The first full proton and carbon assignment of VCR was reported
in 1989 [64]. In this study Rao et al. wrote the following sentence
discussing the 1H NMR properties of the two amid-rotamers:
“the intensity of the most deshielded of the resonances for H-
12 corresponds with the most deshielded of the resonance for
H-2 and the formyl proton”. Although two further studies dealing
with the NMR properties of VCR appeared in the literature
[65], neither of them discussed this ambiguous statement, nor did
they give the assignments of the two isomers. In fact we must
contradict to this statement on both an experimental and a theoretical
basis. Analogously to that described for strychnobrasiline
[66] which contains a similar molecular moiety, protons towards
which the carbonyl oxygen is directed should be deshielded. Thus,
H-12 should be deshielded in the case of the Z-isomer, and H-
2 should be deshielded in the case of the E-isomer. As a result
the three deshielded resonances mentioned above cannot correspond
to the same isomer. In order to clarify this problem
the 1H NMR spectrum of VCR was recorded in CDCl3 on our
800 MHz NMR spectrometer (Fig. 11). The observed amid-rotamer
ratio was ca. 3:2. It is already obvious from the integral values
that the more deshielded formyl proton corresponds to the more
shielded H-12 and to the more deshielded H-2, and vice versa
(Fig. 11).