Reducing Sugars

[supras2kracer]

Could someone clarify this for me? It seems contradictory. This is page 86 of the EK Ochem book (lecture 4).

"Since Tollens reagent must react with the open-chain form of a sugar, glycosides (which are closed ring acetals) do NOT reduce Tollen's reagent, while nonglycosides do."

"Disaccharides and polysaccharides are glycosides where the aglycone is another sugar...A disaccharide or polysaccharides will only react with Tollens reagent if there is an anomeric carbon that is not involved in a glycosidic bond and is free to react"

If disaccharides and polysaccarides are glycosides then according to the first paragraph they can never react with Tollens. Can someone give me a simpler explanation? Maybe I'm reading it wrong. Is a glycosidic bond the linkage between two glycosides or is it the bond that forms the ring?

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[RogueUnicorn]

Could someone clarify this for me? It seems contradictory. This is page 86 of the EK Ochem book (lecture 4).

"Since Tollens reagent must react with the open-chain form of a sugar, glycosides (which are closed ring acetals) do NOT reduce Tollen's reagent, while nonglycosides do."

"Disaccharides and polysaccharides are glycosides where the aglycone is another sugar...A disaccharide or polysaccharides will only react with Tollens reagent if there is an anomeric carbon that is not involved in a glycosidic bond and is free to react"

If disaccharides and polysaccarides are glycosides then according to the first paragraph they can never react with Tollens. Can someone give me a simpler explanation? Maybe I'm reading it wrong. Is a glycosidic bond the linkage between two glycosides or is it the bond that forms the ring?

this is the key to your question.

 

[supras2kracer]

From my understanding the glycosidic bond is the bond between the different sugars. So that would leave one glycoside on the end of the chain of sugars. But that would still be a glycoside and couldn't react with Tollens.

 

[RogueUnicorn]

think about it this way...

can a (1,4) dimer of glucose react with the Tollens reagent?
what about a (1,6)? a (1,1)?