[Oropimente]

I have to purify some boc- and cbz- N- protected aminoacids (not all of them are alpha aminoacids, some of them are gamma and delta aminoacids). Everything seems so easy: take aminoacid, aq. NaHCO3, (Boc)2O in THF, stir overnight. To quench, wash the organic solution with 1M HCl, dry and evaporate the organic solution. No chromatographic column.

But the problem is the rotoevaporation. After removing the solvent, I usually find a viscous, very sticky oil, or sometimes a sticky solid. When I find this:
- I try to redisolve the sticky oil in hexane, and then rotoevaporate. It worked for 3 boc- protected aminoacids.
- I try to redisolve the sticky solid in Et2O, filter it using Celite, then evaporate. If that doesn't work, I redisolve the organic residue in the minimum quantity of Et2O and I left the solution to precipitate. Only worked for 2 boc- protected aminoacids.
- I tried to put them in vacuo overnight, but this doesn't help with any compound.

But for a lot of them this simply doesn't work (or it takes too much time to precipitate, I guess). I know that almost all of them are solids because they are reported in that way. I also tried some things that I saw in the forum (last coment: http://www.chemicalforums.com/index.php?topic=90351.msg325753#msg325753). As you may guess, my compounds are only partially soluble in hexane, and soluble in the usual solvents (CHCl3, DCM, Et2O, EtOAc, Acetone...).

Although maybe some of you didn't work with aminoacids, I would like to see how you dealed with sticky compounds, to learn and try alternative solutions. I would be very appreciate.

[wildfyr]

Can you isolate them as carboxylate salts? Much more likely to be crystalline.

You can also try sticking them in the freezer for a few days. It may allow them to crystallize. Crystallizing cold from a minimum amount of bad solvent can work too.

Unfortunately, this is where we cross from science to art in organic chemistry. There are a lot of trees to bark up (put a seed crystal in? Start dissolved in something nice and add an antisolvent?) that aren't exactly go tos, but refuges of the desperate, do work sometimes.

[rolnor]

If you want to remove trace of water its better to evaporate with toluene.

[Oropimente]

Can you isolate them as carboxylate salts? Much more likely to be crystalline.

You can also try sticking them in the freezer for a few days. It may allow them to crystallize. Crystallizing cold from a minimum amount of bad solvent can work too.

Unfortunately, this is where we cross from science to art in organic chemistry. There are a lot of trees to bark up (put a seed crystal in? Start dissolved in something nice and add an antisolvent?) that aren't exactly go tos, but refuges of the desperate, do work sometimes.

I tried to form the carboxylate salt with some of them and it worked! Thank you very much!

If you want to remove trace of water its better to evaporate with toluene.

So, if some compounds have traces of water can not precipitate? I honestly didn't know that.

[phth]

Can you isolate them as carboxylate salts? Much more likely to be crystalline.

You can also try sticking them in the freezer for a few days. It may allow them to crystallize. Crystallizing cold from a minimum amount of bad solvent can work too.

Unfortunately, this is where we cross from science to art in organic chemistry. There are a lot of trees to bark up (put a seed crystal in? Start dissolved in something nice and add an antisolvent?) that aren't exactly go tos, but refuges of the desperate, do work sometimes.

I tried to form the carboxylate salt with some of them and it worked! Thank you very much!

If you want to remove trace of water its better to evaporate with toluene.

So, if some compounds have traces of water can not precipitate? I honestly didn't know that.

Another trick is to try crystallization from something like DCM/Hexane or Et2O/Hexane, and if it doesn't work put it in the freezer because oils have melting points too close to room temperature.