April 17, 2021, 09:49:57 PM
Forum Rules: Read This Before Posting


Topic: work-up following deacetylation of aromatic esters  (Read 1159 times)

0 Members and 1 Guest are viewing this topic.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
work-up following deacetylation of aromatic esters
« on: February 02, 2021, 08:14:06 PM »
We need to remove two acetyl groups from aromatic oxygen atoms and replace them with hydrogens.  We found a paper that performed a similar deacetylation in the presence of our other functional group; and these workers used three different methods, including hydrazine hydrate.  They used HPLC on the milligram scale to purify their product.  We plan to work on the hundreds of milligrams scale, and preparative HPLC is not realistic for us right now.  Therefore, I was planning to use silica gel chromatography.  What I was wondering about is the work-up prior to chromatography.  We could try extraction (ethyl acetate vs. water, then brine).  I might be inclined to use two or three portions of ethyl acetate.  Another possibility is to remove the volatiles and strip with toluene.  However, acethydrazide is not volatile:  129 °C/18 mmHg (lit.); therefore, I am doubtful that rotary evaporation would do anything.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
Re: work-up following deacetylation of aromatic esters
« Reply #1 on: February 03, 2021, 09:40:42 AM »
There will also be unreacted hydrazine, as well as acethydrazide, at the end of the reaction.  One thing that I just noticed.  The paper that I am following used their NMR sample (the diacetylated aromatic compound) in DMSO as their reactant in the hydrazine step.  I am not sure that I need to use DMSO; maybe THF or DMF would be fine. 

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
Re: work-up following deacetylation of aromatic esters
« Reply #2 on: February 03, 2021, 02:05:56 PM »
I contacted one of the coauthors, and he suggested using an aqueous acid in the extraction, to pull hydrazine into the water phase.  Our other functional group is inert to ammonium chloride or TFA, and I cannot see why it would be acid-labile.  I found a pKa of acetylhydrazide of 3.24, which I presume refers to the ammonium form.  The pKa of hydrazine itself is 8.10.
https://chem.nlm.nih.gov/chemidplus/rn/1068-57-1
« Last Edit: February 03, 2021, 03:20:53 PM by Babcock_Hall »

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
Re: work-up following deacetylation of aromatic esters
« Reply #3 on: February 03, 2021, 10:25:29 PM »
DOI: 10.1002/ejoc.200800232   (M. Lang et al., European Journal of Organic Chemistry, (20), 3544-3551; 2008.  Wolfgang Steglich is a coauthor)
I did a quick SciFinder search and I found a synthesis of benzene-1,2,4-triol from its tris-acetylated precursor.  They worked under argon, and they used a slight excess of hydrazine, with methanol as their solvent.  They used 2 M HCl in their extraction (probably to remove hydrazine and its by-product, as I noted above) and dried the combined ethyl acetate layers with sodium sulfate.
« Last Edit: February 03, 2021, 10:39:05 PM by Babcock_Hall »

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
Re: work-up following deacetylation of aromatic esters
« Reply #4 on: February 04, 2021, 01:15:45 PM »
https://www.sigmaaldrich.com/catalog/product/aldrich/751855?lang=en&region=US
Is there any tendency for hydrazine to add into acetonitrile?  Given that one can buy hydrazine in acetonitrile, I would suspect not, but I get paid to worry about such things.  We need to dissolve our reactant in something, and alcohols (which were my first choice) were not terribly successful.

Regarding the HCl extraction in the Steglich reference, a second function of HCl is presumably to protonate the aromatic hydroxyl groups.
« Last Edit: February 04, 2021, 02:11:55 PM by Babcock_Hall »

Offline Guitarmaniac86

  • Full Member
  • ****
  • Posts: 227
  • Mole Snacks: +27/-2
  • Gender: Male
  • Medicinal Chemist
Re: work-up following deacetylation of aromatic esters
« Reply #5 on: February 04, 2021, 02:33:28 PM »
Regarding hydrazine adding to MeCN I highly doubt it would, and the fact that it can be bought in MeCN means it must be stable in MeCN. The only way I can see it reacting is if you had a strong enough base to deprotonate on the methyl group. I doubt though, that that "enolate" would readily form.

Are you able to use other bases to remove the acetyl group such as NaOMe or Methylamine in methanol?

Don't believe atoms, they make up everything!

Offline rolnor

  • Chemist
  • Sr. Member
  • *
  • Posts: 1311
  • Mole Snacks: +88/-5
Re: work-up following deacetylation of aromatic esters
« Reply #6 on: February 04, 2021, 04:21:51 PM »
I have removed one acetyl group from a ohenolic OH with just NH3 in methanol, I dont think you need hydrazine. The acetamide product can be removed with vacuum.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
Re: work-up following deacetylation of aromatic esters
« Reply #7 on: February 04, 2021, 04:53:04 PM »
Rolnor, I have seen ammonia used to remove two acetyl groups in the same paper from which we originally took the hydrazine method (not the Steglich reference).  Right now that procedure is looking more attractive than it had before.  On the other hand, perhaps we have thought through most of the bugs of the hydrazine method.  Ammonia and its by-product are more volatile.  Hydrazine can be added in only slight excess, but I have not checked the mole to mole ratio in regards to ammonia.

Guitarmaniac, the third method in the paper we are following (which has the same functional group as our molecule) was catalytic methoxide in methanol, but they did not specify exactly how much methoxide.  In effect we flipped a three-sided coin to pick one method.  Methylamine crossed my mind but I did not look into it much.  Does it have anything to recommend it or any drawbacks, in your opinion?

Offline Guitarmaniac86

  • Full Member
  • ****
  • Posts: 227
  • Mole Snacks: +27/-2
  • Gender: Male
  • Medicinal Chemist
Re: work-up following deacetylation of aromatic esters
« Reply #8 on: February 06, 2021, 04:11:33 AM »
Hi Babcock_Hall, sorry for the very delayed reply! I have been very busy at work.

When I use catalytic methoxide I use about 10 mol% and it is a good solid methodology for ester deprotections but methylamine in ethanol or methanol is better because of the nucleophilicity of the amine.

The benefits of methylamine are that the reaction is quick but you do need several equivalents to get full deprotection. For one of my molecules I had two benzoyl ester protecting groups on a di-phenolic compound and I used methylamine in ethanol at 4 equiv. The reaction proceeded well and by LCMS I got full conversion and a yield of 89%.

The main drawback of methylamine is that if the reaction is sluggish, you cant heat the reaction beyond 40 odd degrees (from experience) as the methylamine boils off.
Don't believe atoms, they make up everything!

Offline rolnor

  • Chemist
  • Sr. Member
  • *
  • Posts: 1311
  • Mole Snacks: +88/-5
Re: work-up following deacetylation of aromatic esters
« Reply #9 on: February 06, 2021, 08:54:54 AM »
I have used 5% NH3 in ammonia, its fast att ambient temperature.
I think if you use methoxide you get a minus-charge on the phenol when the first acetyl is hydrolyzed, this will repell the methoxide anion and hinder the second acetyl to be hydrolyzed.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
Re: work-up following deacetylation of aromatic esters
« Reply #10 on: February 06, 2021, 09:45:30 AM »
rolnor,
Good point about repulsion.  Do you mean in water?

Guitarmaniac86,
I wonder if hydrazine is more nucleophilic than ammonia in this reaction, owing to the α-effect.  If so, it might be advantageous in some circumstances.  However, a quick skim of the topic leads me to believe that whether or not one observes enhanced nucleophilicity depends upon the reaction.
https://pubs.acs.org/doi/10.1021/jo301497g

Offline rolnor

  • Chemist
  • Sr. Member
  • *
  • Posts: 1311
  • Mole Snacks: +88/-5
Re: work-up following deacetylation of aromatic esters
« Reply #11 on: February 06, 2021, 12:59:43 PM »
You will get phenoxide anion in any solvent if you hydrolyze with methoxide. The phenoxide anion will repell methoxide from attacing the second acetylester because its so close. Hydrazine is a more powerfull nucleophile but its probably not neccesary.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
Re: work-up following deacetylation of aromatic esters
« Reply #12 on: February 06, 2021, 02:00:12 PM »
333
I have used 5% NH3 in ammonia, its fast att ambient temperature.
Hi rolnor,

Do you mean 5% NH3 in water or in some other solvent?

Offline rolnor

  • Chemist
  • Sr. Member
  • *
  • Posts: 1311
  • Mole Snacks: +88/-5
Re: work-up following deacetylation of aromatic esters
« Reply #13 on: February 06, 2021, 03:30:01 PM »
No, you dissolve ammonia gas in MeOH. You can use a very large excess. You can buy 2M NH3 in MeOH I think.

Offline Babcock_Hall

  • Chemist
  • Sr. Member
  • *
  • Posts: 4616
  • Mole Snacks: +279/-20
Re: work-up following deacetylation of aromatic esters
« Reply #14 on: February 08, 2021, 09:57:38 AM »
rolnor,

Thank you; your comments are always helpful and thought-provoking.  I re-read the paper in question, and they used 2 M ammonia in methanol as one of their three methods.  In my case, the two oxygen atoms are in a 1,4 (para) relationship; therefore, repulsion may not be as bad as when they are in an ortho relationship.

Sponsored Links