Disarmed acceptors

Scheme 6.4) [15, 16], The benzylated thiofucoside 17 was preferentially activated by NIS/TfOH and coupled to the benzoyl (Bz)-containing disarmed acceptor 18. This was followed by subsequent coupling to the linker 20 to afford the protected Lewis X derivative 21 in 71% yield. 

Furthermore, selective activation of differently protected selenoglycosides was also possible with IDCP as promoter. Perbenzylated armed phenyl selenoglycoside 183 was selectively activated in the presence of disarmed acceptor 189 and an < //3 mixture of disaccharide 190 was obtained. With the less-reactive secondary hydroxyl group of 191, the yield of 192 was lower. 

Additionally, the arming and disarming properties of the STaz glycosides were evaluated. It was demonstrated that activated (benzylated) STaz derivatives could be activated over electronically disarmed (partially acylated) STaz glycosyl acceptors in die presence of either AgOTf or Cu(OTf)2 78). For example, a classic armed-disarmed approach (79) allowed chemoselective activation of 54 over disarmed acceptor 61 in the presence of Cu(OTf)2to afford... 

To demonstrate further the generality of latent-active glycosyl strategy in oligosaccharide synthesis, the more difficult p(l 3) linkage was next examined (Scheme 11). Glycosylation of latent-disarmed acceptor 54 with ethyl 1 -thio-p-L-fucopyranoside 57 at -30°C in dichloromethane with 4 A MS, promoted by NIS/ TfOH, gave the desired a-(1 3) disaccharide 58 in 83% yield. Under similar... 

Armed thioglycoside donor 6 was selectively activated over disarmed glycosyl acceptor 7 in the presence of IDCP to provide the disarmed l,2-c -linked disaccharide (8) in 91% yield. The latter was then subjected to a two-step debenzoyla-tion-benzylation sequence, whereupon the resulting disaccharide donor (9) was glycosidated with disarmed acceptor 7, to afford the cis-cis-linked trisaccharide (10) in 72% yield. The conversion of the second generation glycosyl donor 8 into the armed state 9 also allowed for the second coupling step to be performed with the mild promoter IDCP. 

Coupling of the armed phenylthio glycoside 121 with the disarmed acceptor 122 gives the a-disaccharide 123 in 89% yield. Activation of 123 by lithium aluminohy-dride (LAH) reduction of the sulfoxide group and subsequent reacetylation allows to glycosylate an alcohol in an a-selective procedure (124 is obtained in 98% yield when R = cyclohexyl). 

Chemospecific glycosidation of partially benzoylated thiogly-cosides ( disarmed acceptors) with perbenzylated thioglycosides ( armed donors) can be realized in the presence of the promotor IDCP (eq 5). ... 

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