Application of 2-Substituted Benzyl Groups in Stereoselective Glycosylation

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

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.

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.

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

This review is the counterpart of a 2018 Chemical Reviews article (Adero, P. O.; Amarasekara, H.; Wen, P.; Bohé, L.; Crich, D. Chem. Rev. 2018, 118, 8242-8284) that examined the mechanisms of chemical glycosylation in the absence of stereodirecting participation. Attention is now turned to a critical review of the evidence in support of stereodirecting participation in glycosylation reactions by esters from either the vicinal or more remote positions. As participation by esters is often accompanied by ester migration, the mechanism(s) of migration are also reviewed. Esters are central to the entire review, which accordingly opens with an overview of their structure and their influence on the conformations of six-membered rings. Next the structure and relative energetics of dioxacarbeniun ions are covered with emphasis on the influence of ring size. The existing kinetic evidence for participation is then presented followed by an overview of the various intermediates either isolated or characterized spectroscopically. The evidence supporting participation from remote or distal positions is critically examined, and alternative hypotheses for the stereodirecting effect of such esters are presented. The mechanisms of ester migration are first examined from the perspective of glycosylation reactions and then more broadly in the context of partially acylated polyols.

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

Stereoselective formation of glycosidic linkages has been the prime focus for contemporary carbohydrate chemistry. Herein, we report cyanomethyl (CNMe) ether as an efficient and effective participating orthogonal protecting group for the stereoselective synthesis of 1,2-trans-β-O-glycosides. The participating group facilitated good to high β-selective glycosylation with a broad range of electron-rich and electron-deficient glycosyl acceptors. Detailed experimental and theoretical studies reveal the involvement of CNMe ether in the formation of a six-membered imine-type cyclic intermediate for the observed stereoselectivity. Rapid incorporation and selective removal of the CNMe ether group in the presence of benzyl ether and isopropylidene acetal protection have also been reported here. The nitrile group provided an opportunity for the glycodiversification through further derivatizations.

Phenanthroline-Catalyzed Stereoretentive Glycosylations

Carbohydrates are essential moieties of many bioactive molecules in nature. However, efforts to elucidate their modes of action are often impeded by limitations in synthetic access to well-defined oligosaccharides. Most of the current methods rely on the design of specialized coupling partners to control selectivity during the formation of glycosidic bonds. Reported herein is the use of a commercially available phenanthroline to catalyze stereoretentive glycosylation with glycosyl bromides. The method provides efficient access to α-1,2-cis glycosides. This protocol has been performed for the large-scale synthesis of an octasaccharide adjuvant. Density-functional theory calculations, together with kinetic studies, suggest that the reaction proceeds by a double S_N 2 mechanism.

1,2- trans Glycosylation via Neighboring Group Participation of 2- O-Alkoxymethyl Groups: Application to One-Pot Oligosaccharide Synthesis

The use of 2- O-alkoxymethyl groups as effective stereodirecting substituents for the construction of 1,2- trans glycosidic linkages is reported. The observed stereoselectivity arises from the intramolecular formation of a five-membered cyclic architecture between the 2- O-alkoxymethyl substituent and the oxocarbenium ion, which provides the expected facial selectivity. Furthermore, the observed stereocontrol and the extremely high reactivity of 2- O-alkoxymethyl-protected donors allowed development of a one-pot sequential glycosylation strategy that should become a powerful tool for the assembly of oligosaccharides.

Acceptor reactivity in glycosylation reactions

The effect of the reactivity of the glycosyl acceptor on the outcome of glycosylation reactions is reviewed.

Bimodal Glycosyl Donors Protected by 2- O-( ortho-Tosylamido)benzyl Group

A glucosyl donor equipped with C2- o-TsNHbenzyl ether was shown to provide both α- and β-glycosides stereoselectivity, by changing the reaction conditions. Namely, β-glycosides were selectively obtained when the trichloroacetimidate was activated by Tf₂NH. On the other hand, activation by TfOH in Et₂O provided α-glycosides as major products. This "single donor" approach was employed to assemble naturally occurring trisaccharide α-d-Glc-(1→2)-α-d-Glc-(1→6)-d-Glc and its anomers.

Venturing beyond Donor-Controlled Glycosylation: New Perspectives toward Anomeric Selectivity

Glycans are complex compounds consisting of sugars linked glycosidically, existing either as pure polysaccharides or as part of glycoconjugates. They are prevalent in nature and possess important functions in regulating biological pathways. However, their diversity coupled with physiochemical similarities makes it challenging to isolate them in large quantities for biochemical studies, hence hampering progress in glycobiology and glycomedicine. Glycochemistry presents an alternative strategy to obtain pure glycan compounds through artificial synthetic methods. Efforts in glycochemistry have been centered on glycosylation, the key reaction in glycochemistry, especially with regards to anomeric stereoselectivity in polysaccharides and glycoconjugates. In particular, the stereoelectronic and steric properties of glycosyl donors are commonly used to direct the stereoselectivity in glycosylation reactions. Classic glycosylation strategies typically involve saturated glycosyl donors, proceeding either directly using hydrogen bonds and conformational constraints or indirectly by installing moieties covalently through neighboring group participation and intramolecular aglycon delivery. Over the past years, new glycosylation strategies, tapping on the foundations of transition metal catalysis, have emerged. To leverage the power of coordination chemistry, unsaturated glycosyl donors were introduced. Not only are the number of protection/deprotection steps reduced, the resultant unsaturated glycoside provides opportunities for downstream functionalizations, allowing quick access to a variety of sugars, including rare sugars. Alongside the glycosyl donor, an equally important but neglected aspect for targeting stereoselective glycosylation is the glycosyl acceptor. In the case of dual-directing donors, glycosyl acceptors have proved themselves capable of becoming the dominating factor for stereocontrol. Interestingly, rational manipulation or selection of glycosyl acceptors with particular nucleophilicity and p K_a values can lead to different stereoselectivities, thereby proving the tunability of such acceptors to favor the formation of one anomer over the other stereoselectively. By further venturing beyond substrate controlled stereoselectivity, we are presented with the opportunity to effect stereoselective glycosylation through glycosylating reagents. Of the key reagents, stereoselective catalyst stands out as a greener and efficient alternative to direct stereoselective control with stoichiometric substrates. Recently, investigations into this approach of stereocontrol presented an intriguing range of stereoselectivities, achieved by merely varying the nature of catalysts used. Another crucial effort in glycochemistry is enhancing the efficiencies of glycosylations, by reducing the number of preparative steps before or during glycosylation. Through using transient masking groups or one-pot synthetic strategies, these streamlined approaches provide enormous convenience and practicability for oligosaccharide syntheses. This Account presents mainly our advancements beyond the conventional donor-controlled strategies over the past decade, with emphasis placed on mechanistic explanations of anomeric selectivities, thereby providing perspectives to inspire further progress toward a generalized unified strategy for preparing every type of glycan.

Remote Electronic Effects by Ether Protecting Groups Fine-Tune Glycosyl Donor Reactivity

It was established that para-substituted benzyl ether protecting groups affect the reactivity of glycosyl donors of the thioglycoside type with the N-iodosuccinimide/triflic acid promoter system. Having electron donating p-methoxybenzyl ether (PMB) groups increased the reactivity of the donor in comparison to having electron withdrawing p-chloro (PClB) or p-cyanobenzyl ether (PCNB) protecting groups, which decreased the reactivity of the glycosyl donor relative to the parent benzyl ether (Bn) protected glycosyl donor. These findings were used to perform the first armed-disarmed coupling between two benzylated glucosyl donors by tuning their reactivity. In addition, the present work describes a highly efficient palladium catalyzed multiple cyanation and methoxylation of p-chlorobenzyl protected thioglycosides. The results of this paper regarding both the different electron withdrawing properties of various benzyl ethers and the efficient and multiple protecting group transformations are applicable in general organic chemistry and not restricted to carbohydrate chemistry.

Absence of Stereodirecting Participation by 2-O-Alkoxycarbonylmethyl Ethers in 4,6-O-Benzylidene-Directed Mannosylation

The preparation of a series of mannopyranosyl donors carrying 2-O-(2-oxoalkyl) ethers and their use in glycosylation reactions are described. The formation of cyclic products with the simple 2-O-phenacyl ether and with the 2-O-(t-butoxycarbonylmethyl) ether establishes the stereoelectronic feasibility of participation in such systems. The high β-selectivities observed with the bis-trifluoromethyl phenacyl ether indicate that participation can be suppressed through the introduction of electron-withdrawing substituents. The high β-selectivities and absence of cyclic products observed with the 2-O-(methoxycarbonylmethyl) ether exclude the effective participation of esters through six-membered cyclic intermediates in this series. The results are discussed in terms of the conformation of cyclic dioxenium ions (E,E-, E,Z-, or Z,Z-) and in the context of "neighboring group" participation by nonvicinal esters in glycosylation. Methods for the deprotection of the 2-O-phenacyl and 2-O-(methoxycarbonylmethyl) ethers are described.

Achiral 2-Hydroxy Protecting Group for the Stereocontrolled Synthesis of 1,2-cis-α-Glycosides by Six-Ring Neighboring Group Participation

Glycosylation of a fully armed donor bearing a 2-O-(trimethoxybenzenethiol) ethyl ether protecting group is completely α-selective with a range of carbohydrate alcohol acceptors. Low-temperature NMR studies confirm the intermediacy of cyclic sulfonium ion intermediates arising from six-membered β-sulfonium ring neighboring group participation. Selective protecting group removal is achieved in high yield in a single operation by S-methylation and base-induced β-elimination.