From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Tue Dec 14 1999 - 04:24:59 EST
Hola Luis Fernando,
The following are general suggestions without going too deep into the
nature of the system you have in hand. Hopefully they'll point to something
useful in the end.
1. Why not trying to follow the kinetics of the carbonyl-hemiacetal
transformations in solution, using optical rotation and/or possibly UV
spectrophotometry of the CO group? This may point to whether equilibrium
stops at one or two cyclizations, and possibly also which is the preferred
one.
2. Another approach that can be suggested, involving lots of labor, is
trapping the equilibrium mixture by fast freezing of the solution in liquid
nitrogen and getting rid of the solvent by lyophilization. Now you may try
to "fix" the structures with some fast acting reagents such as diazomethane
or dicyclohehylcarbodiimide (or more powerful ones that escape me now),
followed by separation and structural determination. If you are fortunate
enough as to getting no syrup after drying up the solvent, maybe you'll be
able to pick up single crystals of the original stuff for X-ray
crystallography.
All the best,
Jacob
> 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 sa=
y
>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!!
>
>
>==========================
============
>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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Prof. Jacob Zabicky Tel. 972-7-6461271/6461062/6472754
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