Microwave irradiation and COMU: a potent combination for solid-phase peptide synthesis Ramon Subiros-Funosas a,b , Gerardo A. Acosta a,b , Ayman El-Faham a,c,d , Fernando Albericio a,b,e, * a Institute for Research in Biomedicine (IRB Barcelona), Barcelona Science Park, Baldiri Reixac 10, 08028 Barcelona, Spain b CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine, Barcelona Science Park, Baldiri Reixac 10-12, 08028 Barcelona, Spain c Department of Chemistry, College of Science, King Saud University, PO Box 2455, Riyadh, Saudi Arabia d Department of Chemistry, Faculty of Science, Alexandria University, Ibrahimia 21321, Alexandria, Egypt e Department of Organic Chemistry, University of Barcelona, Martí i Franqués 1-11, 08028-Barcelona, Spain article info Article history: Received 2 July 2009 Revised 7 August 2009 Accepted 28 August 2009 Available online 2 September 2009 Keywords: Coupling reagents Microwave Peptide Solid-phase synthesis Oxyma abstract Here we demonstrate the compatibility of Oxyma-based uronium-type coupling reagent COMU with microwave-assisted peptide synthesizers. Consistent with previous reports, COMU displayed higher effi- ciency than benzotriazole classical immonium salts HATU and HBTU in the demanding synthesis of the Aib derivative of Leu-Enkephalin pentapeptide and did not yield Oxyma-based byproducts. Thus, the combination of microwave irradiation and COMU resulted in a similar performance in considerably shorter time to that achieved by manual synthesis. Ó 2009 Elsevier Ltd. All rights reserved. Uronium-type coupling reagent 1-[(1-(cyano-2-ethoxy-2-oxoe- thylideneaminooxy)-dimethylamino-morpholinomethylene)] meth- anaminium hexafluorophosphate, 1 (COMU, Fig. 1) is a more efficient alternative to classical benzotriazole immonium salts in terms of racemization suppression, coupling effectiveness, stability, and solu- bility. 1 In addition to the morpholino moiety, this superiority is attributed to the introduction of ethyl 2-cyano-2-(hydroxyimi- no)acetate, 2 (Oxyma, Fig. 1) in its structure. Oxyma displays high control of optical purity and extension of coupling in demanding sequences. 2 Both Oxyma and its related uronium salt can be handled with a considerably lower risk than its explosive benzotriazole counter- parts, as determined by calorimetry techniques. However, the ther- mal stability of Oxyma is relatively low. 1,2 Nonetheless, no incident has been reported during extreme coupling stability assays, carried out using microwave irradiation at 80 °C. 2 Previous studies examined the stability of Oxyma toward the nucleophilic N-terminus of the growing peptide chain in extreme experiments. To enhance the likelihood of this side reaction, these experiments used a high excess of the additive in the absence of carbodiimide and Fmoc-AA-OH. 2 In the assay conducted at room temperature overnight, no Oxyma-based byproduct was found. In contrast, a similar experiment with microwave irradiation gave rise to some of these undesired byproducts ( Fig. 2). In that demanding experiment, the main byproduct resulted from the formylation of the amino function (3, 47.9%), attack of the amino group on the carboxylate (4) and the carbonyl of the oxime (5), which was also hydrolyzed (6). In all the cases, impurities were de- tected in the range 2–4%. 2 In order to unequivocally determine the compatibility of the Oxyma-derived coupling reagents with microwave-assisted solid- phase peptide synthesis (SPPS), we compared the performance of COMU with that of classical immonium salts HBTU 7 and HATU 8 in the synthesis of a true peptide model conducted in an auto- mated peptide synthesizer with microwave irradiation (Fig. 3). 3 Since its first appearance in 1986, microwave irradiation has been established as a highly useful tool for solid-phase organic 0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2009.08.117 * Corresponding author. Tel.: +11 34 93 402 9058; fax: +11 34 93 339 7878. E-mail address: albericio@pcb.ub.es (F. Albericio). 1 2 PF 6 COOEt N O N N O CN COOEt N CN OH Figure 1. Structure of COMU and Oxyma. Tetrahedron Letters 50 (2009) 6200–6202 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet