Anyone who's familiar with organic synthesis will probably have came across the DMB and PMB protecting groups.

When TLC-ing compounds containing these protecting groups using common stains like vanillin or cerium ammonium molybdate (CAM, the Hanessian stain), its not uncommon for the compounds to appear bright pink/purple. This is a useful phenomenon, but one which I have no explanation for.

I'd assumed that this was related to the stain itself, however even when the TLC plate is simply heated in methanolic hydrochloric acid, the pink color still appears (quite vividly in fact).

I realise that all of the stains mentioned are made up with some acid, however, heating PMB/DMB containing compounds under acidic conditions without the TLC plate (in a flask, for example), doesn't afford this colour.

Any insight into what's forming the color? Specifically when PMB/DMB compounds show it, but simple benzyl ethers etc do not.

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    $\begingroup$ many phenol derivitive are relatively easily oxidized into bright colored products. $\endgroup$ – permeakra Jun 28 '16 at 22:50
  • $\begingroup$ Well, anisaldehyde is technically PMB-carbonyl and colours the TLC pink … And DMB is des-methyl-vanillin, isn’t it? $\endgroup$ – Jan Jun 29 '16 at 13:16

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