Re: ORGLIST:meso and racemic?

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From: ROUSSEL Christian (roussel$##$spi-chim.u-3mrs.fr)
Date: Tue Sep 10 2002 - 08:33:36 EDT


Stephen and all,

We have used with success in many occasion chiral chromatography with
polarimetric detection to decide which of which was a meso versus a d,l
form. In many case it is not neccessay to have a baseline separation
between the enantiomers to observe a positive and negative deviation at the
polarimeter. Of course if you have a full separation it is more convenient.
In fact it is very rare to have no separation at the chirality detection
whereas a single UV trace is observed.
Best wishes
Christian Roussel

At 09:22 10/09/02 +1000, you wrote:
>Stephen and all:
>
>I appreciate the points that Stephen has made - he is certainly correct on
>the issue of availability of chiral compounds for comparison.
>
>The difficulty with chiral chromatography columns is that there are (or were
>when I last looked at this about 10 years ago) at least three different
>mechanisms for binding chiral compounds to chiral column stationary phases.
>In many cases a separation that works with one type of chiral column, e.g.,
>a cyclodextrin column, will fail dismally with a different type. I have seen
>investigators struggle for weeks or even months trying to develop a chiral
>separation, when the real problem was that the column in use was just not
>appropriate for the separation to be attempted.
>
>I suppose that a screen of different column separation mechanisms could be
>used to cover the difficulty raised above, but I am not aware of any in the
>literature.
>
>Regards
>Richard
>
>----- Original Message -----
>From: "Dr. Stephen J. Simpson" <S.J.Simpson$##$salford.ac.uk>
>To: <everybody$##$orglist.net>
>Cc: "Richard Prankerd" <richard.prankerd$##$vcp.monash.edu>
>Sent: Tuesday, September 10, 2002 8:42 AM
>Subject: Re: ORGLIST:meso and racemic?
>
>
> > Richard's points are very valid but only applicable if both the meso
> > compound and the racemic mixture/or one enantiomer are obtainable, as was
> > the case for tartaric acid. We are interested in the situation where only
> > a single compound can be obtained which may be the meso compound or a
> > racemic mixture; i.e an RS(SR) compound versus an RR + SS mixture.
> >
> > The use of an NMR chiral shift reagent which binds say, to only an R
>centre
> > will generate two sets of resonances in either case and so does not solve
> > the problem. Again if both the meso compound and racemic mixture/or single
> > enantiomer are available this technique can solve the problem because the
> > resonances due to the uncomplexed S centre will be different for the two
> > cases..
> >
> > Again in principle a chiral chromatography column which binds only to one
> > centre could work in that a single diastereomer would elute for the meso
> > compound, for the racemic mixture one diastereomer and one unchanged
> > enantiomer would elute, and for a single enantiomer only one compound
>would
> > elute which could be either a diastereomer or an unchanged enantiomer.
> > This is close to solving the problem in a perfect case.
> >
> > >Sylvie and all:
> > >
> > >Physicochemical properties of racemates, enantiomers and meso compounds
>are
> > >very often different. For example, the melting behaviour of the four
> > >tartaric acids are:
> > >
> > >D-tartaric: m.p. 168-170
> > >DL-tartaric: m.p. 206
> > >L-tartaric: m.p. 168-170
> > >meso-tartaric: m.p. 140 (also reported as 159-160, possibly through
> > >polymorphism)
> > >
> > >The D and L forms must always have the same melting point (and enthalpy
>of
> > >fusion, also solubility), but the racemate (and a meso form, if there is
> > >one) are physically different compounds. Other physical differences will
> > >persist in the solid state, although to a greater or lesser degree of
> > >discrimination, e.g., the IR spectrum in a KBr medium, the solubility in
> > >various solvents and the density. See the discussion by Eliel,
> > >Stereochemistry of carbon compounds, McGraw-Hill (1962), pp. 26, 43-47,
> > >which also states that some solution properties of the meso form are
> > >different from the enantiomers, such as the pKa and the dipole moment.
> > >
> > >Regards
> > >Richard
> > >
> > >Richard J. Prankerd, PhD
> > >Senior Lecturer
> > >Victorian College of Pharmacy, Monash University
> > >381 Royal Pde., Parkville VIC 3052
> > >
> > >Phone: INT+613-9903-9003
> > >Phax: INT+613-9903-9583
> > >
> > >Drugs need to be designed with delivery components in mind - Takeru
>Higuchi
> > >
> > >----- Original Message -----
> > >From: "Sylvie Ducki" <sylvie_ducki$##$yahoo.com>
> > >To: <everybody$##$orglist.net>
> > >Cc: <s.j.simpson$##$salford.ac.uk>
> > >Sent: Tuesday, September 10, 2002 2:13 AM
> > >Subject: ORGLIST:meso and racemic?
> > >
> > >
> > >>
> > >> Dear All,
> > >>
> > >> My colleague and I have been discussing a stereochemistry puzzle.
>Appart
> > >from using X-ray , how could you differentiate a meso compound from a
> > >racemic mixture of the two enantiomers? For example, tartaric acid. What
>if
> > >only one compound had been isolated?
> > >>
> > >> Dr Sylvie Ducki (and Dr. Steve Simpson)
> > >>
> > >>
> > >> ***************
> > >> Dr. Sylvie Ducki
> > >> email sylvie_ducki$##$yahoo.com
> > >> http://www.sciences.salford.ac.uk/chemist/Staff/sdres.htm
> > >> ****************
> > >>
> >
>
>
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