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Stereochemical Studies of Fluorescent Tröger’s Bases
David E. Lewis, Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702
The Tröger’s base skeleton is a rigid framework containing two chiral nitrogen atoms
at bridgehead positions. Under acid catalysis, the ring system undergoes inversion,
but two mechanisms for the inversion have been proposed Our primary goal is to
use symmetrically substituted chiral Tröger’s bases to probe the mechanism of this
ring inversion. The synthesis of the required chiral compounds will require the
synthesis of the optically pure 4-amino-3-(1-hydroxyethyl)-N-alkyl-1,8-naphthal-imides,
and then condensing them. The synthesis of the required alcohols continues to be
problematic.
R
N
O
NH2 OH
N
O
N
R
N
O
O
N
R
O
Attempts to incorporate a carbon substituent at C-3
Substitution of the halogen in the 3-bromo-4-alkylamino-1,8naphthalimides has been accomplished by Ullmann-type
displacement with excess cuprous cyanide in refluxing DMF,
and by a photochemical Henry-type reaction in
nitromethane. The yields are modest for both reactions.
Our attempts at coupling the bromide with an alkynide anion
in DMF in the presence or absence of a palladium catalyst
have failed: in the absence of a heterogeneous palladium
(0) catalyst, reduction of the halide occurs; in the presence
of the catalyst, no reaction occurs at all.
During attempts to improve the yield of the Henry
reaction by displacing a nitro group with the
nitromethane anion, we found
that the 3-nitro-4alkylamino-1,8-naphthalimides are unusually acidic, and
that the nitro compound itself is subject to restricted
rotation around the C—N bond. Our attempts to
resolve the two atropisomers have so far yielded
ambiguous results.
If we can resolve these
atropisomers, this would be the first case of stable
atropisomers that do not possess a biaryl skeleton.
O