ORGLIST: Determination of contribution from a single form in an equilibrium

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From: Luis Fernando Garcia Alles (luis.garciaalles$##$ibc.unibe.ch)
Date: Mon Dec 13 1999 - 08:43:22 EST

        Dear all,
        My problem is basically what follows: I have done the oxidation of the
alcoholic group in C6 of D-glucose to the aldehyde form. The newly formed
compound has now two aldehyde functions. As a consequence of the presence
of the remaining OH groups of the sugar squeleton, several forms might
coexist in equilibrium in water. Thus, the sugar can addopt two basic
pyranose hemiacetal structures, depending on whether it is closed over the
C1 or the C6 aldehyde groups (--C5-O-C1--, or --C2-O-C6--). Furthermore, in
every form it exists still the chance to form the alpha or beta anomers. We
should also think that besides the pyranose forms, furanose rings might
also be present in the equilibrium, together with the opened form of the
sugar.
        The fact is that I would like to determine the contribution of one of
these forms to this equilibrium in water, and I do not know exactly how to
approach the problem. I have already thought about several possibilities,
but none of them seems to be completely suitable:
        1) Trap all the forms in equilibrium by a rapid reaction, let us say
permethylation with MeI, and then separate and/or quantitate the
contribution of every form. However, I do not know whether there is any
reaction fast enough as to allow to do such thing without modifying at the
same time the equilibrium (the reaction has to work in water on the other
hand).
        2) Studying the equilibrium by NMR might be easier, but when I see the
1H-NMR and specially 13C-NMR of this compound I feel really discouraged.

        Does anybody have any suggestion about how to solve this problem??

        Well, that is all, thank you very much in advance for any commentary or
suggestion. Have a very good day!!


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Luis Fernando García Alles, Ph.D.
Departement für Chemie und Biochemie
Universität Bern
Freiestrasse 3
CH-3012 Bern, Schweiz
Tel. ++41 (0)31/631 37 92
Fax ++41 (0)31/631 33 83
E-mail :garcia$##$ibc.unibe.ch
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