Revisiting the Armed−Disarmed Concept:  The Importance of Anomeric Configuration in the Activation of <i>S</i>-Benzoxazolyl Glycosides

Crich, David; Li, Ming

doi:10.1021/ol701466u

Cited by 60 publications

(46 citation statements)

References 15 publications

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“…[99] Further investigation is required to determine the precise mechanistic details underlying these results; however, the absence of neighboring-group assistance probably accounts for the low reactivity of 3,4,6-tri-O-acyl 2-O-benzyl SBox glycosides in the presence of a relatively weak promoter. [100] Thioimidate donors have also been activated chemoselectively in the synthesis of sialosides [101] and galactofuranosides. [102] Overall, glycosyl thioimidates are good glycosyl donors for which interesting applications have been found in oligosaccharide synthesis.…”

Section: Glycosyl Thioimidatesmentioning

confidence: 99%

New Principles for Glycoside‐Bond Formation

2009

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Increased understanding of the important roles that oligosaccharides and glycoconjugates play in biological processes has led to a demand for significant amounts of these materials for biological, medicinal, and pharmacological studies. Therefore, tremendous effort has been made to develop new procedures for the synthesis of glycosides, whereby the main focus is often the formation of the glycosidic bonds. Accordingly, quite a few review articles have been published over the past few years on glycoside synthesis; however, most are confined to either a specific type of glycoside or a specific strategy for glycoside synthesis. In this Review, new principles for the formation of glycoside bonds are discussed. Developments, mainly in the last ten years, that have led to significant advances in oligosaccharide and glycoconjugate synthesis have been compiled and are evaluated.

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Section: Glycosyl Thioimidatesmentioning

confidence: 99%

New Principles for Glycoside‐Bond Formation

2009

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“…In combination, these two competing effects result in an overall moderate disarming of glycosyl donor 94. Additionally, Crich and Li [45] recently suggested the importance of the 1,2-trans anomeric configuration for the SBox glycosyl donors of the D-gluco series, in order for this stabilizing participation to occur. In the case of glycosyl donor 95, the O5 disarming effect is only slightly compensated by electron-donating 2-O-benzyl group, whose arming effect is mild.…”

Section: Superarmed Glycosyl Donors In Glycosylation Reactionsmentioning

confidence: 99%

Armed-Disarmed Concept in the Synthesis of Glycosidic Bond

Miljković

2013

Electrostatic and Stereoelectronic Effects in Carbohydrate Chemistry

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-Reid et al. reported [1] that n-pentenyl glycosides undergo chemospecific cleavage 1 ! 2 ! 3, with N-bromosuccinimide under conditions that leave a wide variety of other protecting groups unaffected ( Fig. 5.1).According to the proposed mechanism ( Fig. 5.1), the addition of water to the reaction mixture will lead to hydrolysis but the addition of alcohol or another sugar having a free hydroxyl group will lead to the formation of glycoside or a disaccharide. In the course of these studies, Fraser-Reid et al. [1] observed that sugar molecules having an acyloxy group at the C2 carbon (6 in Fig. 5.2) hydrolyzed much more slowly than if they had an alkoxy group (7 in Fig. 5.2). This observation suggested that the n-pentenyl glycoside could be made more or less reactive (it could be armed or disarmed) by the type of protecting group placed on the C2 oxygen.The ability of the C2 acyloxy group to disarm the n-pentenyl glycoside was rationalized as shown in Fig. 5.3. Thus, it can be assumed that the cyclic halonium ions 8 and 9 are formed reversibly [2] and that the electron density on the glycosidic oxygen is depleted in 2-O-acyl derivatives so that nucleophilic attack of this oxygen on the halonium ion is less favored than in the 2-O-alkyl counterpart 9. The reason for this is that the electron-withdrawing group at the C2 carbon makes the C2 carbon slightly positively charged so that the formation of another positive charge at the C1 carbon is not a favorable process [there cannot exist two positive charges on two neighboring carbons (C1 and C2) (Fig. 5.3).].Consequently, the armed glycosyl donors react with electrophiles faster than the disarmed ones, and therefore in a solution containing both an armed and disarmed glycosyl donor molecules, whereby disarmed glycosyl donor molecules have one free hydroxyl group, the reaction with an electrophile will result in crosscoupling and not self-coupling, i.e., an armed glycosyl donor will react with the disarmed one, whereas a disarmed donor will not react with itself [3] as illustrated in Fig. 5.5. The promoters for the activation of n-pentenyl group [4,5] are iodonium dicollidine perchlorate (IDCP) and N-iodosuccinimide/triethylsilyl triflate (NIS/Et 3 SiOTf).M. Miljkovic, Electrostatic and Stereoelectronic Effects in Carbohydrate Chemistry,

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“…Crich et al . [70]. emphasized that the anchimeric assistance was particular to the 1,2- trans orientation of the 2- O -acyl and 1-SBox leaving group.…”

Section: S-benzoxazolyl Derivativesmentioning

confidence: 99%

Glycosyl thioimidates as versatile building blocks for organic synthesis

Hasty

Demchenko

2012

Chem Heterocycl Comp

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This review discusses the synthesis and application of glycosyl thioimidates in chemical glycosylation and oligosaccharide assembly. Although glycosyl thioimidates include a broad range of compounds, the discussion herein centers on S-benzothiazolyl (SBaz), S-benzoxazolyl (SBox), S-thiazolinyl (STaz), and S-benzimidazolyl (SBiz) glycosides. These heterocyclic moieties have recently emerged as excellent anomeric leaving groups that express unique characteristics for highly diastereoselective glycosylation and help to provide the streamlined access to oligosaccharides.

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Revisiting the Armed−Disarmed Concept: The Importance of Anomeric Configuration in the Activation of S-Benzoxazolyl Glycosides

Cited by 60 publications

References 15 publications

New Principles for Glycoside‐Bond Formation

New Principles for Glycoside‐Bond Formation

Armed-Disarmed Concept in the Synthesis of Glycosidic Bond

Glycosyl thioimidates as versatile building blocks for organic synthesis

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