Glycosides 1-thio-, preparation

The allyl glycoside of a-D-Abe-(l ->3)-a-D-Man 115 was prepared by a different approach.200 The ethyl 1-thio-D-abequopyranoside donor 113 was obtained from methyl (3-D-galactopyranoside derivative 110 according to Scheme 33. The cyclic sulfate intermediate 111 was the precursor for the stereoselective reduction with tetrabutylammonium borohydride to 112 which was further derivatized to the thioglycosyl donor 113. Donor 113 was reacted with acceptor 114 to give 115 after deprotection. 

The disaccharide epitope a-Tyv-(l —>3)-a-o- Man present in the D1 Salmonella serotype was synthesized as the pentenyl glycoside. The tyvelose donor 138 was prepared from ethyl l-thio-p-o-mannopyranoside 137 employing a single dideoxygenation step (Scheme 40). Glycosylation of 138 with the mannopyranosyl... 

An acetal tethered compound can easily be prepared by treatment of equimolar amounts of a 2-propenyl ether derivative of a saccharide with a sugar hydroxyl in the presence of a catalytic amount of acid. Activation of the anomeric thio moiety of the tethered compound with N-iodosuccinimide (NIS) in dichloromethane results in the formation of the p-linked disaccharide. In this reaction, no a-linked disaccharide is usually detected. It is of interest to note that when this reaction was performed in the presence of methanol, no methyl glycosides are obtained. This experiment indicates that the glycosylation proceeds via a concerted reaction and not a free anomeric oxocarbenium ion. 

Activation of thioglucosides. Thioglucosides are useful for glycosidation since they are more reactive than glycosyl halides. They are known to be activated by methyl triflate, but this triflate is a potent carcinogen. Methylsulfenyl trifluoromethanesulfonate (1) is also an activator and has the advantage that it can be prepared in situ. Thus the 1-thio-p-D-glucopyranoside (p—2) couples with a-D-... 

Pyruvated glycosyl donors (halides and trichloroacetimidates) are conveniently prepared from the corresponding alkyl or aryl 0- and S-glycosides or from 1-0 unprotected glycoses using standard methods [39d,41]. In addition, 1-thio-glycosides can be used directly as pyruvated glycosyl donors when adequate thiophilic activations are applied [39b, d]. 

Cox and Owen [42] and Whistler and Campbell [43] provided early synthetic approaches to the 6-thioseptanose system. The latter workers prepared a protected open-chain 6-thio dithioacetal of D-galactose that, upon deprotection at C-1 and S-6, could be cychzed to form the corresponding l,2,3,4,5-penta-0-acetyl-D-galacto-6-thioseptanose 37. From this intermediate, several derivatives, such as both anomers of the thioseptanosyl chloride as well as the methyl thioseptanosides, were synthesized. The methyl thioseptanoside was found to be more prone to hydrolysis than the corresponding sulfur containing pyranoid glycoside. 

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