[Doc Holliday]

I'm doing a reaction where I have completely acetylated GlcNAc and react it with BF3 etherate and HO-CH2CH2N3  in order to add the ethyl azide to the anomeric -OH.  The reaction seems to have worked, the NMR looks great, except for the fact that there appears to be an impurity in it that looks like it has ether groups.  I'm guessing it is the starting material, or maybe some boron complex, I really don't know, since I don't know the mechanism of this reaction.  The work up according to the reference I'm using was simply dilute with ethyl acetate and wash twice with NaHCO3, followed by column chromatography.  I have done all that, but still this impurity persists.  I also took the compound after purifying and washed it with sat. ammonium chloride, took the NMR, and it is still there.  

Does anyone have an idea of what this impurity is--BF3 etherate or some type of boron etherate complex?  Any tricks to getting it out with a wash that I haven't tried yet?   By TLC I don't see any other spots except one. I know the product is present too because I can see the ethyl group in the NMR too.  Would trituation with hexane or ether work?  Thanks in advance for any help.   Much appreciated.

[discodermolide]

I'm doing a reaction where I have completely acetylated GlcNAc and react it with BF3 etherate and HO-CH2CH2N3  in order to add the ethyl azide to the anomeric -OH.  The reaction seems to have worked, the NMR looks great, except for the fact that there appears to be an impurity in it that looks like it has ether groups.  I'm guessing it is the starting material, or maybe some boron complex, I really don't know, since I don't know the mechanism of this reaction.  The work up according to the reference I'm using was simply dilute with ethyl acetate and wash twice with NaHCO3, followed by column chromatography.  I have done all that, but still this impurity persists.  I also took the compound after purifying and washed it with sat. ammonium chloride, took the NMR, and it is still there.  

Does anyone have an idea of what this impurity is--BF3 etherate or some type of boron etherate complex?  Any tricks to getting it out with a wash that I haven't tried yet?   By TLC I don't see any other spots except one. I know the product is present too because I can see the ethyl group in the NMR too.  Would trituation with hexane or ether work?  Thanks in advance for any help.   Much appreciated.

You may have exchanged the etherate for your HO-CH2CH2N3?

Wash the reaction mixture with ethanolamine that should complex the boron if that is still present. I don't think the work-up you are following will remove it properly.

[Doc Holliday]

Thank you for the help.  Will give it a try.  


I do have a synthetic organic background but am not an organic chemist.  I'm just an engineer that needs to do a bit of organic chemistry in order access some molecules that I need.  Again, thanks for the help.

[Dan]

Can you post the chemical shifts of the signals for the anomalous ethyl group? It's not something simple like residual ethyl acetate from the column?

I would actually be pretty surprised if there's any BF₃ after an aqueous wash and a column. I would certainly be very surprised if it's still complexed to diethyl ether after all that.

[Doc Holliday]

So I figured out what was wrong, the person who ordered me the chemical for use actually ordered me HOCH2CH2CH2N3 instead of HOCH2CH2N3.  I didn't even bother looking at the label when I was using it.  However, the NMR integration still seems to be off now. The starting HOCH2CH2CH2N3 signals in CDCl3 are S(1.713, 1proton), m (1.821, 2 proton), s (2.867, 0.33 proton), s (2.944, 0.33 proton), t (3.441, 2 proton), and t (3.743, 2 proton).  I don't understand where those singlets are coming from, impure starting  material out of the bottle? 

Anyway, I still think I was able to make my compound though, since the mass spec is correct.  The signals throwing me off though are for the glycosidic propyl azide.  The signals are m (1.852, 4 protons), triplet (3.457, 4 protons), and triplet (3.763, 5 protons because it is sitting on top of a proton of the carbohydrate ring). 

I can't see why the propyl azide is integrating to 4 protons instead of 2.  Is it due to excess starting material in my compound that needs to be purified out?   I would have guessed that it should have come out in the aqeous layer during work up since it is an alcohol.