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Phenylfluorenyl PG removal + isolation of small aminoacid

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Hey guys,
its somehow related to my previous question but its probably better to start a new topic.
Anyway, Im strugling to remove my protecting group: phenylfluorenyl. I can get the protected intermediate without much trouble but the removal is pain. Using TFA with various scavengers (EtSiH, TIPS, MeOH...) I clearly see the starting material gone (by TLC) but afterwards, I dont see the mass of the product in my LCMS analyssis or only traces to be more specific and the spectrum further shows various unknown impurities. Furthermore, only very weak oftentimes none ninhydrin reaction is observed with TLC analysis.
I tried to deprotect the PhSe intermediate hoping the double bond is the cause of the problems with similar results observing some kind of PhSe transfer to get double selenylated product. Hydrogenation of the PhSe at 2.5 atm did not show any traces of reaction.

Furthermore, I recently started the same sequence with trityl protected aminoacid (trt is more labile to acid) but during first step (KHMDS enolate then PhSe quench) im observing poor reactivity and cleavage of the trityl group. Anyone observed this happening? I did quench the reaction with acetic acid but I dont think trt is that labile.

Im glad for any tips you can give me. Esp. some tricks for LCMS detection of aminoacids are wellcome (i detect by SIR but from that its hard to extimate the amounf of the aminoacid in my sample

thank you

Is it necessary to use TFA? The trityl goes with 80%HOAc, maybe you can try that, maybe some heat? Maybe scavenger is bad in this case, I would try without.

Pf is about 6000x times more stable so I think TFA is needed. I might try HCl in ether and maybe the hydrochloride crashes out. I think I did try without the scavenger and the results are fairly the same. I get rid of the Pf group (TLC) but then im unable to isolate the product.

I think I might be actually getting the pyroglutamic derivative or some kind of michael addition.

I work a good deal with unsaturated amino acids, but I doubt that I can be of much help in this instance.  KMnO4 dipping has worked well for visualizing our TLC plates.  The amino group should not be performing a Michael addition, unless one is under basic conditions; therefore, I am puzzled.  Regarding MS, if I could  spare a small portion of a product with a free amino group, I would at least consider using heptafluorobutyric anhydride to make a derivative.

OK, maybe you can run a deprotection in a NMR-tube to see what goes wrong, if the double-bond is affected this should be easy to spot.


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