Ayman El-Faham

Peptide Coupling Reagents, More than a Letter Soup

Peptide Coupling Reagents, More than a Letter Soup Ayman El-Faham* and Fernando Albericio* Institute for Research in Biomedicine, Barcelona Science Park, Baldiri Reixac 10, 08028-Barcelona, Spain Alexandria University, Faculty of Science, Department of Chemistry, P.O. Box 426, Ibrahimia, 21321 Alexandria, Egypt CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine, Barcelona Science Park, Baldiri Reixac 10, 08028-Barcelona, Spain. Department of Organic Chemistry, University of Barcelona, Martí i Franqu es 1-11, 08028-Barcelona, Spain

Oxyma: An Efficient Additive for Peptide Synthesis to Replace the Benzotriazole‐Based HOBt and HOAt with a Lower Risk of Explosion[1]

Oxyma [ethyl 2-cyano-2-(hydroxyimino)acetate] has been tested as an additive for use in the carbodiimide approach for formation of peptide bonds. Its performance in relation to those of HOBt and HOAt, which have recently been reported to exhibit explosive properties, is reported. Oxyma displayed a remarkable capacity to inhibit racemization, together with impressive coupling efficiency in both automated and manual synthesis, superior to those of HOBt and at least comparable to those of HOAt, and surpassing the latter coupling agent in the more demanding peptide models. Stability assays showed that there was no risk of capping the resin under standard coupling conditions. Finally, calorimetry assays (DSC and ARC) showed decomposition profiles for benzotriazole-based additives that were consistent with their reported explosivities and suggested a lower risk of explosion in the case of Oxyma.

COMU: A Safer and More Effective Replacement for Benzotriazole-Based Uronium Coupling Reagents**

We describe a new family of uronium-type coupling reagents that differ in their iminium moieties and leaving groups. The presence of the morpholino group in conjunction with an oxime derivative--especially ethyl 2-cyano-2-(hydroxyimino)acetate (Oxyma)--had a marked influence on the solubilities, stabilities, and reactivities of the reagents. Finally, the new uronium salt derived from Oxyma (COMU) performed extremely well in the presence of only 1 equiv of base, thereby confirming the effect of the hydrogen bond acceptor in the reaction. COMU also showed a less hazardous safety profile than the benzotriazole-based HDMA and HDMB, which exhibited unpredictable autocatalytic decompositions. Furthermore, the Oxyma moiety contained in COMU suggests a lower risk of explosion than in the case of the benzotriazole derivatives.

Racemization studies during solid-phase peptide synthesis using azabenzotriazole-based coupling reagents

Abstract 1-Hydroxy-7-azabenzotriazole (HOAt) and its corresponding uronium salts are shown to be more effective in avoiding racemization in a model solid-phase peptide segment coupling process than their benzotriazole analogs.

The diisopropylcarbodiimide/ 1-hydroxy-7-azabenzotriazole system: Segment coupling and stepwise peptide assembly

Abstract For a group of model peptide segments, coupling reactions carried out via solution or solid phase techniques have demonstrated the advantages of the system DIC HOAt over DIC HOBt and in addition for systems involving other selected carbodiimides and substituted HOBt derivatives bearing electron-withdrawing substituents. Very little, if any, loss of configuration occurred in DCM regardless of the additive used, although the relative order of efficiency was similar in solvents such as DMF in which more extensive epimerization resulted. In application of DIC HOAt to stepwise peptide assembly by solid phase techniques, it was found that the hindered pyridine base collidine enhanced the step involying preactivation of the carboxylic acid residue in contrast to the normal situation in which bases such as DIEA, NMM, or non-hindered pyridine bases inhibit this step. These results led to development of a stepwise procedure for peptide assembly in which collidine is added to enhance activation and subsequently DIEA is added to enhance coupling.

Advantageous applications of azabenzotriazole (triazolopyridine)-based coupling reagents to solid-phase peptide synthesis

1-Hydroxy-7-azabenzotriazole (HOAt) and its corresponding uronium and phosphonium salts are shown to be superior to their benzotriazole analogs in solid-phase peptide synthesis, thereby making possible the automated synthesis of peptides containing hindered amino acids.

COMU: A third generation of uronium-type coupling reagents

COMU is a third generation of uronium‐type coupling reagent based on ethyl 2‐cyano‐2‐(hydroxyimino)acetate (Oxyma) as well as a morpholino carbon skeleton. The presence of the morpholino group has a marked influence on the solubility, stability and reactivity of the reagent. COMU performed extremely well in the presence of only 1 equiv. of base, thereby confirming the effect of the hydrogen bond acceptor in the reaction. The by‐products of COMU are water soluble and easily removed, making it an excellent choice of coupling reagent for solution‐phase peptide synthesis. Finally, COMU shows a less hazardous safety profile than benzotriazole‐based reagents, such as HATU and HBTU, which in addition exhibit unpredictable autocatalytic decompositions and therefore a higher risk of explosion. Furthermore, in contrast to benzotriazole‐based reagents, COMU is significantly less likely to cause allergic reaction. Copyright

Morpholine-Based Immonium and Halogenoamidinium Salts as Coupling Reagents in Peptide Synthesis1

Here we describe a new family of N-form immonium-type coupling reagents that differ in their carbocation skeleton structure. The N-methylpiperazine derivative failed to form immonium salts, while the thiomorpholine derivative did not give better results than the coupling reagents currently used. The presence of the morpholine had a marked influence on the solubility and stability as well as the reactivity of the reagent. Finally, the fluoroamidinium salt performed extremely well in the presence of only 1 equiv of base, thereby confirming the effect of the proton acceptor in the reaction.

Synthesis and Biological Evaluation of a Teixobactin Analogue

The first synthesis and biological activity of a teixobactin analogue is reported. Substitution of the unusual L-allo-enduracididine residue by the naturally occurring L-arginine was achieved, and the analogue gave an activity trend similar to that of teixobactin (against Gram-postive bacteria) and meropenem, which was approved by the FDA in 1996. The synthetic route used allows for the synthesis of the natural product as well as the development of a program of medicinal chemistry.

Protected amino acid chlorides vs protected amino acid fluorides: Reactivity comparisons.

Abstract Although Fmoc amino acid fluorides are excellent reagents for coupling of moderately hindered amino acids ( e.g. , Aib-to-Aib) they are not suited for significantly more hindered systems ( e.g. , Aib-to-MeAib). While urethane-protected acid chlorides are inherently more reactive than the fluorides they are also ineffective for hindered systems due to competing oxazolone formation. This limitation is by-passed if urethane protection is replaced by arenesulfonyl protection and the Aib-to-MeAib and even the MeAib-to-MeAib couplings are easily achieved via the appropriate acid chlorides but not the acid fluorides.