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Topic: Probable failure to add TBDPS group to a phenolic oxygen  (Read 642 times)

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Offline Babcock_Hall

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Probable failure to add TBDPS group to a phenolic oxygen
« on: October 27, 2023, 02:10:39 PM »
We are attempting to protect the OH group of 2-formyl-5-hydroxypyridine with the tert-butyldiphenylsilyl (TBDPS) group. This is our first time working with TBDPSCl. The pKa of this Pyr-OH group is probably near 7. We tried using DCM, TEA, and catalytic DMAP, and we followed the reaction by TLC. We performed an extraction using water and brine, and we ran a column in the presence of 1% TEA. The UV-active spot gave a yellow-orange color with DNP dipping. We removed the solvent. My first look at the H-1 NMR suggested that the aldehyde hydrogen was only about 4% of the expected integral, relative the the presumed test-butyl peak. There might be a signal from a pyridine H (it has about the same integral). One interpretation is that the actual product and a side product that contains tert-butyl and aromatic groups co-eluted under our conditions. It is surprising that such a small proportion of aldehyde would give a positive reaction to DNP, but I would not rule out the possibility.
I have a milligram of pre-column material that could be checked by NMR. Is it worth trying to flush the column? What might have gone wrong in terms of the low conversion? What should we do differently?
EDT
Is it possible that we formed the hydrate of the aldehyde?  I just found a paper on hydrates of imidazole, and I will keep looking for more information:
https://pubs.acs.org/doi/full/10.1021/jo902588s.  However, the NMR does not suggest the formation of the hydrate to me.
« Last Edit: October 27, 2023, 05:02:25 PM by Babcock_Hall »

Offline Babcock_Hall

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Re: Probable failure to add TBDPS group to a phenolic oxygen
« Reply #1 on: October 28, 2023, 02:44:56 PM »
Update:  There is a intense brownish band at the top of the column that looks to be about the width of the silica and sample that were dry loaded.  I tried to elute with four increasingly polar solvent mixtures, the last one being 4:10:86 acetic acid/methanol/acetone.  Only a little bit of the color has moved; most of the band is stationary.  My reason for using acetic acid is that if we did not add (or if we lost) the TBDPS group, the unprotected hydroxypyridine (pKa estimated to be 7) would deprotonate in the presence of triethylamine and form a salt.

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