Developing a GC assay

26 Nov
Published by RyanPakula

Recent work has been towards developing a GC assay to quickly monitor the sulfur melt reactions.
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In order to optimize the sulfur melt reaction, we've been trying to develop a solid set of GC conditions that'll let us quantitate the reaction progress quickly and accurately (which precludes running a column every time I try the reaction with different conditions).  Initially, PZQ and the dehydrogenated product were found to come off the column at different times, although high temperatures around 320ºC are needed.  A small blip in the PZQ-only sample's chromatogram unfortunately lines up with the product peak.
 
I thought the blip may have been a decomposition product from the high temperatures in the oven because the PZQ looks pure by NMR.  Still, I shot some freshly recrystallized PZQ on and saw the same blip (~10% the area of the main PZQ peak).  We tried an on-column cold injection, which is supposed to prevent decomposition if it is occurring, but the peak persisted.  So it's probably just a contaminant that is present in the batch from the manufacturer and is not possible to remove by recrystallization.  At some point I may try running a column to isolate PZQ from this compound, although the retention time on the column is so similar that it will be hard to separate by column chromatography.
 
Using this rough assay (using the term 'rough' only because exact determination of yields isn't possible due to the blip where the product comes off the column), I tried running the sulfur melt reaction a few different ways: 1) in DMSO, heating to various temperatures; 2) in DMSO, bubbling dinitrogen gas through the solution to drive off formed hydrogen sulfide; 3) melt with 6 eq. of sulfur; 4) melt with <1 eq (i.e., 0.9 eq) of sulfur.
(1) didn't work at all, no matter the temperature.  Longer reaction times may be tried.  (2), same as (1).  (3) Product is formed, but longer reaction times begin to form another product.  This may be a doubly-dehydrogenated product.  Investigations into this product are continuing.  (4) A little cleaner conversion to the desired product, but still some of the possibly doubly-dehydrogenated product showing up after the reaction had been running for a while.
 
So there's potential here, but fine-tuning of the assay needs to continue.