Cyanomethyl Ether as an Orthogonal Participating Group for Stereoselective Synthesis of 1,2-trans-β-O-Glycosides

The effect of neighbouring group participation and possible long range remote group participation in O-glycosylation

Stereoselective glycosylations are one of the most challenging tasks of synthetic glycochemists. The protecting building blocks on the glycosides contribute significantly in attaining the required stereochemistry of the resulting glycosides. Strategic installation of suitable protecting groups in the C-2 position, vicinal to the anomeric carbon, renders neighbouring group participation, whereas protecting groups in the distal C-3, C-4, and C-6 positions are often claimed to exhibit remote group participation with the anomeric carbon. Neighbouring group participation and remote group participation are being widely studied to help the glycochemists design the synthetic protocols for multistep synthesis of complex oligosaccharides and in turn, standardise the process of the glycosylation towards a particular stereochemical output. While neighbouring group participation has been quite effective in achieving the required stereochemistry of the produced glycosides, remote participation exhibits comparatively less efficacy in achieving complete stereoselectivity in the glycosylation reactions. Remote participation is a still highly debated topic in the scientific community. However, implementing the participating role of the remote groups in glycosylation reactions is widely practised to achieve better stereocontrol and to facilitate the formation of synthetically challenging glycosidic linkages.

Catalytic Thioglycoside Activation with Diazo-Derived Copper Carbenes

Traditional glycosylation methods using thioglycosides often require harsh conditions or expensive metal catalysts. This study presents a more sustainable alternative by employing copper, an earth-abundant catalyst. We developed diazo-based thioglycoside donors that, through copper catalysis, undergo intramolecular activation to form glycosyl sulfonium ions, leading to the generation of oxocarbenium ions. This versatile approach efficiently accommodates a variety of O-nucleophiles, including primary, secondary, and tertiary, as well as complex bioactive molecules. It is compatible with various glycosyl donors and protecting groups, including superarmed, armed, and disarmed systems. Notably, the methodology operates orthogonally to traditional thioglycoside and alkyne donors and has been successfully applied to the orthogonal iterative synthesis of trisaccharides. Mechanistic insights were gained by studying the electronic effects of electron-donating (OMe) and electron-withdrawing (NO2) groups on the donors, offering a valuable understanding of the intramolecular reaction pathway.

NFSI mediated C3-ether oxidation of glycals for the synthesis of hex-3-enuloses

Hex-3-enuloses constitute a vital carbohydrate synthetic intermediate that provide access to wide range of chiral molecules through diverse derivatizations. Herein we report synthesis of these fascinating scaffolds by oxidation of C3-ether protections on glycals in presence of N-fluorobenzenesulfonimide (NFSI) under Cu(I) catalysed conditions. Benzyl, methyl and silyl ethers have been efficiently oxidized to the carbonyl group. The oxidation has been found to be highly regioselective where an array of protecting groups were tolerant to the reaction conditions. Pyranosyl glycals from various commercially available sugars have been studied in this work to evaluate the broad substrate scope.

Amino-Acid-Derived Amides as Stereodirecting Leaving Groups for Ferrier Rearrangement via Pd(0)-Catalyzed Tsuji-Trost Reactions

Ferrier rearrangement on glycals is an efficient tool to form 2,3-dideoxy glycosides that provide access to various sugar derivatives through olefin functionalization. The classical acid-mediated transformation delivers the α-O-glycosides selectively. In this protocol, amides obtained from amino acids, glycine and proline, have been utilized as sustainable β-directing leaving groups on glycal substrates. The directing groups facilitate β-selective Ferrier rearrangements for hard alcohol nucleophiles by following the Pd(0)-catalyzed Tsuji-Trost inner sphere pathway.

Unprecedented neighboring group participation of C2 N-imidoxy functionalities for 1,2-trans-selective glycosylation

Stereoselective glycosylation reactions are important in carbohydrate chemistry. The most used method for 1,2-trans(β)-selective glycosylation involves the neighboring group participation (NGP) of the 2-O-acyl protecting group; nevertheless, an alternative stereoselective method independent of classical NGP would contribute to carbohydrate chemistry, despite being challenging to achieve. Herein, a β-selective glycosylation reaction employing unprecedented NGP of the C2 N-succinimidoxy and phthalimidoxy functionalities is reported. The C2 functionalities provided the glycosylated products in high yields with β-selectivity. The participation of the functionalities from the α face of the glycosyl oxocarbenium ions gives stable six-membered intermediates and is supported by density functional theory calculations. The applicability of the phthalimidoxy functionality for hydroxyl protection is also demonstrated. This work expands the scope of functionalities tolerated in carbohydrate chemistry to include O-N moieties.

Efficient Synthesis of 2-OH Thioglycosides from Glycals Based on the Reduction of Aryl Disulfides by NaBH4

An improved method to efficiently synthesize 2-OH thioaryl glycosides starting from corresponding per-protected glycals was developed, where 1,2-anhydro sugars were prepared by the oxidation of glycals with oxone, followed by reaction of crude crystalline 1,2-anhydro sugars with NaBH4 and aryl disulfides. This method has been further used in a one-pot reaction to synthesize glycosyl donors having both “armed” and “NGP (neighboring group participation)” effects.

Recent Advances in Stereoselective Chemical O-Glycosylation Reactions

Carbohydrates involving glycoconjugates play a pivotal role in many life processes. Better understanding toward glycobiological events including the structure–function relationship of these biomolecules and for diagnostic and therapeutic purposes including tailor-made vaccine development and synthesis of structurally well-defined oligosaccharides (OS) become important. Efficient chemical glycosylation in high yield and stereoselectivity is however challenging and depends on the fine tuning of a protection profile to get matching glycosyl donor–acceptor reactivity along with proper use of other important external factors like catalyst, solvent, temperature, activator, and additive. So far, many glycosylation methods have been reported including several reviews also. In the present review, we will concentrate our discussion on the recent trend on α- and β-selective glycosylation reactions reported during the past decade.

Cyanomethyl (CNMe) ether: an orthogonal protecting group for saccharides

Logical manipulation of protecting groups is one of the vital strategies involved in the synthesis of complex oligosachharides. As opposed to the robust permanent protecting groups, the chemoselective protection-deprotection processes on orthogonal protecting groups have facilitated the synthesis of the target molecules with higher effeciency. While the derivatives of benzyl ethers are the most popular orthogonal ether based protecting groups for hydroxyls, the exploration of methyl ethers for similar synthetic application is much limited. We herein report cyanomethyl (CNMe) ether as a readily synthesized orthogonal protecting group for saccharides. The ether moiety was rapidly removed under Na-naphthalenide conditions in good to excellent yields and was found to be compatible with other well-known benzyl/methyl/silyl ether and acetal protecting groups. Additionally, the CNMe group was observed to be tolerant to standard reagents used for the deprotection of ether, ester and acetal protecting groups. The protection and deprotection steps remained unaffected by the position of hydroxyl, the configuration of monosaccharides or the presence of olefins in the skeleton.

Photolabile 2-(2-Nitrophenyl)-propyloxycarbonyl (NPPOC) for Stereoselective Glycosylation and Its Application in Consecutive Assembly of Oligosaccharides

A photolabile protecting group (PPG) 2-(2-nitrophenyl)-propyloxycarbonyl (NPPOC) was explored in glycosylation and applied in the consecutive synthesis of oligosaccharides. NPPOC displays a strong neighboring group participation (NGP) effect to facilitate the construction of 1,2-trans glycosides in excellent yield. Notably, NPPOC could be efficiently removed by photolysis, and the deprotection conditions are friendly to typical protecting groups. A branched and asymmetric oligomannose Man6 was rapidly prepared, and the consecutive assembly of oligosaccharides without intermediate purification was further investigated owing to the compatibility conditions between NPPPOC's photolysis and glycosylation.

o-Cyanobenzoate: A Recyclable and Reusable Stereo-directing Group for β-O-Glycosylation via Pd(0)-catalyzed Ferrier Rearrangement

Inner sphere Tsuji-Trost reaction has found recent application for β-selective Ferrier rearrangement of glycal substrates with alcohol nucleophiles. Herein, we report an efficient and stereoselective synthesis of 2,3-dideoxy-β-O-glycosides from C3-(o-cyanobenzoate) ester protected glycal donors via Ferrier rearrangement under Pd(0)-catalyzed Tsuji-Trost conditions. The synthesized donors indeed reacted with a variety of acceptors to afford the corresponding glycosides in good yields and excellent β-stereoselectivity. The stereochemical outcome of the reactions has been found to be independent of the nature of protecting groups or conformational flexibility of the glycal donors. Furthermore, regeneration of ortho-cyanobenzoic acid post rearrangement makes it a recyclable and reusable stereodirecting group. A preliminary mechanistic study demonstrates the importance of cyano-group for the observed rearrangement and stereoselectivity. Incorporation of the directing group on the benzoate ester has altered the reactivity of the ester group as a leaving group for Tsuji-Trost as well as Ferrier Rearrangement pathway.