Highly diastereoselective alpha-mannopyranosylation in the absence of participating protecting groups

Study of the stereoselectivity of 2-azido-2-deoxygalactosyl donors: remote protecting group effects and temperature dependency

The stereoselectivity of glycosylation reactions is affected by many factors. Synthesis of 1,2-cis glycosidic linkages (such as α linkages in glucose and galactose like monosaccharides) is challenging due to lack of control of the stereoselectivity. Our systematic study of GalN(3) donors with different combination of protecting groups indicated that acetyl groups at the 3- and 4-positions are particularly important for high α-selectivity. Temperature is also recognized as a major factor in control of stereoselectivity. Mechanisms responsible for these experimental results are discussed and explored using computational methods. A remote participation model of the acetyl groups is proposed to explain the directing effects of the acetyl groups.

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.

Configurations and conformations of glycosyl sulfoxides

X-ray crystallographic studies of two axial glycosyl sulfoxides having RS configurations (derivatives of phenyl 2-azido-2-deoxy-1-thio-α-d-galactopyranoside S-oxide) show that they adopt anti conformations in the solid state, in contrast to previous observations and assumptions. Density functional theory (DFT) calculations at the B3lYP6–311G+(d,p)/6–31G(d) level confirm that anti conformations of both phenyl and methyl RS glycosyl sulfoxides of 2-azido-2-deoxy-α-d-pyranosides are more stable than exo-anomeric conformations in the gas phase. 1D NOE measurements indicate that the more polar exo-anomeric conformers are only populated to a slight extent in solution. The anti conformations are distorted so that the glycosyl substituents are closer to being eclipsed with H1. This distortion allows S n → σ* overlap if the sulfur lone pair is a p-type lone pair. Evidence for this overlap comes from short C1–S bond distances, as short as the comparable bond distances in the X-ray crystal structure and in the results from DFT calculations for the SS glycoside, which does adopt the expected exo-anomeric conformation, both in the solid state and in solution, and has normal n → σ* overlap. For 2-deoxy derivatives not bearing a 2-azido group, gas-phase DFT calculations at the same level indicate that the anti- and exo-anomeric conformers have comparable stabilities. Comparison of the results of the two series shows that electronegative substituents in equatorial orientations at C2 destabilize conformations with parallel S–O arrangements, the conformation favored by having an endocyclic C–O dipole antiparallel to the S–O dipole, by about 2.5 kcal mol–1 (1 cal = 4.184 J). An equatorial glycosyl sulfoxide, (SS) phenyl 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-1-thio-β-d-glucopyranoside S-oxide, also adopts an anti conformation in the solid state as shown by X-ray diffraction. It also adopts this conformation in solution, in contrast to studies of other equatorial glycosyl sulfoxides.

Protecting groups in carbohydrate chemistry: influence on stereoselectivity of glycosylations

Saccharides are polyhydroxy compounds, and their synthesis requires complex protecting group manipulations. Protecting groups are usually used to temporarily mask a functional group which may interfere with a certain reaction, but protecting groups in carbohydrate chemistry do more than protecting groups usually do. Particularly, protecting groups can participate in reactions directly or indirectly, thus affecting the stereochemical outcomes, which is important for synthesis of oligosaccharides. Herein we present an overview of recent advances in protecting groups influencing stereoselectivity in glycosylation reactions, including participating protecting groups, and conformation-constraining protecting groups in general.

Mechanism of a chemical glycosylation reaction

Glycosylation is arguably the most important reaction in the field of glycochemistry, yet it involves one of the most empirically interpreted mechanisms in the science of organic chemistry. The beta-mannopyranosides, long considered one of the more difficult classes of glycosidic bond to prepare, were no exception to this rule. A number of logical but circuitous routes for their preparation were described in the literature, but they were accompanied by an even greater number of mostly ineffective recipes with which to access them directly. This situation changed in 1996 with the discovery of the 4,6-O-benzylidene acetal as a control element permitting direct entry into the beta-mannopyranosides, typically with high yield and selectivity. The unexpected nature of this phenomenon demanded study of the mechanism, leading first to the demonstration of the alpha-mannopyranosyl triflates as reaction intermediates and then to the development of alpha-deuterium kinetic isotope effect methods to probe their transformation into the product glycosides. In this Account, we assemble our observations into a comprehensive assessment consistent with a single mechanistic scheme. The realization that in the glucopyranose series the 4,6-O-benzylidene acetal is alpha- rather than beta-directing led to further investigations of substituent effects on the stereoselectivity of these glycosylation reactions, culminating in their explanation in terms of the covalent alpha-glycosyl triflates acting as a reservoir for a series of transient contact and solvent-separated ion pairs. The function of the benzylidene acetal, as explained by Bols and co-workers, is to lock the C6-O6 bond antiperiplanar to the C5-O5 bond, thereby maximizing its electron-withdrawing effect, destabilizing the glycosyl oxocarbenium ion, and shifting the equilibria as far as possible toward the covalent triflate. beta-Selective reactions result from attack of the nucleophile on the transient contact ion pair in which the alpha-face of the oxocarbenium ion is shielded by the triflate counterion. The alpha-products arise from attack either on the solvent-separated ion pair or on a free oxocarbenium ion, according to the dictates of the anomeric effect. Changes in selectivity from varying stereochemistry (glucose versus mannose) or from using different protecting groups can be explained by the shifting position of the key equilibria and, in particular, by the energy differences between the covalent triflate and the ion pairs. Of particular note is the importance of substitutents at the 3-position of the donor; an explanation is proposed that invokes their evolving torsional interaction with the substituent at C2 as the chair form of the covalent triflate moves toward the half-chair of the oxocarbenium ion.

Highly stereoselective synthesis of alpha-D-mannopyranosyl phosphosugars

Alpha-mannopyranosyl phosphosugars are obtained in 61-90% yields from 4,6-O-benzylidene-protected mannosyl thioglycosides bearing ester functionality in the 3-O-position by coupling reactions with ammonium salts of phosphosugars on activation with 1-benzenesulfinyl piperidine, 2,4,6-tri-tert-butylpyrimidine, and trifluoromethanesulfonic anhydride. Due to the presence of the disarming ester group, only the formation of the alpha-isomer was observed.

Sonication-assisted oligomannoside synthesis

We have investigated the use of sonication for the synthesis of oligomannosides. A convenient sonication-mediated glycosylation protocol that is applicable to traditional glycosylation methods has been developed. This protocol can be applicable for activating glycosyl donors that are known to have low reactivity which enable the synthesis of oligomannosides of particular biological interest with the same efficiency.

Does neighboring group participation by non-vicinal esters play a role in glycosylation reactions? Effective probes for the detection of bridging intermediates

Neighboring group participation in glycopyranosylation reactions is probed for esters at the 3-O-axial and -equatorial, 4-O-axial and -equatorial, and 6-O-sites of a range of donors through the use tert-butoxycarbonyl esters. The anticipated intermediate cyclic dioxanyl cation is interrupted for the axial 3-O-derivative, leading to the formation of a 1,3-O-cyclic carbonate ester, with loss of a tert-butyl cation, providing convincing evidence of participation by esters at that position. However, no evidence was found for such a fragmentation of carbonate esters at the 3-O-equatorial, 4-O-axial and -equatorial, and 6-O positions, indicating that neighboring group participation from those sites does not occur under typical glycosylation conditions. Further probes employing a 4-O-(2-carboxy)benzoate ester and a 4-O-(4-methoxybenzoate) ester, the latter in conjunction with an (18)O quench designed to detect bridging intermediates, also failed to provide evidence for participation by 4-O-esters in galactopyranosylation.

Block synthesis of tetra- and hexasaccharides (beta-D-glycero-D-manno-Hep p-(1-->4)-[alpha-l-Rha p-(1-->3)-beta-D-glycero-D-manno-Hep p-(1-->4)]n-alpha-L-Rha p-OMe (n = 1 and 2)) corresponding to multiple repeat units of the glycan from the surface-layer glycoprotein from Bacillus thermoaerophilus

A fully stereocontrolled block synthesis of the title tetra- and hexasaccharides has been achieved taking advantage of the ability of the 4,6-O-benzylidene acetal to control the stereochemistry of the beta-D-glycero-D-mannoheptopyranoside unit and of a 2,3-O-diphenylmethylene acetal to install the alpha-L-rhamnopyranosidic linkages. Comparison of the spectral data for the hexasaccharide with that of the natural isolate confirms the structure of this very unusual and structurally challenging glycan.

Automated solid-phase synthesis of protected oligosaccharides containing beta-mannosidic linkages

For automated oligosaccharide synthesis to impact glycobiology, synthetic access to most carbohydrates has to become efficient and routine. Methods to install "difficult" glycosidic linkages have to be established and incorporated into the overall synthetic concept. Described here is the first automated solid-phase synthesis of oligosaccharides containing the challenging beta-mannosidic linkage. Carboxybenzyl mannoside building blocks proved effective beta-mannosylation agents and resulted in excellent conversion and good to moderate selectivities. [(Triisopropylsilyl)oxy]-methyl ether (Tom), served as an orthogonal, minimally intrusive, and readily cleavable protecting group for the elongation of the C3 position of mannose. The desired oligosaccharide products were readily separated from by-products containing unwanted stereoisomers using reverse-phase HPLC. The methods described here expand the scope of carbohydrates currently accessible by automation as many oligosaccharides of biological interest contain beta-mannosidic linkages.

Convergent synthesis of a beta-(1-->3)-mannohexaose

An as yet unknown beta-(1-->3)-mannohexaose has been synthesized by a block route involving the coupling of two trisaccharides. Comparison of three closely related attempted mannohexaose syntheses reinforces the influence of subtle matching and/or mismatching interactions on the outcome of convergent oligosaccharide synthesis.

Direct stereocontrolled synthesis of 3-amino-3-deoxy-beta-mannopyranosides: importance of the nitrogen protecting group on stereoselectivity

The highly stereocontrolled synthesis of the 3-amino-3-deoxy-beta-mannopyranosides is achieved by means of thioglycoside donors protected with a 4,6-O-benzylidene or alkylidene acetal and a benzylidene imine group. Among the various nitrogen protecting groups investigated only the Schiff's base was found to give high beta-selectivity. N-Phthalimido and N-acetamido protected donors were found to be highly alpha-selective, whereas 3-azido-3-deoxy glycosyl donors gave intermediate selectivity. The reasons for the protecting group dependency are discussed in terms of the change in the O2-C2-C3-N3 torsional interaction on conversion of the covalent glycosyl triflates to the transient oxacarbenium ions.

β-Selective Glucosylation in the Absence of Neighboring Group Participation: Influence of the 3,4-O-Bisacetal Protecting System

A 3,4-O-bisacetal 2,6-di-O-benzyl protected thioglucoside is converted to the corresponding glucosyl triflate with 1-benzenesulfinyl piperidine and trifluoromethanesulfonic anhydride. The moderate to excellent β-selectivity exhibited with this glucosyl triflate with a range of alcohols is generally higher than that observed with the more electronically disarmed corresponding 3,4-O-carbonate, for which a possible reason is advanced.

Synthesis of deoxy and methoxy analogs of octyl alpha-D-mannopyranosyl-(1-->6)-alpha-D-mannopyranoside as probes for mycobacterial lipoarabinomannan biosynthesis

A panel of analogs of the disaccharide alpha-D-Manp-(1-->6)-alpha-D-Manp-OOctyl, a known acceptor substrate for a polyprenol monophosphomannose-dependent alpha-(1-->6)-mannosyltransferase involved in the assembly of the alpha-(1-->6)-linked mannan core of mycobacterial lipoarabinomannan, has been synthesized. Described are synthetic routes to the target deoxy and methoxy analogs in which one of the hydroxyl groups of the parent disaccharide has been modified. All glycosylation reactions involved the use of octyl glycoside acceptors and thioglycoside donors using iodonium-ion activation, and the stereochemistry of the mannopyranoside bond formed was established by measurement of the 1J(C-1,H-1). Depending on the target, the key methylation or deoxygenation reactions were carried out on either mono- or disaccharide substrates. This series of analogs will be useful for probing the substrate specificity of the enzyme, in particular, its steric and hydrogen-bonding requirements.

4,6-O-benzylidene-directed beta-mannopyranosylation and alpha-glucopyranosylation: the 2-deoxy-2-fluoro and 3-deoxy-3-fluoro series of donors and the importance of the O2-C2-C3-O3 interaction

A series of 4,6-O-benzylidene-protected 2-O-benzyl-3-deoxy-3-fluoro- and 3-O-benzyl-2-deoxy-2-fluorogluco- and mannopyranosyl thioglycosides were synthesized and their coupling reactions with a series of alcohols, on preactivation with 1-benzenesulfinylpiperidine and trifluoromethanesulfonic anhydride, investigated. In all cases, the selectivities were lower than those observed with the corresponding simple 4,6-O-benzylidene 2,3-di-O-benzylgluco- and mannopyranosyl thioglycosides. This leads to the conclusion that the high beta-selectivity observed with 4,6-O-benzylidene 2,3-di-O-benzylmannopyranosyl donors under the same conditions is in large part derived from the compression of the O2-C2-C3-O3 torsion angle on going from the intermediate covalent glycosyl triflate to the oxacarbenium ion, as compared to the relaxation of this torsion angle in the gluco series.

The use of cyclic bifunctional protecting groups in oligosaccharide synthesis—an overview

A historical overview is presented on stereo-directing effects of cis- and trans-fused diol protective groups used on both donor and acceptor glycosides. Attention is focused on the use of cyclic carbonates and carbamates, diacetals and acetals and finally the special case of 1,2-O-orthoesters and 1,2-O-cyanoalkylidene functionalised residues.

On the influence of the C2-O2 and C3-O3 bonds in 4,6-O-benzylidene-directed beta-mannopyranosylation and alpha-glucopyranosylation

The synthesis of 4,6-O-benzylidene-protected 2-deoxyarabino-, 3-deoxyarabino-, and 3-deoxyribothioglycosides is described and their glycosylation reactions, with activation by either 1-benzenesulfinyl piperidine/trifluoromethansulfonic anhydride or diphenyl sulfoxide/trifluoromethanesulfonic anhydride, studied. In contrast to the corresponding 4,6-O-benzylidene-protected glucosyl and mannosyl donors, which are alpha- and beta-selective, respectively, poor diastereoselectivity is observed in all cases. The reasons for this poor selectivity are discussed in terms of the interaction between the C2-O2 and C3-O3 bonds in the glucosyl and mannosyl donors and of the influence of this interaction on the ease of formation of the intermediate glycosyl oxacarbenium ions.

On the nitrile effect in L-rhamnopyranosylation

It is shown that the use of 5% acetonitrile or propionitrile in dichloromethane functions to increase the beta-selectivity of a number of L-rhamnopyranosylation reactions conducted by the thioglycoside method with activation by the 1-benzenesulfinyl piperidine/trifluoromethanesulfonic anhydride couple. The use of more significant quantities of acetonitrile or propionitrile results in the formation of complex reaction mixtures containing little coupled product, but from which Ritter-type products can be isolated.

Enhanced diastereoselectivity in beta-mannopyranosylation through the use of sterically minimal propargyl ether protecting groups

[reaction: see text] 2-O-Propargyl ethers are shown to be advantageous in the 4,6-O-benzylidene acetal directed beta-mannosylation reaction. The effect is most pronounced when the O3 protecting group is a bulky silyl ether or a glycosidic bond; however, even with a 3-O-benzyl ether, the use of a 2-O-propargyl ether results in a significant increase in diastereoselectivity. The beneficial effect of the propargyl ether is thought to be a combination of its minimal steric bulk, as determined by a measurement of the steric A-value and of its moderately disarming nature, as reflected in the pKa of propargyl alcohol. Conversely, the application of a 3-O-propargyl ether in the benzylidene acetal directed mannosylation has a detrimental effect on stereoselectivity, for which no explanation is at present available. Deprotection is achieved by base-catalyzed isomerization of the propargyl ether group to the corresponding allenyl ether, followed by oxidative cleavage with N-methylmorpholine N-oxide and catalytic osmium tetroxide.

Stereocontrolled Formation of β-Glucosides and Related Linkages in the Absence of Neighboring Group Participation: Influence of a trans-Fused 2,3-O-Carbonate Group

[reaction: see text] Phenyl 4,6-di-O-benzyl-2,3-O-carbonyl-beta-D-glucothiopyranoside and the regiosiomeric phenyl 2,6-di-O-benzyl-3,4-O-carbonyl-beta-D-glucothiopyranoside were prepared and studied as glucosyl donors at -60 degrees C in dichloromethane with preactivation by 1-benzenesulfinyl piperidine before addition of the acceptor alcohol. The 2,3-O-carbonate protected donor showed moderate to excellent beta-selectivity under these conditions depending on the acceptor employed, thereby providing a means for 1,2-trans-equatorial glycosidic bonds without recourse to neighboring group participation and its associated problem of ortho ester formation. In contrast, the 3,4-O-carbonate protected donor showed moderate to no beta-selectivity under the conditions employed. The results obtained in this study with carbonate protected glucopyranosyl donors are contrasted with those obtained previously in the manno- and rhamnopyranosyl series when the 2,3-O-carbonate protected is alpha-selective and the 3,4-O-carbonate is beta-selective.

An Efficient Synthesis of β-C-Glycosides Based on the Conformational Restriction Strategy: Lewis Acid Promoted Silane Reduction of the Anomeric Position with Complete Stereoselectivity

[reaction: see text] The reduction of glyconolactols having an anomeric carbon substituent by Et(3)SiH/TMSOTf proceeded with complete stereoselectivity to produce the corresponding beta-C-glycosides when the substrates were conformationally restricted in the (4)C(1)-chair form by a 3,4-O-cyclic diketal or a 4,6-O-benzylidene protecting group. Thus, the efficient construction of beta-C-glycosides was achieved on the basis of the conformation restriction strategy.

Oxazolidinone protection of N-acetylglucosamine confers high reactivity on the 4-hydroxy group in glycosylation

[reaction: see text] The preparation of a convenient oxazolidinone protected N-acetyl glucosamine 4-OH derivative is reported. This substance exhibits enhanced reactivity as a glycosyl acceptor in a variety of coupling methods, the products of which are converted to the target N-acetylglucosaminyl saccharides under very mild conditions.

Direct synthesis of the beta-l-rhamnopyranosides

The direct formation of beta-l-rhamnopyranosides by means of thioglycoside donors protected with a 2-O-sulfonate ester and, ideally, a 4-O-benzoyl ester, is reported. Activation is achieved with the combination of 1-benzenesulfinyl piperidine and triflic anhydride in the presence of 2,4,6-tri-tert-butylpyrimidine. Selectivities vary from moderate to good, and the sulfonyl group is easily removed post-glycosylation with sodium amalgam in 2-propanol.

Cyclization of aryl silanes with unexpected retention of silicon

[formula: see text] Intramolecular Friedel-Crafts cyclization of 2-O-benzyl ethers at a model pyranose acetal is activated by incorporation of a trimethylsilyl group, albeit via unexpected, presumably steric means.

Study of interhalogens/silver trifluoromethanesulfonate as promoter systems for high-yielding sialylations

We have studied interhalogen/silver trifluoromethanesulfonate (IX/AgOTf) promoted glycosylations and found differences in the sensitivity of the formed oxocarbenium ions (e.g. from compounds with or without participating groups) toward halide nucleophiles. These differences can be explained using the HSAB theory. By applying this theory on sialylations, we increased the yield for a model reaction from a highly unpredictable 35-46% using ICl to 74% using IBr. We have also showed that the most prominent role of the silver ions is lowering the concentration of the halide nucleophile rather than activating the interhalogen compound and, by increasing the amount of AgOTf from 1 to 1.5 equiv (with respect to IBr), the yield in the model reaction improved from 74% to 89%. A comparison of two different anomeric leaving groups showed that glycal formation can be minimized using a thiophenyl donor instead of xanthate. By combining these observations, we were able to increase the yield of the model reaction to 97%.

Synthesis of the Salmonella type E(1) core trisaccharide as a probe for the generality of 1-(benzenesulfinyl)piperidine/triflic anhydride combination for glycosidic bond formation from thioglycosides

A synthesis of a chromogenic glycoside of the Salmonella anatum group E(1) core trisaccharide is presented in which all three glycosidic bonds, a 1,2-cis-equatorial, a 1,2-trans-axial, and a 1,2-trans-equatorial linkage representing three of the four main classes of glycosidic bond, are formed with thioglycoside donors activated under a single set of conditions by the combination of 1-(benzenesulfinyl)piperidine and trifluoromethanesulfonic anhydride. 2,3-O-Carbonyl- and 2,3-O-isopropylidene-alpha-L-rhamnopyranosyl thioglycosides are found to be highly alpha-selective rhamnosyl donors under these conditions.