Ph. D. THESIS 2009

Figure 22. 13 C NMR APT spectrum of compound syn-5a (300 MHz, DMSO-d 6)
The small signal situated at δ=32.60 ppm is ascribed to the secondary carbon of
the bridge (9-C), whereas the bridgehead carbon atoms 1-C and 5-C appear at
different chemical shifts (δ==27.68 ppm and δ==29.56 ppm). Their
diastereotopicity can be explained in the light of the positioning at opposite
sides relative to the C3-C9-C7 plan, one of them being influenced by the
oximic OH groups and the other one by the lone pair of the nitrogen atoms.
At high frequency appears a low intensity signal assigned to the quaternary
carbon atoms 3-C and 7-C belonging to the oxime groups (δ==154.12 ppm).
Both 1 H NMR spectra of the syn and anti isomers display a 2H doublet in the
region 3.03-2.96 ppm , well separated but exhibiting close chemical shifts so
that allow to establish the sin/anti ratio in a mixture of isomers simply dividing
the integrated value of the doublet for each isomer. This fact give us the
possibility to investigate the synanti isomerisation.
2.3.2. NMR study of kinetics of the synanti isomerisation
Under acidic conditions the syn and anti isomers of dioxime 5 are in a running
equilibrium (Scheme 9; similar equilibria were observed for other dioximes,
too) [35].
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