On the stereoselectivity of glycosidation of thiocyanates, thioimidates, and thioglycosides

On Orthogonal and Selective Activation of Glycosyl Thioimidates and Thioglycosides: Application to Oligosaccharide Assembly

“…15 With the reinvestigation of glycosyl thiocyanates, reactive glycosyl donors introduced by Kochetkov, 26–28 we determined that their activation can be reliably achieved with Cu(OTf) 2 , reaction conditions under which STaz group reacts sluggishly. 29 Indeed, Cu(OTf) 2 -promoted activation of thiocyanate donor 50 over STaz glycosyl acceptor 12 was very efficient and the resulting disaccharide 14 was isolated in 89% yield (Scheme 3). The STaz moiety of disaccharide 14 was then activated over SBox glycosyl acceptor 29 under alkylation conditions to afford the corresponding trisaccharide 51 in 67% yield.…”

Stereocontrolled 1,2-cis glycosylation as the driving force of progress in synthetic carbohydrate chemistry

“…Recently, Demchenko and co-workers have shown that electron-withdrawing acyl protecting groups have a dramatic effect on the stereoselectivity obtained with thiocyanates as glycosyl donors. 88 Thus, when thiocyanate 73 was reacted with acyl-protected acceptors 74 and 75 , the corresponding disaccharides 77 and 78 were obtained with complete α-selectivity (α/β = >25/1, Scheme 9). However, when benzyl-protected acceptor 76 was used instead, the stereoselectivity dropped ( 79 , α/β = 8.3/1).…”

Acceptor reactivity in glycosylation reactions

“…As a first example the model thiodonors 82 and 83 are compared in glycosylation reactions with a set of acceptors (86)(87)(88)(89) of increasing steric demands. 29 Because donors 82 and 83 only carry a single electronwithdrawing substituent, they are rather reactive and substitution reactions on these donors likely proceed through a dissociative mechanism.…”