In 1969 the X-ray structure of VCR-methiodide [1,80] was determined,
leading to the general conclusion that the structure of both
VCR and VLB had thus been fully established. However, the motivation
to find more efficient but less toxic VLB analogues initiated the
exploration of the structure–bioactivity relationship of bisindoles,
leading to attempts to determine their conformational features in
solution. The earliest attempt to gain insight into the solution conformation
of VLB was performed by de Bruyn et al. in 1980 [56]. The
authors in this study gave complete 1H NMR assignments measured
at 360 MHz in different non-polar solvents and binary solvent mixtures
for VLB, the latter being used to avoid spectral congestion
in the 1H NMR spectrum. Based on the determined coupling constants,
the conformation of the velbanamine moiety in VLB using a
Karplus-type equation was deduced. This conformation included a
slightly flattened chair conformation for the piperidine ring with NC(
5) bond being in quasi-equatorial position, while the azanonene
ring was deemed to be in a boat-chair conformation, stabilized by
an intramolecular hydrogen bond between the C(16) COOMe and
the C(20) OH.