DrDevice
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O-methylation and esterification of gallic acid using DMC and PTC
For some time I have been experimenting with the methylation of gallic acid (GA) with dimethyl carbonate. Method is based on literature such as
"O-methylation of phenolic compounds with dimethyl carbonate (DMC) under solid/liquid phase transfer system" (Ouk et al, 2002). I have also noted the
"Successful green PTC-mediated methylation of syringaldehyde to 3,4,5 trimethoxybenzaldehyde using DMC without pressure" thread on SM.
I attempted methylation using these approaches at reflux temperature of DMC (90C) with gallic acid, but no product resulted. I have however had some
success at elevated temperature using a pressure flask. The methylation goes beyond just the -OH groups and also methyl esterifies the carboxylic
acid.
1.87g GA (11mmol) was placed in a 100ml pressure flask (I used an Ace glass 100ml flask), together with 1.77g (5.5mmol) tetrabutylammonium bromide
(TBAB), 20ml DMC and 3.5x molar equivalent of potassium carbonate (5.5g).
The flask was flushed with nitrogen, sealed, and heated with magnetic stirring in an oil bath set to 120C.
After 2 hours, the flask was allowed to cool to room temperature and pressure released carefully. The flask was re-sealed and put back into the oil
bath for a further 3 hours.
After five hours, no trace of GA remained (TLC, 1:1 hexane:ethyl acetate), and a TLC spot appeared at the same Rf as a known good sample of methyl
3,4,5-trimethoxybenzoate.
Liquid was filtered off with suction. The solids were well stirred with hot methanol and re-filtered. The liquids were combined and solvents removed.
The resulting brown crystals were re-crystallized from a minimum amount of hot methanol.
The resulting solids were washed with cold water on the frit under suction, leaving a pure white crystalline substances. MP 82C - 83C (lit. 82C -
84C).
I was a bit surprised at the low melting point when I first ran the experiment, as I wasn't expecting the ester. Subsequent access to "real" methyl
3,4,5-trimethoxybenzoate & the same Rf on the same TLC plate seemed to confirm it.
Yield: yield is complete crap. Obtained 0.71g @ MW = 226.226 = 3.2mmol = 29%. Horrible, but at least my DNA hasn't been methylated...I presume there
is also some mechanical loss during purification that would have dropped yield also.
There are lot of opportunity for exploring different amounts of PTC, base, temperature etc, but I have a reasonable supply of commercial methyl
3,4,5-trimethoxybenzoate so I won't be pursuing this again for a while.
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Sigmatropic
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Nice report, perhaps methanol is not ideal for recrystalization and alot is lost to the mother liquor. were there any other spots on TLC? "I have a
reasonable supply of commercial methyl 3,4,5-trimethoxybenzoate", then why are you taking the hessle to methylate phenols when one can simply
hydrolyze an ester?
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DrDevice
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Posts: 75
Registered: 19-3-2012
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Mood: Incompatible with carbon based lifeforms
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After the recrystalization, I evaporated the rest of the methanol to see if anything remained - there was only a small amount of tarry material that
produced no more crystalization after another attempt with methanol. There was no other spot on the TLC with the recrystalized product. On the "raw"
TLC after the reaction completed but before extraction there was a mess that remained fixed at the starting line, which I guess was all water soluble
compounds that stayed behind in the aqueous portion during extraction.
As to why bother going through the hassle, well, just to see if I could get it to work... I also have plenty of gallic acid, and that can be in turn
extracted from OTC material such as tannic acid.
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