1-O-Methyl Glycosides

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    α-Selective Glycosylation of 3,6-O-o-Xylylene-Bridged Glucosyl Fluoride

    Motoyama, Atsushi; Arai, Tomoki; Ikeuchi, Kazutada; Aki, Kazuya; Wakamori, Shinnosuke; Yamada, Hidetoshi, Synthesis 2018, 50, 282-294

    Keywords: glycosylation · 1,2-cis-selectivity · kinetic control · conformation · o-xylylene bridge

    A 1,2-cis-(α)-selective glycosylation has been developed. An ortho-xylylene group bridged between 3-O and 6-O of d-glucosyl fluoride, which straddles the β-face of the pyranose ring, hinders the ­approach of glycosyl acceptors from that face. The determination of the three-dimensional structure of the bridged glucosyl fluoride, the optimization process of the reaction conditions oriented toward kinetic control to realize the high α-selectivity, and the scope of the reaction are described.

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    Substoichiometric FeCl3 Activation of Propargyl Glycosides for the Synthesis of Disaccharides and Glycoconjugates

    Sun, Guosheng; Wu, Yue; Liu, Anqi; Qiu, Saifeng; Zhang, Wan; Wang, Zhongfu; Zhang, Jianbo, Synlett 2018, 29, 668-672

    Keywords: propargyl glycosides · FeCl3 · disaccharides · glycoconjugates · glycosylation

    Glycosides as glycosyl donors using FeCl3 have been described. Under optimal reaction conditions, three kinds of propargyl glycosides were found to react with steroids and sugar-derived glycosyl acceptors to afford the corresponding disaccharides and glycoconjugates in good to excellent yields (66–91%). Meanwhile, the method can also realize one-pot synthesis of disaccharides, making it an effective, affordable, and green glycosylation procedure.

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    Indium(III) Iodide-Catalyzed Stereoselective Synthesis of β-Glucopyranosides by Using a Glucosyl Fluoride Donor with 2-O-Benzoyl-3,4,6-Tri-O-Benzyl Protection

    Ma, Teng; Li, Changwei; Zhang, Zhan-xin; Wang, Zhaoyan; Yu, Lan; Xue, Weihua, Synlett 2017, 28, 2633-2636

    Keywords: indium triiodide · glucopyranosides · glucosyl fluoride · stereoselectivity · benzoylation

    We have developed a novel protocol for glucosylation by adopting a glucosyl fluoride donor with 2-O-benzoyl-3,4,6-tri-O-benzyl protection. The protocol is useful for the ready assembly of β-linked functional glycoconjugates, and the reaction accommodates a broad range of substrates. Conveniently, water-tolerant and commercially available InI3 is used as a catalyst, and no other additional reagent is required. The method involves an interesting process for glucosyl fluoride activation and, in particular, permits the stereoselective construction of partially benzylated glucopyranosides carrying a selectively removable 2-O-benzoyl group, which hold great potential as glycosyl receptors for building further 1,2-glycosidic linkages.

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    2,6-Lactones as a New Entry in Stereoselective Glycosylations

    Sasaki, Kaname; Hashimoto, Yusuke, Synlett 2017, 28, 1121-1126

    Keywords: glycosylation · 1,2-cis-β-glycosylation · mannosylation · stereoselectivity · glycosyl trichloroacetimidate · glycosyl cation · SN2–SN1 borderline · 2,6-lactone

    The advantages of glycosyl donors bearing a 2,6-lactone moiety in 1,2-cis-β-glycosylation reactions are discussed in the context of recent comprehension on the SN2–SN1 borderline. The 2,6-lactone structure increases the likelihood of the SN2-like reaction, analogous to 4,6-tethered structures or 2-O-electron-deficient substituents, which are known to mound the energetic barrier to SN1 reactions. Furthermore, the glycosyl cation generated from the 2,6-lactone donor seems to direct β-glycosides similar to the torsional and flipped cations generated from 4,6-tethered donors and mannuronate or 3,6-lactone donors, respectively. Overall, 2,6-lactones are suitable for use in 1,2-cis-β-glycosylations, and this novel class of donors is expected to help deepen our global understanding of glycosylation reactions.

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    O-Glycosylation Enabled by Remote Activation

    Spell, Mark L.; Deveaux, Kristina; Bresnahan, Caitlin G.; Ragains, Justin R., Synlett 2017, 28, 751-761

    Keywords: glycosylation · remote activation · oligosaccharides · visible light · photochemistry

    O-Glycosylation is a critically important and recurring step in the synthesis of oligosaccharides and other natural and non-natural products. While many approaches to O-glycosylation have been reported, those strategies involving remote activation are distinguished by the mildness and orthogonality that they often engender. As a result, O-glycosylation using remote activation strategies has been utilized successfully in the synthesis of complex molecules that include oligosaccharides and macrolides. Herein, we discuss a number of contributions that have been made to this area since the 1970s. This includes our own recent contribution involving the visible-light activation of 4-p-methoxyphenyl-3-butenylthioglycosides toward O-glycosylation in the presence of Umemoto’s reagent.

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    Glycosyl Stille Cross-Coupling with Anomeric Nucleophiles – A General Solution to a Long-Standing Problem of Stereocontrolled Synthesis of C-Glycosides

    Zhu, Feng; Yang, Tianyi; Walczak, Maciej A., Synlett 2017, 28, 1510-1516

    Keywords: anomeric stannanes · anomeric nucleophiles · Stille reaction · C-glycosides · glycosylation

    Aryl C-glycosides are common structural motifs found in bioactive natural products and commercially available drugs. Despite their importance, most chemical methods to prepare C-glycosides have relied on the nucleophilic addition/substitution of a glycosyl electrophile, which result in variable anomeric selectivities and yields. Furthermore, these methods are not compatible with saccharides containing free hydroxyl groups. Here, we describe a direct cross-coupling reaction of anomeric nucleophiles (anomeric stannanes) and aryl halides. This method is the first general approach to the synthesis of aryl C-glycosides resulting in exclusive anomeric selectivities for both anomers for a broad range of aryl and carbohydrate coupling partners (including unprotected saccharides).

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    A Novel Glycosyl Donor with a Triisopropylsilyl Nonparticipating Group in Benzyl-Free Stereoselective 1,2-cis-Galactosylation

    Abronina, Polina I.; Zinin, Alexander I.; Malysheva, Nelly N.; Stepanova, Elena V.; Chizhov, Alexander O.; Torgov, Vladimir I.; Kononov, Leonid O., Synlett 2017, 28, 1608-1613

    Keywords: glycosylation · stereoselectivity · stereoselective synthesis · conformation · protecting groups · triisopropylsilyl group · 4-(3-chloropropoxy)phenyl glycosides

    A novel glycosyl donor with a triisopropylsilyl (TIPS) nonparticipating group at O-2 is introduced for use in 1,2-cis-galactosylation. Coupling the 2-O-TIPS-substituted thiogalactoside donor with a series of mono- and disaccharide glycosyl acceptors was found to lead exclusively to α-linked oligosaccharides. The observed exceptionally high α-selectivity was interpreted in terms of conformational changes in the glycosyl cation induced by the bulky 2-O-TIPS group.

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    A Rapid and Diastereoselective Synthesis of 2-Deoxy-2-iodo-α-glycosides and its Mechanism for Diastereoselectivity

    Yuan, Wenjiao; Liu, Yali; Li, Chunbao, Synlett 2017, 28, 1975-1978

    Keywords: 2-deoxy-2-iodo-glycoside · selective glycosylation · glycal · alcohol · iodine · iodobenzene diacetate

    Reductive deiodination of 2-deoxy-2-iodo-glycoside is an efficient and practical approach for the synthesis of 2-deoxyglycosides, which are moieties of bioactive compounds. However, inseparable diastereoisomers are usually formed in the preparation of 2-deoxy-2-iodo-glycosides via glycosylation of glycals with alcohols using current methods. To overcome this problem, a rapid and diastereoselective transformation of glycals and alcohols into 2-deoxy-2-iodo-α-glycosides enabled by I2/PhI(OAc)2 has been developed. 14 glycals, derived from 13 monosaccharides and one disaccharide, diastereoselectively yielded α-glycosides. Only in two cases the diastereoselectivity of the glycosylation was poor. The yields of glycosylation range from 73% to 95%, and the reactions are finished in only five minutes. Investigations for better diastereoselectivity by comparing I2/Ph(OAc)2- with I2/Cu(OAc)2-mediated glycosylations using UV analysis have been conducted.

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    Iron(III)-Catalyzed Prins Cyclization towards the Synthesis of trans-Fused Bicyclic Tetrahydropyrans

    Pérez, Sixto J.; Miranda, Pedro O.; Cruz, Daniel A.; Fernández, Israel; Martín, Víctor S.; Padrón, Juan I., Synthesis 2015, 47, 1791-1798

    Keywords: iron · cyclic ethers · sustainable metal catalysis · bicyclic tetrahydropyrans · Prins cyclization

    trans-Fused bicyclic tetrahydropyrans have been synthesized through an intramolecular Prins cyclization catalyzed by iron(III). The cyclization process is stereoselective, leading exclusively to an all-cis configuration in the newly generated ring. This useful methodology allows for easy access to a variety of bicyclic ethers present in a wide range of bioactive natural products. Remarkably, the cyclization reaction works well when more challenging aldehydes bearing a functional group, such as a double bond or an acetate, are used. In addition, we present a computational study which rationalizes the results and explains the complete cis stereoselectivity in the newly formed tetrahydropyran ring.

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    Synthesis, Characterization, and Crystal Structure of Sodium (Methyl α-d-Mannopyranosid)uronate Monohydrate

    Xu, Peng; Dauter, Zbigniew; Kováč, Pavol, Synthesis 2014, 46, 1073-1078

    Keywords: mannuronic acid · oxidation · chemoselective methylation · TEMPO · HPLC

    TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation of methyl α-d-mannopyranoside with sodium hypochlorite gave sodium (methyl α-d-mannopyranosid)uronate, which was obtained as a crystalline monohydrate in ~70% yield without chromatography. Its purity was proved by NMR spectroscopy, a newly developed HPLC method, combustion analysis, and X-ray crystallography. The crystal structure, solved from synchrotron diffraction data in space group P212121, revealed that the packing of the uronate molecules is connected by an extensive network of hydrogen bonds, and that the Na+ ion is coordinated by six oxygen ligands from one water and three surrounding sugar molecules.