V. Haridas et al. / Tetrahedron Letters 53 (2012) 623–626
625
H
N+
O
O
+
N
+
O
H
N
N
N
N
OCH3
HN
O
N
H
H
O
HN
O
HN
O
O
HN
O
OCH3
HN
OCH3
O
H3CO
O
O
OCH3
OCH3
Boat-Chair
Chair-Chair
Chair-Chair
Figure 3. Various protonation states of 6a.
(a)
In the 1:2 mixture of 6a and methanesulfonic acid, the Gly
methylene proton peaks were at d 3.76 and 4.59 ppm, indicating
a lack of symmetry in the structure. The 13C NMR of diprotonated
6a showed two peaks each for amide and ester carbonyls, while
unprotonated 6a displayed a single peak for each. These observa-
tions and the considerable change in the 1H NMR spectrum of
diprotonated 6a supported a conformational change from chair–
chair to boat–chair. This observation also corroborates previously
reported data on other bis-derivatives of bispidine.14 The repulsion
between the two positively charged nitrogen atoms as a result of
double protonation may potentially be the reason for the change
in conformation from chair–chair to boat–chair. Such a conforma-
tional change can be used as conformational switch that might be
useful in orchestrating biological processes.
H
O
N-Z
N
N
Z-N
H
O
H
O
N-Z
N
N
Z-N
H
O
H
O
N-Z
N
N
Z-N
H
O
In the second design, peptide chains were anchored in a similar
fashion to the bispidine scaffold via amide linkages (Scheme. 2).
The syntheses of 7a–e incorporated this design using amide link-
ages to connect the peptide units to bispidine. Additionally, unlike
the previous design, the bispidine nitrogen atoms are not part of
the amino acid moiety (Scheme 1).
We envisioned that the amide-linked bispidine might provide
additional rigidity as a result of the amide bond, forcing the pep-
tide chain to adopt an extended conformation. The CD spectra of
7b, 7d and 7e in methanol displayed minima at approximately
226 nm, supporting a b-sheet type conformation (Figs. S12–S14).
15 FT-IR studies revealed an amide I band in all of these compounds
at ꢀ1630 cmÀ1 and a large coupling constant (ꢁ8.4 Hz) for the
amide NHs, which are consistent with the extended sheet-like con-
formation. The FT-IR studies in chloroform of 7c showed only one
peak at 3434 cmÀ1, establishing the absence of any intramolecular
hydrogen bonding (Fig. S15).
(b)
Figure 4. (a) Self-assembling pattern observed in the crystal structure of 7b; (b)
Schematic representation of assembly of 7b in the solid state.
ably due to better solvation.12 The internal rotation of the amide
bond of bispidine diamides with appended amino acid units was
studied using dynamic NMR. The rates of exchange of syn–anti
transformation of 7c in CDCl3 were measured using selective and
non-selective inversion techniques.17 The bridgehead CH (d 2.11)
of the major conformer was selectively inverted, and the spectra
were recorded at various times. Analysis of the build up of magne-
tization at the exchanged rate was used to determine the rate con-
stant for the exchange. This was repeated at various temperatures
and the temperature dependent rate constant was determined
using Arrhenius equation. The activation energy barrier (Ea) and
the standard enthalpy of activation (D
Hà) between the syn and anti
Compound 7b was crystallized from methanol, and the X-ray
crystallographic studies (Fig. 4) unambiguously indicated a beta
sheet like arrangement in the solid state. The two piperidine rings
are in the chair conformation with two bispidine nitrogens at a dis-
tance of 2.813 Å. The two carbonyl groups are arranged anti to each
other. Further examination of the crystal structure (Fig. 4) showed
that the molecules of 7b are connected into an infinite sheet
through pairs of N–HÁ Á ÁO@C bonds (NÁ Á ÁO 2.833 Å, HÁ Á ÁO 1.994 Å,
conformer were calculated using Arrhenius plot and determined to
be 92.36 kJ/mol and 89.683 kJ/mol, respectively (Figs. S8–S10).
These values are comparable to the rotational barrier observed in
proline cis/trans isomerization.18 The thermodynamic parameters
obtained from dynamic NMR studies show that the diamides of
bispidine have negligible influence on the rotation of the other
amide and are very close in energy to the rotational barrier of
proline.
angle 164.7°). The Ca to Ca distance between the leucine residues
in 7b is 4.598 Å. The 10-membered hydrogen-bonded rings are
similar to the antiparallel beta sheet-like structure. There are very
few examples of templates that can induce beta sheet through
intermolecular interactions.16
Interestingly, the NMR spectrum of 7b in CDCl3 displayed peaks
corresponding to syn and anti forms, indicative of restricted rota-
tion around N–CO bonds.13 Upon changing the solvent to DMSO-
d6, compound 7b exhibited signals for only the syn conformer.
The N-protecting groups have a significant role in the conformer
ratio, as evident from the Boc-protected derivative 7c that showed
an approximate ratio of 1:3 of syn:anti in CDCl3 compared to the
1:1 syn:anti ratio demonstrated by 7b. The syn conformer, with a
higher dipole moment, is favored in more polar solvent, presum-
The amide-linked bispidine-based peptides do not show any
conformational changes with the addition of methanesulfonic acid,
as evidenced by NMR and CD spectroscopy. This outcome was ex-
pected because the nitrogen atoms are part of the amide bonds and
the lone pairs are delocalized, making the bispidine diamide deriv-
atives (7a–e) stable peptide mimetics.
In conclusion, we have demonstrated that bispidine is a versa-
tile scaffold that can induce specific secondary structure on the at-
tached peptides, depending on the nature of the linkage that
connects the peptide to the bispidine scaffold. The hydrophobic
nature of the scaffold is an additional advantage for designing
membrane active peptide analogs, viral and bacterial inhibitors.
The environmentally triggered conformational change and the