[antracene]

hi all

i'm currently trying to add a BOC group to an aminooxy group on tetra ethylene glycol, where the end opposite to the aminooxy is a carboxylic acid.  
HO2C-CH2-CH2-O-CH2-CH2-O-CH2-CH2-O-CH2-CH2-O-NH2  <- add BOC here

the reaction takes place in THF/water in the presence of NaHCO3 or NaOH, followed by the addition of Boc2O.  the reaction yield seems abysmally low.  the TLC is streaky so it's difficult to monitor it that way (a little AcOH in the mobile phase helps, but not that well).  acidify to pH5, then extract out with DCM.

i'm attributing the problem to the product being still in the aqueous phase (TLC suggests this).  any suggested solvents for removing peg type molecules out of water?  or just tricks dealing with peg type molecules.

thanks

[Polytriazole]

I've had some luck with keeping the aqueous phase as small as possible.  This way, the PEG prefers to be in the organic phase. Can you decrease the amount of water in your reaction solution? 


However, you may need to take my advice with a grain of salt, as I work with higher molecular weight PEG materials (5000 g/mol).

[Dan]

However, you may need to take my advice with a grain of salt

On that note, you could try adding brine to the aqueous layer. That is normally pretty good for persuading relatively polar organics to come out of the aqueous phase. I worked with sugar derivatives for 4 years and regularly got pretty hydrophilic intermediates. Usually worked a charm - but beware that if it doesn't work you will probably end up chucking it, I've never worked with PEGs so I don't really have a practical feel for the polarity.

You might also want to switch DCM for chloroform, I find it is a better extraction solvent for more polar species.

Another generally fail safe method is trituration. Instead of extracting, quench your mixture and remove all the water etc. on a rotavap, then dry thoroughly under high vacuum. Beware that if you acidify before doing this, the Boc might hydrolyse when you concentrate with excess acid. Suspend the dry residue in a suitable organic solvent, this normally requires a good deal of mechanical dispersion (mash it up with a spatula), you can also heat and sonicate to make life a bit easier. Decant and filter the supernatant. Repeat. TLC the supernatant each time to see if you are still extracting anything. Now your organic products will be in the filtrate and the salts should remain in the solid phase.