ABSTRACT
Thioglycosides are among the most popular glycosyl donors for the formation of interglycosidic linkages.1 This popularity stems from the fact that the anomeric thiofunction is stable to most conditions used for protection and functional group manipulations. When treated with an appropriate electrophile, the anomeric thiofunction can be activated to provide a glycosylating species. The stability of thioglycosides, particularly to Lewis acids, has put them at the heart of several sequential glycosylation strategies.2 They can be employed in an orthogonal condensation sequence in which they are combined with other glycosyl donors such as anomeric —uorides and imidates. Alternatively, the difference in reactivity of differentially functionalized thioglycosides can be exploited in a chemoselective glycosylation strategy. Recently, iterative glycosylation procedures have been introduced,2 which are based on the preactivation of thioglycosidic donors. In a preactivation protocol, the thioglycoside donor is transformed into a reactive glycosylating species before the addition of the appropriate acceptor, which in turn can be a second thioglycoside. In 2001, Crich and coworkers introduced the 1-benzenesul¥nyl piperidine (BSP, 1a)-tri—uoromethanesulfonic anhydride (2) (Tf2O) reagent for the (pre-)activation of thioglycosides, which has become a popular thiophilic activating system.3 Subsequently, our laboratory introduced the analogous diphenylsulfoxide (Ph2SO, 1b)-tri—ic anhydride (2) combination.4 This reagent couple was originally developed by Gin and coworkers for the activation of 1-hydroxyl donors,3b,5 and we applied it for the activation of disarmed thioglycosides.4 BSP and Ph2SO are commercially available at €3.3/mmol and €0.23/mmol, respectively. Although the sulfonium bis-tri—ate systems 3a and 3b are rather similar in reactivity,6 the latter is a stronger electrophile and has been shown to activate thioglycosides, which were not reactive enough toward sulfonium tri-—ate 3a.4,7 The Ph2SO-Tf2O reagent combination (3b) has been used in condensation reactions of a variety of thioglycosidic donors, including several uronate esters8 and sialic acids.7a,d Activator 3b can also be used in an iterative glycosylation sequence combining 1-hydroxyl donors and thioglycosides.9 Furthermore, 3b is effective in the stereoselective construction of the challenging β-mannosidic bond using Crich’s benzylidene mannose system6,7b,c or mannuronate ester donors as recently disclosed by our laboratory.10 The opening scheme of this chapter depicts a representative example in which mannuronate ester 4 is preactivated by treatment with diphenylsulfonium bis(tri—ate) 3b and subsequently coupled to glucoside (5) to provide the β-linked disaccharide 6 in 94% yield.
