Synthesis of small glycopeptides by decarboxylative condensation and insight into the reaction mechanism

An in-depth mechanistic study of the p-hydroxyphenylglycine synthetic process using in situ ATR-IR spectroscopy

In this study, an in situ ATR-IR technique was used as a powerful tool to gain insight into the synthetic process of p-hydroxyphenylglycine (p-HPG) by the sulfamic acid-glyoxylic acid-phenol method. Combined with other chemical and instrumental analysis technologies, the reaction sequence and key intermediates of this one-pot reaction were determined, and two concomitant reaction paths have been put forward for the first time. The possible reaction mechanism has been suggested, and the reaction efficiency of each path is discussed in detail. Through the optimization of the experimental parameters, an approximately 40% increase in the final product yield was achieved compared with previous reports. We believe that this study will without a doubt trigger research interest in understanding the industrial production process of important chemicals and pharmaceuticals and as a result will promote the sustainable development and application of novel, efficient chemical reaction routes.

Catalytic asymmetric aza-Michael addition of fumaric monoacids with multifunctional thiourea/boronic acids

Chiral multifunctional thiourea/boronic acid catalysts enabled the synthesis of N-hydroxyaspartate derivatives applicable for KAHA amidation through acid-β selective aza-Michael addition.

Metal-Free Thermal Activation of Molecular Oxygen Enabled Direct α-CH2-Oxygenation of Free Amines

Direct oxidation of α-CH₂ group of free amines is hard to achieve due to the higher reactivity of amine moiety. Therefore, oxidation of amines involves the use of sophisticated metallic reagents/catalyst in the presence or absence of hazardous oxidants under sensitive reaction conditions. A novel method for direct C-H oxygenation of aliphatic amines through a metal-free activation of molecular oxygen has been developed. Both activated and unactivated free amines were oxygenated efficiently to provide a wide variety of amides (primary, secondary) and lactams under operationally simple conditions without the aid of metallic reagents and toxic oxidants. The method has been applied to the synthesis of highly functionalized amide-containing medicinal drugs, such as O-Me-alibendol and -buclosamide.

Obstacles and solutions for chemical synthesis of syndecan-3 (53-62) glycopeptides with two heparan sulfate chains

Proteoglycans play critical roles in many biological events. Due to their structural complexities, strategies towards synthesis of this class of glycopeptides bearing well-defined glycan chains are urgently needed. In this work, we give the full account of the synthesis of syndecan-3 glycopeptide (53-62) containing two different heparan sulfate chains. For assembly of glycans, a convergent 3+2+3 approach was developed producing two different octasaccharide amino acid cassettes, which were utilized towards syndecan-3 glycopeptides. The glycopeptides presented many obstacles for post-glycosylation manipulation, peptide elongation, and deprotection. Following screening of multiple synthetic sequences, a successful strategy was finally established by constructing partially deprotected single glycan chain containing glycopeptides first, followed by coupling of the glycan-bearing fragments and cleavage of the acyl protecting groups.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Chemical Protein Synthesis with the KAHA Ligation

Since the first report of the chemoselective amide bond forming reaction between α-ketoacids and hydroxylamines in 2006, the KAHA (α-ketoacid-hydroxylamine) ligation has advanced to a useful tool for the routine synthesis of small to medium sized proteins and cyclic peptides. In this chapter we introduce the concept of KAHA ligation starting with the synthesis and properties of hydroxylamines and α-ketoacids, methods for their incorporation into peptides, and give an insight into the mechanism of the KAHA ligation. We cover important improvements including sequential ligations with 5-oxaproline, traceless synthesis of peptide α-ketoacids and show their application in chemical protein synthesis and cyclic peptide synthesis. Recent developments of the KAT (potassium acyl trifluoroborate) ligation and its application as fast and chemoselective bioconjugation method are described and an outlook on ongoing work and possible future developments is given at the end of the chapter.

Rethinking amide bond synthesis

One of the most important reactions in organic chemistry--amide bond formation--is often overlooked as a contemporary challenge because of the widespread occurrence of amides in modern pharmaceuticals and biologically active compounds. But existing methods are reaching their inherent limits, and concerns about their waste and expense are becoming sharper. Novel chemical approaches to amide formation are therefore being developed. Here we review and summarize a new generation of amide-forming reactions that may contribute to solving these problems. We also consider their potential application to current synthetic challenges, including the development of catalytic amide formation, the synthesis of therapeutic peptides and the preparation of modified peptides and proteins.

Mixed-phase synthesis of glycopeptides using a N-peptidyl-2,4-dinitrobenzenesulfonamide-thioacid ligation strategy

A strategy for the solid phase peptide synthesis (SPPS) and coupling of N-peptidyl and N-glycopeptidyl 2,4-dinitrobenzenesulfonamides (dNBS) with C-terminal peptidyl thioacids has been developed. The resulting N-dDNBS peptides were coupled to generate longer peptides. Ligation reactions were complete within 15 to 20 min.