July 14, 2020, 08:50:52 AM
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Topic: 2 extremely close spots appear as 1 on TLC  (Read 7994 times)

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Offline rolnor

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #15 on: December 14, 2016, 12:29:28 PM »
If you use tertbutyldiphenylsilyl as PG the disilylated migt be possible to get more selektivly? The disilylated will be stericaly quite hindered.

Offline wildfyr

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #16 on: December 14, 2016, 08:03:38 PM »
Dan,

Hmmm... what about monotosylating or mono acetylating phosphorglucinol? It should be a pretty powerful electron withdrawing group which might make the other alcohols less reactive like you're describing, and favor mono protection. the mono, bi, and tri tosyl or acetate might also be easier to separate by recrystallization or column if they occur than TBDMS. Then you can di-TBDMS protect, and base hydrolyze the tosyl or acetate. Aryl esters and sulfonate are already pretty base hydrolysis unstable, it shouldnt be too hard.

Offline pgk

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #17 on: January 30, 2017, 12:49:50 PM »
Addition of a drop of triethylamine per few ml of such a solvent, may help separating TLC spots that have similar Rf. Unfortunately, this is not reproductible.Ttriethylamine cannot be added in the chromatographic column because it destroys the contained silica or alumina.

Offline hypervalent_iodine

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #18 on: January 30, 2017, 04:49:57 PM »
Addition of a drop of triethylamine per few ml of such a solvent, may help separating TLC spots that have similar Rf. Unfortunately, this is not reproductible.Ttriethylamine cannot be added in the chromatographic column because it destroys the contained silica or alumina.

Really? I didn't think adding low percentages of TEA (1-3 %) to columns was that uncommon. I myself have done it several times to assist in reducing streaking and improve separation of amines.

It is usually a good idea to pretreat plates with the TEA solvent before you run them, also.

Offline wildfyr

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #19 on: January 30, 2017, 08:26:39 PM »
Ive seen the TEA trick in columns in papers before, I dont think it hurts silica gel. You have to wash your product with aqueous acid to get it out afterwards though.

Offline hypervalent_iodine

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #20 on: January 30, 2017, 09:09:10 PM »
Ive seen the TEA trick in columns in papers before, I dont think it hurts silica gel. You have to wash your product with aqueous acid to get it out afterwards though.

It deactivates the silica, which might be what was meant by destroys. However, deactivating it is the point of adding it, so I'm not sure I'd view it as a problem.

Correct about the acid wash, though it is also possible to vac it off if your compound is acid sensitive.

Offline pgk

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #21 on: January 31, 2017, 09:34:17 AM »
The problem is a moderate repeatability.
A little amount of triethylamine increases polarity that helps a better separation; but it simultaneously increases the dielectric constant of the solvent that might lead to partial and random coagulation of silica particles due to electrostatic forces.
Furthermore, triethylamine is rarely free of water traces and as a consequence, a little but significant amount of silica might be dissolved. Thus, a better separation might be achieved, but with moderate repeatability due to the random coagulation and sedimentation of silica particles in the column.
All above is not important, regarding TLC monitoring but it is quite important, regarding the validity of a scientific work and very important, regarding a patent’s validity. 

Offline hypervalent_iodine

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #22 on: January 31, 2017, 09:50:24 AM »
The problem is a moderate repeatability.
A little amount of triethylamine increases polarity that helps a better separation; but it simultaneously increases the dielectric constant of the solvent that might lead to partial and random coagulation of silica particles due to electrostatic forces.
Furthermore, triethylamine is rarely free of water traces and as a consequence, a little but significant amount of silica might be dissolved. Thus, a better separation might be achieved, but with moderate repeatability due to the random coagulation and sedimentation of silica particles in the column.
All above is not important, regarding TLC monitoring but it is quite important, regarding the validity of a scientific work and very important, regarding a patent’s validity.

I wasn't aware of that, thank you for sharing. Working in the realm of patents is only something I have recently become involved in. Most of previous work where I had used TEA was in the context of the purification of an extremely recalcitrant and sensitive cyclic amine. Admittedly, TEA wasn't successful there (though it had been with compounds earlier in the sequence).

Offline rolnor

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Re: 2 extremely close spots appear as 1 on TLC
« Reply #23 on: January 31, 2017, 09:55:55 AM »
I have used TEA succesfully, I know others that use a small amount of pyridine instead, that is more mild to the silica.

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