Thioglycosides from anomeric acetates

Thioglycosides are readUy prepared using a variety of methods, but mostly by nucleophilic substi-mtions at the anomeric center. They are prepared most commonly from anomeric l,2-franj -acetates... 

Mercuric acetate Glycosides from thioglycosides with anomerization... 

In the first phase the vinyl ether 51 is prepared from the acetate 50 by methylene-transfer from the Tebbe-reagent. Next, the thioglycoside 51 and the aglycone 52 are connected by acid-catalyzed addition of the latter to the vinyl ether moiety to afford the acetal-tethered intermediate 53. In the final phase, NIS/TfOH-induced anomeric activation proceeds in concert with the attack of the tethered oxygen at the anomeric carbon to afford the P-linked disaccharide 54 (Scheme 16) [114]. 

Notes and discussion. This reaction can be utilized for the preparation of a-allyl, ethyl, phenyl and substituted aryl thioglycosides of sialic acid. NMR and NMR spectra of the crude reaction mixtures have confirmed that complete anomeric inversion occurs under the PTC condition. Adoption of tetrabutylammoniumhydro-gen sulfide as a PTC is critical for mild reaction conditions, otherwise, if triethylbenzylammonium chloride is employed, more harsh conditions are required. The use of ethyl acetate is advantageous over dichloromethane owing to the production of PhSCH2Cl and (PhS)2CH2 resulting from nucleophilic displacement of one or two chlorides of dichloromethane with thiophenol in the model experiment. 

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