I am trying to synthesise a sugar (peracetylated 6-azidofucose) and
have at long last reached the final step of peracetylation. The protocol I am following suggests using Hexane:Ethyl acetate 2:1 but my product gets stuck on top of the silica if I use it (even though it runs at an Rf of about 0,4 on a TLC). I therefore thought of using a more polar eluent like Dichloromethane:Methanol 98:2 (which makes the product run at about 0,5 on TLC) but surprisingly, it stayed stuk on top of the silica again. For curiosity's sake, I eluted the product with pure methanol (at least to see if it was indeed my product) and it seems to be. But of course, no seperation occurred so my product
is not purer than before I started the column!
I nevertheless evaporated the methanol and obtained an oil. For some reason, added pure dichloromethane to the flask. To my surprise, the oil dissolved in the DCM but a white precipitate formed. I checked it on a TLC and it seems to be my sugar (it is now at the NMR for confirmation).
I would like to understand better what has been going on in order to start my synthesis again and purify it in a more controlled manner Would any of you have an explanation to this phenomenon? And would somebody have a suggestion to help me "unstuck" my product AND get a sepration?
Thank you very much in advance!
Thank you for your help"