagents in a demanding solid phase synthesis procedure.

Table 1: Yields for the solid phase synthesis of the pentapeptide H-Tyr-Aib-Aib-Phe-Leu-NH2 with different coupling reagents, and the amount of undesired deletion sequence des-Aib (H-Tyr-Aib-Phe-Leu-NH2) formed.


Coupling reagent Pentapeptide yield [%] Des-Aib [%]
HBTU 47.0 53
HATU 83.0 17
HOTU 99.0 1.0
COMU 99.7 0.26


COMU features optimal properties as a peptide coupling reagent. In addition to its high and fast coupling efficiency, it shows very low or non-existent tendencies for racemization. Epimerization during fragment coupling appears to be lessened with COMU than with HOBt or HATU. COMU is easily soluble with remarkable stability in most commonly employed peptide coupling solvents, such as DMF or NMP, which makes it ideally suited for solid phase peptide synthesis. It is equally attractive for solution phase synthesis since by-products formed by COMU are water-soluble and can be separated by simple extraction. A color change during the reaction allows visual or colorimetric reaction monitoring. DSC and ARC data indicate that COMU can be rated as non-explosive.

COMU can be used practically with the same protocols that apply for common coupling reagents such as HBTU, TBTU, PyBOP, or HATU. In more challenging circumstances where racemization is a major concern, one equivalent of base can be used with COMU as the coupling reagent as the polar morpholino group contributes as internal base.


  • Equal or even superior performance to HATU
  • Non-explosive (does not contain benzotriazole moiety)
  • Suitable for solution phase & solid phase peptide synthesis
  • Utmost retention of configuration – low to non-existent racemization
  • High solubility and stability in typical solvents
  • Visual or colorimetric reaction monitoring possible
  • Easy removal of water-soluble by-products

Sigma-Aldrich is proud to offer the most comprehensive range of products for peptide synthesis: all common coupling reagents and additives, more than 2400 amino acid building blocks, as well as resins, solvents, products for peptide analysis, and tools for PEGylation or fluorous-phase peptide synthesis. To browse conveniently through Sigma-Aldrich’s application based online catalog, please visit Chem Product Central.

Materials List



  1. El-Faham, A.; Albericio, F. J. Org. Chem. 2008, 73, 2731.
  2. “Oxyma: An Efficient Additive for Peptide Synthesis to Replace the Benzotriazole-Based HOBt and HOAt with a Lower Risk of Explosion”, Subirós-Funosas, R.; Prohens, R.; Barbas, R.; El-Faham, A.; Albericio, F. Chem. Eur. J. 2009, 15, 9394.
  3. “COMU: A Safer and More Effective Replacement for Benzotriazole-Based Uronium Coupling Reagents”, El-Faham, A.; Subirós-Funosas, R.; Prohens, R.; Albericio, F. Chem. Eur. J. 2009, 15, 9404.
  4. Carpino, L.A. et al. Angew. Chem. Int. Ed. 2002, 41, 441.