[someonefromasia]

Hi guys,
I've just ran a reaction and my desired product (~0.02g) is a lactam which contains an OtBu group. Unfortunately I suspect the mixture contains DMAP (from TLC with large light spot) which is in excess as my reaction condition. Trouble is I can't really use acid to wash away DMAP since my product is water soluable and it contains an OtBu group which I'm sure when in contact with acid and water will be lost. Anyidea of how to do it?
TLC of my mixture actually doesn't show my product but because it is very polar I suspect it is covered by the DMAP spot.
Since I used a small scale I can't really recrystallise it, any idea?

[Dan]

a lactam which contains an OtBu group.

Are these the only functional groups present? Lactam and ether?

Trouble is I can't really use acid to wash away DMAP since my product is water soluable and it contains an OtBu group which I'm sure when in contact with acid and water will be lost. Anyidea of how to do it?
TLC of my mixture actually doesn't show my product but because it is very polar I suspect it is covered by the DMAP spot.

What's your mobile phase for TLC? If you can move it in a more polar mobile phase, you can column it on silica.

If not, you can turn to reverse phase chromatography.

You might get away with quickly passing your aqueous crude mixture through a short column of acidic resin - that should absorb DMAP and probably won't deprotect the ^tBu ether (a wash with dil. acid would probably not either, but the water solubility of the product is a problem).

Could you triturate out the DMAP with diethylether or some other low polarity solvent? I don't know enough about the solubility of DMAP or your lactam to know if that's likely to work - do you?

[someonefromasia]

a lactam which contains an OtBu group.

Are these the only functional groups present? Lactam and ether?

Trouble is I can't really use acid to wash away DMAP since my product is water soluable and it contains an OtBu group which I'm sure when in contact with acid and water will be lost. Anyidea of how to do it?
TLC of my mixture actually doesn't show my product but because it is very polar I suspect it is covered by the DMAP spot.

What's your mobile phase for TLC? If you can move it in a more polar mobile phase, you can column it on silica.

If not, you can turn to reverse phase chromatography.

You might get away with quickly passing your aqueous crude mixture through a short column of acidic resin - that should absorb DMAP and probably won't deprotect the ^tBu ether (a wash with dil. acid would probably not either, but the water solubility of the product is a problem).

Could you triturate out the DMAP with diethylether or some other low polarity solvent? I don't know enough about the solubility of DMAP or your lactam to know if that's likely to work - do you?

Cheers for replying.
The name of my product is 3-(tert-butoxymethyl)-6-isobutylpiperazine-2,5-dione, it's a 6-ring with two amides (opposite of each other) plus a CH₂O^tBu and an isopropyl in between them, so I guess the ether and amide are the functional groups that matter.
I've already ran TLC on phases such as DCM:MeOH 10:1 but still no signs of seeing two separate spots from the one I suspect contains my product and DMAP.
As for acidic treatment, I've indeed tried using acidic resin like DOWEX to neutralise it but the NMR does not show any likes of the ^tBu. A quick run though a short column sounds good, so is the titration bit but because I only used 0.02g of starting material is it still reasonable to run a column? Lot of my product will be lost in the column, also titration would take ages since I have to check TLC of the titrate for each 0.1cm³ of acid added.

[Dan]

Hmmm. I think I'd investigate recrystallisation or trituration (N.B. not titration).

The other option is to modify the conditions for synthesis the compound - avoid DMAP and you may solve the problem. Pyridine might do the job and it's much easier to remove.

[someonefromasia]

Cheers for that. I need something that is basic enough to remove Fmoc group but not nucleophilic enough to attack thiolester. Any suggestions?

[Dan]

I've never done it myself, but on paper (Green & Wuts) pyridine can work, or triethylamine (the latter is a stronger base). Perhaps DIPEA for a bit more of a kick? Obviously there are many possible bases, but those I mentioned are all volatile options.

[someonefromasia]

Pyridins is too nucleophilic for it and so is NEt₃ so I guess I'll have to think of some other route to my product. Cheers.

[Dan]

Surely pyridine is less nucleophilic and less basic than DMAP? If the thioester is stable in the presence of DMAP, I can't see why it would not be stable in pyridine. Personally, I'd try it.

[someonefromasia]

Yes I'll try it. Cheers.