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Parametric Analysis of Donor Activation for Glycosylation Reactions. Chemistry 2024:e202400479. [PMID: 38545936 DOI: 10.1002/chem.202400479] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2024] [Indexed: 04/18/2024]
Abstract
The chemical synthesis of complex oligosaccharides relies on efficient and highly reproducible glycosylation reactions. The outcome of a glycosylation is contingent upon several environmental factors, such as temperature, acidity, the presence of residual moisture, as well as the steric, electronic, and conformational aspects of the reactants. Each glycosylation proceeds rapidly and with a high yield within a rather narrow temperature range. For better control over glycosylations and to ensure fast and reliable reactions, a systematic analysis of 18 glycosyl donors revealed the effect of reagent concentration, water content, protecting groups, and structure of the glycosyl donors on the activation temperature. With these insights, we parametrize the first step of the glycosylation reaction to be executed reliably and efficiently.
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2
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Direct Synthesis of α- and β-2'-Deoxynucleosides with Stereodirecting Phosphine Oxide via Remote Participation. J Am Chem Soc 2024; 146:8768-8779. [PMID: 38483318 DOI: 10.1021/jacs.4c01780] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/28/2024]
Abstract
2'-Deoxynucleosides and analogues play a vital role in drug development, but their preparation remains a significant challenge. Previous studies have focused on β-2'-deoxynucleosides with the natural β-configuration. In fact, their isomeric α-2'-deoxynucleosides also exhibit diverse bioactivities and even better metabolic stability. Herein, we report that both α- and β-2'-deoxynucleosides can be prepared with high yields and stereoselectivity using a remote directing diphenylphosphinoyl (DPP) group. It is particularly efficient to prepare α-2'-deoxynucleosides with an easily accessible 3,5-di-ODPP donor. Instead of acting as a H-bond acceptor on a 2-(diphenylphosphinoyl)acetyl (DPPA) group in our previous studies for syn-facial O-glycosylation, the phosphine oxide moiety here acts as a remote participating group to enable highly antifacial N-glycosylation. This proposed remote participation mechanism is supported by our first characterization of an important 1,5-briged P-heterobicyclic intermediate via variable-temperature NMR spectroscopy. Interestingly, antiproliferative assays led to a α-2'-deoxynucleoside with IC50 values in the low micromole range against central nervous system tumor cell lines SH-SY5Y and LN229, whereas its β-anomer exhibited no inhibition at 100 μM. Furthermore, the DPP group significantly enhanced the antitumor activities by 10 times.
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3
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Characterization of elusive rhamnosyl dioxanium ions and their application in complex oligosaccharide synthesis. Nat Commun 2024; 15:2257. [PMID: 38480691 PMCID: PMC10937939 DOI: 10.1038/s41467-024-46522-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2023] [Accepted: 02/29/2024] [Indexed: 03/17/2024] Open
Abstract
Attaining complete anomeric control is still one of the biggest challenges in carbohydrate chemistry. Glycosyl cations such as oxocarbenium and dioxanium ions are key intermediates of glycosylation reactions. Characterizing these highly-reactive intermediates and understanding their glycosylation mechanisms are essential to the stereoselective synthesis of complex carbohydrates. Although C-2 acyl neighbouring-group participation has been well-studied, the reactive intermediates in more remote participation remain elusive and are challenging to study. Herein, we report a workflow that is utilized to characterize rhamnosyl 1,3-bridged dioxanium ions derived from C-3 p-anisoyl esterified donors. First, we use a combination of quantum-chemical calculations and infrared ion spectroscopy to determine the structure of the cationic glycosylation intermediate in the gas-phase. In addition, we establish the structure and exchange kinetics of highly-reactive, low-abundance species in the solution-phase using chemical exchange saturation transfer, exchange spectroscopy, correlation spectroscopy, heteronuclear single-quantum correlation, and heteronuclear multiple-bond correlation nuclear magnetic resonance spectroscopy. Finally, we apply C-3 acyl neighbouring-group participation to the synthesis of complex bacterial oligosaccharides. This combined approach of finding answers to fundamental physical-chemical questions and their application in organic synthesis provides a robust basis for elucidating highly-reactive intermediates in glycosylation reactions.
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4
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Exploring the Dual Functions of Distal Acyl Group Direction in Various Nucleophilic Environments. J Org Chem 2024; 89:2375-2396. [PMID: 38288704 DOI: 10.1021/acs.joc.3c02397] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/17/2024]
Abstract
A universal glycosylation strategy could significantly simplify glycoside synthesis. One approach to achieving this goal is through acyl group direction for the corresponding 1,2-, 1,3-, 1,4-, or 1,6-trans glycosylation; however, this approach has been challenging for glycosidic bonds that require distal equatorial-acyl group direction. We developed an approach in weakly nucleophilic environments for selective 1,4-trans glycosylation directed by the equatorial-4-O-acyl group. Here, we explored this condition in other distal acyl groups and found that, besides 1,n-trans direction, acyl groups also mediated hydrogen bonding between acyl groups and alcohols. The latter showed a diverse effect and classified the acyl group direction into axial and equatorial categories. Corresponding glycosylation conditions were distinguished as guidance for acyl group direction from either category. Hence, acyl group direction may serve as a general glycosylation strategy.
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5
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Anomeric Triflates versus Dioxanium Ions: Different Product-Forming Intermediates from 3-Acyl Benzylidene Mannosyl and Glucosyl Donors. J Org Chem 2024; 89:1618-1625. [PMID: 38235652 PMCID: PMC10845153 DOI: 10.1021/acs.joc.3c02262] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2023] [Revised: 12/19/2023] [Accepted: 12/28/2023] [Indexed: 01/19/2024]
Abstract
Minimal structural differences in the structure of glycosyl donors can have a tremendous impact on their reactivity and the stereochemical outcome of their glycosylation reactions. Here, we used a combination of systematic glycosylation reactions, the characterization of potential reactive intermediates, and in-depth computational studies to study the disparate behavior of glycosylation systems involving benzylidene glucosyl and mannosyl donors. While these systems have been studied extensively, no satisfactory explanations are available for the differences observed between the 3-O-benzyl/benzoyl mannose and glucose donor systems. The potential energy surfaces of the different reaction pathways available for these donors provide an explanation for the contrasting behavior of seemingly very similar systems. Evidence has been provided for the intermediacy of benzylidene mannosyl 1,3-dioxanium ions, while the formation of the analogous 1,3-glucosyl dioxanium ions is thwarted by a prohibitively strong flagpole interaction of the C-2-O-benzyl group with the C-5 proton in moving toward the transition state, in which the glucose ring adopts a B2,5-conformation. This study provides an explanation for the intermediacy of 1,3-dioxanium ions in the mannosyl system and an answer to why these do not form from analogous glucosyl donors.
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6
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Synthesis of sialyl halides with various acyl protective groups. Carbohydr Res 2024; 536:109033. [PMID: 38295530 DOI: 10.1016/j.carres.2024.109033] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2023] [Revised: 12/31/2023] [Accepted: 01/05/2024] [Indexed: 02/02/2024]
Abstract
Glycosyl halides are historically one of the first glycosyl donors used in glycosylation reactions, and interest in glycosylation reactions involving this class of glycosyl donors is currently increasing. New methods for their activation have been proposed and effective syntheses of oligosaccharides with their participation have been developed. At the same time, the possibilities of using these approaches to the synthesis of sialosides are restricted by the limited diversity of known sialyl halides (previously, mainly sialyl chlorides, less often sialyl bromides and sialyl fluorides, with acetyl (Ac) groups at the oxygen atoms and AcNH, Ac2N and N3 groups at C-5 were used). This work describes the synthesis of six new N-acetyl- and N-trifluoroacetyl-sialyl chlorides and bromides with O-chloroacetyl and O-trifluoroacetyl protective groups. Preparation of N,O-trifluoroacetyl protected derivatives was made possible due to development of the synthesis of sialic acid methyl ester pentaol with N-trifluoroacetyl group.
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7
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Total Synthesis of a Conjugation-Ready Tetrasaccharide Repeating Unit of Vibrio cholerae O:3 O-antigen Polysaccharide. Org Lett 2024; 26:745-750. [PMID: 38198674 DOI: 10.1021/acs.orglett.3c04225] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2024]
Abstract
Herein, we report the first total synthesis of the tetrasaccharide repeating unit of Vibrio cholerae O:3 O-antigen polysaccharide. The highly complex tetrasaccharide contains rare amino sugars such as d-bacillosamine and l-fucosamine, highly labile sugar ascarylose, and higher carbon sugar d-d-heptose. Stereoselective glycosylation of the notoriously reactive ascarylose with d-d-heptose, poor nucleophilicity of the axial C4-OH of l-fucosamine, and amide coupling are the key challenges encountered in the total synthesis, which was completed via a longest linear sequence of 23 steps in 4.2% overall yield.
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8
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Detection and Characterization of Rapidly Equilibrating Glycosylation Reaction Intermediates Using Exchange NMR. J Am Chem Soc 2023; 145:26190-26201. [PMID: 38008912 DOI: 10.1021/jacs.3c08709] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2023]
Abstract
The stereoselective introduction of glycosidic bonds (glycosylation) is one of the main challenges in the chemical synthesis of carbohydrates. Glycosylation reaction mechanisms are difficult to control because, in many cases, the exact reactive species driving product formation cannot be detected and the product outcome cannot be explained by the primary reaction intermediate observed. In these cases, reactions are expected to take place via other low-abundance reaction intermediates that are in rapid equilibrium with the primary reaction intermediate via a Curtin-Hammett scenario. Despite this principle being well-known in organic synthesis, mechanistic studies investigating this model in glycosylation reactions are complicated by the challenge of detecting the extremely short-lived reactive species responsible for product formation. Herein, we report the utilization of the chemical equilibrium between low-abundance reaction intermediates and the stable, readily observed α-glycosyl triflate intermediate in order to infer the structure of the former species by employing exchange NMR. Using this technique, we enabled the detection of reaction intermediates such as β-glycosyl triflates and glycosyl dioxanium ions. This demonstrates the power of exchange NMR to unravel reaction mechanisms as we aim to build a catalog of kinetic parameters, allowing for the understanding and eventual prediction of glycosylation reactions.
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9
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Advances in glycoside and oligosaccharide synthesis. Chem Soc Rev 2023; 52:7773-7801. [PMID: 37830906 DOI: 10.1039/d3cs00321c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/14/2023]
Abstract
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.
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10
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Unraveling the promoter effect and the roles of counterion exchange in glycosylation reaction. SCIENCE ADVANCES 2023; 9:eadk0531. [PMID: 37851803 PMCID: PMC10584349 DOI: 10.1126/sciadv.adk0531] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/31/2023] [Accepted: 09/14/2023] [Indexed: 10/20/2023]
Abstract
The stereoselectivity of glycosidic bond formation continues to pose a noteworthy hurdle in synthesizing carbohydrates, primarily due to the simultaneous occurrence of SN1 and SN2 processes during the glycosylation reaction. Here, we applied an in-depth analysis of the glycosylation mechanism by using low-temperature nuclear magnetic resonance and statistical approaches. A pathway driven by counterion exchanges and reaction byproducts was first discovered to outline the stereocontributions of intermediates. Moreover, the relative reactivity values, acceptor nucleophilic constants, and Hammett substituent constants (σ values) provided a general index to indicate the mechanistic pathways. These results could allow building block tailoring and reaction condition optimization in carbohydrate synthesis to be greatly facilitated and simplified.
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11
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Neighboring-Group Participation by C-2 Acyloxy Groups: Influence of the Nucleophile and Acyl Group on the Stereochemical Outcome of Acetal Substitution Reactions. Chemistry 2023; 29:e202301894. [PMID: 37410662 PMCID: PMC10592418 DOI: 10.1002/chem.202301894] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2023] [Revised: 07/06/2023] [Accepted: 07/06/2023] [Indexed: 07/08/2023]
Abstract
A single acyloxy group at C-2 can control the outcome of nucleophilic substitution reactions of pyran-derived acetals, but the extent of the neighboring-group participation depends on a number of factors. We show here that neighboring-group participation does not necessarily control the stereochemical outcome of acetal substitution reactions with weak nucleophiles. The 1,2-trans selectivity increased with increasing reactivity of the incoming nucleophile. This trend suggests the intermediacy of both cis-fused dioxolenium ions and oxocarbenium ions in the stereochemistry-determining step. In addition, as the electron-donating ability of the neighboring group decreased, the preference for the 1,2-trans products increased. Computational studies show how the barriers for the ring-opening reaction on the dioxolenium ions and the transition states to provide the oxocarbenium ions change with the electron-donating capacity of the C-2-acyloxy group and the reactivity of the nucleophile.
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12
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Selective reactivity of glycosyl cation stereoisomers: the role of intramolecular hydrogen bonding. Phys Chem Chem Phys 2023; 25:26737-26747. [PMID: 37779496 DOI: 10.1039/d3cp03326k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/03/2023]
Abstract
The impact of the stereochemistry of the glycosyl cation species upon its dynamic properties is examined together with their vibrational spectra in order to gain insight into the effects of configurational isomerism on conformer dynamics and proton mobility. Ab initio molecular dynamics (AIMD) simulations and infrared multiple photon dissociation (IRMPD) spectroscopy explore the conformational and reactive dynamics of two pairs of glycosyl cation isomers: (1) protonated α- and β- anomers of methyl-D-galactopyranoside and (2) the oxocarbenium ions of the D-aldohexose C2 epimers galactose and talose. Analysis of these simulations together with experimental spectroscopy, interpreted by anharmonic calculations, points to the key role played by the intramolecular hydrogen bonds which are present in a unique pattern and extent in each isomer. We find that the reactivity of galactoside stereoisomers toward acid-catalyzed nucleophilic substitution, as gauged by the ability to form free oxocarbenium ions, differs markedly in a way that agrees with experimental measurements in the condensed phase. Other properties such as conformer stability and vibrational transitions were also found to reflect the characteristic hydrogen bonding interactions present in each isomer. In both systems, the stereochemistry is shown to determine the strength of intramolecular hydrogen bonding as well as between which substituents proton transfer is possible. We expect that the critical impact of non-covalent interactions on stereoisomer selectivity may be a widely found phenomenon whose effects should be further investigated.
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β-l-Rhamnosylation and β-d-Mannosylation Mediated by 4- O-Ester Groups in a Weakly Nucleophilic Environment. Org Lett 2023; 25:7120-7125. [PMID: 37738091 DOI: 10.1021/acs.orglett.3c02566] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/23/2023]
Abstract
eq-4-O-Acyl group directed β-rhamnosylation and β-mannosylation are achieved in a carborane or BARF anion formed weakly nucleophilic environment with the assistance of a 2,3-orthocarbonate group. The 4-O-acyl group plays a critical role in directing the β-selectivity, and the weakly coordinating anion is essential to amplify this direction. The orthocarbonate group could be readily removed with 1,3-propanediol in the presence of BF3·Et2O.
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14
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Combination of 3- O-Levulinoyl and 6- O-Trifluorobenzoyl Groups Ensures α-Selectivity in Glucosylations: Synthesis of the Oligosaccharides Related to Aspergillus fumigatus α-(1 → 3)-d-Glucan. J Org Chem 2023; 88:12542-12564. [PMID: 37593939 DOI: 10.1021/acs.joc.3c01283] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 08/19/2023]
Abstract
Stereospecific α-glucosylation of primary and secondary OH-group at carbohydrate acceptors is achieved using glucosyl N-phenyl-trifluoroacetimidate (PTFAI) donor protected with an electron-withdrawing 2,4,5-trifluorobenzoyl (TFB) group at O-6 and the participating levulinoyl (Lev) group at O-3. New factors have been revealed that might explain α-stereoselectivity in the case of TFB and pentafluorobenzoyl (PFB) groups at O-6. They are of conformational nature and confirmed by DFT calculations. The potential of this donor, as well as the orthogonality of TFB and Lev protecting groups, is showcased by the synthesis of α-(1 → 3)-linked pentaglucoside corresponding to Aspergillus fumigatus α-(1 → 3)-d-glucan and of its hexasaccharide derivative, bearing β-glucosamine residue at the non-reducing end.
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15
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Origin of Stereoselectivity in S E 2' Reactions of Six-membered Ring Oxocarbenium Ions. Chemistry 2023; 29:e202203490. [PMID: 36511875 DOI: 10.1002/chem.202203490] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2022] [Revised: 12/09/2022] [Accepted: 12/13/2022] [Indexed: 12/15/2022]
Abstract
Oxocarbenium ions are key reactive intermediates in organic chemistry. To generate a series of structure-reactivity-stereoselectivity principles for these species, we herein investigated the bimolecular electrophilic substitution reactions (SE 2') between allyltrimethylsilane and a series of archetypal six-membered ring oxocarbenium ions using a combined density functional theory (DFT) and coupled-cluster theory approach. These reactions preferentially proceed following a reaction path where the oxocarbenium ion transforms from a half chair (3 H4 or 4 H3 ) to a chair conformation. The introduction of alkoxy substituents on six-membered ring oxocarbenium ions, dramatically influences the conformational preference of the canonical 3 H4 and 4 H3 conformers, and thereby the stereochemical outcome of the SE 2' reaction. In general, we find that the stereoselectivity in the reactions correlates to the "intrinsic preference" of the cations, as dictated by their shape. However, for the C5-CH2 OMe substituent, steric factors override the "intrinsic preference", showing a more selective reaction than expected based on the shape of the ion. Our SE 2' energetics correlate well with experimentally observed stereoselectivity, and the use of the activation strain model has enabled us to quantify important interactions and structural features that occur in the transition state of the reactions to precisely understand the relative energy barriers of the diastereotopic addition reactions. The fundamental mechanistic insight provided in this study will aid in understanding the reactivity of more complex glycosyl cations featuring multiple substituents and will facilitate our general understanding of glycosylation reactions.
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Mapping the effect of configuration and protecting group pattern on glycosyl acceptor reactivity. Chem Sci 2023; 14:1532-1542. [PMID: 36794180 PMCID: PMC9906709 DOI: 10.1039/d2sc06139b] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2022] [Accepted: 01/12/2023] [Indexed: 01/15/2023] Open
Abstract
The reactivity of the acceptor alcohol can have a tremendous influence on the outcome of a glycosylation reaction, both in terms of yield and stereoselectivity. Through a systematic survey of 67 acceptor alcohols in glycosylation reactions with two glucosyl donors we here reveal how the reactivity of a carbohydrate acceptor depends on its configuration and substitution pattern. The study shows how the functional groups flanking the acceptor alcohol influence the reactivity of the alcohol and show that both the nature and relative orientation play an essential role. The empiric acceptor reactivity guidelines revealed here will aid in the rational optimization of glycosylation reactions and be an important tool in the assembly of oligosaccharides.
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Involvement of an Oxonium Ion Intermediate in Controlling the Diastereoselectivity of Nucleophilic Substitution Reactions of Septanoses. Org Lett 2023; 25:152-157. [PMID: 36599094 PMCID: PMC9850862 DOI: 10.1021/acs.orglett.2c03963] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
The alkoxy substituents at C4 and C2 of septanoses control the stereochemical outcomes of O-glycosylation reactions of these seven-membered-ring intermediates. Isolation of a bicyclic acetal byproduct in some substitution reactions suggests that the C4 benzyloxy substituent engaged in long-range participation, stabilizing intermediates by the formation of an oxonium ion intermediate. Inductive destabilization of the carbocationic intermediate provided by the C2 substituent is crucial to the participation of the remote alkoxy group.
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Stereoselective synthesis of α-glucosides with glucosyl (Z)-Ynenoates as donors. Carbohydr Res 2023; 523:108710. [PMID: 36370627 DOI: 10.1016/j.carres.2022.108710] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2022] [Revised: 10/21/2022] [Accepted: 10/26/2022] [Indexed: 11/11/2022]
Abstract
A SPhosAuNTf2-promoted DMF-modulated glycosylation approach with glycosyl (Z)-ynenoates as donors was developed for highly α-selective synthesis of various linkage types of α-glucans. The substituent groups were also found to play a significant role in the α-selective glucosylation reactions. The glycosylation approach was effectively applied to the stereospecific synthesis of the α-1,6-linked triglucoside.
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Abstract
Uronic acids are carbohydrates carrying a terminal carboxylic acid and have a unique reactivity in stereoselective glycosylation reactions. Herein, the competing intramolecular stabilization of uronic acid cations by the C-5 carboxylic acid or the C-4 acetyl group was studied with infrared ion spectroscopy (IRIS). IRIS reveals that a mixture of bridged ions is formed, in which the mixture is driven towards the C-1,C-5 dioxolanium ion when the C-5,C-2-relationship is cis, and towards the formation of the C-1,C-4 dioxepanium ion when this relation is trans. Isomer-population analysis and interconversion barrier computations show that the two bridged structures are not in dynamic equilibrium and that their ratio parallels the density functional theory computed stability of the structures. These studies reveal how the intrinsic interplay of the different functional groups influences the formation of the different regioisomeric products.
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α-Selective Glucosylation Can Be Achieved with 6- O- para-Nitrobenzoyl Protection. J Org Chem 2022; 87:13763-13789. [PMID: 36206491 DOI: 10.1021/acs.joc.2c01475] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
A systematic study of the effect of various 6-O-acyl groups on anomeric selectivity in glucosylations with thioglycoside donors was conducted. All eight different esters were found to induce moderate-to-high α-selectivity in glucosylation with l-menthol with the best being 6-O-p-nitrobenzoyl. The effect appears to be general across various glucosyl acceptors, glucosyl donor types, and modes of activation. No evidence was found in favor of distal participation.
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The Influence of the Electron Density in Acyl Protecting Groups on the Selectivity of Galactose Formation. J Am Chem Soc 2022; 144:20258-20266. [PMID: 36289569 PMCID: PMC9650713 DOI: 10.1021/jacs.2c05859] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
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The stereoselective formation of 1,2-cis-glycosidic
bonds is a major bottleneck in the synthesis of carbohydrates. We
here investigate how the electron density in acyl protecting groups
influences the stereoselectivity by fine-tuning the efficiency of
remote participation. Electron-rich C4-pivaloylated galactose building
blocks show an unprecedented α-selectivity. The trifluoroacetylated
counterpart with electron-withdrawing groups, on the other hand, exhibits
a lower selectivity. Cryogenic infrared spectroscopy in helium nanodroplets
and density functional theory calculations revealed the existence
of dioxolenium-type intermediates for this reaction, which suggests
that remote participation of the pivaloyl protecting group is the
origin of the high α-selectivity of the pivaloylated building
blocks. According to these findings, an α-selective galactose
building block for glycosynthesis is developed based on rational considerations
and is subsequently employed in automated glycan assembly exhibiting
complete stereoselectivity. Based on the obtained selectivities in
the glycosylation reactions and the results from infrared spectroscopy
and density functional theory, we suggest a mechanism by which these
reactions could proceed.
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Recent advances in stereoselective 1,2-cis-O-glycosylations. Front Chem 2022; 10:972429. [PMID: 36059876 PMCID: PMC9437320 DOI: 10.3389/fchem.2022.972429] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2022] [Accepted: 07/08/2022] [Indexed: 02/03/2023] Open
Abstract
For the stereoselective assembly of bioactive glycans with various functions, 1,2-cis-O-glycosylation is one of the most essential issues in synthetic carbohydrate chemistry. The cis-configured O-glycosidic linkages to the substituents at two positions of the non-reducing side residue of the glycosides such as α-glucopyranoside, α-galactopyranoside, β-mannopyranoside, β-arabinofuranoside, and other rather rare glycosides are found in natural glycans, including glycoconjugate (glycoproteins, glycolipids, proteoglycans, and microbial polysaccharides) and glycoside natural products. The way to 1,2-trans isomers is well sophisticated by using the effect of neighboring group participation from the most effective and kinetically favored C-2 substituent such as an acyl group, although high stereoselective synthesis of 1,2-cis glycosides without formation of 1,2-trans isomers is far less straightforward. Although the key factors that control the stereoselectivity of glycosylation are largely understood since chemical glycosylation was considered to be one of the useful methods to obtain glycosidic linkages as the alternative way of isolation from natural sources, strictly controlled formation of these 1,2-cis glycosides is generally difficult. This minireview introduces some of the recent advances in the development of 1,2-cis selective glycosylations, including the quite recent developments in glycosyl donor modification, reaction conditions, and methods for activation of intermolecular glycosylation, including the bimodal glycosylation strategy for 1,2-cis and 1,2-trans glycosides, as well as intramolecular glycosylations, including recent applications of NAP-ether-mediated intramolecular aglycon delivery.
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Abstract
Glycoscience assembles all the scientific disciplines involved in studying various molecules and macromolecules containing carbohydrates and complex glycans. Such an ensemble involves one of the most extensive sets of molecules in quantity and occurrence since they occur in all microorganisms and higher organisms. Once the compositions and sequences of these molecules are established, the determination of their three-dimensional structural and dynamical features is a step toward understanding the molecular basis underlying their properties and functions. The range of the relevant computational methods capable of addressing such issues is anchored by the specificity of stereoelectronic effects from quantum chemistry to mesoscale modeling throughout molecular dynamics and mechanics and coarse-grained and docking calculations. The Review leads the reader through the detailed presentations of the applications of computational modeling. The illustrations cover carbohydrate-carbohydrate interactions, glycolipids, and N- and O-linked glycans, emphasizing their role in SARS-CoV-2. The presentation continues with the structure of polysaccharides in solution and solid-state and lipopolysaccharides in membranes. The full range of protein-carbohydrate interactions is presented, as exemplified by carbohydrate-active enzymes, transporters, lectins, antibodies, and glycosaminoglycan binding proteins. A final section features a list of 150 tools and databases to help address the many issues of structural glycobioinformatics.
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Stabilization of Glucosyl Dioxolenium Ions by "Dual Participation" of the 2,2-Dimethyl-2-( ortho-nitrophenyl)acetyl (DMNPA) Protection Group for 1,2- cis-Glucosylation. J Org Chem 2022; 87:9139-9147. [PMID: 35748115 PMCID: PMC9295149 DOI: 10.1021/acs.joc.2c00808] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
Abstract
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The stereoselective
introduction of glycosidic bonds is of paramount
importance to oligosaccharide synthesis. Among the various chemical
strategies to steer stereoselectivity, participation by either neighboring
or distal acyl groups is used particularly often. Recently, the use
of the 2,2-dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA)
protection group was shown to offer enhanced stereoselective steering
compared to other acyl groups. Here, we investigate the origin of
the stereoselectivity induced by the DMNPA group through systematic
glycosylation reactions and infrared ion spectroscopy (IRIS) combined
with techniques such as isotopic labeling of the anomeric center and
isomer population analysis. Our study indicates that the origin of
the DMNPA stereoselectivity does not lie in the direct participation
of the nitro moiety but in the formation of a dioxolenium ion that
is strongly stabilized by the nitro group.
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Characterization of Elusive Reaction Intermediates Using Infrared Ion Spectroscopy: Application to the Experimental Characterization of Glycosyl Cations. Acc Chem Res 2022; 55:1669-1679. [PMID: 35616920 PMCID: PMC9219114 DOI: 10.1021/acs.accounts.2c00040] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
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A detailed
understanding of the reaction mechanism(s) leading to
stereoselective product formation is crucial to understanding and
predicting product formation and driving the development of new synthetic
methodology. One way to improve our understanding of reaction mechanisms
is to characterize the reaction intermediates involved in product
formation. Because these intermediates are reactive, they are often
unstable and therefore difficult to characterize using experimental
techniques. For example, glycosylation reactions are critical steps
in the chemical synthesis of oligosaccharides and need to be stereoselective
to provide the desired α- or β-diastereomer. It remains
challenging to predict and control the stereochemical outcome of glycosylation
reactions, and their reaction mechanisms remain a hotly debated topic.
In most cases, glycosylation reactions take place via reaction mechanisms
in the continuum between SN1- and SN2-like pathways.
SN2-like pathways proceeding via the displacement of a
contact ion pair are relatively well understood because the reaction
intermediates involved can be characterized by low-temperature NMR
spectroscopy. In contrast, the SN1-like pathways proceeding
via the solvent-separated ion pair, also known as the glycosyl cation,
are poorly understood. SN1-like pathways are more challenging
to investigate because the glycosyl cation intermediates involved
are highly reactive. The highly reactive nature of glycosyl cations
complicates their characterization because they have a short lifetime
and rapidly equilibrate with the corresponding contact ion pair. To
overcome this hurdle and enable the study of glycosyl cation stability
and structure, they can be generated in a mass spectrometer in the
absence of a solvent and counterion in the gas phase. The ease of
formation, stability, and fragmentation of glycosyl cations have been
studied using mass spectrometry (MS). However, MS alone provides little
information about the structure of glycosyl cations. By combining
mass spectrometry (MS) with infrared ion spectroscopy (IRIS), the
determination of the gas-phase structures of glycosyl cations has
been achieved. IRIS enables the recording of gas-phase infrared spectra
of glycosyl cations, which can be assigned by matching to reference
spectra predicted from quantum chemically calculated vibrational spectra.
Here, we review the experimental setups that enable IRIS of glycosyl
cations and discuss the various glycosyl cations that have been characterized
to date. The structure of glycosyl cations depends on the relative
configuration and structure of the monosaccharide substituents, which
can influence the structure through both steric and electronic effects.
The scope and relevance of gas-phase glycosyl cation structures in
relation to their corresponding condensed-phase structures are also
discussed. We expect that the workflow reviewed here to study glycosyl
cation structure and reactivity can be extended to many other reaction
types involving difficult-to-characterize ionic intermediates.
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26
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Influence of 3-Thio Substituents on Benzylidene-Directed Mannosylation. Isolation of a Bridged Pyridinium Ion and Effects of 3- O-Picolyl and 3- S-Picolyl Esters. European J Org Chem 2022; 2022:e202200320. [PMID: 36340645 PMCID: PMC9632450 DOI: 10.1002/ejoc.202200320] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2022] [Indexed: 08/08/2023]
Abstract
The influence on glycosyl selectivity of substituting oxygen for sulfur at the 3-position of 4,6-O-benzylidene-protected mannopyranosyl thioglycosides is reported and varies considerably according to the protecting group employed at the 3-position. The substitution of a thioether at the 3-position for the more usual 3-O-benzyl ether results in a significant loss of selectivity. The installation of a 3-S-picolinyl thioether results in a complex reaction mixture, from which a stable seven-membered bridged bicyclic pyridinium ion is isolated, while the corresponding 3-O-picolinyl ether affords a highly α-selective coupling reaction. A 3-O-picolyl ester provides excellent β-selectivity, while the analogous 3-S-picolyl thioester gives a highly α-selective reaction. The best β-selectivity is seen with a 3-deoxy-3-(2-pyridinyldisulfanyl) system. These observations are discussed in terms of the influence of the various substituents on the central glycosyl triflate - ion pair equilibrium.
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27
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Glycosyl Dioxalenium Ions as Reactive Intermediates of Automated Electrochemical Assembly. European J Org Chem 2022. [DOI: 10.1002/ejoc.202200135] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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28
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More than a Leaving Group: N-Phenyltrifluoroacetimidate as a Remote Directing Group for Highly α-Selective 1,2-cis Glycosylation. Angew Chem Int Ed Engl 2022; 61:e202201510. [PMID: 35266604 DOI: 10.1002/anie.202201510] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2022] [Indexed: 12/31/2022]
Abstract
The anomeric configuration can greatly affect the biological functions and activities of carbohydrates. Herein, we report that N-phenyltrifluoroacetimidoyl (PTFAI), a well-known leaving group for catalytic glycosylation, can act as a stereodirecting group for the challenging 1,2-cis α-glycosylation. Utilizing rapidly accessible 1,6-di-OPTFAI glycosyl donors, TMSOTf-catalyzed glycosylation occurred with excellent α-selectivity and broad substrate scope, and the remaining 6-OPTFAI group can be cleaved chemoselectively. The remote participation of 6-OPTFAI is supported by the first characterization of the crucial 1,6-bridged bicyclic oxazepinium ion intermediates by low-temperature NMR spectroscopy. These cations were found to be relatively stable and mainly responsible for the present stereoselectivities. Further application is highlighted in glycosylation reactions toward trisaccharide heparins as well as the convergent synthesis of chacotriose derivatives using a bulky 2,4-di-O-glycosylated donor.
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Diastereoselective Substitution Reactions of Acyclic β-Alkoxy Acetals via Electrostatically Stabilized Oxocarbenium Ion Intermediates. Org Lett 2022; 24:3217-3222. [PMID: 35446592 PMCID: PMC9817112 DOI: 10.1021/acs.orglett.2c01004] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
Substitution reactions of acyclic β-alkoxy acetals proceeded with generally high diastereoselectivities (>90:10) to form the anti product. Mechanistic experiments supplemented with computational studies suggest that, upon activation of the acetal, the resulting oxocarbenium ion is electrostatically stabilized by the β-alkoxy group. This stabilization defines the conformation of the reactive intermediate, which can be attacked preferentially from the more exposed face, leading to the observed products.
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30
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Abstract
Cells encode information in the sequence of biopolymers, such as nucleic acids, proteins, and glycans. Although glycans are essential to all living organisms, surprisingly little is known about the "sugar code" and the biological roles of these molecules. The reason glycobiology lags behind its counterparts dealing with nucleic acids and proteins lies in the complexity of carbohydrate structures, which renders their analysis extremely challenging. Building blocks that may differ only in the configuration of a single stereocenter, combined with the vast possibilities to connect monosaccharide units, lead to an immense variety of isomers, which poses a formidable challenge to conventional mass spectrometry. In recent years, however, a combination of innovative ion activation methods, commercialization of ion mobility-mass spectrometry, progress in gas-phase ion spectroscopy, and advances in computational chemistry have led to a revolution in mass spectrometry-based glycan analysis. The present review focuses on the above techniques that expanded the traditional glycomics toolkit and provided spectacular insight into the structure of these fascinating biomolecules. To emphasize the specific challenges associated with them, major classes of mammalian glycans are discussed in separate sections. By doing so, we aim to put the spotlight on the most important element of glycobiology: the glycans themselves.
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31
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Towards a Systematic Understanding of the Influence of Temperature on Glycosylation Reactions. Angew Chem Int Ed Engl 2022; 61:e202115433. [PMID: 35032966 PMCID: PMC9306470 DOI: 10.1002/anie.202115433] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2021] [Indexed: 11/08/2022]
Abstract
Glycosidic bond formation is a continual challenge for practitioners. Aiming to enhance the reproducibility and efficiency of oligosaccharide synthesis, we studied the relationship between glycosyl donor activation and reaction temperature. A novel semi-automated assay revealed diverse responses of members of a panel of thioglycosides to activation at various temperatures. The patterns of protecting groups and the thiol aglycon combine to cause remarkable differences in temperature sensitivity among glycosyl donor building blocks. We introduce the concept of donor activation temperature to capture experimental insights, reasoning that glycosylations performed below this reference temperature evade deleterious side reactions. Activation temperatures enable a simplified temperature treatment and facilitate optimization of glycosyl donor usage. Isothermal glycosylation below the activation temperature halved the equivalents of building block required in comparison to the standard "ramp" regime used in solution- and solid-phase oligosaccharide synthesis to-date.
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32
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Neighboring Group Participation of Benzoyl Protecting Groups in C3‐ and C6‐Fluorinated Glucose. European J Org Chem 2022; 2022:e202200255. [PMID: 35915640 PMCID: PMC9321577 DOI: 10.1002/ejoc.202200255] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2022] [Revised: 03/23/2022] [Indexed: 11/17/2022]
Abstract
Fluorination is a potent method to modulate chemical properties of glycans. Here, we study how C3‐ and C6‐fluorination of glucosyl building blocks influence the structure of the intermediate of the glycosylation reaction, the glycosyl cation. Using a combination of gas‐phase infrared spectroscopy and first‐principles theory, glycosyl cations generated from fluorinated and non‐fluorinated monosaccharides are structurally characterized. The results indicate that neighboring group participation of the C2‐benzoyl protecting group is the dominant structural motif for all building blocks, correlating with the β‐selectivity observed in glycosylation reactions. The infrared signatures indicate that participation of the benzoyl group in enhanced by resonance effects. Participation of remote acyl groups such as Fmoc or benzyl on the other hand is unfavored. The introduction of the less bulky fluorine leads to a change in the conformation of the ring pucker, whereas the structure of the active dioxolenium site remains unchanged.
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More than a Leaving Group: N‐Phenyltrifluoroacetimidate as a Remote Directing Group for Highly α‐Selective 1,2‐cis Glycosylation. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202201510] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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34
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Small tools for sweet challenges: advances in microfluidic technologies for glycan synthesis. Anal Bioanal Chem 2022; 414:5139-5163. [PMID: 35199190 DOI: 10.1007/s00216-022-03948-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2021] [Revised: 01/26/2022] [Accepted: 01/31/2022] [Indexed: 11/01/2022]
Abstract
Glycans, including oligosaccharides and glycoconjugates, play an integral role in modulating the biological functions of macromolecules. Many physiological and pathological processes are mediated by interactions between glycans, which has led to the use of glycans as biosensors for pathogen and biomarker detection. Elucidating the relationship between glycan structure and biological function is critical for advancing our understanding of the impact glycans have on human health and disease and for expanding the repertoire of glycans available for bioanalysis, especially for diagnostics. Such efforts have been limited by the difficulty in obtaining sufficient quantities of homogenous glycan samples needed to resolve the exact relationships between glycan structure and their structural or modulatory functions on a given glycoconjugate. Synthetic strategies offer a viable route for overcoming these technical hurdles. In recent years, microfluidics have emerged as powerful tools for realizing high-throughput and reproducible syntheses of homogenous glycans for the potential use in functional studies. This critical review provides readers with an overview of the microfluidic technologies that have been developed for chemical and enzymatic glycan synthesis. The advantages and limitations associated with using microreactor platforms to improve the scalability, productivity, and selectivity of glycosylation reactions will be discussed, as well as suggested future work that can address certain pitfalls.
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35
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Zu einem Systematischen Verständnis des Einflusses der Temperatur auf Glykosylierungsreaktionen. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202115433] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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36
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Abstract
A novel protocol is established for the long-standing challenge of stereoselective geminal bisglycosylations of saccharides. The merger of PPh3 as a traceless glycosidic leaving group and 1,2-boronate migration enables the simultaneous introduction of C-C and C-B bonds at the anomeric stereogenic center of furanoses and pyranoses. The power of this method is showcased by a set of site-selective modifications of glycosylation products for the construction of bioactive conjugates and skeletons. A scarce metal-free 1,1-difunctionalization process of alkenes is also concomitantly demonstrated.
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37
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Characterization of Mannosyl Dioxanium Ions in Solution Using Chemical Exchange Saturation Transfer NMR Spectroscopy. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202109874] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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38
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Research Advances in Functional Group-Directed Stereoselective Glycosylation. CHINESE J ORG CHEM 2022. [DOI: 10.6023/cjoc202204050] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
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39
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Gas-phase infrared spectroscopy of glycans and glycoconjugates. Curr Opin Struct Biol 2021; 72:194-202. [PMID: 34952241 DOI: 10.1016/j.sbi.2021.11.006] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2021] [Revised: 11/02/2021] [Accepted: 11/21/2021] [Indexed: 11/28/2022]
Abstract
Glycans are intrinsically complex biomolecules that pose particular analytical challenges. Standard workflows for glycan analysis are based on mass spectrometry, often coupled with separation techniques such as liquid chromatography and ion mobility spectrometry. However, this approach does not yield direct structural information and cannot always distinguish between isomers. This gap might be filled in the future by gas-phase infrared spectroscopy, which has emerged as a promising structure-sensitive technique for glycan fingerprinting. This review highlights recent applications of gas-phase infrared spectroscopy for the analysis of synthetic and biological glycans and how they can be integrated into mass spectrometry-based workflows.
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40
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Abstract
N‐Acyliminium ions are highly reactive intermediates that are important for creating CC‐bonds adjacent to nitrogen atoms. Here we report the characterization of cyclic N‐acyliminium ions in the gas phase, generated by collision induced dissociation tandem mass spectrometry followed by infrared ion spectroscopy using the FELIX infrared free electron laser. Comparison of DFT calculated spectra with the experimentally observed IR spectra provided valuable insights in the conformations of the N‐acyliminium ions.
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41
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Total Synthesis of a Chimeric Glycolipid Bearing the Partially Acetylated Backbone of Sponge-Derived Agminoside E. J Org Chem 2021; 86:15357-15375. [PMID: 34672576 DOI: 10.1021/acs.joc.1c01907] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We describe the total synthesis of a chimeric glycolipid bearing both the partially acetylated backbone of sponge-derived agminoside E and the (R)-3-hydroxydecanoic acid chain of bacterial rhamnolipids. The branched pentaglucolipid skeleton was achieved using a [3 + 2] disconnection approach. The β-(1 → 2) and β-(1 → 4)-glycosidic bonds were synthesized through a combination of NIS/Yb(OTf)3- and TMSOTf-mediated stereoselective glycosylations of thiotolyl, N-phenyltrifluoroacetimidate, and trichloroacetimidate donors. Late-stage pentaacetylation, Staudinger reduction of a (2-azidomethyl)benzoyl group, followed by continuous-flow microfluidic hydrogenolysis completed the total synthesis of the structurally simplified glycolipid, whose partial acetylation pattern on the glycan part was identical to agminoside E. Our study lays the foundation for the total synthesis of sponge-derived agminosides and the understanding of their biological functions in sponges.
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42
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Abstract
A multifunctional O-phenyl thiocarbonyl (O(C═S)OPh) group was introduced in glycosylation reactions. This auxiliary group exhibits three features (1) C6-long-range participation effect, (2) relay activation, and (3) switchable promoter-controlled carbonylation, which enables the facile synthesis of both 6-deoxy glucoside and 6-alcohol glucoside. In addition, we successfully quantified the extent of the C6-acyl participation effect and developed its application toward the α-trisaccharide motif.
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Characterization of Mannosyl Dioxanium Ions in Solution Using Chemical Exchange Saturation Transfer NMR. Angew Chem Int Ed Engl 2021; 61:e202109874. [PMID: 34519403 PMCID: PMC9305821 DOI: 10.1002/anie.202109874] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2021] [Indexed: 11/12/2022]
Abstract
The stereoselective introduction of the glycosidic bond remains one of the main challenges in carbohydrate synthesis. Characterizing the reactive intermediates of this reaction is key to develop stereoselective glycosylation reactions. Herein we report the characterization of low-populated, rapidly equilibrating, mannosyl dioxanium ions that arise from participation of a C-3 acyl group using chemical exchange saturation transfer (CEST) NMR spectroscopy. Dioxanium ion structure and equilibration kinetics were measured under relevant glycosylation conditions and highly α -selective couplings were observed suggesting glycosylation took place via this elusive intermediate.
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Abstract
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Automated synthesis
of DNA, RNA, and peptides provides quickly
and reliably important tools for biomedical research. Automated glycan
assembly (AGA) is significantly more challenging, as highly branched
carbohydrates require strict regio- and stereocontrol during synthesis.
A new AGA synthesizer enables rapid temperature adjustment from −40
to +100 °C to control glycosylations at low temperature and accelerates
capping, protecting group removal, and glycan modifications using
elevated temperatures. Thereby, the temporary protecting group portfolio
is extended from two to four orthogonal groups that give rise to oligosaccharides
with up to four branches. In addition, sulfated glycans and unprotected
glycans can be prepared. The new design reduces the typical coupling
cycles from 100 to 60 min while expanding the range of accessible
glycans. The instrument drastically shortens and generalizes the synthesis
of carbohydrates for use in biomedical and material science.
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How do Various Reaction Parameters Influence Anomeric Selectivity in Chemical Glycosylation with Thioglycosides and NIS/TfOH Activation? European J Org Chem 2021. [DOI: 10.1002/ejoc.202100260] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
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Abstract
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Polysaccharides are
Nature’s most abundant biomaterials
essential for plant cell wall construction and energy storage. Seemingly
minor structural differences result in entirely different functions:
cellulose, a β (1–4) linked glucose polymer, forms fibrils
that can support large trees, while amylose, an α (1–4)
linked glucose polymer forms soft hollow fibers used for energy storage.
A detailed understanding of polysaccharide structures requires pure
materials that cannot be isolated from natural sources. Automated
Glycan Assembly provides quick access to trans-linked
glycans analogues of cellulose, but the stereoselective installation
of multiple cis-glycosidic linkages present in amylose
has not been possible to date. Here, we identify thioglycoside building
blocks with different protecting group patterns that, in concert with
temperature and solvent control, achieve excellent stereoselectivity
during the synthesis of linear and branched α-glucan polymers
with up to 20 cis-glycosidic linkages. The molecules
prepared with the new method will serve as probes to understand the
biosynthesis and the structure of α-glucans.
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Abstract
Carbohydrates (glycans, saccharides, and sugars) are essential molecules in all domains of life. Research on glycoscience spans from chemistry to biomedicine, including material science and biotechnology. Access to pure and well-defined complex glycans using synthetic methods depends on the success of the employed glycosylation reaction. In most cases, the mechanism of the glycosylation reaction is believed to involve the oxocarbenium ion. Understanding the structure, conformation, reactivity, and interactions of this glycosyl cation is essential to predict the outcome of the reaction. In this Account, building on our contributions on this topic, we discuss the theoretical and experimental approaches that have been employed to decipher the key features of glycosyl cations, from their structures to their interactions and reactivity.We also highlight that, from a chemical perspective, the glycosylation reaction can be described as a continuum, from unimolecular SN1 with naked oxocarbenium cations as intermediates to bimolecular SN2-type mechanisms, which involve the key role of counterions and donors. All these factors should be considered and are discussed herein. The importance of dissociative mechanisms (involving contact ion pairs, solvent-separated ion pairs, solvent-equilibrated ion pairs) with bimolecular features in most reactions is also highlighted.The role of theoretical calculations to predict the conformation, dynamics, and reactivity of the oxocarbenium ion is also discussed, highlighting the advances in this field that now allow access to the conformational preferences of a variety of oxocarbenium ions and their reactivities under SN1-like conditions.Specifically, the ground-breaking use of superacids to generate these cations is emphasized, since it has permitted characterization of the structure and conformation of a variety of glycosyl oxocarbenium ions in superacid solution by NMR spectroscopy.We also pay special attention to the reactivity of these glycosyl ions, which depends on the conditions, including the counterions, the possible intra- or intermolecular participation of functional groups that may stabilize the cation and the chemical nature of the acceptor, either weak or strong nucleophile. We discuss recent investigations from different experimental perspectives, which identified the involved ionic intermediates, estimating their lifetimes and reactivities and studying their interactions with other molecules. In this context, we also emphasize the relationship between the chemical methods that can be employed to modulate the sensitivity of glycosyl cations and the way in which glycosyl modifying enzymes (glycosyl hydrolases and transferases) build and cleave glycosidic linkages in nature. This comparison provides inspiration on the use of molecules that regulate the stability and reactivity of glycosyl cations.
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Merging Reagent Modulation and Remote Anchimeric Assistance for Glycosylation: Highly Stereoselective Synthesis of α‐Glycans up to a 30‐mer. Angew Chem Int Ed Engl 2021. [DOI: 10.1002/ange.202103826] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
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49
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Metal-free glycosylation with glycosyl fluorides in liquid SO 2. Beilstein J Org Chem 2021; 17:964-976. [PMID: 33981367 PMCID: PMC8093551 DOI: 10.3762/bjoc.17.78] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2021] [Accepted: 04/17/2021] [Indexed: 11/23/2022] Open
Abstract
Liquid SO2 is a polar solvent that dissolves both covalent and ionic compounds. Sulfur dioxide possesses also Lewis acid properties, including the ability to covalently bind Lewis basic fluoride ions in a relatively stable fluorosulfite anion (FSO2 -). Herein we report the application of liquid SO2 as a promoting solvent for glycosylation with glycosyl fluorides without any external additive. By using various temperature regimes, the method is applied for both armed and disarmed glucose and mannose-derived glycosyl fluorides in moderate to excellent yields. A series of pivaloyl-protected O- and S-mannosides, as well as one example of a C-mannoside, are synthesized to demonstrate the scope of the glycosyl acceptors. The formation of the fluorosulfite species during the glycosylation with glycosyl fluorides in liquid SO2 is proved by 19F NMR spectroscopy. A sulfur dioxide-assisted glycosylation mechanism that proceeds via solvent separated ion pairs is proposed, whereas the observed α,β-selectivity is substrate-controlled and depends on the thermodynamic equilibrium.
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Merging Reagent Modulation and Remote Anchimeric Assistance for Glycosylation: Highly Stereoselective Synthesis of α-Glycans up to a 30-mer. Angew Chem Int Ed Engl 2021; 60:12597-12606. [PMID: 33763930 DOI: 10.1002/anie.202103826] [Citation(s) in RCA: 34] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/19/2021] [Indexed: 12/12/2022]
Abstract
The efficient synthesis of long, branched, and complex carbohydrates containing multiple 1,2-cis glycosidic linkages is a long-standing challenge. Here, we report a merging reagent modulation and 6-O-levulinoyl remote anchimeric assistance glycosylation strategy, which is successfully applied to the first highly stereoselective synthesis of the branched Dendrobium Huoshanense glycans and the linear Longan glycans containing up to 30 contiguous 1,2-cis glucosidic bonds. DFT calculations shed light on the origin of the much higher stereoselectivities of 1,2-cis glucosylation with 6-O-levulinoyl group than 6-O-acetyl or 6-O-benzoyl groups. Orthogonal one-pot glycosylation strategy based on glycosyl ortho-alkynylbenzoates and ortho-(1-phenylvinyl)benzoates has been demonstrated in the efficient synthesis of complex glycans, precluding such issues as aglycon transfer inherent to orthogonal one-pot synthesis based on thioglycosides.
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