[clemi2310]

Hello everyone,

I am currently working on the biological studies of diketo esters on a certain therapeutic target.
I perfectly manage to synthetize them as shown in the synthetic pathway in the picture.

However, I found out these compounds are not stable at all in DMSO, which is a real problem since we conserve our aliquots of final compounds in DMSO.

Depending on concentration and temperature (it degrades quickly at RT, slowly at -20°C), after one or few days, my sample (diketo ester in solution in anhydrous DMSO) initially colorless, become dark blue.

And when I analyse my samples by LC-MS, I see that the ester is partially hydrolyzed in the corresponding acid.

However, when I do perform proper hydrolysis my diketo acid compound in solution is colorless and not blue.

I am wondering about the role of DMSO in a potential hydrolysis of diketo ester, and this weird blue color.
The DMSO I use is anhydrous.

I also tried to inactivate DMSO by passing through some Alumina Al2O3, but degradation remains the same.

Any ideas of a possible mechansim with DMSO?

We plan to do a stability study regarding, temperature and concentration.
This phenomenon happens, for all the scaffolds I synthetized (>20).

Thank you for your help,

Clémence

[phth]

methyl esters will deprotect in the precence of NaCl+DMSO i.e. ubiquitous things.  The α-diketo struture is inherently unstable.  The ester can deprotect, and then react to form an carbon anion that is colored.  Those colors are observing charge transfer bands. I would just avoid DMSO.  Did you make the diketo compound or did you buy it??

[wildfyr]

Its gonna be tough for him to avoid DMSO because that is really the only organic solvent compatible with biology. I think the answer here is to isolate it in another organic solvent, then aliquot it out in small amounts and store it dry and cold, then dissolve it in DMSO and add it to your biological sample right when the experiment starts.

[clemi2310]

Thank you for your answers.

Indeed, wildfyr is right, I cannot avoid the use of DMSO for biological tests, it is the universal solvent used because of its capacity to solubilize a wide range of compounds...

I have to precise this degradation is also observed for ETHYL esters, but there's not salt in the medium only DMSO.

I synthetized the diketo compound starting from the corresponding methyl ketone.

I agree with you the only solution is to keep it in powder form dry and cold.

My questions was more like mechanistic to explain this blue color, so you think it is a carbon anion ?
Because when I put my diketo acid properly synthetized in DMSO, it is stable and does not turn into blue color

i wondered if it was possible that DMSO form some adducts or particular species that make the whole thing blue !

[hypervalent_iodine]

Definitely just store it as the powder. I work with compounds that are similarly unstable and this is really the only way around it for us.

Not sure about the blue. How sure are you that the compound in the stored solution is the acid?

[clemi2310]

I checked it by LC-MS. Same retention time and MS than the corresponding acid. Still it could be something else...

A student from my lab talked about a Krapcho decarboxylation but I think it requires high temperature and salt which is not the case.

Thanks for your answer

[phth]

I checked it by LC-MS. Same retention time and MS than the corresponding acid. Still it could be something else...

A student from my lab talked about a Krapcho decarboxylation but I think it requires high temperature and salt which is not the case.

Thanks for your answer

That person is thinking about the question incorrectly.  The mechanism has significantly different energetics compared to β-diketo stuctures for the Krabc decarbox.  For example, Oxalyl chloride decomposition pathway proceeds at cold temperature. 

@Wildfyr DMF, MeCN, NMP, etc don't work ?

[hypervalent_iodine]

DMSO just happens to be especially non toxic. The fungal work I do can tolerate up to 10% v/v. Not sure about the other solvents listed.

[wildfyr]

[/quote] 

@Wildfyr DMF, MeCN, NMP, etc don't work ?
[/quote]

Well, "dont work" isn't the issue, they will dissolve stuff and help it get into water obviously, its just that DMSO has especially low toxicity compared to other highly polar aprotic solvents. I think its even less toxic than low alcohols. Its a very standard thing in biology to dissolve some hydrophobic molecule like a dye in DMSO, add it to an aqueous suspension then agitate it to get it to label or whatever. Using DMF, MeCN, etc would be unusual.

[clemi2310]

Wildfyr is totally right, DMSO is not a parameter we can modify, it is the universal solvent for biological tests. My questions was more to explain the degradation. Specially because I do not use any salt when I solubilize, and I use anhydrous DMSO.

Other idea, after the synthesis with tBuOLi, I run a column. do you think there could be residual lithium that go through the column and in my sample?

Thanks for your help

[hypervalent_iodine]

Wildfyr is totally right, DMSO is not a parameter we can modify, it is the universal solvent for biological tests. My questions was more to explain the degradation. Specially because I do not use any salt when I solubilize, and I use anhydrous DMSO.

Other idea, after the synthesis with tBuOLi, I run a column. do you think there could be residual lithium that go through the column and in my sample?

Thanks for your help

Not if you do any sort of aqueous work up prior to columning it.

[clemi2310]

okay... I do perform an aqueous work-up quenching with 1M HCl and extraction with AcOEt so it is not lithium.