Brain&Force
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Production of chiral alcohol enantiomers
I think it'd be fun to prepare some enantiomerically enriched (or preferably pure!) 2-butanol, but I'm trying to find some ways to do it.
One way I've run into is the baker's yeast reduction of methyl ethyl ketone, but this leaves a few problems. First, is there reasonable
enantioselectivity? Second, what isomer is selected for? That can probably be settled by an optical rotation experiment.
The other method I'm considering is chromatography of diastereomeric esters. And there's an easily available source of enantiomerically pure
carboxylic acids - amino acids! Quite a few are available, including L-threonine, L-tryptophan, L-aspartic acid, D-aspartic acid, L-lysine,
L-tyrosine, and L-methionine. I'm thinking L-tyrosine and L-tryptophan will be the best options for anyone with TLC plates (I may be able to get some
with the useful F254 indicator), though this may not be applicable to everyone.
Either way, I'd be more interested in the second route, because it would then be possible to separate both enantiomers, and I'd like to be able to
demonstrate optical rotation with them.
At the end of the day, simulating atoms doesn't beat working with the real things...
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Metacelsus
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Tartric acid was used historically (you can get it from cream of tartar)
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CharlieA
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I am curious: do you have a reference to the MEK reduction with yeast? My guess (and it is just that) is that a racemic mixture (equal amounts of each
diasteriomer) would be produced or just one of the diasteriomers would be produced. I would guess it depends on the site on the yeast where the
reduction takes place; if the site of the reaction is chiral, would this produce just 1 of the possible enantiomers?
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NitreRat
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https://en.wikipedia.org/wiki/Biocatalysis#Asymmetric_biocat...
I realize the wiki doesn't give a proper reference but asymmetric reduction of ketones with yeast is definitely a thing.
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CharlieA
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Thanks for the link. I saved it and now I just have to read (and understand) it! Bugs (my generic name for enzymes) are remarkable. (Is there anything
that is not on Wikipedia? Just an idle speculation...) It will be interesting to hear how your experiments turn out. Good luck with them.
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Dr.Bob
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There are many ways to make or separate the isomers of chiral alcohols. For smaller ones, like 2-butanol, chiral reductions are tough, but some
enzymatic ones will work, one example uses whole yeast, which is messier, but works to some degree. Another method is to use lipase enzymes to
either make or hydrolyze esters of the alcohol. That can work pretty well, but not trivial to do. The yeast is likely the simplest route to do at
home, try googling "chiral yeast reduction of ethyl acetoacetate" in "google scholar" to find some papers. Not exactly the same, but this is a well
documented example of the concept.
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karlos³
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If you want to use yeast, it is advised to immobilise it with alginate, makes it easier to remove the yeast afterwards, less product gets trapped in
the biomass, and also the yeast can stand a higher concentration of substrate without getting harmed.
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AvBaeyer
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The chromatographic method you suggest is probably the best way to go if you have access to the right equipment. Instead of making an ester with an
amino acid, consider instead making an ester using ibuprofen. With an amino ester you have the problem of the amino group polarity which will make
your chromatography more difficult though you could block the amino group via acylation. With ibuprofen you won't have that problem. Note that in the
US commercial ibuprofen tablets are a single isomer so you get a chiral acid to start with. The diastereomeric esters from 2-butanol should be readily
separable on silica gel. Moreover, the ibuprofen esters will be detectable by uv. You could also consider naproxen as the chiral acid. Just some
thoughts.
AvB
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