Glycosyl activation, chemoselective

Chemoselective Glycosyl Activation Discriminating Between NPGs and NPOEs... 

Glycosyl esters with remote functionality constitute a relatively new class of O-carbonyl glycosyl donors, which fulfill the prospect of mild and chemoselective activation protocols (Scheme 3.22). For example, Kobayashi and coworkers have developed a 2-pyridine carboxylate glycosyl donor 134 (Y = 2-pyridyl), which is activated by the coordination of metal Lewis acid (El+) to the Lewis basic pyridine nitrogen atom and ester carbonyl oxygen atom [324]. In the event, 2-pyridyl (carbonyl) donor 134 and the monosaccharide acceptor were treated with copper(II) triflate (2.2 equiv) in diethyl ether at —50 °C, providing the disaccharide 136 in 70% (a P,... 

Besides conformational disarming, one-pot methods also benefit from building blocks that are electronically disarmed. Biao Yu and coworkers utilized this effect in the one-pot assembly of the 19-membered macrolactone-containing tricolorin A (16) (Scheme 6.3) [13, 14], The key design of their synthesis is the chemoselective activation and coupling of the perbenzylated donor 7 to the bis-acyl-containing disarmed bifunctional acceptor 12. The subsequent glycosylation of macrolactone 14 in the same flask formed the protected tricolorin A15 in 43% overall yield, which was deprotected to provide tricolorin A (16). 

SCHEME 6.11 Kobayashi s one-pot synthesis of trisaccharide 63 through chemoselective activation of glycosyl iV-trichloroacetylcarbamate. 

One-Pot Glycosylation Based on Chemoselective Activation of Different Types of Glycosyl Donors... 

SCHEME 6.14 One-pot glycosylation based on chemoselective activation of different types of glycosyl donors. 

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