Oxacarbenium intermediate

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... 

A Lewis-acid-mediated reaction of tungsten linked alkynes 209 with aldehydes forms the intermediate tungsten-r 1 -oxacarbenium salts 210 treatment with base and subsequent demetalation affords 3,4-dihydropyrans 211 that are classed as oxacyclic dienes (Scheme 62) <1997JA4404, 1998JOC7289, 1998JA4520, 2000JOC3761>. 

Whereas most transformations are selfexplanatory, the conversion to the tetra-hydrofuran-3-carboxylate 178 should be regarded with special attention. This highly stereoselective reaction proceeds with an oxacarbenium ion as intermediate and could also be extended to certain silylated C-nucleophiles (vide infra) 93). 

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