Hello,
I am working on a sugar chemistry project and one of reactions was reacting one of my sugar derivatives which contains an aldehyde, ketone and an imine with Lawesson's reagent. The reaction was refluxed for 24hrs and was checked via TLC with the solvent system of 10% MeOH/DCM in which the product was slightly more polar than than the starting material and the TLC plate was charred with KMnO4.
Workup, extraction with ETOAc and Brine solution was performed and the organic and the aqueous layers were collected. A TLC was performed for both layers and the product was found in the aqueous layer and was lipholyzed. A TLC showed two products in this layer in which one spot was at the baseline(4-methoxybenzenecarbothioic acid)-a by product of the reaction and slightly above was the desired product. NMR of the crude product showed desired compound and by-product.
Recrystallization with Ethanol, Methanol and MeOH/DCM was performed and was not successful. The next I was thinking was Silica Gel Chromatography. Please advise on how to proceed with tjis purification method, I would greatly appreciate it.
Well, the only way is to try to resolve using preparative chromatography - something like flash chromatography with silica gel. If You don't have flash or a preparative chromatograph, the only way is to go old school. Personally, I love dry column vacuum chromatography (link below) as a preparative technique without equipment.
http://www.pittelkow.kiku.dk/synmet/synmet2012/DCVC.pdf
The best way to separate compounds using preparative chromatography is to find such solvent mixture, which elutes Your compounds of interest and does not have enough elution strength that impurities stay on the column. Such cases are when compounds differs in polarity or one of the compounds have different characteristic moieties that can be used as an advantage (like the acidic or basic group, which gives option to use mobile phase adjectives like acid or bases).
You can also maybe try using different modified silica, like NH2, CN or diol or different stationary phase (alumina or florisil) in order to enhance selectivity and for that making the separation easier.
However before You start the preparative chromatography, the best way to find an appropriate mobile phase is to use TLC for that. Also, remember when You adsorb You compounds mixture make sure that they are dissolved in a solvent which does have as less elution strength as it can be.
Thank you Adam. What solvent system should I use? I was going use MeOH/DCM since my compound moves somewhat in this system
If the MeOH/DCM seems to be fine than You should use these solvents. You have written that You use 10% of MeOH so for preparative chromatography I would use a little less like 5%. You need to remember that TLC, even very old and forgotten, is a technique with quite good performance. The column chromatography will be for sure less efficient. The solvent for Your compounds solubilization I would try pure DCM or dissolve in a little volume of MeOH and dilution it with DCM.