the preferred conformation of the nine-membered ring was also
Fig. 13. Tube representation of the molecular structure of the vindoline half (left)
and the velbanamine half (right) of VLB reproduced from the X-ray structure [82].
deduced (Fig. 12) and the spatial relationship of the velbanamine
and vindoline halves was determined, showing that the characteristic
C(17) C(18) C(15) C(16) dihedral angle was in the range of
140–170◦. This was the first indication that the structure of VLB in
solution was very similar to that found in solid state [1,82] (Fig. 13).
Investigating further the influence chemical modifications on
conformation, in 1986 a 500 MHz 1H NMR study was reported on
C(20)-deoxy C(20)-epivindesine [83]. In this work, the solution
conformation of this bisindole derivative (Fig. 1) was discussed on
the basis of 1H, COSY and NOESY data. The overall conclusion was
that the loss of the OH function and the epimerization of C(20)
resulted only in a slight change in the conformation as compared to
that described for VLB. The piperidine ring in this structure shifted
from being half-chair towards a boat conformation. It should be
noted that, in accordance with X-ray analysis conducted later,
Hunter et al. [81] described, as it was already mentioned, an almost
chair conformation for this piperidine ring. Moreover the authors
in this work did not consider a conformation where the N lone pair
was in an equatorial position.
Continuing with these investigations, an analysis of the solution
behaviour of AVLB and its borane complex was conducted by
Szántay et al. [84] and by Andrews and co-workers [85]. In the
former paper, an NMR investigation including the use of 1H 1H
NOE measurements in the presence of 11B decoupling were performed
together with COSY and 13C 1H heterocorrelation methods
for AVLB and its borane complex using a 400 MHz spectrometer.
It was found as a part of this work that the borane complex is
surprisingly stable as compared to AVLB, which is easily oxidized
by air to leurosine. Besides this, the detailed NMR investigation of
the borane complex revealed that the conformation taken by this
complex in solution is markedly different from that described for
its “monomeric” analogue, i.e. the borane complex of cleavamine
[86]. For cleavamine it was shown that two isomers differing in
the position occupied by the N(4) lone pair exist and they show
very different thermodynamical stability. The “natural” one (the
N lone pair is in an axial position) is thermodynamically more