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Purifying an aromatic glycoside from tetrabutylammonium ion
Babcock_Hall:
We have two impure samples of an aromatic glycoside. In both cases we used eight equivalents of tetrabutylammonium fluoride (TBAF) to remove tert-butyldimethylsilyl (TBDMS) groups from the four oxygen atoms of the glycosidic portion of the molecule. Our first two attempts at using Dowex-50/calcium carbonate to remove the TBA cations and to protonate the oxygen atoms went well, with over 99% being removed. This protocol was found in a 2007 J. Org. Chem. paper (9(4):723-726) written by Kaburagi and Kishi (10.1021/ol063113h). Our second two attempts removed roughly 90% of the TBA, which still leaves roughly one equivalent of TBA present. The TBA is easily observed in the H-1 NMR spectrum. We do not know why there was this decline in efficiency. We are not sure of the identity of the anion paired with TBA (it might be fluoride or bicarbonate, I suppose). Right now my thinking is to repeat the Kaburagi-Kishi step on one sample, perhaps doubling the amounts of both the Dowex-50 resin and calcium carbonate.
If this attempt fails, we can try reverse phase flash chromatography on a Teledyne ISCO system on one or both samples. Chapter 4 of their book has an example (Figure 60 on page 69 of the fifth edition of Effective Organic Compound Purification, which is Teledyne's book) in which two aromatic glycosides are purified nearly to baseline using water and acetonitrile as the mobile phase, but few details are given. We presently have a 26-gram C18 reverse phase column, and the samples are very roughly in the range of 100 milligrams, going on memory. We have no experience in preparative RP-chromatography, and I am not sure how to detect TBA during a chromatographic run (which are reasons to favor trying the Kaburagi-Kishi method first). We did obtain some information from Teledyne on changing from normal to reverse phase and back again.
Is there any point in doing preliminary TLC on RP plates to get a sense of what solvent ratio will move our compound? I never had much luck with them. Can anyone inform us about possible pitfalls that we might encounter in preparative RP chromatography? Thanks for any help or suggestions.
MOTOBALL:
You may recall from my response to one of your earlier posts, that I am a strong proponent of the use of TLC with silica gel/microscope slides to monitor carbohydrate reactions.
I would start with EtOAc/Hexane (2:1, v/v) and modify the mobile phase from there.
I used to aim for an Rf value of about 0.2-0.3 (IIRC) from a single run, and then perform three runs (letting the slide dry each time) before spraying with 5% conc H2SO4 / EtOH and heating to char the organics. This will give you a picture of how the reaction mixture will elute from a Silica column.
Use that mobile phase for dry-column chromatography.
Good Luck!
Babcock_Hall:
I no longer see the TBDMS portions of the molecule (the protecting groups) by NMR, and there are four hydroxyl groups in the product. I strongly doubt that this product will move on silica or be soluble in ethyl acetate/hexane mixtures. Thank you for reminding me about sulfuric acid as a detection method.
MOTOBALL:
You are correct Re solubility and lack of movement on silica gel; I had misunderstood the status of your product.
Suitable developing phases could be,
1) EtOAc/EtOH/H2O (10:3:2 v/v/v)
or
2) 45:5:3
Regards.
wildfyr:
Water on acidic silica gel seems bad for TBDMS. Definitely do 2D TLC first.
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