Neptun
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Road to Ethyl Maltol
Ethyl Maltol is a compound widely used in perfumery industry and as a fragrance and aroma in e-liquids, confectionary and other consumer products.
It is a synthetic aroma compound which is said to have a smell reminiscient of cotton candy and caramel. Furthermore, it has a very interesting
chemical structure, because instead of common ester motifs of pleasantly-smelling compounds, Ethyl Maltol incorporates a 4-Pyranone moiety,
substituted with a hydroxy- and ethyl group on position 2 and 3, respectively.
Taking all of that into account, I just felt that I had to synthesize it! This is a huge undertaking for me, because I just started organic chemistry
as a hobby. Anyway, I feel like I have enough literature I can adapt for my project. Oh, by the way, feedback from all of you would be hugely
appreciated!
As to the synthetic route, I'll start with frufuryl acetate, a common fragrance. I plan to cleave the ester to the furfuryl alcohol and acetate.
Furfuryl alcohol can then be subjected to an Achmatowicz-rearrangement in methanolic NBS, forming a dihydropyran with an alpha,beta unsaturated ketone
motif, which can be oxidised to the epoxide via conjugate addition of basic hydrogen peroxide. Said epoxide rearranges in aqueous acid to pyromeconic
acid. This intermediate can undergo alpha-hydroxyethylation in the 3-position to form a secondary alcohol, which is subsequently reduced to the
alkane, forming ethyl maltol in 6 steps.
A few days ago, I started the project and made a bit of furfuryl alcohol, albeit impure and with a horrible yield. I haven't weighed my product yet,
but I expect it to be about 20-30%, which is why I see this synthesis as a failure. I'll share my synthesis anyway, but I plan to refine it to get a
yield of at least 80 or 85%. Now to the writeup:
My (horrible) synthesis of furfuryl alcohol
Disclaimer: Furfuryl alcohol and methanol are poisonous and cancerous upon inhalation! Proper safety measures have to be adequately met! I wore
goggles, a lab coat, latex gloves and a gas mask rated for organic solvents.
In a beaker, about 8g (doesn't matter too much because it is in excess anyway) of KOH were weighed out and dissolved in 20 mL of MeOH under stirring
on a hot plate. The solution heated pretty much, so a larger beaker (min. 100mL) is recommended. A round bottom boiling flask is charged with 9.98g of
commercial furfuryl acetate as a yellow oil, the beaker in which it was weighed out is washed out with a bit from the KOH/MeOH solution which
immedately discolours to a scarlet red. The entirety of methanolic KOH were addded at once into the boiling flask. The mixture takes on a cherry red
colour. I'm not really sure as to where the colour came from, but I was expecting a lot of coloured impurities.
A graham condenser was attached to the flask and heating in a sand bath and under strong heating was applied until reflux. The condenser was
water-cooled with tap water. I planned on refluxing for about an hour, but a few minuted in, the exhaust hose of the condenser slipped out of the
water reservoir beaker, and spilled water all over my hotplate. Luckily, the FI-switch (I think in english its called rCD?) caused a power outtage,
which is why I could clean up the water and resume the refluxing. apart from this little mishap the reflux was unspectacular, the solution stayed
exactly the same, colour wise. A few mL of water were added, until I realized how counterproductive this is. I then wanted to distill off all the
solvents via vacuum distillation. A vacuum pump was connected to a gas washing bottle with a adjustable gas inlet connected to the distillation setup.
This extra measurre was to ensure there was no suckback of liquid into the pump. A vacuum of about 0.1 bar was applied to the setup and the red liquid
started bubbling. I'm afraid that this was way to much vacuum, because no liquid condensed in the liebig. The methanol probably directly evaporated
out of the pump. The solvents were removed until all solids crashed out instantly as a brown crystalline mass. This was mostly KOAc and residual KOH,
as well as some colored impurities.
This sludge was left to dry a bit until it was broken up and washed out with 3 portions of about 20mL of MTBE, the salts and impurities were filtered
off using vacuum filtration and washed with a little more MTBE. The filtrate was collected in the same erlenmeyer flask. All colored impurities were
gone, and only a bit of yellow oil, maybe residual furfuryl acetate, remained in the crystal clear MTBE solution. To this, two teaspoons (very much
now that I think of it..) of anhydrous sodium sulfate were added and the flask was swirled around a bit; the insoluble yellow oil seems to have been
absorbed by the sodium sulfate. This emulsion was passed through a cotton filter to remove the drying agent andwas collected in the (now washed) 250
mL flask . The MTBE was removed using strong vacuum and heating until only an oil remained wich turned darker every moment. This was crude furfuryl
alcohol (I suppose) and I collected about 1.8 mL of it. I don't really have a way to confirm its identity tho, maybe I can get my hands on a some TLC
plates soon.
I see multiple flaws with this preparation now. Maybe a solvent free approach might be suited? I'd love to hear your thoughts on this preparation. I
personally couldn't find any literature on furfuryl-alcohol saponification, and the normal approach of water/organic seperation doesn't really work
here bc furfuryl alcohol is miscible with water.
I plan to produce at least 10mL of furfuryl alcohol until I proceed with the next reaction in my Ethyl Maltol synthesis.
Attachment: Ethyl Maltol structure (11kB)
This file has been downloaded 45 times Attachment: mixture before reflux (898kB)
This file has been downloaded 52 times Attachment: sludge after evaporation of most of the solvents (614kB)
This file has been downloaded 48 times
 
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Fery
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Location: Czechoslovakia
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Hi Neptun and welcome here!
Interesting experiment and thanks for posting. Ester hydrolysis is typically almost quantitative reaction with very good yields, wastes should be only
during workup procedure. I've bought furfuryl acetate from https://shop.es-drei.de/ester/694/essigsaeurefurfurylester-m... and it had horseradish scent.
Furfuryl alcohol tends to polymerize but I do not know how much quickly either at which temperature. I have one very old bottle of commercial furfuryl
alcohol which content is very viscous brown syrup so it is very likely suitable for liquidation only (looks like few decades old).
Magpie and later Boffis synthesized furfuryl aldehyde: https://www.sciencemadness.org/whisper/viewthread.php?tid=15... and here a method from corn cobs https://www.orgsyn.org/demo.aspx?prep=CV1P0280 then it is possible from cherry tree rosin, ribose, xylitol, various pentoses and so on. You can do
Cannizzaro reaction from furfuryl aldehyde into furfuryl alcohol and acid or even better crossed Cannizzaro reaction furfyl aldehyde + formaldehyde
-> furfuryl alcohol.
Keep up good work and let us informed about your cool synthesis of ethyl maltol.
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Neptun
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Thanks Fery for your nice words!
I actually planned on using that route to furfuryl alcohol, I found a paper describing the simultaneous production of furfural and formaldehyde via
dehydration of fructose. This procedure employs a simple ferric sulfate catalyst. I actually wanted to conduct a crossed cannizaro in the receiving
flask of that distillation (containig water, furfural, formaldehyde and some other byproducts) to yield furfuryl alcohol, but I then found that very
source of furfuryl acetate you stated in your post. So, my thought process was, instead of making ferric sulfate, doing a tedious dehydration at over
170°C and working with formaldehyde vapour in my first synthesis, I could just easily hydrolyse that ester, saving me a bit of work here.
Speaking of, today I obtained a lot of TLC plates and will do the testing of my first furfuryl alcohol sample. Still pondering what to use as a stain
and eluents tho, because I only found plates without fluorescent markers. I'm new to TLC, so I just thought a mixture of MTBE/MeOH does the trick? I'd
love to hear your opinions on that matter.
The plates are from 1985 btw 
Back to the ester hydrolysis, I thought of maybe just grinding some furfuryl acetate with solid KOH with a mortar and pestle? Gonna try that out soon,
maybe even today, run a TLC, check the results. Maybe I'll get some conversion there.
I'll keep y'all updated.
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chempyre235
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Welcome to the forum! Be sure to keep us posted on your progress.
Just a side note, aren't pyrans like these typically good antioxidants (e.g., kojic acid)?
"However beautiful the strategy, you should occasionally look at the results." -Winston Churchill
"I weep at the sight of flaming acetic anhydride." -@Madscientist
"...the elements shall melt with fervent heat..." -2 Peter 3:10
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Neptun
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Yeah, I think they are. Speaking of kojic acid, my initital synthesis plan started there, but I quickly scrapped this route due to reported incredibly
low yields in the oxidation step to comenic acid.
Anyway, I came around to run some TLCs on the product from my original synthesis. I whipped together a KMnO4 staining solution with 1.5g
KMnO4, 7.6g Na2CO3 and 0.125g NaOH in 200mL distilled water and a little more from the beaker washings. I used a
cheap spray bottle to apply this to my TLC plates.
So, I finally got to use my '85 TLC plates! Also, since I don't have any capillary tubes, I figured that Q-Tips would do the trick. So I bought some
and cut off one cotton end; the other end I dipped slightly into pure furfuryl acetate (abbreviated as FFOAc) and another one my crude product
(abbreviated as FFOH). Then, the Q-tips were used to stir a few mLs of EtOAc in seperate beakers to dissolve the respective organic compounds. The
other (cut off) side was the dipped into the solution and used to apply the sample onto the plate. Just as a side note for people out there with no
access to capillary tubes.
Anyway, here are my results:
FFOAc in EtOAc: Rf = 0.778 (first); 0.72 (second), averages out to about 0.75
FFOH in EtOAc: Rf = 0.66
I thought that this seperation was way to low, hence I decided to use another, less polar solvent. At first sight, the spots looked the same, which
made me really uneasy. Upon further inspection, I could see the difference in Rf values, but they were too close together for a proper
proof. Unfortunately, I'm not able to access pure alkanes that easily and I don't really want to go to the trouble of destilling various hardware
store products for that matter. The only solvents less polar than EtOAc in my possession are THF, MTBE and Toluene.
My TLC with MTBE failed completely, probably because of the spots getting submerged in the solvent. In hindsight, I should've used less MTBE and apply
the samples a little higher.
My MTBE/Toluene 1:2 solution didn't work either; it didn't mix very well but I suppose that was because of residual water in the beaker that I didn't
really dry.. The solvent front didn't really climb up at some point anymore, which is why I decided to end this TLC prematurely and discard it.
My last try was with pure Toluene as the eluent. Although the solvent front wasn't very straight (I blame my poor plate cutting skills and the dull
pair of scissors), I could obtain some promising results:
FFOAc in Toluene: Rf = 0.75 (funnily almost the same as the EtOAc one)
FFOH in Toluene: Rf = 0.2
Although the FFOH spot is kinda stretched, indicating some impurities there, I'm very happy with these results. This is a proof that I synthesized
some drastically more polar substance than FFOAc, and the only one I could think of that meets that chemical and physical properties such as the
slightly brown color from impurities and a high boiling point is furfuryl alcohol. Please correct me if I'm missing something there, but I'M now
pretty sure that I suceeded in the production of FFOH. My next goal is to find a better process with better yields.
I'm not really sure how to properly insert pictures here, so pictures of the TLC are attached. Note that they are only that ugly because I took the
pictures too late and the pencil was too sharp..
Attachment: FFOAc in EtOAc (767kB)
This file has been downloaded 48 times Attachment: FFOAc and FFOH in EtOAc (1.2MB)
This file has been downloaded 50 times Attachment: Failed MTBE attempt (1MB)
This file has been downloaded 53 times Attachment: FFOH and FFOAc in Toluene (404kB)
This file has been downloaded 41 times 
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Neptun
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Solvent-free saponification attempt
So, I came around to do the saponification without tedious extractions and solvent removals. My try was in a small scale (~1g furfuryl acetate used)
but tests seem to give green light on this procedure.
So what I did was to add a five-fold excess of KOH to some FFOAc and stir vigurously. This was done until a brown paste is obtained and the mixture
stops evolving heat.
Experimental
1g of Furfurfyl acetate were added to a beaker and ~2g of KOH (about 5 eq.) were dumped in at once. Immediately, the mixture took on a deep wine-red
colour. A Glass stirring rod was used to break up chunks of KOH and give the mixture a good stir. Significant heat and discolouration was observed
during this step.
A small sample was taken from this brown goo and dissolved in Ethyl acetate. The KOH and the brown impurity didn't dissolve in the solvent. From this
mixture, a TLC was done using Toluene as Eluent. Two spots could be observed, with the FFOAc spot being very slim and the bigger FFOH spot much more
visible.
The remaining brown mass was turned into a slurry with MTBE and stirred vigorously. The majority of solids did not dissolve in the solvent. From this,
TLC with Toluene showed a uniform FFOH spot.
From this experimental data, I'm very sure that the solvent-free saponification of furfuryl acetate is very promising as a less PITA way to obtain
furfuryl alcohol. In the future, I'll scale this process up and compare yields between the two runs.
Attachment: initial rxn mixture (1MB)
This file has been downloaded 42 times  Attachment: first tlc during rxn (577kB)
This file has been downloaded 42 times Attachment: tlc after rxn (866kB)
This file has been downloaded 43 times
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