Anomeric triflate intermediate

Besides stereospecific reactions on anomeric triflate intermediates, furanosyl oxocarbenium ions have also been exploited for the introduction of cis-furanosidic... 

Crich and co-workers have demonstrated elegantly that the stereoselective synthesis of fi-mannosides with glycosyl sulfoxides proceeds via anomeric a-triflate intermediates (51). Crich... 

The key intermediate 14, a stable tetrasubstituted tetrahydropyran derivative, was obtained from commercially available methyl o-glucopyranoside 12 via a three-step reaction sequence (Scheme 4). First the primary hydroxyl group was protected as thexyldimethylsilyl (TDS) ether, then the other OH groups were methylated under standard conditions to give 13. The latter glycoside was deoxy-genated at the anomeric position by treatment at room temperature with a Lewis acid (trimethylsilyl triflate) and a hydride donor (triethylsilane). This treatment also... 

Starting with phosphite 59 stannous triflate catalyzes the formation of an oxenium ion 60, which is stabilized by the solvent (61). The complex 61 is more favored than 63, justified by the reverse anomeric effect until now. But as it is shown there is no possibility for 61 to react in an SN2 reaction to 62. The intermediate 63 can be attacked by the alcohol functionality of the glycosyl acceptor, and with MeCN as leaving group you get 64. Ultimately the equilibrium between 61 and 63 shifts completely to 63. This kind of stereocontrol during a glycosylation is known as the nitrile effect.28... 

Another popular promoter for glycosylation with thioglycosides is the reagent dimethyl(methylthio)sulfonium triflate (DMTST).60 A comparison between DMTST and NIS/TfOH for the formation of sialoglycoconjugates demonstrated that the iodine system provided superior yields and anomeric selectivity in nearly all cases.61 The yields and stereoselectivity also increased with an increase in solvent polarity, suggesting that enhanced stabilization of the oxacarbenium intermediate, the rate-limiting step in glycosylation, was responsible for the increased... 

In both approaches, the required diastereomeric purity of the glycosylated intermediates 6 and 9, respectively, is reached in a laborious way. Nicolaou et al. obtain 6 as a mixture of anomers from which the desired yS-form is separated by column chromatography (a 28 %, / 54 %). Danishefsky et al. stereospecifically couple the anomerically pure arabinosyloxymethyl stannane 7 to the vinyl triflate 8, but they achieve a yield of only 40-50 %. In addition, 7 has to be separated from its a-anomer prior to use. 

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