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Topic: unexpected precipitate in the alkylation of cysteine  (Read 4692 times)

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Offline Babcock_Hall

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unexpected precipitate in the alkylation of cysteine
« on: July 04, 2016, 11:05:46 AM »
We have previously alkylated cysteine with bromoacetylpyridinium bromide and two equivalents of base (this alkylation apparently takes several days at room temperature).  A typical protocol in our laboratory is to run the reaction in water at a concentration of about 80 mM in each reactant and to follow the reaction by means of the DTNB assay (Ellman's reagent).  I had to set this reaction up again this weekend, and so I thought I would try to alkylate cysteine with 2-Chloro-3′,4′-dihydroxyacetophenone (chloroacetylcatechol) and one equivalent with base, at the same time.  The reaction started showing precipitation in less than 30 minutes.

I have not done an exhaustive search of the literature, but there is a 1958 paper from B. R. Baker's laboratory on the alkylation of cysteine with chloroacetophenone (Goodman et al, JOC 23, 1251)  "A suspension of 1.0 g. (6.35 mmoles) of L-cysteine hydrochloride and 0.97 g. (6.35 mmoles) of a-chloroacetophenone in a mixture of 4ml. of water and 2 ml. of 95% ethanol was prepared. To the well- stirred mixture was added, dropwise, a solution of 0.84 g. (12.7 mmoles) of potassium hydroxide in 5 ml. of water. Upon complete addition of the base, 1.07 g. (68% yield) of compound II precipitated as a crystalline, slightly yellow compound. It could be recrystallized in poor yield from water on a small scale but decomposed rapidly on prolonged heating in water.  Purification of the material wm carried out by washing thoroughly with cold water and acetone. The material melted at 90-95' (dec.) and was homogeneous on paper using the t-butyl alcohol/formic acid/water system.  Compound II gave a faint yellow color with ninhydrin."

I am surprised that our reaction showed precipitation.  Could the precipitate be some other compound entirely?  The chemistry of alkylating cysteine is not without its surprises, in my experience.  For example, attempts to alkylate cysteine with chloroacetone (which sounds pretty straightforward) gives a reddish substance of unknown composition.  Even given the results above and assuming that the precipitate is the desired product, our reaction was performed at a lower concentration that the one from the Baker laboratory, and I would have expected our compound (a catechol) to be more soluble with the two hydroxyl groups than without.

We often purify amino acid derivatives with ion-exchange chromatography, but solubility may be an issue if we attempt that here.  However, I suppose we can cross that bridge when we come to it.

Offline wildfyr

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Re: unexpected precipitate in the alkylation of cysteine
« Reply #1 on: July 04, 2016, 11:45:33 AM »
Catechol oxidizes quickly in the presence of base and oligomerizes. Upon deprotonation of the phenol, a quinone forms and radical chemistry ensues. It is generally stable under acidic conditions. As I mentioned on your last post, I highly suggest you protect your catechol group. More hindered silyl ethers such as TBDMS, TIPS, and dimethylphenyl silyl ethers are all pretty water and base stable. They will damage your water solubility though, but a heterogenous reaction at moderate temperature in water might work. Or you could use MeCN or THF.

Offline Babcock_Hall

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Re: unexpected precipitate in the alkylation of cysteine
« Reply #2 on: July 04, 2016, 02:03:23 PM »
Thanks; I had forgotten about that suggestion.  Do you recall at what pH the oligomerization of catechol begins to be a serious issue? I have begun searching...

Offline Babcock_Hall

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Re: unexpected precipitate in the alkylation of cysteine
« Reply #3 on: July 05, 2016, 10:24:50 AM »
In the presence of up to 80 mM cysteine, I would think that a catechol-derived free radical would get consumed.

Offline Babcock_Hall

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Re: unexpected precipitate in the alkylation of cysteine
« Reply #4 on: July 05, 2016, 05:50:04 PM »
I checked both reactions today.  Both were nearly complete by DTNB assay, and the pH values of both were near 4.

Offline wildfyr

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Re: unexpected precipitate in the alkylation of cysteine
« Reply #5 on: July 05, 2016, 10:25:12 PM »
Great! Well keep in mind the pH dependence. Also catechol has oxygen stability issues.

This paper and its references (http://pubs.acs.org/doi/abs/10.1021/ma401594z) might be of use. I know the first author and can pass a question on if you have questions.

Offline orthoformate

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Re: unexpected precipitate in the alkylation of cysteine
« Reply #6 on: July 22, 2016, 01:37:06 PM »
There are lots of references for the Catecholic phenoxide to substitute for the chlorine.

Ti/Ni-Based Multimetallic System for the Efficient Allylation of Carbonyl Compounds
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By Martinez-Peragon, Angela et al From European Journal of Organic Chemistry, 2012(8), 1499-1503, S1499/1-S1499/18; 2012

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