Recent Progress in 1,2-cis glycosylation for Glucan Synthesis

Controlling the stereoselectivity of 1,2-cis glycosylation is one of the most challenging tasks in the chemical synthesis of glycans. There are various 1,2-cis glycosides in nature, such as α-glucoside and β-mannoside in glycoproteins, glycolipids, proteoglycans, microbial polysaccharides, and bioactive natural products. In the structure of polysaccharides such as α-glucan, 1,2-cis α-glucosides were found to be the major linkage between the glucopyranosides. Various regioisomeric linkages, 1→3, 1→4, and 1→6 for the backbone structure, and 1→2/3/4/6 for branching in the polysaccharide as well as in the oligosaccharides were identified. To achieve highly stereoselective 1,2-cis glycosylation, including α-glucosylation, a number of strategies using inter- and intra-molecular methodologies have been explored. Recently, Zn salt-mediated cis glycosylation has been developed and applied to the synthesis of various 1,2-cis linkages, such as α-glucoside and β-mannoside, via the 1,2-cis glycosylation pathway and β-galactoside 1,4/6-cis induction. Furthermore, the synthesis of various structures of α-glucans has been achieved using the recent progressive stereoselective 1,2-cis glycosylation reactions. In this review, recent advances in stereoselective 1,2-cis glycosylation, particularly focused on α-glucosylation, and their applications in the construction of linear and branched α-glucans are summarized.

The most common and linear example of a stereoisomeric β-D-glucans is cellulose, composed of β-D-Glcp, which plays a fundamental role as a structural component of the cell wall [75,76]. As physiologically active biological response modifiers (BRMs), the structure of glucans and the biological activity relationship of β-D-glucans have been reported to be adjuvants in bacterial, viral, or protozoan infections, and potent antitumor drugs, depending on the molecular weight, degree of branching, conformation, and intermolecular associations of glucans [76][77][78][79][80][81]. In the case of the synthesis of β-D-glucans, a common methodology such as stereoselective β-D-glucopyranosylation via the effect of neighboring group participation from the 2-O-acyl group can be effectively used [82][83][84].

Bimodal Glycosylation Approach for 1,2-Cis α-Glucosylation
Because of the structural diversity of glucans, a unified strategy for the assembly of pure glucans is yet to be developed. For the stereocontrolled synthesis of both α-and βglycosides, a general strategy that applies to the construction of all types of glucans by exploiting a bimodal [195][196][197][198][199] glucosyl donor equipped with C2-o-TAB ether [200][201][202][203][204] by the simple switching of the reaction conditions was developed in our laboratory [191,192] (Scheme 1). The synthesis of the glycosyl trichloroacetimidate donor with C2-O-TAB ether was carried out through a five-step transformation from the C2-OH of the thioglycoside derivative via C2-O-ether formation with o-azidobenzyl bromide [205,206] and NaH, reduction in the azide moiety by triphenylphosphine, and tosylation of the resultant amine. This was followed by the hydrolysis of the thioglycoside and subsequent treatment with trichloroacetonitrile in the presence of 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) (Scheme 1a).

Bimodal Glycosylation Approach for 1,2-cis α-glucosylation
Because of the structural diversity of glucans, a unified strategy for the assembly of pure glucans is yet to be developed. For the stereocontrolled synthesis of both αand β-glycosides, a general strategy that applies to the construction of all types of glucans by exploiting a bimodal [195][196][197][198][199] glucosyl donor equipped with C2-o-TAB ether [200][201][202][203][204] by the simple switching of the reaction conditions was developed in our laboratory [191,192] (Scheme 1). The synthesis of the glycosyl trichloroacetimidate donor with C2-O-TAB ether was carried out through a five-step transformation from the C2-OH of the thioglycoside derivative via C2-O-ether formation with o-azidobenzyl bromide [205,206] and NaH, reduction in the azide moiety by triphenylphosphine, and tosylation of the resultant amine. This was followed by the hydrolysis of the thioglycoside and subsequent treatment with trichloroacetonitrile in the presence of 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) (Scheme 1a).
The selective formation of β-glucosides was achieved when the activation of trichloroacetimidate was carried out by bis(trifluoromethanesulfonyl)imide (Tf 2 NH) [207][208][209][210] in propionitrile (EtCN) at low temperatures (−40 to −78 • C) (β-directing conditions). Using the same glucosyl donor, an alternative activation by triflic acid (TfOH) in Et 2 O under diluted conditions at room temperature predominantly provided α-glucosides as the major product (α-directing conditions) (Scheme 1b). After glucosylation, the selective liberation of the 3-, 4-, or 6-OH functionality in the presence of the TAB group at the C2 position and deprotection of the TAB group to liberate the 2-hydroxy group allowed for further glycosylations. The versatility of the bimodal glucosylation method was demonstrated by effectively assembling fragments of natural and non-natural glucans [191].
When the PhSO 2 group of an equatorially oriented TAB group at the 2-O-position interacts with the glycosyl cation through neighboring group participation in the presence of Tf 2 NH in EtCN, β-glycosides are predominantly formed (Scheme 1c). The stereodirecting effects of the TAB group have been explained by the contribution of hydrogen bonding between tosylamide and benzylic oxygen, forming a quasi-bicyclic form, such as the 2phthalimide (NPhth) group as a 1,2-trans directing group [211][212][213]. The activation of the donor moiety to initiate the formation of the oxocarbenium ion results in subsequent NGP by the sulfonamide oxygen to provide β-glycosides. Contrary to 1,2-trans-glycosylation, in ether solvents, the disruption of the intramolecular hydrogen bonding may result in the interaction with the incoming acceptor via intermolecular hydrogen bonding, controlling the 1,2-cis attack to afford α-glucosides selectively. Reactions using the perbenzylglucosyl trichloroacetimidate donor without the NHTs group in the presence of Tf 2 NH in EtCN provided the corresponding glycosides with diminished stereoselectivity (α/β = 11/ 89 compared with β-only for 6a), whereas the stereoselectivity was similar (α/β = 83/17) to 6a (84/16) in the presence of TfOH in Et 2 O [191]. These results also support the proposed mechanism.
Molecules 2023, 28 The selective formation of β-glucosides was achieved when the activation of trichloroacetimidate was carried out by bis(trifluoromethanesulfonyl)imide (Tf2NH) [207][208][209][210] in propionitrile (EtCN) at low temperatures (−40 to −78 °C) (β-directing conditions). Using the same glucosyl donor, an alternative activation by triflic acid (TfOH) in Et2O under diluted conditions at room temperature predominantly provided α-glucosides as the major product (α-directing conditions) (Scheme 1b). After glucosylation, the selective liberation of the 3-, 4-, or 6-OH functionality in the presence of the TAB group at the C2 position and deprotection of the TAB group to liberate the 2-hydroxy group allowed for further glycosylations. The versatility of the bimodal glucosylation method was demonstrated by effectively assembling fragments of natural and non-natural glucans [191].
When the PhSO2 group of an equatorially oriented TAB group at the 2-O-position interacts with the glycosyl cation through neighboring group participation in the presence of Tf2NH in EtCN, β-glycosides are predominantly formed (Scheme 1c). The stereodirecting effects of the TAB group have been explained by the contribution of hydrogen bonding between tosylamide and benzylic oxygen, forming a quasi-bicyclic form, such as the 2-phthalimide (NPhth) group as a 1,2-trans directing group [211][212][213]. The activation of the donor moiety to initiate the formation of the oxocarbenium ion results in subsequent NGP by the sulfonamide oxygen to provide β-glycosides. Contrary to 1,2-trans-glycosylation, in ether solvents, the disruption of the intramolecular hydrogen bonding may result The bimodal αand β-glycosylations were simply applied to the stereoselective synthesis of both αand β-galactosides using a bimodal galactosyl donor with C2-O-TAB ether by the simple switching of the αand β-directing reaction conditions, respectively, optimized for glucosylation [191]. The galactosyl donor has equatorial C2-O-TAB, which should similarly induce αand β-selectivity as the glucosyl donor (Scheme 2a).
An overstoichiometric amount of the Zn 2+ salt (2 equiv.) is required for β-mannosylation using a mannosyl donor with an imidate or phosphite as the leaving groups. Under these conditions, oxygen atoms at the 2-and 3-position coordinate with Zn 2+ , cleaving the intramolecular hydrogen bonding [237,238] (Scheme 2c). Afterward, the liberated NH group will be able to interact with an incoming nucleophile in an intermolecular fashion, reversing the stereocontrolling effect of the TAB group. The use of Cu(OTf) 2 in toluene, especially at elevated temperatures with the same phosphite donor, afforded the α-isomer predominantly. As shown in the case of bimodal α-and β-mannosylations using the 2-O-TAB group that interacted with/without the acceptor (ROH), the effect of Zn 2+ salt (2 equiv.) was revealed for β-mannosylation, using the mannosyl donor with imidate or phosphite as the leaving group. It has been observed that the Zn 2+ cation not only activates the donor leaving group but also coordinates with oxygens at the 2-and 3-positions to induce the effective interaction of TAB with an incoming nucleophile during 1,2-cis-β-mannosylation [214]. Combined with the enhancement of the fixed conformation of the pyranose ring by the 4,6-O-cyclic protection reported by Crich [244,245], a simple ZnI2-mediated procedure involving activation and direction to control the stereoselectivity for glucosylation has

ZnI 2 -mediated 1,2-cis α-glucosylation
As shown in the case of bimodal αand β-mannosylations using the 2-O-TAB group that interacted with/without the acceptor (ROH), the effect of Zn 2+ salt (2 equiv.) was revealed for β-mannosylation, using the mannosyl donor with imidate or phosphite as the leaving group. It has been observed that the Zn 2+ cation not only activates the donor leav-ing group but also coordinates with oxygens at the 2-and 3-positions to induce the effective interaction of TAB with an incoming nucleophile during 1,2-cis-β-mannosylation [214]. Combined with the enhancement of the fixed conformation of the pyranose ring by the 4,6-O-cyclic protection reported by Crich [244,245], a simple ZnI 2 -mediated procedure involving activation and direction to control the stereoselectivity for glucosylation has been developed as a novel general synthetic strategy for the construction of α-glucoside as one of the most abundant 1,2-cis-glycosidic bonds in nature [194]. To the best of our knowledge, the effective use of ZnI 2 for 1,2-cis glycosylation using a simple trichloroacetimidate donor has not been reported until recently. Using various acceptors, ZnI 2 -mediated α-glucosylation was demonstrated using 4,6-O-naphthylidene (NapCH<)-protected donors (6a) to demonstrate its versatility and effectiveness (Figure 3, Scheme 3(a-1)). The modular synthesis of various α-glucans with both linear and branched backbone structures using this simple approach was successfully achieved, as described in Section 3.2.1.
Molecules 2023, 28, x FOR PEER REVIEW 10 of 25 modular synthesis of various α-glucans with both linear and branched backbone structures using this simple approach was successfully achieved, as described in Section 3.2.1.   TS structure for 1,2-cis α-glucosylation; (c-1) stereoselective cis β-galactosylation and (c-2) TS structure for 1,2-cis α-glucosylation. TS structures were proposed by DFT calculations. For Ar, P, and R, CH 3 -, CH 3 -, and CH 3 CH 2 -were used for the calculation instead of Nap, Bn, and an acceptor molecule, respectively.
In addition to the experimental investigations and theoretical calculations, the ZnI 2mediated 1,2-cis glycosylation was analyzed (Scheme 3(a-2)) [194]. Theoretically, ZnI 2 activates the anomeric leaving groups on the donor molecule as Lewis acids and enhances glycosyl iodide formation. Subsequent activation of glycosyl iodide by another ZnI 2 leads to an intermediate that is also coordinated with the first ZnI 2 , which effectively coordinates with both hydroxyl groups on the acceptor, forming a six-membered structure with a trichloroacetimidate ion and the O-2 of the donor. Subsequent stereocontrolled nucleophilic attacks from the same side to the O-2 of the donor afford a 1,2-cis linkage, which then dissociates to the desired products.

Conclusions
In this review, recent advances in stereoselective 1,2-cis glycosylation, focusing on αglucosylation by bimodal glycosylation using o-TsNHbenzyl ether and ZnI 2 -mediated αglucosylation, and their applications in the construction of various types of linear branched glycans, are discussed. These enable a systematic investigation of the glucan structurebiological activity relationships with a whole series of possible structural isomers that would become simpler and more facile. In addition, recent approaches toward cyclic α-glucans such as cyclodextrins with a small ring size (down to three glucose residues in the ring) used conformationally counterbalanced donors between equatorial-and axial-rich forms. The automated α-glucan synthesis of up to 20 glucose residues was reported by Yamada [259] and by Seeberger [260], respectively. As Yu reported very recently [199], the synthesis and structural analysis of α-glucans could be possible with MD calculations to allow a more reliable estimation of the van der Waals volumes of α-glucans. Further structural investigations are valuable and may enable various applications, such as biotechnology for medicine and cosmetics, functional foods, drug delivery, and immunological responses.