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conditions for formation of Schiff-Base

Frank Fuerst ffrank at rz.uni-potsdam.de
Mon Dec 13 10:26:39 EST 1999

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Hi, I'm a biochemist who has problems synthesizing a fluorescently
labeled sugar molecule.

I want to make a Schiff-Base (imine) of a saccharide (the aldehyde
form of the terminal Glc of Maltose) and the amino group of L-aspartic
acid-4-(4-amino-7-methylcoumarin) (and later I want to reduce it with
NaCNBH3). The coumarin is the fluorophor, and I need the aspartic acid
as a spacer and because of it's charge.

Problem: The receipt I have been working with (according to Prakash-C,
Vijay-IK (1983), Analytical Biochemistry _128_, 41-46) is designed for
aromatic amines which are stronger nucleophiles. Additionally, the
flourescent label is very poorly soluble.

The old conditions for the reaction: Solvent is methanol, 4 % (v/v) as
catalyst, 1h at 40°C

This aspartyl-aminocoumarin is nearly insoluble in methanol, H20,
MeCN. I managed to dissolve it by adding conc. HCl (some %v/v). I
assume this helps because the zwitterion is protonated to a cation by
HCl, and a zwitterion with a hydrophobic moiety is poorly soluble. For
clarity, here comes the 90°-ASCII-formula of the molecule, without the
zwitterionic protonation/deprotonation:

                 |   ||    |  (please apologize for the rect-
                 C=C-C--C==C   angular six-membered rings and 
                        |      the strange double bonds...)

Well, now I've got the molecule in solution, but since it's
protonated, it is no longer nucleophilic (I assume that it's the N on
the left that would react, am I right?). I have tested that, with HCl
the yield is 0 %.

But with the old conditions (HOAc), one of the reagents would stay
undissolved, which would slow down the reaction very much. But at
least, in a pretest, I got a minimal yield with that method.

What conditions would you recommend? The old ones, only incubating
much longer? Or something else, perhaps phase transfer catalysis?

Thank you in advance, Frank

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