I have dehydroleucodine (DHL) and parthenolide (PTL) in powder form. This powder was left at -4C for a year. When the powder is redissolved in ethyl acetate or methylene chloride it rapidly forms a clear, rubber like substance that is insoluble in water and organic solvents. TLC Rf reveals that DHL and PTL are still present in mixture. What might be the source of the polymerization and how might I recover any unreacted PTL and DHL from the mixture?
Both of those have strained rings with double bonds and parthenolide also has epoxides. These are very reactive - might react even at -4 C, being open to the air would increase this. The original sesquiterpene lactones probably dissolve pretty well in dichloromethane or chloroform - if you wash the rubbery substance with these solvents you should be able to extract any unreacted materials.
Dr. Hanson
I tried that method, but it mostly fails.
1. I dissolved in ethyl acetate and filtered through a 0.2uM PTFE filter. The rubbery film reemerged.
2. I tried to purify on a silica column with ethyl acetete/hexanes. The collected fraction with parthenolide or DHL was clear initially, but the rubbery film reemerged after rotovaporation.
3. I dissolved in benzene and added activated charcoal. Then I filtered through Celite. After rotovaporation, thins crystalline material forms on the sides of the flask. I tried to recrystallize with hot ethanol (60C) or hot ethyl acetate (60C). This will dissolve the crystals on the sides of the flasks, but I also get white, insoluble, rubbery precipitate in the solution.
James E Hanson ,
I attached pictures with their descriptions of the different procedures I used to recover PTL and DHL from the polymeric material.
It appears that I am not successfully separating the pure molecules of PTL and DHL from the material that is initiating their polymerization. I have isolated these sesquiterpene lactones from plants and have had no problem recrystallizing them at this stage. It is only when I have taken these compounds from long term storage that I have trouble purifying them from polymeric material. Once I filter the polymeric material from the pure compounds, more of it will form. I need to remove the initiating source from the mixture.
I appreciate your help in uncovering the problem. I have been struggling with this issue for a long time.
David
Either there are soluble fractions of the polymerized/oligomerized material - here the best way to recover is to run a column using your TLC conditions This will separate the soluble oligomer from the unreacted monomer
or
The material is reacting further after isolation - you are separating the starting material but then it polymerizes further. If that is happening, you have to think of an inhibitor to prevent this. This is a harder problem to solve.
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