[ThePostDoc]

I have been trying to recrytallize a compound that crashes out of the reaction.  It is a benzylidene protected thiophenyl glucoside.  Not a crazy compound by any means.  All solvents have lead to the same result... a formation of crystals that gets completely engulfed by some gel.  I mean it was a 5g reaction, using 4-5 mls of benzyaldehyde dimethyl acetal.  I have taken a about a gram to try some recrystallizations on.  EtOH, EtOAc/pentane, toluene, Chloroform/hexane have all been used.  The entire solution becomes a gel when it cools.  If I increase the solvent, I get more gel. 

The compound isn't terribly soluble and a 5g column on it wouldn't be terrible if I could get it dissolved to load it on silica gel in a reasonable volume.  I tried to solid load it but it didn't adsorb well onto the silica.. craziest compound I had to deal with.  I think I am going to take it on to the next few steps and see if I can purify it then but I would like to solve this problem.  Anyone have any rxl techniques that may help?

[mst]

Why don't you filter it off if it crashes out of the solution anyway? Have you tried using the solvent of the reaction for recrystallization? What's your exact recrystallization procedure? Have you tried scratching the glass to produce nucleation sides? Have you tried using some of the crashed out product of the reaction for impregnation of crystal growth? Have you tried ultrasonic bath to initiate crystal growth?

[ThePostDoc]

I am not having any trouble growing the crystals so I haven't tried seeding or scratching.  The gel formation is very rapid, crystals start forming and then in a matter of minutes the gel forms.  I have tried triturating with the reaction solvent and some others but the  NMR is kind of gross still.  I have tried rxl from the reaction solvent as well but it is MeCN and kind of expensive to be using like this. 

The closest I got was a strange combination that I thought would be different.  I was thinking the gel was something to do with the extra eq of benzaldehyde DMA (I use 2 eq in the reaction) so I thought let me heat it up with MeOH and hexane.  My product will be insoluble in hexane and the benzaldehyde DMA will be in the hexane layer as it is quite non polar.  Well strangley, my compound facilities a miscibility between hexane and methanol.  My product quickly crashes out into these nuggets, not very crystal like but hey if it is pure... then in seconds the gel froms.  It takes on the exact volume of the solvent no matter how much solvent I have used.  I guess I could go for super dilute but I mean I took one gram and rxl from 75 ml of toluene and I formed 75 ml of gel s#*$ with crystals trapped in it. 

[ThePostDoc]

Problem solved.  Move on to next step, that product crashes out without gel bs

[billnotgatez]

If it is not a secret you want to let us know what you changed

[ThePostDoc]

I just took the impure product and went on to the next step.  So the compound I had was phenyl 4,6-O-benzylidene-ß-D-thioglucopyrannoside (okay, sugar nomenclature can suck it) and I think there was some issue with RXL with the xs benzaldehyde forming some sort of hydrogel like substance, some sort of solvent inclusive matrix.  I benzoylated the remaining 2,3 diol with BzCl, Et3N in CH2Cl2.  When I quenched the xs BzCl with MeOH, my product crashed out so I filtered and washed with MeOH.  Took NMR and it was super clean so essentially I just skipped the RXL and moved on with the crude product.