Activation of Thioglycosides

The ability to arm and disarm thioglycosides in a manner similar to the NPG glycosyl acceptors has been reported. As with n-pentenyl glycosyl acceptors, the use of an electron-withdrawing substituent at C-2 was reported to inactivate 

Iodine has been used to activate methyl thioglycosides for the synthesis of disaccharides.49 Similarly, it has been reported that the use of IBr allowed for the 

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 

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General Procedure for Activation of Thioglycosides with Ph2S0/Tf20 [87,99]... 

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... 

We were attracted to the use of iodonium ion-based activation of thioglycosides and we were drawn to conclude that molecular iodine might serve as an effective source of this, or related, thiophilic species (Fig. 2). 

IODINE, A VERSATILE REAGENT IN CARBOHYDRATE CHEMISTRY ACTIVATION OF THIOGLYCOSIDES AND GLYCOSYL SULFOXIDES K. P. Ravindranathan Kartha, Mahmoud Aloui,... 

Duron SG, Polat T, Wong C-H. N-(phenylthio)-epsilon-capro-lactam A new promoter for the activation of thioglycosides. Org. 

SCHEME 4.15 Activation of thioglycosides for glycosylations by electrophilic reagents. 

Another concept for the activation of thioglycosides is their reaction with soft sulfur electrophiles. The first reagents of this type were dimethyl(methylthio)sulfonium triflate (DMTST, 83) and the corresponding tetrafluoroborate (DMTSF, 84) (Figure 4.4) [86,183-185], which are powerful methylsulfenylating agents (E = SMe) (Scheme 4.15), and are still extensively used. 

Related to the activation of thioglycosides by sulfenylation are selenylation type reactions (E = SeR) (Scheme 4.15). Arylselenylating type promoters used with thioglycosides include benzeneselenenyl triflate [199,200] or the Wphenylselenylphtalimide-trimethylsilyltriflate [201] and Wphenylselenylphtalimide-magnesium perchlorate [202] systems. 

Kihlberg, J O, Leigh, D A, Bundle, D R, The in situ activation of thioglycosides with bromine an improved glycosylation method, ]. Org. Chem., 55, 2860-2863, 1990. 

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