Catalyst-Controlled, Regioselective Reactions of Carbohydrate Derivatives

Site-Selective Acylation of Pyranosides with Oligopeptide Catalysts

Herein, we report the oligopeptide-catalyzed site-selective acylation of partially protected monosaccharides. We identified catalysts that invert site-selectivity compared to N-methylimidazole, which was used to determine the intrinsic reactivity, for 4,6-O-protected glucopyranosides (trans-diols) as well as 4,6-O-protected mannopyranosides (cis-diols). The reaction yields up to 81% of the inherently unfavored 2-O-acetylated products with selectivities up to 15:1 using mild reaction conditions. We also determined the influence of protecting groups on the reaction and demonstrate that our protocol is suitable for one-pot reactions with multiple consecutive protection steps.

Total Syntheses of Resin Glycosides Murucoidins IV and V

Murucoidins IV and V, two bioactive resin glycosides with complex yet similar structures isolated from the morning glory family, were synthesized in a convergent manner. All of the glycosylations in these syntheses including the key [3 + 2] coupling were achieved by our recently developed interrupted Pummerer reaction mediated (IPRm) glycosylations. The broad functional group compatibility of IPRm glycosylation allowed us to employ a latent-active concept and a single-pot transient protection-glycosylation-deprotection strategy which significantly improved the global synthetic efficiency.

Recent advances in site-selective functionalization of carbohydrates mediated by organocatalysts

As one of the four fundamental building blocks of life, carbohydrates assume varied and expansive roles in biological contexts. More in-depth understanding of carbohydrates and their interactions, however, is often restricted by our inability to synthesize and subsequently functionalize them in a site-selective manner. This review will summarize recent advances in the site-selective functionalization of carbohydrates using organocatalysts, including achiral catalysts, chiral nucleophilic bases, chiral N-heterocyclic carbenes, and chiral phosphoric acids, with an emphasis on the catalytic nature in each case. As in many endeavors, taking an alternative approach can often lead to success, and selected examples of these achievements will be highlighted as well.

Rapid phenolic O-glycosylation of small molecules and complex unprotected peptides in aqueous solvent

Glycosylated natural products and synthetic glycopeptides represent a significant and growing source of biochemical probes and therapeutic agents. However, methods that enable the aqueous glycosylation of endogenous amino acid functionality in peptides without the use of protecting groups are scarce. Here, we report a transformation that facilitates the efficient aqueous O-glycosylation of phenolic functionality in a wide range of small molecules, unprotected tyrosine, and tyrosine residues embedded within a range of complex, fully unprotected peptides. The transformation, which uses glycosyl fluoride donors and is promoted by Ca(OH)2, proceeds rapidly at room temperature in water, with good yields and selective formation of unique anomeric products depending on the stereochemistry of the glycosyl donor. High functional group tolerance is observed, and the phenol glycosylation occurs selectively in the presence of virtually all side chains of the proteinogenic amino acids with the singular exception of Cys. This method offers a highly selective, efficient, and operationally simple approach for the protecting-group-free synthesis of O-aryl glycosides and Tyr-O-glycosylated peptides in water.

Second-Order Biomimicry: In Situ Oxidative Self-Processing Converts Copper(I)/Diamine Precursor into a Highly Active Aerobic Oxidation Catalyst

A homogeneous Cu-based catalyst system consisting of [Cu(MeCN)₄]PF₆, N,N'-di-tert-butylethylenediamine (DBED), and p-(N,N-dimethylamino)pyridine (DMAP) mediates efficient aerobic oxidation of alcohols. Mechanistic study of this reaction shows that the catalyst undergoes an in situ oxidative self-processing step, resulting in conversion of DBED into a nitroxyl that serves as an efficient cocatalyst for aerobic alcohol oxidation. Insights into this behavior are gained from kinetic studies, which reveal an induction period at the beginning of the reaction that correlates with the oxidative self-processing step, EPR spectroscopic analysis of the catalytic reaction mixture, which shows the buildup of the organic nitroxyl species during steady state turnover, and independent synthesis of oxygenated DBED derivatives, which are shown to serve as effective cocatalysts and eliminate the induction period in the reaction. The overall mechanism bears considerable resemblance to enzymatic reactivity. Most notable is the "oxygenase"-type self-processing step that mirrors generation of catalytic cofactors in enzymes via post-translational modification of amino acid side chains. This higher-order function within a synthetic catalyst system presents new opportunities for the discovery and development of biomimetic catalysts.