Oligomers of β-amino acid bearing non-planar amides form ordered structures

Article abstract of DOI:10.1016/j.tet.2006.09.062

In this report, we explore the feasibility of using bicyclic chiral β-amino acids, (1R,2R,4S)- and (1S,2S,4R)-7-azabicyclo[2.2.1]heptane-2-carboxylic acid (R-Ah2c and S-Ah2c, respectively), to prepare novel peptides with unique properties. Facile cis-trans isomerization of the non-planar amide bonds of these β-amino acids should result in great flexibility of the backbone structure of β-peptides containing them. Indeed, oligomers of these amino acids showed thermostability and characteristic CD absorptions, which were not concentration-dependent, suggesting that the oligomers remained monomeric. The results indicated the formation of self-organized monomeric structures with chain-length-dependent stabilization. Energy calculations suggested that the peptides can take helical structures in which the energy barriers to cis-trans isomerization are greater for the central amide bonds than for the terminal amides.

Full text of DOI:10.1016/j.tet.2006.09.062

Tetrahedron 62 (2006) 11635–11644
Oligomers of b-amino acid bearing non-planar
amides form ordered structures
b
b
c
Yuko Otani,a Shiroh Futaki,b, Tatsuto Kiwada, Yukio Sugiura, Atsuya Muranaka,
*
Nagao Kobayashi,c, Masanobu Uchiyama, Kentaro Yamaguchi and Tomohiko Ohwada
a
d
a,
*
*
aGraduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
bInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
cDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
dFaculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan
Received 22 July 2006; revised 19 September 2006; accepted 19 September 2006
Available online 19 October 2006
Abstract—In this report, we explore the feasibility of using bicyclic chiral b-amino acids, (1R,2R,4S)- and (1S,2S,4R)-7-azabicyclo[2.2.1]-
heptane-2-carboxylic acid (R-Ah2c and S-Ah2c, respectively), to prepare novel peptides with unique properties. Facile cis–trans isomeriza-
tion of the non-planar amide bonds of these b-amino acids should result in great flexibility of the backbone structure of b-peptides containing
them. Indeed, oligomers of these amino acids showed thermostability and characteristic CD absorptions, which were not concentration-
dependent, suggesting that the oligomers remained monomeric. The results indicated the formation of self-organized monomeric structures
with chain-length-dependent stabilization. Energy calculations suggested that the peptides can take helical structures in which the energy
barriers to cis–trans isomerization are greater for the central amide bonds than for the terminal amides.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
have been employed. Such structures would diminish the
loss of entropy of the protein when an ordered structure is
formed by enthalpy-driven intramolecular interaction.4
Amide bonds in a peptide backbone usually take a planar
structure, and this structural feature can be considered to
compensate, in some respects, for the entropic loss upon
the formation of higher-order structures.1,2 On the other
hand, the planar amide structure also restricts the structural
design of peptides, because if we could employ a non-planar
amide backbone structure, novel high-order structures might
be designable.
Recently, design of b-peptides, i.e., peptides containing
b-amino acids, has been attracting attention because of the
feasibility of derivatization with various side chains at either
or both of the a- and b-positions.1,2 This extends the range of
peptide scaffolds potentially available for the creation of
novel functional molecules and materials. Since b-peptides
are not readily susceptible to proteolysis, they are also prom-
ising platforms forthedesign of new pharmaceuticals, includ-
ing enzyme inhibitors, receptor agonists, and antagonists.3
b-Peptides, comprised of b-amino acids, have an extra
methylene group along the backbone, compared with natural
proteins. This extra methylene group provides more scope
for the introduction of functional side chains on the peptide
backbone, but simultaneously results in greater flexibility of
the peptide backbone. Therefore, in order to create a spatially
ordered structure, methods to reduce the flexibility of the
peptide backbone, such as introduction of 2-aminocyclohex-
anecarboxylic acid,2c b-prolines2d or bulky substituents,1a
Bicyclic 7-azabicyclo[2.2.1]heptane amides (Fig. 1a) take
intrinsically non-planar structures, i.e., they exhibit nitrogen-
pyramidalization and twisting with respect to the N–C(]O)
bond, in the solid,5a solution,5c and gas (computation)
phases.5b,c The tilt angles (a) of N-aroyl-7-azabicyclo[2.2.1]-
heptanes range from 26 to 31ꢀ in the X-ray crystal structures.5
Non-planarity of these amides in solution has also been sug-
gested on the basis of the reduction of rotational barriers
to the cis–trans isomerization of the amide bonds (see
Fig. 4a). These dynamic features of this skeleton have been
proposed to stem from: (i) angle strain of the bicyclic ring
structure and (ii) 1,3-allylic type strain induced by the repul-
sion between the bridgehead protons and the amide moiety.
The rigid bicyclic ring structure should also be beneficial to
diminish the entropic loss if oligomers of these amino acids
form higher-order structures. Although employment of
Keywords: Non-planar amide; b-Amino acids; Oligopeptides; Circular
dichroism; Ordered structure.
* Corresponding authors. Tel.: +1 81 3 5841 4730; fax: +1 81 3 5841 4735
(T.O.); fax: +81 22 795 7719 (N.K.); fax: +81 774 32 3038 (S.F.); e-mail
addresses: ohwada@mol.f.u-tokyo.ac.jp; nagaok@mail.tains.tohoku.ac.
jp; futaki@scl.kyoto-u.ac.jp
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.09.062

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Y. Otani et al. / Tetrahedron 62 (2006) 11635–11644
such amino acids,7 we have investigated in this study the
(a)
R1
(b)
R1
R
N
O
N
properties of oligomers of chiral b-amino acids based on
the 7-azabicyclo[2.2.1]heptane skeleton, i.e., (1R,2R,4S)-7-
azabicyclo[2.2.1]heptane-2-carboxylic acid (R-Ah2c) and
(1S,2S,4R)-7-azabicyclo[2.2.1]heptane-2-carboxylic acid (S-
Ah2c) (Fig. 1b). We show that oligopeptides of our b-amino
acids, having enhanced nitrogen inversion and facile cis–trans
amide isomerization, do form ordered monomeric structures,
suggesting the feasibility of employing peptides composed
of amino acids bearing non-planar amide bonds as novel
peptide scaffolds.
O
H
H
H
N
H
R2
O
R2
n
n
7-Azabicyclo[2.2.1]heptane
amide
(R-Ah2c)n
(S-Ah2c)n
(c) cis-(S-Ah2c)8-NH2
2. Results and discussion
2.1. Synthesis of bicyclic b-amino acids and their
homopeptides
(d) trans-(S-Ah2c)8-NH2
Racemic amino acid 5 was synthesized by modifications of
the method of Zhang and Trudell,8 and the enantiomers 6a
and 6b were each obtained in enantiopure form by repeated
recrystallization after introducing Oppolzer’s camphorsul-
tam on the carboxylic acid moiety as a chiral auxiliary
(Scheme 1).9
Figure 1. (a) 7-Azabicyclo[2.2.1]heptane amide and (b) Ah2c peptides.
n¼1, 2, 3, 4, 5; R1¼H$HCl, R2¼OH; HCl$H-(S-Ah2c)n-OH. n¼4, 5, 8;
R1¼H, R2¼NH2; H-(S-Ah2c)n-NH2. n¼8; R1¼H, R2¼NH2; H-(R-
Ah2c)n-NH2. Side view and top views of (c) cis- and (d) trans-H-(S-
Ah2c)8-NH2 obtained by B3LYP/6-31G* calculations.
For the synthesis of the oligomers of this amino acid, the Boc
derivatives (5-2R and 2S) and the Fmoc derivatives (9-2R
and 2S) were prepared. Since these amino acids have highly
hindered and strained structures, we first examined whether
construction of oligomers was possible by using solution-
and solid-phase methods. Solution-phase synthesis was
employed for the preparation of the dimer to pentamer of
S-Ah2c. The benzyl ester of S-Ah2c was obtained by remov-
ing the Boc group from 7-2S, and this was condensed with
Boc derivatives of S-Ah2c (5-2S) using the hydrochloride
such non-planar-amide amino acids is expected to broaden
the scope for design of novel peptide-based scaffolds, few
studies have been reported on the structures of homo-
oligomers with enforced non-planar amides.6 Thus, in order
to obtain basic information about the influence of amide non-
planarity on the formation of high-order structures as a step
toward the design of functional molecules composed of
Scheme 1. Synthesis of bicyclic amino acids.

Y. Otani et al. / Tetrahedron 62 (2006) 11635–11644
11637
salt of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDCI$HCl) in DMF (n¼2, 3, 4, 5, R1¼tert-butoxycar-
bonyl, R2¼OCH2Ph). The repetitive removal of the N-termi-
nus of the oligomer and introduction of the Boc derivative of
S-Ah2c, followed by the final deprotection of the N-terminal
tert-butoxycarbonyl and C-terminal benzyl groups (n¼1, 2,
3, 4, 5, R1¼H$HCl, R2¼OH) with catalytic hydrogenation
and 4 N HCl treatment, yielded the desired oligomers of
S-Ah2c. Alternatively, solid-phase synthesis using the Rink
amide resin and the benzotriazolyloxotris(pyrrolidino)-phos-
phonium hexafluorophosphate (PyBOP)–N-hydroxybenzo-
triazole (HOBt) coupling procedure was employed for the
preparation of the 4-, 5-, and 8-mers of S-Ah2c (n¼4, 5, 8,
R1¼H, R2¼NH2).9 After construction of the peptide chains,
the peptide-resin was treated with trifluoroacetic acid in the
presence of ethanedithiol, followed by HPLC purification to
give highly pure peptides. The octamer of R-Ah2c was simi-
larly prepared by the solid-phase procedure (n¼8, R1¼H,
R2¼NH2). All syntheses proceeded without difficulty. This
suggested that the conventional solution- and solid-phase
methods are applicable to this amino acid, in spite of the steric
hindrance due to the rigid side chain.
2.2. CD and UV spectroscopic studies of oligomers:
chain-length dependence of spectra
In order to study whether the oligomers of R-Ah2c can gen-
erate structure in solution, the circular dichroism (CD) spec-
tra in methanol (100 mM) were recorded. A characteristic and
intense CD spectrum of the octamer H-(R-Ah2c)8-NH2 was
obtained in the far-UV region (190–260 nm) (Fig. 2a). The
mean residue ellipticity of H-(R-Ah2c)8-NH2 shows a maxi-
mum at 198 nm and a minimum at 217 nm. This spectrum
does not coincide with those of typical structures of natural
peptides (composed of a-amino acids), but resembles
those observed for helical b-peptide oligomers based on
b-proline2d and those of helical b-peptide oligomers with
amide protons based on trans-2-aminocyclopentanecarboxy-
lic acid2c or b2,3-peptide (compound 7c in Ref. 1a), reported
previously.
Figure 2. (a) CD spectra of H-(R-Ah2c)8-NH2, HCl$H-(S-Ah2c)1–5-OH, and H-(S-Ah2c)8-NH2 measured at 100 mM in MeOH at 20 ꢀC. (b) Dependence of CD
intensity at 217 nm on the number of oligomers of (S-Ah2c)n, derived from (a). Band deconvolution of the UV and CD spectra of (c) HCl$H-(S-Ah2c)-OH
(monomer) measured at 100 mM in MeOH and (d) HCl$H-(S-Ah2c)4-OH (tetramer) measured at 100 mM in MeOH. Sample cells of 1-mm path length
were used for CD measurement. Black line: experimental spectra of Ah2c monomer or tetramer measured at 100 mM in MeOH at 20 ꢀC (CD) and at rt
(UV), red line: p–p* transition, blue line: n–p* transition, pink circles: simulated spectra.

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Y. Otani et al. / Tetrahedron 62 (2006) 11635–11644
The octamer peptide of the enantiomeric configuration,
H-(S-Ah2c)8-NH2, showed a symmetric spectrum with
respect to that of H-(R-Ah2c)8-NH2 in terms of the Cotton
effects. This suggests that, even though these enantiomeric
amino acids have flexibility in the amide structure, their
oligomers converge in terms of higher-order structures;
this suggests the induction of thermodynamically stable con-
formations. Of particular interest is the observation of an
apparent length-dependent trend in the CD spectra of the
S-Ah2c oligomers: in the CD spectra of the trimer, tetramer,
pentamer, and octamer of S-Ah2c, the positions of the
minimum (198 nm) and the maximum (217 nm) are similar,
and the intensity per residue (values normalized for con-
centration and the number of residues) at both the minimum
and the maximum increase as the number of monomer units
increases (Fig. 2b).10 These oligomers also showed an isodi-
chroic point (see below). This observation strongly suggests
that CD-active regular secondary structures are induced and
are increasingly favored as the peptide chain is elongated.
Note that H-(S-Ah2c)n-NH2 and H-(S-Ah2c)n-OH (n¼4
and 5) showed practically identical CD spectra in methanol,
indicating that the difference in C-terminal structure
does not significantly affect the higher-order structure for-
mation. Similar chain-length-dependent increase in CD
intensities has also been observed in other ordered sys-
tems.3,10 In the case of water-soluble amphiphilic peptides,
aggregation stimulated by hydrophobic interaction would
be a cause of such stabilization. However, this is not so in
the present case; essentially identical CD spectra were
obtained at peptide concentrations in the range of 50–
1000 mM for H-(S-Ah2c)5-NH2 and HCl$H-(S-Ah2c)4-OH
with no significant change in molecular ellipticity at
217 nm ([q]217) (Fig. 3). Since methanol is effective to re-
duce hydrophobic interactions, inhibiting possible aggrega-
tion of peptides,11 these results are suggestive of monomeric
structures of the oligomers.
Therefore, the Ah2c peptides are considered to adopt a self-
organized monomeric structure. The question arises, what is
the major driving force for the structure formation? In a hy-
drophobic environment, intramolecular hydrogen bonding
or ion-pair interaction is effective. However, this is not the
case here, since there is no amide proton or charged group
in these peptides; thus, presumably the increase of the
inter-residue van der Waals attraction3,12 is a factor in this
structural organization. Support for this interpretation comes
from the results of the study of the thermal stability of these
peptides. A gradual decrease in [q]217 in the CD spectra of
the octamer, pentamer, and tetramer peptides was observed
when the temperature was raised to 60 ꢀC: decreases of 11,
20, and 31% compared with the [q]217 values at 20 ꢀC
were obtained, respectively, and no significant thermal tran-
sition of these peptides was observed. The degree of the
reduction of [q]217 is smaller in longer peptides, which
suggests stabilization of the secondary structures in longer
peptides.
2.3. Structure of the oligomers
The above results suggested that, even when a peptide lacks
a planar amide structure and has flexibility in the amide
bonds, it can show a preference for ordered structures, pre-
sumably with the contribution of conformationally strained
side chains to reduce entropic loss upon the thermodynami-
cally driven formation of the high-order structures. NMR
study of the tetramer only gave broad signals for all the pro-
tons (see Supplementary data, Fig. S3), which is consistent
with the presence of a mixture of isomers with respect to
the cis–trans amides in the oligomers. This possible cis–
trans isomerization was confirmed by NMR measurement
1
of the terminal-protected dimer of Ah2c; in the H NMR
spectrum (CDCl3) of the N-Boc-S-dimer-benzyl ester, a
mixture of cis- and trans-amide (cis:trans¼47:53) structures
with respect to the central amide bond was observed (see
Supplementary data, Fig. S2) (the trans-amide crystal
structure was obtained for t-Boc-(R-Ah2c)2-OBn; Fig. 4
(CCDC 621064), see Supplementary data for a summary
of the data). This observed non-biased mixture of the iso-
mers is compatible with the calculated small energy differ-
ence (Fig. 5). Thus, there is an equilibrium mixture of
cis- and trans-amide structures in solution. Since the oligo-
mers have several amide bonds, overlapping of the signals
from various cis–trans isomerization states of the peptide
would give broad and complex signals, making structural
characterization difficult. However, it is plausible that oligo-
merization of Ah2c may increase the preference for the ulti-
mate, thermodynamically stable ordered structures. This
would be consistent with the observed chain-length depen-
dency and thermostability of the peptides.
The possible formation of helical structure was next exam-
ined. Firstly, the UV and CD spectra of the oligopeptides
were compared with those of the corresponding monomeric
amino acid. As shown in Figure 2c, the monomer exhibited
an intense absorption band at ca. 200 nm and a shoulder at
longer wavelength. As is usual for naturally occurring pep-
tides, these bands can be assigned, respectively, to the elec-
tric dipole allowed p–p* transition and the forbidden n–p*
transition of the peptide chromophore.13 The UVabsorption
bands of the oligomers broadened systematically toward
Figure 3. Concentration dependence study. CD spectra of HCl$H-(S-
Ah2c)4-OH at 50, 100, 500, and 1000 mM (1 mM) in MeOH at 20 ꢀC. There
is a normal concentration dependency in the intensity of the CD spectra be-
tween 50 and 1000 mM in methanol; that is, after concentration normaliza-
tion, the CD spectra are consistent.

Y. Otani et al. / Tetrahedron 62 (2006) 11635–11644
11639
Figure 4. ORTEP diagram of trans-amide structure of dimer, t-Boc-(R-Ah2c)2-OBn (at 150 K).
the longer-wavelength region, with increasing number of
monomeric units. While the CD signals of the oligopeptides
appear to be dispersion-type signals, an absorption-
shaped CD signal was observed for the monomeric amino
acid.
In order to estimate the number of transitions in the UV re-
gion, a band deconvolution analysis of the experimental data
was carried out (Fig. 2c and d). This approach rests on the
fact that a single set of Gaussian components can be used
for fitting of both the UV and CD spectra (Table 1).14 In
the case of the monomer, three Gaussian bands were used,
two of which can be associated with the p–p* (red line,
lmax¼200 nm, f¼0.063) (red line) and n–p* (blue line,
lmax¼218 nm, f¼0.022) (blue line) transitions (Fig. 2c). In
contrast, at least four Gaussian bands were required to fit
the spectra of the tetramer. The two higher-energy bands
(lmax¼199 nm, f¼0.374 and lmax¼211 nm, f¼0.182) should
be associated with the splitting of the p–p* transitions (red
line) as a result of the strong exciton interactions, while the
lower-energy band (lmax¼225 nm, f¼0.066) was assigned to
the n–p* band (blue line) (Fig. 2d). As a result, the intense
CD signals observed for the oligomers are considered to
arise from couplings between the n–p* transition and the
p–p* transitions on nearby amides,15 as well as couplings
between the p–p* transitions of the amide units,16 similar
to those of a-helical polypeptides. Since these CD signals
would depend strongly on the transition moment directions,
it can be concluded that the oligomers adopt well-defined
structures, plausibly helical structures, under the experimen-
tal conditions.
Figure 5. (a) cis- and trans-Amide dimers, H-(S-Ah2c)2-OH, of 7-azabicy-
clo[2.2.1]heptane amide and their syn- and anti-inversions. (b) Scans of the
potential surface at the B3LYP/6-31G* level for the cis- (yellow) and trans-
conformers (green). The potential surface was scanned with respect to the
j torsion for cis-H-(S-Ah2c)2-OH (yellow) and trans-H-(S-Ah2c)2-OH
(green). Inset: energies (MP2/6-31G* level) are relative to the global mini-
mum, cis (ꢁ65ꢀ). Two additional minima for cis-(S-Ah2c)2 are located at j
values of ꢁ95ꢀ and 165ꢀ. Three minima for trans-H-(S-Ah2c)2-OH are
located at ꢁ90ꢀ, ꢁ65ꢀ, and 160ꢀ.
Table 1. Results of the Gaussian fit analysis of the monomer and tetramer
Compound
Band
no.
Assignment
Wavelength
(nm)
Energy
(cmꢁ1
Intensity
Bandwidth
)
Oscillator
strength ( f )
m (D)
)
(dm3/mol cm)
(cmꢁ1
Monomer
1
2
3
262
218
200
38,140
45,970
50,121
274
874
3332
6666
5390
4103
0.008
0.022
0.063
0.09
0.19
0.51
n–p*
p–p*
Tetramer
1
2
3
4
271
225
211
199
36,908
44,529
47,454
50,157
1373
2721
9475
19,299
5258
5282
4169
4209
0.033
0.066
0.182
0.374
0.37
0.61
1.57
3.05
n–p*
p–p*
p–p*

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Y. Otani et al. / Tetrahedron 62 (2006) 11635–11644
2.4. Model study
the trans-amide helical structure by 7.8 kcal/mol by means
of MP2/6-31G* calculations. This result, together with the
presence of an isodichroic point for oligomers with n¼3–8
(Fig. 2a), allows us to propose a possible ultimate structure
of the oligomers of bicyclic b-amino acids with 7-azabicy-
clo[2.2.1]heptane, i.e., a helical structure with cis-amide
and with four residues per turn, in the presence of the trans
conformer to some extent. It was also shown that the energy
barriers with respect to the cis–trans isomerization increase
in the center of the oligomers as compared with the terminal
amides in the hexamer (H-(S-Ah2c)6-NH2): in the model
cis-helix structure of the hexamer (H-(S-Ah2c)6-NH2) ob-
tained similarly, the cis-to-trans rotational barriers were as
follows: with respect to the first amide bond from the N-ter-
minal: HF/6-31G*, 10.65 kcal/mol (HF/3-21G*, 9.28 kcal/
mol); the second amide bond: HF/6-31G*, 11.32 kcal/mol
(HF/3-21G*, 9.08 kcal/mol); the third amide bond: HF/
6-31G*: 12.29 kcal/mol (HF/3-21G*:11.52 kcal/mol). The
rotational barrier increased toward the center along the helix.
This is consistent with the length-dependent increase of the
robustness of the ordered structures deduced from the CD
spectra.
Assuming that the oligomers prefer helical structures, then
what kind of structure is probable? To address this question,
quantum chemical calculations were carried out first for the
dimer, H-(S-Ah2c)2-OH, at the B3LYP/6-31G* level. In the
case of b-amino acids such as those in this study, there are
three valuable parameters.17 The f, q, and j torsions are de-
fined as [C(iꢁ1)–Ni–Cai–Cbi], [Ni–Cai–Cbi–Ci], and [Cai–
Cbi–Ci–N(i+1)], respectively. In the present case, f¼ꢁ91ꢀ
(in the case of syn) or ꢁ151ꢀ (in the case of anti) and
q¼ꢁ162ꢀ due to the rigid bicyclic 7-azabicyclo[2.2.1]-
heptane structure, so that the most stable structure(s) can
be obtained only by changing j. We have considered two
types of isomerism of amide (cis and trans) and two direc-
tions of inversion (syn and anti) (Fig. 5a). The j value was
changed stepwise every five degrees and the obtained struc-
tures were optimized. For each j, both syn- and anti-
inversion cases were calculated for the cis-amide and
trans-amide structures and the energetically more stable
cases with respect to the inversion were chosen (Fig. 5b).
A calculated inversion barrier was small (2.5 kcal/mol).5c
Three energetically minimum structures were obtained for
both the cis- and trans-amide structures. The results indicate
that the amide bonds are not planar and that they take cis- or
trans-conformation. Further calculations at the MP2/6-31G*
level in the vicinity of the above three minima revealed that
the most stable structure is the cis-amide at j¼ꢁ65ꢀ attained
via syn-inversion (the most stable trans-amide was obtained
at j¼ꢁ90ꢀ via syn-inversion, but the energy is higher than
that of the most stable cis-amide by 0.69 kcal/mol). The
same result was obtained when the solvent (water) was taken
into account. The f, q, and j values in the most stable struc-
ture, after full optimization, were ꢁ86ꢀ, ꢁ162ꢀ, and ꢁ65ꢀ,
respectively.
3. Conclusions
In this study, we have shown that homooligomers of bicyclic
b-amino acids containing 7-azabicyclo[2.2.1]heptane Ah2c,
which lacks amino hydrogen and has a spatially hindered
structure, can be synthesized without difficulty. Although
coexistence of the cis and trans isomers with respect to the
non-planar amide bonds has been suggested, oligomeriza-
tion of Ah2c significantly promoted the formation of
higher-order structures, yielding characteristic CD spectra.
Deconvolution and modeling studies suggest the formation
of unique helical structures without intramolecular hydrogen
bonds. These results support the idea that b-amino acids
bearing non-planar amide bonds can be used to generate
novel protein scaffolds. The ordered structure may be at-
tained owing to the spatially strained structure of the side
chain, which would compensate for the loss of entropy and
simultaneously gain enthalpy to form the ordered structures
in the presence of cis–trans isomerizable amide bonds. On
the other hand, the non-planarity of the amide bond should
endow the resulting scaffolds with unique features, which
may yield properties different from those of scaffolds com-
posed of units with typical planar amide bonds. The results
obtained in this study suggest that it may also be feasible to
construct protein scaffolds comprised of other N-substituted
b-amino acids. Because of the interest in chemical features
of amino acid derivatives based on the 7-azabicyclo[2.2.1]-
heptanes, the possibility of derivatization at amide nitrogen,
in addition to a- and b-carbon atoms, should offer great
scope to design novel functional molecules and materials
showing unique chemical and physical properties. Thus,
we believe our results make feasible a novel approach to pro-
tein scaffold design.
A single crystal structure of the trans-amide of the enantio-
tropic dimer, t-Boc-(R-Ah2c)2-OBn was obtained (Fig. 4).
The central amide bond is pyramidalized (a¼21.5ꢀ), and
the nitrogen atom is syn-inverted. The observed torsion
angles, j, f and q were +82.9 (3)ꢀ, +98.3 (3)ꢀ, and +153.2
(2)ꢀ, respectively. The absolute magnitudes of these values
were consistent with the computationally estimated values
(ꢁ90ꢀ, ꢁ91ꢀ, and ꢁ162ꢀ, respectively) of the most stable
trans-amide structure of the S-dimers (that is, syn-inverted).
This supports the validity of the present computational struc-
tural analysis.
In the next step, a Monte-Carlo conformation search was
performed for the all cis- or trans-amide forms of the
octamer (H-(S-Ah2c)8-NH2) by means of MMFF94s force-
field calculations, and the resultant most stable structure
was further optimized at the B3LYP/6-31G* level. It was
found that the cis-amide conformer takes a left-handed heli-
cal structure with four residues per turn (Fig. 1c), while the
trans-amide conformer takes an elongated helical structure
with roughly two residues per turn (Fig. 1d). The average
values of the torsion angle j (Cai–Cbi–Ci–N(i+1)) of the op-
timized structures of the octamer are ꢁ68ꢀ for the cis-amide
helix, and ꢁ90ꢀ for the trans-amide helix. These j values are
consistent with those for the energy minimum structures of
the cis- and trans-dimers, respectively (Fig. 5). The cis-
amide helix structure was estimated to be more stable than
4. Experimental
4.1. General methods
All the melting points were measured with a Yanagimoto
Micro Melting Point Apparatus and are uncorrected. Proton

Y. Otani et al. / Tetrahedron 62 (2006) 11635–11644
11641
(400 MHz) NMR spectra were measured on a JEOL Caliber-
GX400 NMR spectrometer with TMS as an internal refer-
ence in CDCl3 as the solvent, unless otherwise specified.
Chemical shifts d are shown in parts per million. Coupling
constants are given in hertz. Low-resolution EI mass spectra
(MS, EI+) and high-resolution EI mass spectra (HRMS, EI+)
were recorded on a JEOL JMS-O1SG-2 spectrometer.
Low-resolution FAB mass spectra (MS, FAB+) and high-
resolution FAB mass spectra (HRMS, FAB+) were
recorded on a JEOL MStation JMS-700 spectrometer.
Matrix-assisted laser desorption ionization time-of-flight
mass spectra (MALDI-TOF MS) were recorded on an
Applied Biosystems Voyager-DE STR instrument. ESI-TOF
MS (ESI+) were recorded on a Bruker Daltonics, micro-
TOF-LC, and Micromass, micromass-LCT instruments.
Column chromatography was carried out on silica gel (silica
gel 60 N (100–210 mm), Merck). The combustion analyses
were carried out in the microanalytical laboratory of this
faculty.
(n-hexane/CHCl3¼1/6, then CHCl3 only). Hydrogenated
compound 4 (262 mg, 92% yield) was obtained as a yellow
oil.
1H NMR: 4.39 (br s, 1H), 4.21 (br s, 1H), 3.70 (s, 3H), 3.04
(m, 1H), 2.00–1.50 (m, 6H), 1.47 (s, 9H).
4.3.4. 7-(tert-Butoxycarbonyl)-7-azabicyclo[2.2.1]hep-
tane-2-carboxylic acid (5). To a solution of 4 (7.90 g,
30.95 mmol) in THF (80 mL), a solution of lithium hydrox-
ide monohydrate (1.60 g, 38.13 mmol) in 20 mL of water
was added, and the mixture was stirred at rt for 11 h.
The organic solvent was evaporated and the residue was
acidified to pH¼2. Then the whole was extracted with
AcOEt, the organic phase was washed with brine, and the
solvent was evaporated to afford 5 (7.25 g, quant.) as a
brown oil.
MS (m/z): 241 (M+H+). HRMS (EI+, M+H+) calcd for
C12H19NO4: 241.1313; obsd: 241.1308. 1H NMR: 4.45
(br s, 1H), 4.23 (br s, 1H), 3.13–3.10 (m, 1H), 2.00–1.46
(m, 6H), 1.46 (s, 9H).
4.2. Materials
Racemic bicyclic amino acid (5) was synthesized by modi-
fications of the method of Zhang and Trudell and Singh
et al.,8,18–20 and each optically pure enantiomer was obtained
by repeated recrystallization after introducing Oppolzer’s
camphorsultam on the carboxylic acid moiety as a chiral
auxiliary.
4.3.5. 7-(tert-Butoxycarbonyl)-2-endo-bornane-2,10-sul-
tam-7-azabicyclo[2.2.1]heptane (6). To a stirred solution
of 5 (936 mg, 3.79 mmol) in dry CH2Cl2 was added drop-
wise a solution of oxalyl chloride (0.8 mL, 9.1 mmol) in
dry CH2Cl2 at ꢁ70 ꢀC. When the addition was finished, a
catalytic amount of dry DMF (two drops) was added, and
the reaction mixture was stirred for 24 h at ꢁ40 ꢀC. The sol-
vent and unreacted oxalyl chloride were removed in vacuum
to afford the acid chloride. Under Ar atmosphere, a solution
of 1S-(ꢁ)-2,10-camphorsultam (1000 mg, 4.64 mmol) in
THF was added to a solution of NaH (65% in oil, washed
with n-hexane, 383 mg, 10.37 mmol) in THF at 0 ꢀC and
the reaction mixture was stirred for 20 min at 0 ꢀC. Then
to the mixture was added dropwise a solution of the acid
chloride in THF at 0 ꢀC and the whole was stirred for 12 h
at rt. Addition of water, followed by 2 N aqueous HCl,
quenched the reaction, and the whole was extracted with
AcOEt, and the organic layer was washed with brine and
dried over Na2SO4. The solvent was evaporated to give a
pale yellow solid, which was chromatographed (n-hexane/
AcOEt¼4/1) to give a mixture of 6a and 6b (1.13 g, 66%
yield). The mixture was stirred in n-hexane/Et2O/AcOEt to
give a suspension and the precipitate was filtered to afford
6b as a colorless solid. Pure 6a was obtained as colorless
needles by recrystallization of the residue from n-hexane/
CH3CN/iPr2O.
4.3. Synthesis of chiral bicyclic amino acids
4.3.1. Methyl 3-bromopropiolate (2). To a magnetically
stirred solution of methyl propionate 1 (25 mL, 281 mmol)
in acetone (300 mL) at rt, silver nitrate (4.71 g, 27.7 mmol)
was added, followed by the addition of N-bromosuccinimide
(58.26 g, 327.3 mmol) in one portion. The homogeneous
solution turned cloudy within 5 min, then a grayish solid
was precipitated and the whole was stirred for 1 h. Acetone
was removed carefully under reduced pressure at rt and the
residue was distilled at rt to afford 2 as a pale yellow oil
(45.2 g as a mixture of acetone, 67% NMR yield).
1H NMR: 3.79 (s, 3H).
4.3.2. Methyl 2-bromo-7-(tert-butoxycarbonyl)-7-azabi-
cyclo[2.2.1]hepta-2,5-diene-2-carboxylate (3). A solution
of 2 (188 mmol) and tert-butyl 1-pyrrolecarboxylate
(100 mL, 598 mmol) in acetone was heated at 90 ꢀC for
30 h with stirring under Ar atmosphere. The resulting mix-
ture was subjected to column chromatography (n-hexane/
AcOEt 9:1). Compound 3 (33.92 g, 37% yield) was obtained
as a yellow oil.
4.3.5.1. 6aD6b. Anal. Calcd for C22H34N2O5S: C, 60.25;
H, 7.81; N, 6.39. Found: C, 59.95; H, 7.58; N, 6.32.
1H NMR: 7.13 (br s, 2H), 5.50 (br s, 1H), 5.15 (br s, 1H),
3.79 (s, 3H), 1.41 (s, 9H).
Compound 6a: mp 182.9–183.1 ꢀC (colorless needles). Anal.
Calcd for C22H34N2O5S: C, 60.25; H, 7.81; N, 6.39. Found:
C, 60.55; H, 7.86; N, 6.40. H NMR: 4.71–4.69 (1H, m),
1
4.3.3. Methyl 7-(tert-butoxycarbonyl)-7-azabicyclo-
[2.2.1]heptane-2-carboxylate (4). A solution of 3 (367 mg,
1.11 mmol) in MeOH was hydrogenated catalytically (10%
Pd/C) in the presence of triethylamine (218 mg, 2.15 mmol).
The mixture was filtrated, the solvent was evaporated,
and the residue was subjected to column chromatography
4.25–4.21 (1H, m), 3.91–3.87 (1H, m), 3.58–3.53 (1H, m),
3.51 (1H, d, J¼14.0 Hz), 3.46 (1H, d, J¼14.0 Hz), 2.10–
1.63 (13H, m), 1.47 (9H, s), 1.15 (3H, s), 0.98 (3H, s).
Compound 6b: mp 208.0–208.5 ꢀC (colorless solid). Anal.
CalcdforC22H34N2O5S:C, 60.25;H, 7.81;N, 6.39. Found:C,

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Y. Otani et al. / Tetrahedron 62 (2006) 11635–11644
60.10; H, 7.76; N, 6.40. 1H NMR: 4.66–4.64 (1H, m), 4.24–
4.22 (1H, m), 3.91–3.89 (1H, m), 3.62–3.55 (1H, m), 3.52
(1H, d, J¼14.0 Hz), 3.44 (1H, d, J¼13.6 Hz), 2.10–1.80
(7H, m), 1.80–1.65 (2H, m), 1.60–1.40 (4H, m), 1.47 (9H,
s), 1.15 (3H, s), 0.99 (3H, s).
0 ꢀC and the mixture was stirred for 1.5 h at 0 ꢀC. Then
the solvent was evaporated to give a crude oil. This residue
was dissolved in 7 mL of water, and to this solution Na2CO3
(320 mg, 3.01 mmol) was added, followed by the addition of
a solution of 9-fluorenylmethyl succinimidyl carbonate
(471 mg, 1.40 mmol) in 7 mL of dry DMF at 0 ꢀC. The re-
sultant viscous solution was stirred vigorously for 12 h at
rt. The aqueous layer was acidified to pH¼3 with 1 N aque-
ous HCl and the whole was extracted with AcOEt. The
organic layer was washed with brine, dried over Na2SO4,
and the solvent was evaporated to give a crude oil, which
was chromatographed (n-hexane/AcOEt¼2/1) to give 8-2R
as a red-brown oil (461 mg, 96% yield). MS (m/z): 453
(M+H+). HRMS (EI+, M+H+) calcd for C29H27NO4:
4.3.6. (1R,2R,4S)-7-(tert-Butoxycarbonyl)-7-azabicy-
clo[2.2.1]heptane-2-carboxylic acid (5-2R). A solution of
6a (2.29 g, 5.21 mmol) in 40 mL of MeOH/H2O (3:1) mix-
ture containing LiOH$H2O (220 mg, 5.24 mmol) was
warmed to 45 ꢀC with stirring. After 1 h, the mixture was
cooled and the resulting solution was extracted with AcOEt
to remove the chiral auxiliary sultam. The aqueous layer was
acidified to pH¼3 with 1 N aqueous HCl under an ice-cold
condition, and the whole was extracted with CHCl3. The
crude acid 5-2R (641 mg, yield 51%) was obtained as
a pale yellow oil after evaporation of the organic solvent.
1
453.1939; obsd: 453.1931. H NMR: 7.71–7.69 (2H, m),
7.58–7.55 (2H, m), 7.38–7.26 (9H, m), 5.13 (2H, s), 4.49–
4.30 (2H, m), 4.38 (1H, br s), 4.19 (2H, t, J¼6.2 Hz),
3.00–2.80 (1H, br s), 1.84–1.82 (2H, m), 1.70–1.67 (1H,
m), 1.63–1.58 (1H, m), 1.46–1.37 (2H, m).
4.3.7. (1S,2S,4R)-7-(tert-Butoxycarbonyl)-7-azabicy-
clo[2.2.1]heptane-2-carboxylic acid (5-2S). Compound 5-
2S was obtained in a similar manner to 5-2R, described above.
4.3.12. (1S,2S,4R)-7-(Fluorenylmethyl)-7-azabicyclo-
[2.2.1]heptane-2,7-dicarboxylic acid 2-benzyl ester (8-
2S). Compound 8-2S was obtained in a similar manner to
8-2R, described above.
4.3.8. Determination of the absolute stereochemistry of
5-2R. The absolute configuration of the resolved material
was established by the optical rotation, and crystallo-
graphically.
4.3.13. (1R,2R,4S)-7-(Fluorenylmethyl)-7-azabicyclo-
[2.2.1]heptane-2,7-dicarboxylic acid (9-2R). Debenzyla-
tion of 8-2R (400.0 mg, 0.88 mmol) was carried out by
hydrogenation over Pd/C (94.7 mg) in MeOH (10 mL) at rt
for 24 h. The reaction mixture was filtered and the filtrate
was evaporated to give a crude oil, which was chromato-
graphed (n-hexane/CHCl3¼1/6, then a small amount of ace-
tic acid was added to the eluent) to give 9-2R as a pale yellow
oil (190 mg, 60% yield). Pale yellow amorphous solid. MS
(m/z): 363 (M+H+). HRMS (EI+, M+H+) calcd for
Carboxylic acid 5-2R (8 mg, 0.033 mmol) was converted to
methyl ester by using TMS–diazomethane in benzene. Then
the reaction mixture was evaporated to give quantitatively
10 mg of white solid (4-2R).
[a]2D3 ꢁ20.80 (c 0.50, CHCl3), lit.20 [a]2D3 ꢁ22.6 (c 1.01,
CHCl3).
4.3.9. (1R,2R,4S)-7-(tert-Butoxycarbonyl)-7-azabicy-
clo[2.2.1]heptane-2-carboxylic acid 2-benzyl ester (7-2R).
To a solution of 5-2R (713 mg, 2.95 mmol), benzyl alcohol
(0.95 mL, 9.11 mmol), and dimethylaminopyridine (290 mg,
2.37 mmol) in 5 mL of CH2Cl2 was added dicyclohexylcar-
bodiimide (671 mg, 3.25 mmol) at 0 ꢀC. The whole was
warmed to rt and stirred for 3 h. The precipitate was filtered
and washed with ether. The combined organic layer was
washed with 0.5 N aqueous HCl, saturated aqueous NaHCO3,
dried over Na2SO4, and the solvent was evaporated. The
residue was chromatographed (n-hexane/AcOEt¼2/1) to
give 7-2R as a yellow oil (513 mg, 52% yield).
1
C22H21NO4: 363.1471; obsd: 363.1496. H NMR: 7.78–
7.76 (2H, m), 7.60–7.57 (2H, m), 7.42–7.38 (2H, m),
7.34–7.30 (2H, m), 4.49 (2H, d, J¼6.2 Hz), 4.45 (1H, br
s), 4.23 (1H, br s, overlap), 4.22 (1H, t, J¼6.2 Hz), 2.93
(1H, br s), 1.95–1.45 (6H, m). [a]2D1 ꢁ14.4 (c 0.46, CHCl3).
4.3.14. (1S,2S,4R)-7-(Fluorenylmethyl)-7-azabicyclo-
[2.2.1]heptane-2,7-dicarboxylic acid (9-2S). Compound
9-2S was obtained in a similar manner to 9-2R, described
above. [a]2D1 +14.6 (c 0.48, CHCl3).
4.3.15. Boc-(S-Ah2c)2-OBn. Compound 7-2S (252 mg,
0.76 mmol) was dissolved in 4 N HCl in dioxane (1.7 mL)
and stirred for 1 h. The solvent was then removed in vacuum,
and the residue was dried in vacuum. Compound 5-2S
(171 mg, 0.74 mmol) and DMAP (129.0 mg, 1.06 mmol)
were added to the flask, followed by the addition of DMF
(3 mL). Then EDCI$HCl (284 mg, 1.48 mmol) was added,
and the whole was stirred for 48 h under Ar. DMF was re-
moved in vacuum and the residue was washed with 1 N
aqueous HCl, the whole was extracted with CHCl3, then
the solvent was evaporated to give a white sticky solid,
which was chromatographed (n-hexane/AcOEt¼1/1) to
give a colorless amorphous solid (270 mg, 78% yield). MS
(m/z): 455 (M+H+). HRMS (FAB+, M+H+) calcd for
MS (m/z): 331 (M+H+). HRMS (EI+, M+H+) calcd for
C19H25NO4: 331.1782; obsd: 331.1797. H NMR: 7.40–
7.33 (5H, m), 5.14 (2H, s), 4.42–4.40 (1H, m), 4.21–4.19
(1H, m), 3.13–3.06 (1H, m), 1.97–1.91 (1H, m), 1.90–1.85
(1H, m), 1.85–1.80 (1H, m), 1.73–1.67 (1H, m), 1.45 (9H,
s), 1.45 (2H, m, overlap).
1
4.3.10. (1S,2S,4R)-7-(tert-Butoxycarbonyl)-7-azabicyclo-
[2.2.1]heptane-2-carboxylic acid 2-benzyl ester (7-2S).
Compound 7-2S was obtained in a similar manner to 7-2R,
described above.
4.3.11. (1R,2R,4S)-7-(Fluorenylmethyl)-7-azabicyclo-
[2.2.1]heptane-2,7-dicarboxylic acid 2-benzyl ester (8-
2R). To a solution of 7-2R (351 mg, 1.06 mmol) in 2 mL
of dry CH2Cl2 was added 2 mL of trifluoroacetic acid at
1
C26H35N2O5: 455.2541; obsd: 455.2548. H NMR: 7.38–
7.35 (5H, m), 5.17–5.15 (2H, a mixture of conformers),
(amide cis–trans mixture) 4.88 (0.5H, m), 4.67 (0.5H, m),

Y. Otani et al. / Tetrahedron 62 (2006) 11635–11644
11643
4.52 (0.5H, m), 4.35 (0.5H, m), 3.10–3.03 (2H, m), 2.08–
1.50 (12H, m), 1.48 (9H, m).
(0.5 mL) for 2 h at rt. The resin was filtered and the reaction
mixture was evaporated. The precipitate formed upon addi-
tion of cold Et2O (15 mL) to the oily residue was separated
by decanting the solvent. The precipitate was lyophilized
to yield 25 mg of the crude peptide. Purification of the
crude peptide by RP-HPLC (15–40% B in 25 min) afforded
the TFA salt of H-(R-Ah2c)8-NH2, 4.91 mg (4.1%) as
a white fluffy powder. MS (m/z): 1003.75 (M+H+). MS
(m/z) (ESI+, [M+H]+) calcd for C56H76N9O8: 1002.5811;
obsd: 1002.5804.
4.3.16. HCl$H-(S-Ah2c)2-OH. Debenzylation of Boc-(S-
Ah2c)2-OBn (77 mg, 0.17 mmol) was carried out by hydro-
genation over Pd/C (60.1 mg) in MeOH (1.5 mL) at rt for
1.5 h. The reaction mixture was filtered and the filtrate was
evaporated to give a crude oil. The residue was dissolved
in 4 N HCl in dioxane (0.4 mL) and the resultant solution
was stirred for 1 h. The solvent was then removed by a stream
of Ar, and the residue was dried in vacuum to afford HCl$H-
(S-Ah2c)2-OH (63 mg, quant.). HRMS (FAB+, M+H+) calcd
for C14H21N2O3: 265.1552; obsd: 265.1587.
4.4.2.2. H-(S-Ah2c)4-NH2. The preparation of H-(S-
Ah2c)4-NH2 was carried out in a similar manner to (R-
Ah2c)8-NH2. Colorless oil. MS (m/z): 511 (MALDI-TOF
MS, [M+H]+).
4.3.17. HCl$H-(S-Ah2c)3-OH. The preparation of HCl$H-
(S-Ah2c)3-OH was carried out in a similar manner to the
dimer HCl$H-(S-Ah2c)2-OH. HRMS (FAB+, M+H+) calcd
for C21H30N3O4: 388.2236; obsd: 388.2207.
4.4.2.3. H-(S-Ah2c)5-NH2. The preparation of H-(S-
Ah2c)5-NH2 was carried out in a similar manner to H-(R-
Ah2c)8-NH2. Colorless oil. MS (m/z): 633 (MALDI-TOF
MS, [M+H]+).
4.3.18. HCl$H-(S-Ah2c)4-OH. The preparation of HCl$H-
(S-Ah2c)4-OH was carried out in a similar manner to the
dimer HCl$H-(S-Ah2c)2-OH. HRMS (FAB+, M+H+) calcd
for C28H39N4O5: 511.2921; obsd: 511.2830.
4.4.2.4. H-(S-Ah2c)8-NH2. The preparation of H-(S-
Ah2c)8-NH2 was carried out in a similar manner to H-(R-
Ah2c)8-NH2. White fluffy powder. MS (m/z) (ESI-TOF,
[M+H]+) calcd for C56H76N9O8: 1002.5811; obsd:
1002.5770.
4.3.19. HCl$H-(S-Ah2c)5-OH. The preparation of HCl$H-
(S-Ah2c)5-OH was carried out in a similar manner to the
dimer HCl$H-(S-Ah2c)2-OH. HRMS (FAB+, M+H+) calcd
for C28H39N4O5: 634.3605; obsd: 634.3559.
4.5. CD measurements
4.4. Solid-phase synthesis of Ah2c oligomers
Dry peptide samples were weighed and dissolved in an ap-
propriate amount of spectroscopic grade methanol. Sample
cells of 1-mm path length were used. Data were collected
on a JASCO J-820 spectrometer at 20 ꢀC. Data are
represented in terms of mean residue ellipticity, [q]
(deg cm2 decimolꢁ1 residueꢁ1).
4.4.1. Reversed-phase (RP) HPLC analysis and purifica-
tion. RP-HPLC purification was performed on a Cosmosil
5C18-AR-II column (10ꢂ250 mm, Nacalai Tesque) with
a linear gradient of A (0.1% TFA in H2O) and B (0.1%
TFA in MeCN) at a flow rate of 1 mL minꢁ1 (Hitachi
HPLC system); UV detection at 215 nm.
The mean residue ellipticities for H-(S-Ah2c)8-NH2 are
ca. ꢁ3.5ꢂ104 deg cm2 decimolꢁ1 residueꢁ1 at 198 nm (a
minimum) and 4.0ꢂ104 at 217 nm (a maximum), while
those for H-(R-Ah2c)8-NH2 are 4.0ꢂ104 at 198 nm and
ꢁ3.1ꢂ104 at 217 nm. The two oligomers have the crossover
point at about 206 nm.
4.4.2. Peptide synthesis.
4.4.2.1. H-(R-Ah2c)8-NH2. The Rink amide resin
(200 mg, 0.04 mmol; loading 0.21 mmol gꢁ1) was swelled
in DMF (5 mL) for 90 min and Fmoc was deprotected using
20% piperidine in DMF (2 mL, 15 min). The resin was fil-
tered and washed with DMF (5ꢂ4 mL). A solution of Fmoc-
R-Ah2c-OH (9-2R) (43 mg, 3 equiv), PyBOP (3 equiv) and
HOBt (3 equiv) in DMF (0.9 mL), and N-methylmorpholine
(6 equiv) were added successively to the resin and the sus-
pension was mixed for 1.5 h at rt. The resin was then filtered
and washed with DMF (5ꢂ4 mL) prior to the following
Fmoc deprotection step. The Fmoc group was removed
using 20% piperidine in DMF (2 mL, 15 min). The resin
was filtered and washed with DMF (5ꢂ4 mL). For each cou-
pling, a solution of the Fmoc-amino acid (3 equiv), PyBOP
(3 equiv) and HOBt (3 equiv) in DMF (0.9 mL), and
N-methylmorpholine (6 equiv) were added successively to
the resin and the suspension was mixed for 1.5 h at rt. The
resin was then filtered and washed with DMF (5ꢂ4 mL).
After coupling the last amino acid, the Fmoc group was
cleaved and the resin was washed with DMF (5ꢂ4 mL),
MeOH (5ꢂ4 mL), Et2O (5ꢂ4 mL). Drying overnight under
vacuum afforded the Fmoc-deprotected peptide-resin
(209 mg). The dry Fmoc-deprotected peptide-resin was
treated with a mixture of TFA (10 mL) and ethanedithiol
The positions and intensities of the minimum and maximum
peaks of H-(S-Ah2c)4-NH2 and HCl$H-(S-Ah2c)4-OH in
the CD spectra are practically identical, H-(S-Ah2c)4-NH2:
ca. ꢁ1.9ꢂ104 deg cm2 decimolꢁ1 residueꢁ1 at 197 nm
(minimum) and 2.4ꢂ104 at 216 nm (maximum); HCl$H-
(S-Ah2c)4-OH: ca. ꢁ2.0ꢂ104 deg cm2 decimolꢁ1 residueꢁ1
at 197 nm (minimum) and 2.4ꢂ104 at 216 nm (maximum).
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research (Category S, No. 17109001) to T.O., and in part
by a Grant-in-Aid (No. 17350063) for Scientific Research
and the COE project, Giant Molecules and Complex Sys-
tems 2005, from the Ministry of Education, Science, Sports,
Culture, and Technology to N.K. A part of the calculations
was carried out at the Computer Center of the Institute for
Molecular Science and the Computer Center of the Univer-
sity of Tokyo. We thank these computational facilities for

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Fischer, S.; Dunbrack, R. L.; Karplus, M. J. Am. Chem. Soc.
1994, 116, 11931–11937; (e) Head-Gordon, T.; Head-
Gordon, M.; Frisch, M. J.; Brooks, C. L., III; Pople, J. J. Am.
Chem. Soc. 1991, 113, 5989–5997; (f) Guo, H.; Karplus, M.
J. Phys. Chem. 1992, 96, 7273–7287; (g) Sulzbach, H. M.;
Schleyer, P. v. R.; Schefer, H. F., III. J. Am. Chem. Soc. 1995,
117, 2632–2637; (h) MacArthur, M. W.; Thornton, J. M.
J. Mol. Biol. 1996, 264, 1180–1195; (i) Hu, J.-S.; Bax, A.
J. Am. Chem. Soc. 1997, 119, 6360–6368.
generous allotments of computer time. We also thank Masa-
toshi Kawahata for assistance in crystal structure analysis.
Supplementary data
Supporting information, additional structural data, and
1H NMR spectra are available. Supplementary data associ-
ated with this article can be found in the online version, at
doi:10.1016/j.tet.2006.09.062.
7. An example of the use of the present bicyclic structure as a
peptidomimetic was presented by Avenoza, A.; Busto, J. H.;
Peregrina, J. M.; Rodriguez, F. J. Org. Chem. 2002, 67,
4241–4249.
References and notes
8. Zhang, C.; Trudell, M. L. J. Org. Chem. 1996, 61, 7189–7191.
9. The structure of 7-(tert-butoxycarbonyl)-2-endo-bornane-
2,10-sultam-7-azabicyclo[2.2.1]heptane (6b) was determined
by X-ray crystallographic analysis.
10. This kind of molecular weight dependence of the CD spectra is
one of the criteria for judging the helical nature of polymers
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