Chemistry of 1-Alkoxy-1-glycosyl Radicals: The Manno- and Rhamnopyranosyl Series. Inversion of alpha- to beta-Pyranosides and the Fragmentation of Anomeric Radicals

Remote Functionalization of Inert C(sp3)-H Bonds via Dual Catalysis Driven by Alkene Hydrofluoroalkylation Using Industrial Feedstocks

We have developed a dual-catalytic system capable of site-selective azidation of inert C(sp3)-H bonds with concomitant and modular anti-Markovnikov alkene fluoroalkylation. The protocol leverages the synergetic cooperation of both the photocatalyst and earth-abundant iron catalyst to deliver two radical species in succession to minimally functionalized alkenes. This powerful catalyst system exhibits broad scope, mild conditions, and excellent regioselectivity for a variety of substrates and fluoroalkyl fragments. The key to this C-centered radical relay is the matched rate of both photocatalytic and iron catalytic cycles, ensuring selective azidofluoroalkylation with a broad array of fluoroalkyl sources from trivial reagents.

Urea-catalyzed N-Glycosylation of Amides/Azacycles with Glycosyl Halides

The efficient synthesis of N-glycosides via direct N-glycosylation of amides/azacycles has been reported. The glycosylation of amides/azacycles with glycosyl halides in the presence of a catalytic amount of urea proceeded smoothly to provide the corresponding N-glycosylated amides or nucleosides in good to excellent yields with 1,2-trans-stereoselectivity. Moreover, by the addition of terpyridine, the 1,2-cis-stereoselectivity was achieved.

Ferrier/Aza-Wacker/Epoxidation/Glycosylation (FAWEG) Sequence to Access 1,2-Trans 3-Amino-3-deoxyglycosides

3-Amino-3-deoxyglycosides constitute an essential class of nitrogen-containing sugars. Among them, many important 3-amino-3-deoxyglycosides possess a 1,2-trans relationship. In view of their numerous biological applications, the synthesis of 3-amino-3-deoxyglycosyl donors giving rise to a 1,2-trans glycosidic linkage is thus an important challenge. Even though glycals are highly polyvalent donors, the synthesis and reactivity of 3-amino-3-deoxyglycals have been little studied. In this work, we describe a new sequence, involving a Ferrier rearrangement and subsequent aza-Wacker cyclization that allows the rapid synthesis of orthogonally protected 3-amino-3-deoxyglycals. Finally a 3-amino-3-deoxygalactal derivative was submitted for the first time to an epoxidation/glycosylation with high yield and great diastereoselectivity, highlighting FAWEG (Ferrier/Aza-Wacker/Epoxidation/Glycosylation) as a new approach to access 1,2-trans 3-amino-3-deoxyglycosides.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Exploring the Biochemical Foundations of a Successful GLUT1-Targeting Strategy to BNCT: Chemical Synthesis and In Vitro Evaluation of the Entire Positional Isomer Library of ortho-Carboranylmethyl-Bearing Glucoconjugates

Boron neutron capture therapy (BNCT) is a noninvasive binary therapeutic modality applicable to the treatment of cancers. While BNCT offers a tumor-targeting selectivity that is difficult to match by other means, the last obstacles preventing the full harness of this potential come in the form of the suboptimal boron delivery strategies presently used in the clinics. To address these challenges, we have developed delivery agents that target the glucose transporter GLUT1. Here, we present the chemical synthesis of a number of ortho-carboranylmethyl-substituted glucoconjugates and the biological assessment of all positional isomers. Altogether, the study provides protocols for the synthesis and structural characterization of such glucoconjugates and insights into their essential properties, for example, cytotoxicity, GLUT1-affinity, metabolism, and boron delivery capacity. In addition to solidifying the biochemical foundations of a successful GLUT1-targeting approach to BNCT, we identify the most promising modification sites in d-glucose, which are critical in order to further develop this strategy toward clinical use.

C-H functionalization reactions enabled by hydrogen atom transfer to carbon-centered radicals

Selective functionalization of ubiquitous unactivated C-H bonds is a continuous quest for synthetic organic chemists. In addition to transition metal catalysis, which typically operates under a two-electron manifold, a recent renaissance in the radical approach relying on the hydrogen atom transfer (HAT) process has led to tremendous growth in the area. Despite several challenges, protocols proceeding via HAT are highly sought after as they allow for relatively easy activation of inert C-H bonds under mild conditions leading to a broader scope and higher functional group tolerance and sometimes complementary reactivity over methods relying on traditional transition metal catalysis. A number of methods operating via heteroatom-based HAT have been extensively reported over the past few years, while methods employing more challenging carbon analogues have been less explored. Recent developments of mild methodologies for generation of various carbon-centered radical species enabled their utilization in the HAT process, which, in turn, led to the development of remote C(sp3)-H functionalization reactions of alcohols, amines, amides and related compounds. This review covers mostly recent advances in C-H functionalization reactions involving the HAT step to carbon-centered radicals.

Iron-catalyzed remote functionalization of inert C(sp3)-H bonds of alkenes via 1,n-hydrogen-atom-transfer by C-centered radical relay

As an alternative approach to traditional C-H activation that often involved harsh conditions, and vicinal or primary C-H functionalization, radical relay offers a solution to these long-held problems. Enabled by 1,n (n = 5, 6)-hydrogen atom transfer (HAT), we use a most prevalent moiety, alkene, as the precursor to an sp³ C-centered radical to promote selective cleavage of inert C(sp³)-H bonds for the generation of azidotrifluoromethylated molecules. Mild conditions, broad scope and excellent regioselective control (>20 : 1) are observed in the reactions. Deuterium labelling studies disclose the kinetic characteristics of the transformations and verify a direct 1,n-HAT pathway. The key to this C-centered radical relay is that iron plays a dual role as a radical initiator and terminator to incorporate the azide functionality through radical oxidation via azido-ligand-transfer. The methods and the later derivatization promise expeditious synthesis of CF₃-containing organic azides, γ-lactam and triazoles that are widely used in designing new fluorescent tags and functional materials.

Total Synthesis of (+)-Polyoxamic Acid via Visible-Light-Mediated Photocatalytic β-Scission and 1,5-Hydrogen Atom Transfer of Glucose Derivative

A total synthesis of polyoxamic acid has been achieved. The key feature of the synthetic route is a visible-light-mediated β-scission and carbon-to-carbon 1,5-hydrogen atom transfer (1,5-HAT) to provide the functionalized alditol under mild conditions. This type of carbon-to-carbon 1,5-HAT initiated by C(sp³)-centered radicals has been scarcely reported. Furthermore, the reaction was adapted for flow chemistry, facilitating the total synthesis of polyoxamic acid.

Applications of Radical Carbonylation and Amine Addition Chemistry: 1,4-Hydrogen Transfer of 1-Hydroxylallyl Radicals

1,4-Hydrogen transfer from the 1-hydroxyallyl radical to give the enoxyl (α-keto) radical is an exothermic process with a high activation energy based on DFT calculations. The lack of experimental examples of such 1,4-H shift reactions lies in the difficulty of generating the 1-hydroxyallyl radical. We have shown that radical carbonylation of alkenyl radicals with CO followed by nucleophilic trapping of the carbonyl portion of the resulting radical by amines gives rise to 1-amino-substituted 1-hydroxyallyl radicals in situ. At the outset of this chemistry, we examined intramolecular trapping reactions via radical carbonylation of alkynylamines mediated by tributyltin hydride. Consequently, α-methylene lactams were obtained, for which the initially formed 1-amino-substituted 1-hydroxyallyl radical underwent a 1,4-H shift followed by subsequent β-scission, which led to the expulsion of a tributyltin radical. A competing pathway of the 1,4-H shift of 1-amino-substituted 1-hydroxyallyl radicals involving hydrogen abstraction was observed, which led to the formation of α-stannylmethylene lactams as a major byproduct. However, in contrast, when intermolecular trapping of α-ketenyl radicals by amines was carried out, the 1,4-H shift from the 1-amino-substituted 1-hydroxyallyl radical became the major pathway, which gave good yields of α,β-unsaturated amides. Thus, we were able to develop three-component reactions comprising terminal alkynes, CO, and amines that led to α,β-unsaturated amides via the 1,4-H shift reaction. DFT calculations support the observation that the 1,4-H shift is more facile when 1-hydroxyallyl radicals have both 1-amino and 3-tin substituents. The choice of substituents on the amine nitrogen is also important, since N-C bond cleavage via an S_H2-type reaction can become a competing pathway. Such an unusual S_H2-type reaction at the amine nitrogen is favored when the leaving alkyl radicals are stable, such as PhC(•)H(CH₃) and t-Bu•. Interestingly, even nucleophilic attack of tertiary amines onto α-ketenyl radicals causes cleavage of the C-N bond. For this reaction, DFT calculations predict an indirect homolytic substitution mechanism involving expulsion of alkyl radicals through the zwitterionic radical intermediate arising from nucleophilic amine addition onto the α-ketenyl radical. In contrast, the carbonylation of aryl radicals, generated from aryl iodides, in the presence of amines gave aromatic carboxylic amides in good yields. It is proposed that radical anions originating from acyl radicals and amines undergo electron transfer to aryl iodides to give aminocarbonylation products.

Remote C-H Functionalization via Selective Hydrogen Atom Transfer

The selective functionalization of remote C-H bonds via intramolecular hydrogen atom transfer (HAT) is transformative for organic synthesis. This radical-mediated strategy provides access to novel reactivity that is complementary to closed-shell pathways. As modern methods for mild generation of radicals are continually developed, inherent selectivity paradigms of HAT mechanisms offer unparalleled opportunities for developing new strategies for C-H functionalization. This review outlines the history, recent advances, and mechanistic underpinnings of intramolecular HAT as a guide to addressing ongoing challenges in this arena.

Recent advances in the synthesis of fungal antigenic oligosaccharides

The driving force for the constant improvement and development of new synthetic methodologies in carbohydrate chemistry is a growing demand for biologically important oligosaccharide ligands and neoglycoconjugates thereof for numerous biochemical investigations such as cell-to-pathogen interactions, immune response, cell adhesion, etc. Here we report our syntheses of the spacer-armed antigenic oligosaccharides related to three groups of the polysaccharides of the fungal cell-wall including α- and β-mannan, α- and β-glucan and galactomannan chains, which include new rationally designed synthetic blocks, efficient solutions for the stereoselective construction of glycoside bonds, and novel strategy for preparation of furanoside-containing oligosaccharides based on recently discovered pyranoside-into-furanoside (PIF) rearrangement.

Phosphine-catalyzed remote α-C–H bond activation of alcohols or amines triggered by the radical trifluoromethylation of alkenes: reaction development and mechanistic insights

An efficient radical protocol for concomitant functionalization of both alkenes and remote α-C–H bonds of alcohols or amines via 1,5(6,7)-HAT was realized in a highly controlled site-selective manner.

Mechanistic Investigations into the Application of Sulfoxides in Carbohydrate Synthesis

The utility of sulfoxides in a diverse range of transformations in the field of carbohydrate chemistry has seen rapid growth since the first introduction of a sulfoxide as a glycosyl donor in 1989. Sulfoxides have since developed into more than just anomeric leaving groups, and today have multiple roles in glycosylation reactions. These include as activators for thioglycosides, hemiacetals, and glycals, and as precursors to glycosyl triflates, which are essential for stereoselective β-mannoside synthesis, and bicyclic sulfonium ions that facilitate the stereoselective synthesis of α-glycosides. In this review we highlight the mechanistic investigations undertaken in this area, often outlining strategies employed to differentiate between multiple proposed reaction pathways, and how the conclusions of these investigations have and continue to inform upon the development of more efficient transformations in sulfoxide-based carbohydrate synthesis.

Stereoselective Construction of β-Mannopyranosides by Anomeric O-Alkylation: Synthesis of the Trisaccharide Core of N-linked Glycans

A new and efficient approach for direct and stereoselective synthesis of β-mannopyranosides by anomeric O-alkylation has been developed. This anomeric O-alkylation of mannopyranose-derived lactols is proposed to occur under synergistic control of a kinetic anomeric effect and metal chelation. The presence of a conformationally flexible C6 oxygen atom in the sugar-derived lactol donors is required for this anomeric O-alkylation to be efficient, probably because of its chelation with cesium ion. In contrast, the presence of a C2 oxygen atom plays a minor role. This glycosylation method has been successfully utilized for the synthesis of the trisaccharide core of complex N-linked glycans.

Studies on the diastereoselective oxidation of 1-thio-β-D-glucopyranosides: synthesis of the usually less favoured R(S) sulfoxide as a single diastereoisomer

A detailed study on the diastereoselective oxidation of 1-thio-β-D-glucopyranosides is reported. It has been shown that the sense and the degree of stereochemical outcome of the oxidation are highly dependent on the substituent of the sulfur and on the protective group of the C2-OH. In the case of thioglycosides with a bulky aglycone, the mesylation of C2-OH has a significant effect on the stereochemical outcome of the oxidation, affording the usually less favoured RS sulfoxide as a single diastereoisomer. The absolute configuration of the final sulfinyl glycosides was ascertained by NMR analysis and corroborated by X-ray crystallography.

Scope and limitations of 2-deoxy- and 2,6-dideoxyglycosyl bromides as donors for the synthesis of β-2-deoxy- and β-2,6-dideoxyglycosides

It is shown that 2-deoxy- and 2,6-dideoxyglycosyl bromides can be prepared in high yield (72-94%) and engaged in glycosylation reactions with β:α selectivities ≥6:1. Yields of product are 44-90%. Fully armed 2-deoxyglycoside donors are viable, while 2,6-dideoxyglycosides require one electron-withdrawing substituent for high efficiency and β-selectivity. Equatorial C-3 ester protecting groups decrease β-selectivity, and donors bearing an axial C-3 substituent are not suitable. The method is compatible with azide-containing donors and acid-sensitive functional groups.

Methodology development and physical organic chemistry: a powerful combination for the advancement of glycochemistry

This Perspective outlines work in the Crich group on the diastereoselective synthesis of the so-called difficult classes of glycosidic bond: the 2-deoxy-β-glycopyranosides, the β-mannopyranosides, the α-sialosides, the α-glucopyranosides, and the β-arabinofuranosides with an emphasis on the critical interplay between mechanism and methodology development.

Influence of the O3 protecting group on stereoselectivity in the preparation of C-mannopyranosides with 4,6-O-benzylidene protected donors

α-C-Glucopyranosides and mannopyranosides are obtained in 65-85% yields from 4,6-O-benzylidene-protected glucosyl and mannosyl thioglycosides bearing ester functionality at the 3-O-position by a coupling reaction with C-nucleophiles on activation with diphenyl sulfoxide, 2,4,6-tri-tert-butylpyrimidine, and trifluoromethanesulfonic anhydride.

Intramolecular nucleophilic carbonyl trapping of α-ketenyl radicals by an amino group

Free-radical carbonylation of omega-alkynylamines with tributyltin hydride gives a mixture of alpha-methylene lactams and alpha-stannylmethylene lactams. Nucleophilic addition of an internal amino group to the carbonyl group of alpha-ketenyl radicals is proposed as the cyclization step. The subsequent unusual 1,4-H shift from the resulting 1-hydroxyallyl radical, followed by elimination of the beta-tributyltin radical leads to the formation of alpha-methylene lactams.