[freetoair2007]

Hi all,

The scenario:
Took D-glucose, fully benzylated the whole compound to give you a mixture of alpha- and beta-anomers of this perbenzylated compound, then recrystallised it from ethanol giving only the beta-anomer as determined by 1H NMR spectroscopy. No alpha-anomer was detected.

Question: Why is this? That is, why does recrystallisation only give the beta-anomer? I assume the alpha is formed during the reaction, but due to recrystallisation only the beta-anomer crashes out and the alpha-anomer stays in solution. I've read some carbohydrate text books that say "recrystallisation of XXX gives only the beta-anomer of XXX" but it does not state why.

The alpha- and beta-anomer obviously don't differ by much in solubility and structurally they are "pretty much the same".

If some one can answer this, it would be much appreciated.
Thanks a bunch.

[discodermolide]

Hi all,

The scenario:
Took D-glucose, fully benzylated the whole compound to give you a mixture of alpha- and beta-anomers of this perbenzylated compound, then recrystallised it from ethanol giving only the beta-anomer as determined by 1H NMR spectroscopy. No alpha-anomer was detected.

Question: Why is this? That is, why does recrystallisation only give the beta-anomer? I assume the alpha is formed during the reaction, but due to recrystallisation only the beta-anomer crashes out and the alpha-anomer stays in solution. I've read some carbohydrate text books that say "recrystallisation of XXX gives only the beta-anomer of XXX" but it does not state why.

The alpha- and beta-anomer obviously don't differ by much in solubility and structurally they are "pretty much the same".

If some one can answer this, it would be much appreciated.
Thanks a bunch.

How did you perbenzylate  glucose?
"structurally they are pretty much the same":
This is not exactly true, obviously their solubility in your crystallization solvent is completely different!
Look up the anomeric effect

[AWK]

Pure D-glucose when crystallized from methanol also gives pure beta-anomer. But when acetic acid is used as a solvent only alpha-anomer is formed.


http://books.google.pl/books?id=2VkuzMjDFDoC&pg=PA468&lpg=PA468&dq=crystallization+of+glucose+anomers&source=bl&ots=3I7gqx-REN&sig=XEQV3Hf9vZnV75h_igQFXNYiknw&hl=pl&ei=fYV5TvbFAdSa1AWyrKHUAQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CDYQ6AEwAg#v=onepage&q=crystallization%20of%20glucose%20anomers&f=false

[Dan]

Can you confirm whether by "perbenzylated glucose" you mean 1,2,3,4,6-penta-O-benzyl-glucopyranoside or 2,3,4,6-tetra-O-benzyl-glucopyranoside?

I would assume you mean the former, I just ask because there are hints of mutarotation in these posts and the pentabenzyl compound is a glycoside and does not mutarotate.

[freetoair2007]

"How did you perbenzylate  glucose?
"structurally they are pretty much the same":
This is not exactly true, obviously their solubility in your crystallization solvent is completely different!
Look up the anomeric effect"

Benzylation was carried out using NaH and BnBr. So fairly typical. Basically nuked the s#*$ out of the reaction by adding heaps of NaH and BnBr. ~12 equivs of each.

I thought the anomeric effect only covers why the alpha-anomer is also formed during the reaction (hyperconjugation), along with the beta-anomer. Not why, one anomer is crystillised over the other. Also, is there really such a major difference in solubilities between alpha- an beta-anomers of the same compound?

Pure D-glucose when crystallized from methanol also gives pure beta-anomer. But when acetic acid is used as a solvent only alpha-anomer is formed.

This doesn't really answer my question. I know that different solvents can cause the alpha- or beta-anomer to crash out. But Why is that? maybe solubility differences in the alpha-anomer compared to the beta-anomer? As mentioned previously. Though I wouldn't have thought it would be that great!

Can you confirm whether by "perbenzylated glucose" you mean 1,2,3,4,6-penta-O-benzyl-glucopyranoside or 2,3,4,6-tetra-O-benzyl-glucopyranoside?

I would assume you mean the former, I just ask because there are hints of mutarotation in these posts and the pentabenzyl compound is a glycoside and does not mutarotate.

Yes it was the former: 1,2,3,4,6-penta-O-benzyl-glucopyranoside.

[Dan]

In that case mutarotation is irrelevant and we are just talking about recrystallisation as a method of purification. Your contaminant (minor anomer) is still completely soluble in the recryst solvent, or to put it another way the recryst solution is not saturated with minor anomer (but the solution is saturated with the major anomer).

[freetoair2007]

Cheers for all the help everyone, especially to Dan. I guess the simple answer is just to do with the basic principles of recrystallisation. As the beta-anomer would be the major product from benzylation.