Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 74, Issue 5, Pages 1886-1896Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jo802278w
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Funding
- The University of Toledo deArce Memorial Endowment Fund
- Elsa U. Pardee Foundation
- The Ohio Cancer Research Associates
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The chemical synthesis of homogeneous glycoproteins and glycopeptides facilitates progress toward understanding the functional role of carbohydrates attached to proteins and is important in the preparation of glycopeptide-based therapeutics. A series of protected and unprotected glycosyl dipeptides, glycopeptide 1, which contained the alpha-ketoacid moiety at the C-terminus, were synthesized and ligated with a series of O-tert-butyl-protected N-hydroxylamino acids to afford O-tert-butyl-protected glycosyl tripeptides, glycopeptide II. The reactions were carried out under both anhydrous and aqueous conditions at neutral pH to produce glycopeptide products in yields ranging from 15% to 86% depending on the amino acids present at the ligation junction. The best yields were obtained when both the alpha-ketoacid and the N-hydroxylamino acid contained medium-sized side chains. In addition to the expected tripeptide product, 2,5-substituted oxazoles were isolated when O-tert-butyl protected N-hydroxylamines of glycine were employed in the reaction. The formation of the oxazole is believed to result from an intramolecular cyclization of the O-tert-butyl ester on a nitrilium ion intermediate followed by aromatization. A decarboxylative condensation between O-18-labeled phenyl pyruvic acid and N-hydroxyphenethylamine oxalate salt resulted in amide products lacking the O-18-label, providing further support for the nitrilium ion in the reaction pathway.
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