Protecting glycosylation stereoselectivity

An alternative stereoselective synthesis of chiral heterocycles based on carbohydrate-induced stereodifferentiation includes nucleophilic addition reactions on heterocyclic systems already bound to the carbohydrate auxiliary. An example of this strategy has been shown by stereoselective addition of Grignard reagents to carbohydrate-linked 4-pyridones [61]. For this purpose, trimethylsiloxypyridine was glycosylated regioselectively using pivaloyl-protected glycosyl fluorides. 

Seward, CMP, Cumpstey, I, Aloui, M, Ennis, S C, Redgrave, A J, Fairbanks, A J, Stereoselective cis glycosylation of 2-0-allyl protected glycosyl donors by intramolecular aglycon delivery (lAD), Chem. Commun., 1409-1410, 2000. 

Kim and co-workers have developed a highly a- and /)- stereoselective (dual stereoselective) [210] method for the synthesis of 2-deoxyglycosides by employing CB 2-deoxyglyco-sides as glycosyl donors. Glycosylation of the 4,6-0-benzylidene-protected glycosyl donor... 

Glycosylaiion. Two laboratories have reported that protected glycosyl fluorides undergo stereoselective glycosylation in CH2CI2 with alcohols or silyl ethers in the presence of or Bp3-etherate. The liberated HF is trapped with N(QH5)3. 

In the case of amino sugars with the hydroxyl group at position 2, the use of the benzyl group as the hydroxyl protective group leads to stereoselective formation of 1,2-cw-glycoside. Thus, the 2-O-benzyl-protected glycosyl bromide 40 was coupled with the alcohol derivative 41 to produce conveniently the a-glycosidic product 42 in 44% yield [45] (compoimds 40-42 ). 

The stereoselective synthesis of N-(g-L-aspartyl)-a-D-gluco-pyranosylamlne from 2,3,, 6-tetra- -benzyl-a-D-glycopyranosyl azide has been detailed, as has the synthesis of the protected glycosyl trlpeptlde (1), representing the partial unit of bovine deoxyrlbo-... 

Protecting Groups Effects on Reactivity, Glycosylation Stereoselectivity, and Coupling Efficiency... 

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