Equatorial thioglycosides

A subsequent report by Kwan Soo Kim and coworkers indicated that remote participation from an equatorial C3 position is indeed possible [16]. In this case, a tri-chloroacetimidate was used as a nucleophilic trap. Upon low-temperature activation of thioglycoside 11 using 1-benzenesulfinylpiperidine (BSP) and Tf20, bicyclic... 

Disaccharides of 1-thioglycosides, an important class of glycomimics, can be synthesized by 5-alkylation in exceptionally high yields when iminophosphorane bases are employed. The reaction conditions employed appear to be general and stereospecific. Axial and equatorial 4-triflates and primary tosylates of alkyl pyranosides provided excellent yields of... 

Interestingly, this transformation was not possible using acceptors with equatorial 3,4-substituents. Acetolysis of the anhydro bridge followed by conversion into the ethyl thioglycoside afforded a ttisaccharide donor, which, in NlS/TfOH-promoted couplings to the spacer and to a Kdo acceptor followed by deprotection, efficiently gave the target compounds. 

Benzylidene acetals have found broad application in synthetic carbohydrate chemistry. A new chiral center (PhC H<) is formed during their formation, wherein the bulky phenyl substituent adopts predominantly the thermodynamically more stable equatorial orientation. Our laboratory has previously developed a simple protocol for the preparation of methyl 4,6-0-benzylidene-a-D-glucopyranoside (2) from commercially available methyl a-o-glucopyranoside (1). The purification of the product was achieved by simple precipitation without the need for chromatographic separation. Herein, we describe the application of this procedure to the synthesis of benzylidene acetals of ethyl thioglycosides 3 and 5. All product purification can be achieved by crystallization/precipitation only. Additional quantities and hence higher yields can be achieved by subsequent chromatography of the mother liquor. 

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