Purification of alpha/beta anomeric sugars

[gravenewworld]

If I purify monosaccharide alpha/beta isomers, which I can clearly see by TLC, will they just isomerize again once I put that sugar into solution? In other words, I'd just be wasting my time purifying them out correct?

 

[MrSid]

The inversion barrier is lowered by acid catalysis in H2O, yet you are seeming to see two populations in the TLC and can posit the alpha /beta anomers...What are the conditions of TLC being used?
I would try to see the time that each spot reverts (maybe 2D TLC at various stop times for the first direction...) or isolate both spots and do several NMRs as time evolves in D2O.
If the barrier is found to be low then a change in one of the PG's to introduce more steric hindrance may prove useful to enhance a 'locked conformation".

 

[gravenewworld]

TLC- EtOAC/Hex 50%. Right out the bottle the sugars are mixtures of anomers. Most of the time hexoses exist naturally as mixtures of roughly 66% and 33% (beta vs alpha) correct? The steric hindrace idea would be interesting. What about possibly replacing the anomeric -OH with either -S or -F (if possible)?

 

[MrSid]

Sorry not to have come back right away gravenewworld. Your TLC conditions have me thrown as "native hexoses" shouldn't move on NP silica gel under those conditions. I am suspecting that you are working with one of the penta-acetates as these are more amenable to typical organic reactions on sugars. A quick look at some of the typical hexoses in Sigma for C6H12O6 has some of them amenable to being sold as the alpha or the beta and with little pricing differential. Take this as evidence of significant barrier against reforming the equilibrium mixture of anomers without a catalyst. Others only specify one anomer and predominantly pure; this could be evidence that rather than a 66:33 mixture the sugar has enough steric and electronic factors to be seen as only one anomer. Always a good place to research your material is in the references Beilstein, Merck Index that are often in the catalog description.

 

[MrSid]

I am wondering if you are seeing the concept of using 2-D TLC to look for evidence that the two spots either retain their RF values at 90 Degrees offset? If they do, then the anomers may be able to be isolated, and tested for the relative stability against re-equilibrating under the conditions that you want to investigate further. If however, the two spots when developed at 90 degrees show that they resolve to two spots with RFs in line with anaomers then starting from either anomer, only a short time is necessary for re-equilibration.

With regards to steric locking, many reactions such as making a glycoside with an acid catalyst and excess alcohol, uses up the anomer that is most reactive and tends to form the single thermodynamically stable anomer of the glycoside- Le Chatliers principle has the removal of the one anomer shifting through acid catalysis the other anomer until everything creates the equilibrium concentration of products. Many times a single anomer is seen for the pentaacetates that is mostly steric. The anomeric stabilizing effect of alpha halo substituted sugars also can form a lock and these are often exploited to do interesting synthetic transformation. There is actually some broad shoulders to stand on if you'll take some time in the library!

 

[gravenewworld]

I am wondering if you are seeing the concept of using 2-D TLC to look for evidence that the two spots either retain their RF values at 90 Degrees offset? If they do, then the anomers may be able to be isolated, and tested for the relative stability against re-equilibrating under the conditions that you want to investigate further. If however, the two spots when developed at 90 degrees show that they resolve to two spots with RFs in line with anaomers then starting from either anomer, only a short time is necessary for re-equilibration.

With regards to steric locking, many reactions such as making a glycoside with an acid catalyst and excess alcohol, uses up the anomer that is most reactive and tends to form the single thermodynamically stable anomer of the glycoside- Le Chatliers principle has the removal of the one anomer shifting through acid catalysis the other anomer until everything creates the equilibrium concentration of products. Many times a single anomer is seen for the pentaacetates that is mostly steric. The anomeric stabilizing effect of alpha halo substituted sugars also can form a lock and these are often exploited to do interesting synthetic transformation. There is actually some broad shoulders to stand on if you'll take some time in the library!

Hmm thanks for the comments Mr. Sid. Looks like I have some researching to do. You're right about the pentaacetate analogs I'm working with. I forgot to mention that we cap the -OHs, which is how we can get them to move under the TLC conditions I described.