[AR] Re: ... Coronavirus
- From: Peter Fairbrother <peter@xxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 24 Mar 2020 20:27:29 +0000
On 24/03/2020 19:04, Barry Jolly wrote:
At the time thalidomide was synthesized, researchers did not know how to
chemically separate a racemic mixture of stereoisomers.
We did know how (yes, I am that old), but it was expensive to do - eg
you made a salt of a racemic mixture with natural tartaric acid, which
is d (or l, or R or S, I forget - the point is that the natural product
is is only one of two possible optical isomers) so that you get two
compounds which are
d-thalidomide d-tartrate and
l-thalidomide d-tartrate.
Because these diastereoisomers are not mirror images of each other -
think a right hand with a right foot and a left hand with a right foot -
they will have slightly different physical properties, and you can
separate them with eg fractional crystallisation or something like that.
Which takes ages and is hard to do, as the emphasis here is on
_slightly_ different physical properties. There were other methods
including things like getting bacteria to selectively eat one isomer,
but with few exceptions all these methods were difficult and time
consuming, which made single stereoisomer products very expensive.
It is still hard to separate stereoisomers, though less so than it was.
However stereospecific synthesis, where you only make one stereoisomer
of thalidomide or whatever, has been all the rage in drug manufacture
for the last twenty or thirty years.
I heard there was something about stereospecific synthesis in Breaking
Bad, but I have never seen it (I do not have a TV, though if I am to be
stuck inside for some months I might buy one). But for sure chemists
have developed lots of stereospecific syntheses for methedrine, and many
other compounds, over the last 20 or 30 years.
Thalidomide is nowadays again used for some niche applications. I do not
know the isomeric composition, but it is very definitely not given to
possibly-becoming-pregnant women.
Peter Fairbrother
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