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

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journal contribution
posted on 04.04.2008, 00:00 by Ayman El-Faham, Fernando Albericio
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.