From: Richard Prankerd (richard.prankerd$##$vcp.monash.edu)
Date: Mon Sep 09 2002 - 19:22:25 EDT
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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