Protecting Groups at the Secondary Positions of Carbohydrates

Vidal, Sébastien; Goekjian, Peter G.

doi:10.1002/9783527697014.ch3

Cited by 7 publications

(6 citation statements)

References 317 publications

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“…Beyond the concept of ester migration during participation, it is commonly known that acyl groups in organic molecules are prone to intramolecular migration between the hydroxy groups, as first demonstrated for carbohydrates by Fischer in 1920 . Several studies providing closer insights into the actual migration process have been performed since, mostly in monosaccharide derivatives, although also larger carbohydrate molecules have been addressed, albeit less frequently. − The often spontaneous migration, particularly under basic conditions, significantly hampers the use of partially acylated carbohydrate derivatives in synthesis and must always be considered in all isolation, characterization and identification processes of both synthetic compounds and natural products. ,− …”

Section: Introductionmentioning

confidence: 99%

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

et al. 2020

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This review is the counterpart of a 2018 Chemical Reviews article that examined the mechanisms of chemical glycosylation in the absence of stereodirecting participation. Attention is now turned to a critical review of the evidence in support of stereodirecting participation in glycosylation reactions by esters from either the vicinal or more remote positions. As participation by esters is often accompanied by ester migration, the mechanism(s) of migration are also reviewed. Esters are central to the entire review, which accordingly opens with an overview of their structure and their influence on the conformations of six-membered rings. Next the structure and relative energetics of dioxacarbeniun ions are covered with emphasis on the influence of ring size. The existing kinetic evidence for participation is then presented followed an overview of the various intermediates either isolated or characterized spectroscopically. The evidence supporting participation from remote or distal positions is critically examined and alternative hypotheses for the stereodirecting effect of such esters is presented. The mechanisms of ester migration are first examined from the perspective of glycosylation reactions, and then more broadly in the context of partially acylated polyols.

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Section: Introductionmentioning

confidence: 99%

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

et al. 2020

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“…Another benzyl ether in the form of Nap ethers gained momentum as their deprotection conditions are orthogonal to the benzyl ethers . In view of this, alcohol 7 was treated with Nap carbonate 30 in the presence of catalytic quantities of Au phosphite ( 2 ) and AgOTf in CH 2 Cl 2 to note the formation of Nap ether 31 in 62% yield.…”

Section: Results and Discussionmentioning

confidence: 99%

Stable Benzylic (1-Ethynylcyclohexanyl)carbonates Protect Hydroxyl Moieties by the Synergistic Action of [Au]/[Ag] Catalytic System

et al. 2019

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Chemical syntheses of oligosaccharides and glycosides call utilization of many protecting groups that can be installed or deprotected without affecting other functional groups present. Benzyl ethers are routinely used in the synthesis of glycans as they can be subjected to hydrogenolysis under neutral conditions. However, installation of benzyl ethers is often carried out under strong basic conditions using benzyl halides. Many a times, strongly basic conditions will be detrimental for some of the other sensitive functionalities (e.g., esters). Later introduced reagents such as benzyl trichloroacetimidate and BnOTf are not shelf-stable, and hence, a new method is highly desirable. Taking a cue from the [Au]/[Ag]-catalyzed glycosidations, we have identified a method that enables protection of hydroxyl groups as benzyl, p-methoxybenzyl, or naphthylenemethyl ethers using easily accessible and stable carbonate reagent. A number of saccharide-derived alcohols were subjected to the benzylation successfully using a catalytic amount of gold phosphite and silver triflate. Furthermore, the protocol is suitable for even protecting menthol, cholesterol, serine, disaccharide OH, and furanosyl-derived alcohol easily. The often-utilized olefins and benzoates, as well as benzylidene-, silyl-, Troc-, and Fmoc-protecting groups do not get affected during the newly identified protocol. Regioselective protection and one-pot installation of benzyl and p-methoxybenzyl ethers are demonstrated.

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“…4). 120 Selecting the best nitrogen protecting group for a particular target involves often trial and error experimentation (see Section 6.4, Scheme 3). 115,117,121–124…”

Section: Glycan Connectivitymentioning

confidence: 99%