Ruthenium catalyzed synthesis of 2,3-unsaturated C-glycosides from glycals

Ferrier Glycosylation Mediated by the TEMPO Oxoammonium Cation

The TEMPO oxoammonium cation has been proven to be both an efficient oxidizing reagent and an electrophilic substrate frequently found in organic reactions. Here, we report that this versatile chemical reagent can also be used as an efficient promoter for C- and N-glycosylation reactions through a Ferrier rearrangement with moderate to high yields. This unprecedented reactivity is explained in terms of a Lewis acid activation of glycal by TEMPO+ forming a type of glycal-TEMPO+ mesomeric structure, which occurs through an extended vinylogous hyperconjugation toward the π*(O═N+) orbital [LP(O1) → π*(C1═C2), π*(C1═C2) → σ*(C3-O3), and LP(O6) → π*(O═N+)]. This enables the formation of the respective Ferrier glycosyl cation, which is trapped by various nucleophiles. The extended hyperconjugation (or double hyperconjugation) toward the π*(O═N+) orbital, which confers the Lewis acid character of the TEMPO cation, was supported by natural bond orbital analysis at the M06-2X/6-311+G** level of theory.

N-Si Heterolysis by Chiral (BOX)Cu(OTf)2 Catalysts for the Synthesis of Indole and Carbazole Glycosides

Chiral Cu(II) bisoxazolines have been shown to catalyze the coupling of acetyl-protected carbohydrates with N-silylated indoles to give the corresponding N-glycosides. Preliminary mechanistic experiments indicated that catalysis occurs through formation of a Cu-indolide complex with concomitant formation of TMS-OTf which together activate the sugar and deliver the indole nucleophile.

Reagent-controlled chemo/stereoselective glycosylation of ʟ-fucal to access rare deoxysugars

2,6-Dideoxy sugars constitute an important class of anticancer antibiotics natural products and serve as essential medicinal tools for carbohydrate-based drug discovery and vaccine development. In particular, 2-deoxy ʟ-fucose or ʟ-oliose is a rare sugar and vital structural motif of several potent antifungal and immunosuppressive bioactive molecules. Herein, we devised a reagent-controlled stereo and chemoselective activation of ʟ-fucal, enabling the distinctive glycosylation pathways to access the rare ʟ-oliose and 2,3-unsaturated ʟ-fucoside. The milder oxo-philic Bi(OTf)3 catalyst induced the direct 1,2-addition predominantly, whereas B(C6F5)3 promoted the allylic Ferrier-rearrangement of the enol-ether moiety in ʟ-fucal glycal donor, distinguishing the competitive mechanisms. The reagent-tunable modular approach is highly advantageous, employing greener catalysts and atom-economical transformations, expensive ligand/additive-free, and probed for a diverse range of substrates comprising monosaccharides, amino-acids, bioactive natural products, and drug scaffolds embedded with susceptible or labile functionalities.

Stereoselective synthesis of glycosyl azides from anomeric hydroxides via protecting group manipulations

Herein, we report the direct conversion of anomeric hydroxides to glycosyl azides in one step using diphenylphosphoryl azide. Protecting group manipulations on the hexose sugars have enabled the stereoselective synthesis of either the α-glycosyl azides or the β-anomeric azides in moderate to very good yields. The reaction has also been successfully used to enable the synthesis of β-2-deoxy-2-aminoglucosyl azides.

Protecting group enabled stereocontrolled approach for rare-sugars talose/gulose via dual-ruthenium catalysis

We herein report a convenient and highly stereocontrolled approach for rare and vital ᴅ-talo and ᴅ-gulo sugars directly from economical ᴅ-galactal through dual ruthenium-catalysis. The stereo-divergent strategy involves Ru(III)Cl₃-catalyzed Ferrier glycosylation of ᴅ-galactal to give 2,3-unsaturated ᴅ-galactopyranoside, further selective functionalization of C-4 and C-6 position with diverse protecting groups and dihydroxylation with Ru(VIII)O₄ generated in situ providing access to talo/gulo isomers. The α-anomeric stereoselectivity and syn-diastereoselectivity in glycosylation-dihydroxylation steps have been predominantly achieved by judicious selection of stereoelectronically diverse protecting groups. The synthetic utility of the dual-ruthenium catalysis was demonstrated for efficiently assembling the ᴅ-talose and/or ᴅ-gulose sugars in natural products and bioactive scaffolds.

Me3SI-promoted chemoselective deacetylation: a general and mild protocol

A catalytic and practical approach for the selective removal of acetyl groups using various substrates bearing orthogonal moieties has been demonstrated under ambient conditions.

Copper(ii)-catalyzed stereoselective 1,2-addition vs. Ferrier glycosylation of "armed" and "disarmed" glycal donors

Selective activation of "armed' and ''disarmed" glycal donors enabling the stereo-controlled glycosylations by employing Cu(ii)-catalyst as the promoter has been realized. The distinctive stereochemical outcome in the process is mainly influenced by the presence of diverse protecting groups on the donor and the solvent system employed. The protocol is compatible with a variety of aglycones including carbohydrates, amino acids, and natural products to access deoxy-glycosides and glycoconjugates with high α-anomeric selectivity. Notably, the synthetic practicality of the method is amply verified for the stereoselective assembling of trisaccharides comprising 2-deoxy components. Mechanistic studies involving deuterated experiments validate the syn-diastereoselective 1,2-addition of acceptors on the double bond of armed donors.

Plant-derived Glycosides with α-Glucosidase Inhibitory Activity: Current Standing and Future Prospects

Background:

The α-glucosidase (EC 3.2.1.20), a calcium-containing intestinal enzyme which is positioned in the cells which cover the intestinal microvilli brush border. The carbohydrates require metabolism by α-glucosidase before being absorbed into the small intestine, and as a result, this enzyme represents a significant drug target for the effective management of diabetes. There are few α- glucosidase inhibitors in the clinical practice that is challenged by several limitations. Thus, new effective and safe therapeutic agents in this class are required. In this regard, plant secondary metabolites are a very promising source to be investigated. Herein in this review, we have focused on the preclinical studies on various glycosides with in vitro α-glucosidase inhibitory activity.

Methods:

The literature available on various websites such as GoogleScholar, PubMed, Scopus. All the peer-reviewed articles were included without considering the impact factor.

Results:

The surveyed literature revealed marked inhibitory profile of various glycosides derived from plants, and some of them were extremely potent relatively to the standard, acarbose in preclinical trials and exhibited multiple targeted effects.

Conclusion:

Keeping in view the results, these glycosides are strong candidates for further, more detailed studies to ascertain their clinical potential and for effective contribution in effective management of diabetes, where multiple targets are required to address

Robust perfluorophenylboronic acid-catalyzed stereoselective synthesis of 2,3-unsaturated O-, C-, N- and S-linked glycosides

A convenient protocol was developed for the synthesis of 2,3-unsaturated C-, O-, N- and S-linked glycosides (enosides) using 20 mol % perflurophenylboronic acid catalyst via Ferrier rearrangement. Using this protocol, D-glucals and L-rhamnals reacted with various C-, O-, N- and S-nucleophiles to give a wide range of glycosides in up to 98% yields with mainly α-anomeric selectivity. The perflurophenylboronic acid successfully catalyzed a wide range of substrates (both glucals and nucleophiles) under very mild reaction conditions.

Homogeneous Catalysis: A Powerful Technology for the Modification of Important Biomolecules

Homogeneous catalysis plays an important and ubiquitous role in the synthesis of simple and complex molecules, including drug compounds, natural products, and agrochemicals. In recent years, the wide-reaching importance of homogeneous catalysis has made it an indispensable tool for the modification of biomolecules, such as carbohydrates (sugars), amino acids, peptides, nucleosides, nucleotides, and steroids. Such a synthetic strategy offers several advantages, which have led to the development of new molecules of biological relevance at a rapid rate relative to the number of available synthetic methods. Given the powerful nature of homogeneous catalysis in effecting these synthetic transformations, this Focus Review has been compiled to highlight these important developments.

Chemoselective and stereospecific iodination of alkynes using sulfonium iodate(i) salt

The sulfonium iodate(i) electrophilc reagent efficiently promoted the stereodivergent iodination of alkynes. The chemoselective iodination was effectively realized by switching the solvent system to access the 1-iodoalkyne and (E)-vicinal-diiodoalkenes.

C-Glycopyranosyl Arenes and Hetarenes: Synthetic Methods and Bioactivity Focused on Antidiabetic Potential

This Review summarizes close to 500 primary publications and surveys published since 2000 about the syntheses and diverse bioactivities of C-glycopyranosyl (het)arenes. A classification of the preparative routes to these synthetic targets according to methodologies and compound categories is provided. Several of these compounds, regardless of their natural or synthetic origin, display antidiabetic properties due to enzyme inhibition (glycogen phosphorylase, protein tyrosine phosphatase 1B) or by inhibiting renal sodium-dependent glucose cotransporter 2 (SGLT2). The latter class of synthetic inhibitors, very recently approved as antihyperglycemic drugs, opens new perspectives in the pharmacological treatment of type 2 diabetes. Various compounds with the C-glycopyranosyl (het)arene motif were subjected to biological studies displaying among others antioxidant, antiviral, antibiotic, antiadhesive, cytotoxic, and glycoenzyme inhibitory effects.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Regioselective vicinal functionalization of unactivated alkenes with sulfonium iodate(i) reagents under metal-free conditions

Metal-free, molecular iodine-free direct 1,2-difunctionalization of unactivated alkenes has been reported. The sulfonium iodate(i) reagent efficiently promoted the intermolecular vicinal iodo-functionalization of a diverse range of olefins in a stereo and regioselective manner. This method enables the divergent and straightforward preparation of synthetically useful functionalities; β-iodocarboxylates, β-iodohydrins, and β-iodoethers in a one-step process. Further interconversion of iodo-functionalized derivatives allows easy access to valuable synthetic intermediates en route to biologically active molecules.

A mild and efficient Zn-catalyzed C-glycosylation: synthesis of C(2)–C(3) unsaturated C-linked glycopyranosides

A mild and efficient protocol for the C-glycosylation from glycals is described by using Zn-catalysis.