The Efficient Preparation of Di- and Tripeptides by Coupling N-(Cbz- or Fmoc-α-aminoacyl)benzotriazoles with Unprotected Amino Acids

Abstract

N-(Cbz- or Fmoc-α-aminoacyl)benzotriazoles 2 and N-protected peptidoylbenzotriazoles 6 are coupled in aqueous acetonitrile solution with free amino acids or dipeptides to prepare: (i) 22 chirally pure dipeptides 5a-v (in an average yield of 82%) from N-(Cbz- or Fmoc-α-aminoacyl)benzotriazoles 2 and unprotected amino acids, (ii) five chiral tripeptides 7a-e (in an average yield of 75%) from N-protected peptidoylbenzotriazoles 6 and unprotected amino acids, (iii) one chiral tripeptide 7g (62%) from N-(Cbz- or Fmoc-α-aminoacyl)benzotriazole 2a and the free dipeptide 8. In all, N-(Cbz- or Fmoc-α-aminoacyl)benzotriazole derivatives of 17 of the 20 naturally occurring amino acids were used, including those containing the following unprotected side chain functionalities: alcoholic -OH (Ser), indole -NH (Trp), imidazole -NH, phenolic -OH (Tyr), -CONH₂ (Gln, Asn), -SH (Cys), -CO₂H (Glu, Asp), and -S-S (Cystine). Support for the complete retention of chirality was obtained by parallel experiments involving d-Ala, l-Ala, and dl-Ala for the preparation of di- and tripeptides. This and other evidence for chiral integrity was supported by NMR and HPLC analyses.

Key words

N-(protected α-aminoacyl)benzotriazole - acylating reagent - dipeptides - N-Cbz-peptidoylbenzotriazole - tripeptides

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