Helical structures of bicyclic α-amino acid homochiral oligomers with the stereogenic centers at the side-chain fused-ring junctions

Article abstract of DOI:10.1002/hlca.201200403

Chiral bicyclic α-amino acid (R,R)-Ab_5,6=c with stereogenic centers at the γ-position of fused-ring junctions, and its enantiomer (S,S)-Ab_5,6=c, were synthesized. The CD spectra of (R,R)-Ab_5,6=c oligomers indicated that th

Full text of DOI:10.1002/hlca.201200403

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Helical Structures of Bicyclic a-Amino Acid Homochiral Oligomers with the
Stereogenic Centers at the Side-Chain Fused-Ring Junctions
by Kosuke Anana), Yosuke Demizub), Makoto Obac), Masaaki Kuriharab), Mitsunobu Doid),
Hiroshi Suemunea), and Masakazu Tanaka*c)
a) Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan
b) Division of Organic Chemistry, National Institute of Health Sciences, 1-18-1, Kamiyoga, Setagaya,
Tokyo 158-8501, Japan
c) Graduate School of Biomedical Sciences, Nagasaki University, 1-14 Bunkyo-machi,
Nagasaki 852-8521, Japan (phone/fax: þ 81-95-819-2423; e-mail: matanaka@nagasaki-u.ac.jp)
d) Osaka University of Pharmaceutical Sciences, Osaka 569-1094, Japan
Dedicated to Professor Dr. Dieter Seebach on the occasion of his 75th birthday
Chiral bicyclic a-amino acid (R,R)-Ab5,6¼c with stereogenic centers at the g-position of fused-ring
junctions, and its enantiomer (S,S)-Ab5,6¼c, were synthesized. The CD spectra of (R,R)-Ab5,6¼c oligomers
indicated that the (R,R)-Ab5,6¼c hexapeptide formed a mixture of right-handed (P)- and left-handed
(M)-310-helices, while, in the (R,R)-Ab5,6¼c nonapeptide, a right-handed (P)-310-helix slightly dominated
over the (M)-helix. X-Ray crystallographic analyses of (S,S)-tripeptide and (R,R)-hexapeptide revealed
that both the tripeptide and hexapeptide formed a mixture of (P)- and (M)-310-helices, respectively.
These results indicated that the side-chain environments around the stereogenic centers are particularly
important to control the helical-screw handedness of foldamers.
Introduction. – Helices are chiral, and, therefore, two helical screw senses, right-
handed (P) and left-handed (M), are possible. The relationship between (P)- and (M)-
helices is enantiomeric with regard to the helical backbone structure. The a-helices in
proteins almost always form a right-handed (P) screw sense, but not a left-handed (M)
screw sense. This is because a-l-amino acids have stereogenic a-C-atoms and,
therefore, right-handed (P)- and left-handed (M)-helices of peptides become
diastereoisomers. If proteinogenic a-l-amino acid peptides formed left-handed (M)-
helices, there would be steric repulsion between the C¼O group and the side-chain b-C-
atom, and, therefore, right-handed (P)-helices are preferred [1].
Among a-l-amino acids, isoleucine and threonine possess an additional stereogenic
center at the side-chain b-position besides a stereogenic center at the a-position;
however, little attention has been paid as to how the stereogenic centers of the side
chain affect the secondary structure of their oligomers [2]. We have previously reported
that chiral cyclic a,a-disubstituted a-amino acid (Ac5cdOM) [3], in which stereogenic
centers are only the side-chain g-C-atoms, controls the helical-screw sense of its
homochiral oligomers (Fig. 1) [4][5]; however, it is not clear how strongly the side-
chain stereogenic centers affect the screw sense of helical oligomers. Thus, herein we
designed a chiral bicyclic a,a-disubstituted a-amino acid with stereogenic centers at the
g-side-chain fused-ring junctions, i.e., (1R,6R)-8-aminobicyclo[4.3.0]non-3-ene-8-car-
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Fig. 1. Homochiral oligomers Cbz-[(S,S)-Ac5cdOM]n-OMe and Boc-[(R,R)-Ab5,6¼c]n-OEt, in which
stereogenic centers are only the side-chain g-C-atoms
boxylic acid ((R,R)-Ab5,6¼c)1) and its enantiomer (S,S)-Ab5,6¼c, and studied the effect
of side-chain stereogenicity on the secondary structure of their homochiral oligomers in
solution and in the crystal state. Furthermore, we demonstrated the chemical
modifications of olefin in amino acid (R,R)-Ab5,6¼c and its oligomer [6].
Results. – Synthesis of Optically Active Bicyclic Amino Acid. The optically active
bicyclic amino acid Boc-[(R,R)-Ab5,6¼c]-OEt ((R,R)-4) was synthesized from (S,S)-
cyclohex-4-ene-1,2-dicarboxylic acid ((S,S)-1) [7] as follows: reduction of the
dicarboxylic acid (S,S)-1 gave diol (S,S)-2, followed by replacement of OH groups
with I, to afford (S,S)-3. Bisalkylation of ethyl isocyanoacetate (CNCH2COOEt) with
(S,S)-3 [8], followed by hydrolysis of isocyanate under acidic conditions and protection
with Boc2O, gave the bicyclic amino acid ester Boc-[(R,R)-Ab5,6¼c]-OEt ((R,R)-4). For
the peptide preparation, the N-terminal-free amino acid ester H-[(R,R)-Ab5,6¼c]-OEt
((R,R)-5) was prepared by acidic deprotection, and the C-terminal-free amino acid
Boc-[(R,R)-Ab5,6¼c]-OH ((R,R)-6) was obtained by alkaline hydrolysis from (R,R)-4.
Also, we synthesized the enantiomer of bicyclic amino acid, Boc-[(S,S)-Ab5,6¼c]-OEt
((S,S)-4), starting from (R,R)-1 in the same manner described for the synthesis of
(R,R)-4 (Scheme 1).
Modification of Bicyclic Amino Acid. We reasoned that the olefin function at the
side chain of the bicyclic amino acid could be converted to several functional groups.
Actually, after ozonolysis of the olefin in (R,R)-4, reduction with NaBH4 gave a
dihydroxy amino acid ester (R,R)-7, oxidation with Oxoneꢂ afforded a dicarboxy amino
acid ester (R,R)-8, and reductive amination with BnNH2 afforded a bicyclic seven-
membered ring amino acid ester (R,R)-9. Moreover, hydrogenation of the olefin in
(R,R)-4 furnished a saturated bicyclic amino acid ester Boc-[(R,R)-Ab5,6c]-OEt (10).
1
)
The descriptor ꢃ5,6 ¼ ꢄ represents five-membered ring, six-membered ring, and olefin.

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Scheme 1. Synthesis and Modification of Bicyclic Amino Acid Boc-[(R,R)-Ab5,6¼c]-OEt ((R,R)-4), and
Its Enantiomer (S,S)-4
Preparation of Ab5,6¼c Homochiral Oligomers and Modification. Homochiral
oligomers Boc-(Ab5,6¼c)n-OEt (up to nonapeptide; n ¼ 9 for (R,R)-enantiomer, and
hexapeptide; n ¼ 6 for (S,S)-enantiomer) were prepared by solution-phase methods
(Scheme 2); that is, coupling between H-[(R,R)-Ab5,6¼c]-OEt ((R,R)-5) and Boc-
[(R,R)-Ab5,6¼c]-OH ((R,R)-6) using O-benzotriazol-1-yl-N,N,N’,N’-tetramethyluroni-
um hexafluorophosphate (HBTU) and EtNiPr2 (DIPEA) led to a dipeptide Boc-
[(R,R)-Ab5,6¼c]2-OEt ((R,R)-11) in 62% yield. Removal of the Boc group in 11,
followed by coupling with (R,R)-6, afforded a tripeptide Boc-[(R,R)-Ab5,6¼c]3-OEt
((R,R)-12) in 79% yield. Hydrolysis of C-teminal ester in 12 under alkaline conditions

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Scheme 2. Synthesis and Modification of (R,R)-Ab5,6¼c Homochiral Oligomers
gave C-terminal-free tripeptide Boc-[(R,R)-Ab5,6¼c]3-OH ((R,R)-13). Removal of the
Boc group in 12 and subsequent coupling with the C-terminal free tripeptide (R,R)-13
using HBTU and DIPEA produced a hexapeptide (R,R)-15 in 50% yield. The (R,R)-
Ab5,6¼c nonapeptide (R,R)-16 was prepared by coupling between N-terminal-free
hexapeptide and the C-terminal-free tripeptide (R,R)-13 in 32% yield. The enantio-
meric (S,S)-Ab5,6¼c hexapeptide (S,S)-15 was also prepared in a manner similar to that
described for (R,R)-15.
The six olefins in (R,R)-Ab5,6¼c hexapeptide 15 could be hydrogenated by H2/
20%Pd(OH)2-C in one step to give the saturated peptide Boc-[(R,R)-Ab5,6c]6-OEt (17)
in 70% yield.
Conformational Study of Homochiral Oligomers in Solution. The preferred
1
conformation of homochiral oligomers in solution was studied by FT-IR, H-NMR,
and CD spectroscopy. In the FT-IR spectra of oligomers Boc-[(R,R)-Ab5,6¼c]n-OEt
(n ¼ 1, 2, 3, 6, 9) in the NH-stretching region (amide A 3250 – 3500 cmꢀ1, in CDCl3

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solution), the weak bands around the 3430 cmꢀ1 region were assigned to free (solvated)
peptide NH groups, and the strong bands around the 3320 – 3360 cmꢀ1 region were
assigned to peptide NH groups with NꢀH ··· O¼C intramolecular H-bonds of different
strength (data are not shown; see [6]). As the peptide-chain length increases, the band
at 3360 cmꢀ1 in tripeptide 12 (n ¼ 3) shifts to a slightly lower wavenumber (3320 cmꢀ1 in
nonapeptide 16 (n ¼ 9)), and the relative intensity of the bands in the 3320 – 3360 cmꢀ1
region gradually increases. These IR spectra were very similar to those of helical
peptides, such as aminocyclopentanecarboxylic acid (Ac5c) [9] and a-aminoisobutyric
acid (Aib) homopeptides [10], but different from those of peptides such as
diethylglycine (Deg) [11] and (S)-butylethylglycine [12] homopeptides, which form
the fully extended planar C5-conformation.
To obtain further conformational information on homochiral oligomers, the
1H-NMR spectra of (R,R)-Ab5,6¼c hexapeptide 15 and nonapeptide 16, and saturated
(R,R)-Ab5,6c hexapeptide 17 were recorded in CDCl3 solution (data are not shown; see
[6]). In the 1H-NMR spectra of 15, 16, and 17, N(1)H signals of the urethane type at the
N-terminus were unambiguously assigned by their high-field positions, but the
remaining NH H-atoms could not be attributed at this stage. Solvent-perturbation
experiments involving the addition of the strong H-bond acceptor solvent DMSO (0 –
10% (v/v)) or the paramagnetic free radical (2,2,6,6-tetramethylpiperidin-1-yl)oxyl
(TEMPO; 0 – 5 ꢁ 10ꢀ2% (w/v)) were performed [10a][10b]. Two NH chemical shifts in
hexapeptides 15 and 17, and nonapeptide 16 were sensitive to the addition of the
perturbing reagent DMSO. Also, the addition of the TEMPO broadened the
bandwidth of the two NH signals. These two NH H-atoms were those of the first two
N-terminal residues, as demonstrated by 2D-NMR experiments. These results revealed
that the two NH H-atoms are solvent-exposed, suggesting that they are not intra-
molecularly H-bonded. These results are in accord with a 310-helical structure, in which
two NH groups at the N-terminus of the peptide are freely solvated (not intra-
molecularly H-bonded).
The nuclear Overhauser effect spectroscopy (NOESY) 1H-NMR spectra of helical
peptides show a series of strong sequential NH(i ! i þ 1) dipolar interactions.
Furthermore, in peptides and proteins based on coded a-amino acids, there are two
NOE constraints, [daN (i ! i þ 2)] and [daN (i ! i þ 4)], which are characteristic of the
310- and the a-helical structure, respectively [13]. Unfortunately, these latter
interactions do not occur in homopeptides composed of a,a-disubstituted a-amino
1
acids, because their residues lack a-CH H-atoms. Fig. 2 shows the ROESY H-NMR
spectra of hexapeptides 15 (a), 17 (b), and nonapeptide 16 (c) in CDCl3 solution. The
spectra of 15 and 17 displayed a complete series of sequential NH(i ! i þ 1) dipolar
interactions from the N-terminal N(1)H to the C-terminal N(6)H, which is character-
istic of a helical secondary structure (Fig. 2,a and b), while the spectrum of
nonapeptide 16 showed only a partial series of sequential NH(i ! i þ 1) cross-peaks
from NH(1) to NH(5) and from NH(6 or 7) to NH(8 or 9) (Fig. 2,c).
The CD spectra of (R,R)- and (S,S)-Ab5,6¼c hexapeptides, (R,R)- and (S,S)-15
respectively, (R,R)-Ab5,6c hexapeptide 17, and (R,R)-Ab5,6¼c nonapeptide 16 were
recorded in 2,2,2-trifluoroethanol (TFE) solution to obtain information about their
helical screw senses. The spectrum of (R,R)-15 does not show characteristic maxima for
a helical structure (208 and 222 nm), suggesting the existence of roughly equal amounts

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1
Fig. 2. ROESY H-NMR (CDCl3) Spectra of hexapeptides 15 (a) and 17 (b), and nonapeptide 16 (c).
Peptide concentration: 5.0 mm.
of both (P)- and (M)-helices (Fig. 3). The CD spectrum of the enantiomeric (S,S)-15
shows the mirror image shape of the CD spectrum of (R,R)-15. These results suggest
that the (R,R)- and (S,S)-15 form enantiomeric global secondary structures. The

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Fig. 3. CD Spectra of Boc-[(R,R)-Ab5,6¼c]n-OEt ((R,R)-15: n ¼ 6 (circle), (R,R)-16: n ¼ 9 (square)),
enantiomeric Boc-[(S,S)-Ab5,6¼c]6-OEt ((S,S)-15 (triangle)), and Boc-[(R,R)-Ab5,6c]6-OEt ((R,R)-17
(cross)) in TFE soln. Peptide concentration: 50 mm.
spectrum of nonapeptide 16 shows weak negative maxima at 208 and 222 nm, and a
positive maximum at 192 nm, indicating that the (P)-helix would be slightly
predominant. The conversion of 15 to the saturated (R,R)-Ab5,6c hexapeptide 17 by
hydrogenation changed the shape of the CD spectrum, suggesting that the dominant
conformation may slightly be changed.
X-Ray Crystallographic Analyses. X-Ray crystallographic analyses unambiguously
revealed the secondary structures of (S,S)-Ab5,6¼c tripeptide (S,S)-12 and (R,R)-
Ab5,6¼c hexapeptide (R,R)-15 in the crystal state. Both the tripeptide (S,S)-12 and the
hexapeptide (R,R)-15 provided suitable crystals for X-ray crystallographic analyses by
slow evaporation of CHCl3/EtOH solutions at room temperature. X-Ray data
collections were performed using RIGAKU VariMax with Saturn (confocal) for
(S,S)-12, and RIGAKU RAPID and graphite-monochromated MoKa radiation for
(R,R)-15. The structures of the peptides were solved using the SHELXS 97 direct
method [14] and expanded using the Fourier technique [15]. All non-H-atoms were
given anisotropic thermal parameters, some H-atoms were refined isotropically, and
the remaining H-atoms were placed at the calculated positions. Relevant backbone and
side-chain torsion angles and the intra- and intermolecular H-bond parameters are
compiled in Tables 1 – 4, respectively.
The (S,S)-Ab5,6¼c tripeptide (S,S)-12 crystallized in space group P1 to form four b-
turn conformers (310-helices) A, B, C, and D in the asymmetric unit. Two molecules A
and D were (P)-b-turns, and two molecules B and C were (M)-b-turns (Fig. 4). In four

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Table 1. Selected Torsion Angles w, f, and y [8] for Tripeptide (S,S)-12, as Determined by X-Ray
Crystallographic Analysis
Torsion angle
A
B
C
D
w0
f1
y1
w1
f2
y2
w2
f3
y3
w3
ꢀ 171.1
ꢀ 64.6
ꢀ 26.0
ꢀ 178.5
ꢀ 48.2
ꢀ 47.5
ꢀ 174.7
43.9
171.6
62.5
29.1
179.6
48.8
47.9
ꢀ 179.5
ꢀ 56.7
ꢀ 36.6
ꢀ 175.5
170.1
63.4
26.2
ꢀ 171.9
ꢀ 60.6
ꢀ 30.1
ꢀ 179.2
ꢀ 46.1
ꢀ 45.6
ꢀ 178.1
55.4
ꢀ 177.9
48.1
47.8
178.3
ꢀ 43.2
ꢀ 47.1
175.6
46.5
ꢀ 174.0
35.2
174.8
Table 2. Intra- and Intermolecular H-Bond Parameters for Tripeptide (S,S)-12
Peptidea)
Donor
Acceptor
Distance [ꢅ]
Angle [8]
Symmetry operations
Molecule A
N3aꢀH
N1aꢀH
O0a
O1b’
3.13
2.87
139.9
164.2
x, y, z
1 þ x, y, z
Molecule B
Molecule C
Molecule D
N3bꢀH
O0b
O1a
3.11
2.89
133.2
168.0
x, y, z
x, y, z
N1bꢀH
N3cꢀH
N1cꢀH
O0c
O1d’
3.09
2.91
135.5
163.0
x, y, z
ꢀ 1 þ x, ꢀ 1 þ y, z
x, y, z
x, 1 þ y, z
N3dꢀH
O0d
O1c
3.06
2.82
140.9
164.0
N1d’ꢀH
a) The number of amino acid residues begins at the N-terminus of the peptide chain.
b-turn conformers, the f and y torsion angle signs at the C-terminus residue (3) were
opposite of those of the preceding residues (1 and 2). The peptide-backbone structures
of (P)-b-turns A and D, and also (M)-b-turns B and C were similar, respectively
(Fig. 5).
In the b-turn structures A – D, one intramolecular H-bond of the i i þ 3 type was
observed between HꢀN(3) and C(0) ¼ O(0) (3.13 ꢅ (a), 3.11 ꢅ (b), 3.09 ꢅ (c), 3.06 ꢅ
(d)), respectively. In the packing mode, the b-turn structures of ··· A(P) ··· B(M) ···
A(P) ··· B(M) ··· and ··· C(M) ··· D(P) ··· C(M) ··· D(P) ··· were formed, by one
intermolecular H-bond, respectively.
In the asymmetric unit of (R,R)-Ab5,6¼c hexapeptide (R,R)-15, there were four
crystallographically independent molecules E, F, G, and H, along with two EtOH
molecules. Two molecules E and H were (P)-310-helices, and two molecules F and G
were (M)-310-helices (Fig. 6). The main-chain backbone structures of (P)-helical
molecules E and H, and also (M)-helical molecules F and G were well matched,
respectively, except for small differences in the conformations of their side chains
(Fig. 7). The mean values of the f, y torsion angles of the amino acid residues (1 – 5)

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Table 3. Selected Torsion Angles w, f, and y [8] for Hexapeptide (R,R)-15, as Determined by X-Ray
Crystallographic Analysis
Torsion angle
E
F
G
H
w0
f1
y1
w1
f2
y2
w2
f3
y3
w3
f4
y4
w4
f5
y5
w5
f6
y6
w6
ꢀ 176.1
ꢀ 61.1
ꢀ 25.2
ꢀ 179.5
ꢀ 60.9
ꢀ 17.2
176.0
ꢀ 58.4
ꢀ 20.6
177.2
ꢀ 59.9
ꢀ 23.8
175.4
ꢀ 55.7
ꢀ 32.7
ꢀ 175.3
51.2
172.7
66.1
19.0
178.8
59.1
20.7
172.5
61.8
26.7
178.7
56.8
19.0
ꢀ 171.8
ꢀ 66.8
ꢀ 19.9
179.4
ꢀ 59.1
ꢀ 19.9
174.9
ꢀ 56.2
ꢀ 24.8
179.9
ꢀ 59.8
ꢀ 25.2
ꢀ 179.0
ꢀ 53.9
ꢀ 37.7
ꢀ 173.4
48.6
ꢀ 178.3
ꢀ 178.8
53.7
56.3
29.4
177.5
59.2
23.6
ꢀ 179.2
54.1
22.2
27.1
179.2
59.1
ꢀ 178.7
52.9
37.5
27.4
171.5
ꢀ 48.4
ꢀ 47.7
179.4
170.2
ꢀ 54.6
ꢀ 44.3
179.6
46.6
157.9
45.1
179.8
Fig. 4. a) X-Ray diffraction structure of Boc-[(S,S)-Ab5,6¼c]3-OEt ((S,S)-12); b) the structure of molecule
A, and c) molecule C, as viewed along the helical axis
were ꢀ 59.38, ꢀ 23.98 for E, þ 58.28, þ 25.88 for F, þ 57.68, þ 24.88 for G, and ꢀ 59.28,
ꢀ 25.58 for H, respectively, which are close to those for an ideal 310-helical structure
(ꢂ608 and ꢂ308) [16]. Reversal of the torsion angle signs at the C-terminus residue
occurred, i.e., the values of the f, y torsion angles of the residue (6) were þ 51.28, þ
46.68 for E, ꢀ 48.48, ꢀ 47.78 for F, ꢀ 54.68, ꢀ 44.38 for G, and þ 48.68, þ 45.18 for H.

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Table 4. Intra- and Intermolecular H-Bond Parameters for Hexapeptide (R,R)-15
Peptidea)
Donor
Acceptor
Distance [ꢅ]
Angle [8]
Symmetry operations
Molecule E
N3eꢀH
N4eꢀH
N5eꢀH
N6eꢀH
N1eꢀH
N2eꢀH
O0e
O1e
O2e
O3e
O5f
O6f
2.99
3.03
2.95
2.93
2.98
2.96
167.2
173.3
157.8
155.6
168.2
149.5
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
Molecule F
Molecule G
N3fꢀH
N4fꢀH
N5fꢀH
N6fꢀH
N1fꢀH
N2fꢀH
N3gꢀH
N4gꢀH
N5gꢀH
N6gꢀH
N1gꢀH
N2gꢀH
O0f
O1f
O2f
O3f
O5e’
O6e’
3.16
2.96
2.93
3.05
2.87
3.15b)
171.8
166.1
159.5
158.0
167.6
131.7
x, y, z
x, y, z
x, y, z
x, y, z
x, y, ꢀ 1 þ z
x, y, ꢀ 1 þ z
O0g
O1g
O2g
O3g
O5h’
O6h’
3.01
3.01
2.95
2.92
3.03
2.95
169.1
172.3
163.9
167.4
171.5
147.5
x, y, z
x, y, z
x, y, z
x, y, z
x, y, 1 þ z
x, y, 1 þ z
Molecule Hc) N3hꢀH
O0h
O1h
O2h
O3h
O5g
3.12
2.93
2.95
3.06
2.85
167.7
165.1
158.9
152.8
160.1
x, y, z
x, y, z
x, y, z
x, y, z
x, y, z
N4hꢀH
N5hꢀH
N6hꢀH
N1hꢀH
a) The number of amino acid residues begins at the N-terminus of the peptide chain. b) The D ··· A
distance is slightly long for an intermolecular H-bond. c) The D ··· A distance of N2h ··· O6g (3.48 ꢅ) is too
long for an intermolecular H-bond.
Fig. 5. Overlaid structures of Boc-[(S,S)-Ab5,6¼c]3-OEt ((S,S)-12). a) Molecules A (dark) and B
(bright), and b) molecules C (dark) and D (bright).

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Fig. 6. X-Ray diffraction structure of a) Boc-[(R,R)-Ab5,6¼c]6-OEt ((R,R)-15) (EtOH molecules are
omitted); b) the structure of molecule E and c) molecule F, as viewed along the helical axis
Fig. 7. Overlaid structures of Boc-[(R,R)-Ab5,6¼c]6-OEt ((R,R)-15). a) Molecules E (dark) and H
(bright), and b) molecules F (dark) and G (bright)
In the 310-helical molecules E – H, four consecutive H-bonds of the i i þ 3 type
were observed between HꢀN(3) and C(0)¼O(0) (N(3) ··· O(0) 2.99 (e), 3.16 (f), 3.01
(g), and 3.12 ꢅ (h); NꢀH ··· O 167.28 (e), 171.88 (f), 169.18 (g), and 167.78 (h)),
HꢀN(4) and C(1)¼O(1) (N(4) ··· O(1) 3.03 (e), 2.96 (f), 3.01 (g), and 2.93 ꢅ (h); NꢀH

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··· O 173.38 (e), 166.18 (f), 172.38 (g), and 165.18 (h)), HꢀN(5) and C(2)¼O(2) (N(5)
··· O(2) 2.95 (e), 2.93 (f), 2.95 (g), and 2.95 ꢅ (h); NꢀH ··· O 157.88 (e), 159.58 (f),
163.98 (g), and 158.98 (h)), and HꢀN(6) and C(3)¼O(3) (N(6) ··· O(3) ¼ 2.93 (e), 3.05
(f), 2.92 (g), and 3.06 ꢅ (h); NꢀH ··· O 155.68 (e), 158.08 (f), 167.48 (g), and 152.88
(h)).
In the packing mode, the 310-helical chains of ··· E(P) ··· F(M) ··· E(P) ··· F(M) ···
and ··· G(M) ··· H(P) ··· G(M) ··· H(P) ··· mode were formed in head-to-tail align-
ment. Four intermolecular H-bonds between molecules E and F, i.e., HꢀN(1e) and
C(5f)¼O(5f), HꢀN(2e) and C(6f)¼O(6f), HꢀN(1f) and C(5e’)¼O(5e’), and HꢀN(2f)
and C(6e’)¼O(6e’) were observed, and three intermolecular H-bonds between
molecules G and H, i.e., HꢀN(1g) and C(5h’)¼O(5h’), HꢀN(2g) and C(6h’)¼O(6h’),
and HꢀN(1h) and C(5g)¼O(5g) were observed, as shown in Table 4 and Fig. 8.
Fig. 8. Packing of hexapeptide 15 (dark color for molecules E and H, and bright color for molecule F and
G) in the crystalline state. Intermolecular H-bonds are indicated as dashed lines.
Molecular-Mechanics Calculation. A conformational search calculation of (R,R)-
Ab5,6¼c hexapeptide (R,R)-15 was performed using the Monte Carlo Multiple
Minimum (MCMM) method of MacroModel (v. 8.1, Schrçdinger, Inc.) with the
AMBER* force field. As initial structures, extended, 310- and a-helical structures were
used, and more than 50,000 structures were optimized. The calculated structure of
(R,R)-15 produced a (P)-310-helix as the global minimum-energy conformation (0 kcal/
mol), and an (M)-310-helix was obtained as a local minimum-energy conformation,
which exhibited an energy of þ 1.60 kcal/mol. The peptide main-chain structures of
(P)- and (M)-310-helical conformers produced by the calculations were similar to those
observed in the crystalline state, with some differences in the side-chain and the C-
terminal ester conformations, as depicted by their superimposition in Fig. 9.
Discussion. – The dominant conformations of (R,R)- and (S,S)-Ab5,6¼c, and
saturated (R,R)-Ab5,6c homopeptides in solution were found to be 310-helical structures
by FT-IR and 1H-NMR analyses. Judging from the CD spectra in TFE solution, roughly

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Fig. 9. Superimposition of the conformation determined by X-ray crystallographic analysis and of the
calculated minimum-energy conformation of (R,R)-15. Overlaid X-ray (dark) and calculated (bright)
structure of right-handed (P)-helix as viewed a) perpendicular to and b) along the helical axes. Overlaid
X-ray (dark) and calculated (bright) structure of left-handed (M)-helix as viewed c) perpendicular to
and d) along the helical axes.
equal amounts of both (P)- and (M)-helices seemed to occur of two (R,R)-Ab5,6¼c and
(R,R)-Ab5,6c hexapeptides 15 and 17, respectively, while the enantiomeric (R,R)-
Ab5,6¼c and (S,S)-Ab5,6¼c hexapeptides showed the mirror images of the CD spectra,
suggesting that one enantiomeric global structure may be slightly predominant.
Furthermore, the CD spectrum of the (R,R)-Ab5,6¼c nonapeptide 16 showed character-
istic maxima at 192, 208, and 222 nm for the right-handed (P)-helical structure,
although the intensities of ellipticity were weak. These results might be attributed to the
fact that, by the increase of the number of stereogenic centers (eighteen) in 16, the (P)-
helical conformer would become more stable than the (M)-conformers.
In the crystalline state, X-ray crystallographic analyses of (S,S)-12 and (R,R)-15
revealed the existence of both diastereoisomeric (P)- and (M)-b-turns, or 310-helical
conformations in a 1:1 ratio, respectively.
We have reported that the chiral cyclic a-amino acid (S,S)-Ac5cdOM, in which
stereogenic centers are only at the side-chain g-C-atoms, controls the helical screw
sense of its homopeptides [4]. That is to say, the (S,S)-Ac5cdOM hexapeptide forms a
(M)-310-helix, and the (S,S)-Ac5cdOM octapeptide and decapeptide form (M)-a-helices
both in solution and in the crystalline state.
The g-side-chain stereogenic centers in bicyclic a-amino acid (R,R)-Ab5,6¼c are less
flexible than those in monocyclic a-amino acid (S,S)-Ac5cdOM because the stereogenic
centers exist at the fused-ring junctions, and thus, the helical-screw handedness of
(R,R)-Ab5,6¼c hexapeptide (R,R)-15 would not be controlled.
Fig. 10 shows the mean values of torsion angles (CaꢀCbꢀCgꢀCd) of (R,R)-Ab5,6¼
c
and those of torsion angles (CaꢀCbꢀCgꢀO) of (S,S)-Ac5cdOM in their crystalline states.
In (S,S)-Ac5cdOM, the MeO substitutents occupy the vertical region of the cyclopentane

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ring and would strongly affect the peptide-backbone conformation. On the other hand,
in (R,R)-Ab5,6¼c, the cyclohexane substituent occupies the horizontal region of the
cyclopentane ring, and might slightly affect the peptide-backbone conformation.
Taking these results into account, the side-chain stereogenic centers of a-amio acids
affect the secondary structure of oligomers, and the side-chain environments around
the stereogenic centers, such as bulkiness and flexibility, are important to control the
screw sense of their helical oligomers.
Fig. 10. Torsion angles of side chain at the g-position. a) Mean values of torsion angles (CaꢀCbꢀCgꢀCd) of
(R,R)-Ab5,6¼c, and b) those of torsion angles (CaꢀCbꢀCgꢀO) of (S,S)-Ac5cdOM
.
Conclusions. – We have synthesized chiral bicyclic a-amino acids (R,R)- and (S,S)-
Ab5,6¼c, and modified them to various cyclic a,a-disubstituted amino acids. The (R,R)-
Ab5,6¼c homochiral hexapeptide (R,R)-15, having twelve stereogenic centers at the
side-chain bicyclic skeleton formed both diastereoisomeric right-handed (P)- and left-
handed (M)-310-helices. These results indicate that the side-chain stereogenic centers
affect the secondary structure of their peptides, but the side-chain environments of the
stereogenic centers (bulkiness and flexibility) are important to control the screw sense
of helical oligomers (foldamers).
Experimental Part
General. (S,S)- and (R,R)-cyclohex-4-ene-1,2-dicarboxylic acids (1) were prepared according to the
methods described in [7]. Optical rotations: Jasco DIP-316 polarimeter, with a 1.0-dm cell. Circular
dichroism (CD) spectra: Jasco J-720W spectropolarimeter, with 1.0-mm path length cell. IR Spectra:
Nicolet Avatar-320 spectrometer for conventional measurement (KBr, or neat), and the soln. (CDCl3)
method with 0.1-mm path length of NaCl cells. 1H-NMR Spectra: at 400 or 500 MHz (Varian Unity). EI-
MS and FAB-MS spectra: Jeol JMS 610 H or Jeol JMS-SX 102 spectrometer. Elemental analyses were
performed at the Analytical Center of the Faculty of Sciences, Kyushu University.
Synthesis and Modification of Bicyclic Amino Acids.
(1S,2S)-Cyclohex-4-ene-1,2-dicarboxylic Acid ((S,S)-1) [7]. Data of (S,S)-1. Colorless crystals. M.p.
147 – 1488. [a]2D6 ¼ þ142.0 (c ¼ 1.10, EtOH) ([7]: [a]D ¼ þ160.0 (c ¼ 2.70, EtOH)). IR (KBr): 3100 (br.),
2933, 1698, 1215. 1H-NMR (CDCl3): 10.17 (br., 2 H); 5.73 (m, 2 H); 2.83 (m, 2 H); 2.45 (m, 2 H); 2.28
(m, 2 H). FAB-MS: 171 ([M þ H]þ). Anal. calc. for C8H10O4 (170.16): C 56.47, H 5.92; found: C 56.49, H
5.83.
Data of (R,R)-1. [a]D23 ¼ ꢀ149.0 (c ¼ 0.97, EtOH).
(4S,5S)-4,5-Bis(hydroxymethyl)cyclohexene (¼(1S,2S)-Cyclohex-4-ene-1,2-diyldimethanol; (S,S)-
2). The carboxylic acid (S,S)-1 (3.6 g, 21.2 mmol) was added portionwise to a rigorously stirred

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suspension of LiAlH4 (1.80 g, 46.5 mmol) in THF (160 ml) at 08. After stirring at r.t. for 12 h, the reaction
was quenched with 15% aq. NaOH, and the resulting suspension was filtered off through Celite. The
filtrate was dried (MgSO4), and evaporated in vacuo to leave a white solid, which was purified by CC
(SiO2). The fraction eluted with 5% MeOH in CHCl3 afforded diol (S,S)-2 (2.60 g, 86%). Colorless
crystals. M.p. 53 – 548. [a]2D6 ¼ þ66.5 (c ¼ 1.00, EtOH). IR (KBr): 3300 (br.), 3025, 2959, 2916. 1H-NMR
(400 MHz, CDCl3): 5.65 (m, 2 H); 3.83 (br., 2 H); 3.70 (d, J ¼ 11.0, 2 H); 3.56 (dd, J ¼ 5.0, 11.0, 2 H); 2.02
(m, 2 H); 1.85 (m, 2 H); 1.68 (br. s, 2 H). FAB-MS: 143 ([M þ H]þ). Anal. calc. for C8H14O2 (142.20): C
67.57, H 9.92; found: C 67.12, H 9.94.
Data of (R,R)-2 Yield: 92%. M.p. 55 – 568. [a]2D5 ¼ ꢀ66.7 (c ¼ 1.02, EtOH).
(4S,5S)-4,5-Bis(iodomethyl)cyclohexene ((S,S)-3). A soln. of (S,S)-2 (2.50 g, 17.6 mmol), PPh3
(18.4 g, 70.3 mmol), 1H-imidazole (7.2 g, 105.5 mmol), and I2 (22.3 g, 87.9 mmol) in THF (200 ml) was
stirred at r.t. for 3 h. The soln. was diluted with AcOEt, washed with H2O, sat. aq. Na2S2O3, aq. 1N HCl,
5% aq. NaHCO3, brine, and dried (MgSO4). After removal of the solvent, the residue was purified by CC
(SiO2; hexane) to give (S,S)-3 (6.20 g, 97%). Colorless crystals. M.p. 39 – 408. [a]2D6 ¼ þ49.4 (c ¼ 1.13,
EtOH). IR (KBr): 3023, 2951, 2898, 2833, 1257, 1202, 1173. 1H-NMR (400 MHz, CDCl3): 5.62 (m, 2 H);
3.37 (dd, J ¼ 4.3, 10.3, 2 H); 3.28 (d, J ¼ 10.3, 2 H); 2.02 – 2.08 (m, 4 H); 1.42 (m, 2 H). FAB-MS: 362
([M þ H]þ). Anal. calc. for C8H12I2 (361.99): C 26.54, H 3.34; found: C 26.52, H 3.27.
Data of (R,R)-3 Yield: 91%. M.p. 39 – 408. [a]2D5 ¼ ꢀ54.2 (c ¼ 0.81, EtOH).
Ethyl (1R,6R)-8-[(tert-Butoxycarbonyl)amino]bicyclo[4.3.0]non-3-ene-8-carboxylate (¼ Ethyl
(3aR,7aR)-2-[(tert-Butoxycarbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-indene-2-carboxylate; Boc-
[(R,R)-Ab5,6¼c]-OEt; (R,R)-4). A soln. of CNCH2COOEt (756 ml, 6.92 mmol) and (S,S)-3 (2.27 g,
6.29 mmol) in Et2O/DMF (10 ml; 9 :1) was added dropwise to the stirred suspension of NaH (423 mg,
17.61 mmol) in Et2O/DMF (40 ml) at 08. After stirring at 08 for 3 h, the soln. was diluted with H2O,
extracted with Et2O, and dried (MgSO4). Removal of the solvent afforded an oily residue, which was
purified by CC (SiO2; 30% AcOEt in hexane) to give the alkylated isonitrile (882 mg, 64%). Colorless
oil. [a]2D6 ¼ þ98.8 (c ¼ 1.25, EtOH). IR (neat): 3023, 2980, 2895, 2836, 2135, 1744, 1443, 1275, 1195.
1H-NMR (400 MHz, CDCl3): 5.70 – 5.71 (m, 2 H); 4.27 (q, J ¼ 7.1, 2 H); 2.75 (dd, J ¼ 7.7, 13.0, 1 H); 2.46
(dd, J ¼ 11.8, 18.0, 1 H); 2.27 – 2.34 (m, 2 H); 1.71 (m, 1 H); 1.80 – 1.98 (m, 5 H); 1.34 (t, J ¼ 7.1, 3 H ) .
FAB-MS: 219 ([M þ H]þ).
A soln. of the alkylated isonitrile (1.70 g, 7.75 mmol) and conc. aq. HCl (4 ml) in EtOH (50 ml) was
stirred at 08 for 3 h. Then, the soln. was neutralized with 5% aq. NaHCO3, and EtOH was evaporated.
The aq. residue was extracted with AcOEt and dried (MgSO4). Removal of the solvent afforded a crude
amine, which was solved in dioxane (80 ml), and then (Boc)2O (2.03 g, 9.30 mmol) and Et3N (1.3 ml,
9.32 mmol) were added. After stirring at r.t. for 24 h, the soln. was evaporated, and diluted with AcOEt.
The soln. was washed with 1n aq. HCl, 5% aq. NaHCO3, brine, and dried (MgSO4). After removal of the
solvent, the oily residue was purified by CC (SiO2). The fraction eluted with 30% AcOEt in hexane
afforded (R,R)-4 (2.21 g, 92%). Colorless crystals. M.p. 101 – 1038. [a]2D6 ¼ þ60.8 (c ¼ 0.99, EtOH). IR
(KBr): 3350 (br.), 3021, 2981, 1712, 1216. 1H-NMR (400 MHz, CDCl3): 5.63 – 5.66 (m, 2 H); 5.01 (br. s,
1 H); 4.16 (q, J ¼ 7.0, 2 H); 2.62 (dd, J ¼ 6.0, 13.3, 1 H); 2.22 – 2.26 (m, 2 H); 2.10 (m, 1 H); 1.79 – 1.93 (m,
5 H); 1.49 (m, 1 H); 1.42 (s, 9 H); 1.24 (t, J ¼ 7.0, 3 H). FAB-MS: 310 ([M þ H]þ). HR-FAB-MS:
310.2006 ([M þ H]þ, C17H28NO4þ ; calc. 310.2013). Anal. calc. for C17H27NO4 (309.40): C 65.99, H 8.80, N
4.53; found: C 65.90, H 8.73, N 4.50.
Data of (S,S)-4 Yield: 77%. M.p. 101 – 1038. [a]2D8 ¼ ꢀ62.1 (c ¼ 0.975, EtOH).
Ethyl (1R,6R)-8-Aminobicyclo[4.3.0]non-3-ene-8-carboxylate (¼ Ethyl (3aR,7aR)-2-Amino-
2,3,3a,4,7,7a-hexahydro-1H-indene-2-carboxylate; H2N-[(R,R)-Ab5,6¼c]-OEt; (R,R)-5}. A soln. of
(R,R)-4 (190 mg, 0.615 mmol) in conc. aq. HCl (3 ml) and AcOEt (9 ml) was stirred at r.t. for 5 h.
The soln. was neutralized with 5% aq. NaHCO3, extracted with AcOEt, and dried (MgSO4). After
removal of the solvent, the residue was purified by CC (SiO2; 10% MeOH in CHCl3) to give (R,R)-5
(127 mg, > 99%). Colorless oil. [a]2D9 ¼ þ93.3 (c ¼ 0.30, EtOH). IR (neat): 3370 (br.), 3302, 3020, 2958,
2906, 2832, 1727, 1441, 1266, 1189. 1H-NMR (400 MHz, CDCl3): 5.68 – 5.69 (m, 2 H); 4.17 (q, J ¼ 7.1, 2 H ) ;
2.50 (dd, J ¼ 7.3, 12.9, 1 H); 2.24 – 2.29 (m, 2 H); 1.68 – 1.89 (m, 8 H); 1.28 (t, J ¼ 7.1, 3 H); 1.21 (m, 1 H).
HR-FAB-MS: 210.1530 ([M þ H]þ, C12H20NOþ2 ; calc. 210.1489).
Data of (S,S)-5 Yield: > 99%. [a]2D8 ¼ ꢀ102.7 (c ¼ 1.60, EtOH).

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(1R,6R)-8-[(tert-Butoxycarbonyl)amino]bicyclo[4.3.0]non-3-ene-8-carboxylic Acid (¼(3aR,7aR)-
2-[(tert-Butoxycarbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-indene-2-carboxylic Acid; Boc-[(R,R)-
Ab5,6¼c]-OH; (R,R)-6). A soln. of (R,R)-4 (186 mg, 0.602 mmol) and 1n aq. NaOH (2.0 ml, 2.0 mmol)
in MeOH (12 ml) was stirred at r.t. for 12 h. Then, the soln. was neutralized with 1n aq. HCl, and MeOH
was evaporated. The aq. soln. was extracted with AcOEt and dried (Na2SO4). Removal of the solvent
afforded (R,R)-6 (170 mg, > 99%). Colorless crystals, which were used for the next reaction without
further purification.
Data of (R,R)-6. M.p. 88 – 908. [a]2D4 ¼ þ49.0 (c ¼ 1.02, EtOH). IR (KBr): 3333, 3023, 2981, 1710,
1681, 1529, 1172. 1H-NMR (400 MHz, CDCl3): 5.65 – 5.68 (m, 2 H); 5.09 (br. s, 1 H); 2.74 (m, 1 H); 2.29
(m, 1 H); 2.25 (m, 1 H); 2.13 (m, 1 H); 1.99 (m, 1 H); 1.76 – 1.84 (m, 4 H); 1.44 (br., 1 H); 1.44 (s, 9 H).
HR-FAB-MS: 282.1747 ([M þ H]þ, C15H24O4Nþ; calc. 282.1705).
Data of (S,S)-6 Yield: > 99%. M.p. 90 – 918. [a]2D9 ¼ ꢀ53.1 (c ¼ 0.91, EtOH).
Ethyl (3R,4R)-1-[(tert-Butoxycarbonyl)amino]-3,4-bis(2-hydroxyethyl)cyclopentanecarboxylate
(7). Ozone gas was bubbled into a soln. of (R,R)-4 (60 mg, 0.194 mmol) in MeOH (4 ml) and CH2Cl2
(6 ml) at ꢀ 788, and the reaction was monitored by TLC. NaBH4 (32 mg, 0.86 mmol) was added
portionwise to the soln. at ꢀ 788. After stirring at ꢀ 788 for 30 min, the soln. was gradually warmed to r.t.
and neutralized with 1n aq. HCl. After evaporation of MeOH and CH2Cl2, the aq. soln. was extracted
with CHCl3 and dried (MgSO4). Removal of the solvent afforded a colorless oil, which was purified by
CC (SiO2). The fraction eluted with 5% MeOH in CHCl3 gave 7 (59 mg, 63%). Colorless oil. [a]D25
¼
ꢀ35.3 (c ¼ 1.00, EtOH). IR (neat): 3350 (br.), 2977, 2932, 1720, 1699, 1519, 1168, 1050. 1H-NMR
(400 MHz, CDCl3): 5.18 (br. s, 1 H); 4.17 (q, J ¼ 7.1, 2 H); 3.59 – 3.74 (m, 4 H); 2.57 (dd, J ¼ 8.1, 13.7,
1 H); 1.81 – 2.20 (m, 9 H); 1.58 (m, 1 H); 1.48 (m, 1 H); 1.43 (s, 9 H); 1.26 (t, J ¼ 7.1, 3 H). HR-FAB-MS:
346.2199 ([M þ H]þ, C17H32NOþ6 ; calc. 346.2224).
2,2’-[(1R,2R)-4-[(tert-Butoxycarbonyl)amino]-4-(ethoxycarbonyl)cyclopentane-1,2-diyl]diacetic
Acid (8). Ozone gas was bubbled into a soln. of (R,R)-4 (50 mg, 0.162 mmol) in MeOH (9 ml) and
CH2Cl2 (1 ml) at ꢀ 788, and the reaction was monitored by TLC. Me2S (0.2 ml) was added to the soln. at
ꢀ 788, and the soln. was warmed to r.t. Removal of the solvent afforded a crude dialdehyde. The crude
dialdehyde and Oxoneꢂ (104 mg, 0.169 mmol) in DMF (4 ml) was stirred at r.t. for 5 h. The soln. was
acidified with 1n aq. HCl, extracted with AcOEt, and dried (Na2SO4). After removal of the solvent, the
residue was purified by CC (SiO2; 5% MeOH in CHCl3) to give 8 (24 mg, 40%). Colorless oil. [a]D25
¼
ꢀ16.9 (c ¼ 0.72, CHCl3). IR (neat): 3368 (br.), 2980, 1713, 1369, 1167. 1H-NMR (400 MHz, CDCl3): 5.13
(br., 1 H); 4.19 (q, J ¼ 7.1, 2 H); 1.45 – 2.61 (m, 12 H); 1.43 (s, 9 H); 1.27 (t, J ¼ 7.1, 3 H). EI-MS: 300.3 (4,
t
[M ꢀ BuO]þ), 241.2 (19), 149.1 (15), 83.0 (100).
Ethyl (1R,7R)-N-Benzyl-9-[(tert-butoxycarbonyl)amino]-4-azabicyclo[5.3.0]decane-9-carboxylate
(¼ Ethyl (5aR,8aR)-3-Benzyl-7-[(tert-butoxycarbonyl)amino]decahydrocyclopenta[d]azepine-7-car-
boxylate; 9). Crude aldehyde, which was prepared from (R,R)-4 (50 mg, 0.162 mmol) by the same
procedure, was dissolved in CH2Cl2 (3 ml) and MeOH (3 ml). BnNH2 (44 ml, 0.40 mmol) was added to
the soln., and the soln. was stirred at r.t. for 30 min, and then NaBH3CN (25 mg, 0.40 mmol) was added.
After stirring at r.t. for 12 h, the soln. was diluted with H2O, extracted with CHCl3, and dried (MgSO4).
Removal of the solvent afforded an oily residue, which was purified by CC (SiO2 (10% AcOEt in hexane)
to give 9 (35 mg, 53%). Colorless oil. [a]2D4 ¼ ꢀ6.90 (c ¼ 0.17, CHCl3). IR (neat): 3366 (br.), 3026, 2976,
1
2930, 1716 (br.), 1495, 1453, 1366, 1170. H-NMR (400 MHz, CDCl3): 7.21 – 7.35 (m, 5 H); 4.95 (br. s,
1 H); 4.18 (q, J ¼ 7.1, 2 H); 3.65 (s, 2 H); 2.54 – 2.74 (m, 5 H); 1.82 – 2.12 (m, 7 H); 1.55 (m, 1 H); 1.44 (s,
9 H); 1.36 (m, 1 H); 1.27 (t, J ¼ 7.1, 3 H). HR-FAB-MS: 417.2767 ([M þ H]þ, C24H37N2O4þ ; calc.
417.2753).
Ethyl (1R,6R)-8-[(tert-Butoxycarbonyl)amino]bicyclo[4.3.0]nonane-8-carboxylate (¼ Ethyl
(3aR,7aR)-2-[(tert-Butoxycarbonyl)amino]octahydro-1H-indene-2-carboxylate; Boc-[(R,R)-Ab5,6c]-
OEt; 10). A suspension of (R,R)-4 (23 mg, 0.074 mmol) and 5% Pd/C (50 mg) in MeOH (5 ml) was
stirred at r.t. under H2 for 24 h. Then, the catalyst was filtered off, and the filtrate was evaporated. The
residue was purified by CC (SiO2; 10% AcOEt in hexane) to give 10 (24 mg, > 99%). Colorless crystals.
M.p. 53 – 548. [a]2D6 ¼ ꢀ6.35 (c ¼ 1.14, EtOH). IR (KBr): 3346, 2977, 2930, 2851, 1728, 1686, 1522, 1289,
1183. 1H-NMR (400 MHz, CDCl3): 5.01 (br. s, 1 H); 4.18 (q, J ¼ 7.1, 2 H); 2.51 (m, 1 H); 1.71 – 2.00 (m,
7 H); 1.43 (s, 9 H); 1.35 – 1.42 (m, 2 H); 1.26 (t, J ¼ 7.1, 3 H); 1.15 – 1.25 (m, 2 H); 0.98 – 1.15 (m, 2 H).

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FAB-MS: 312 ([M þ H]þ). Anal. calc. for C17H29NO4 (311.42): C 65.57, H 9.39, N 4.50; found: C 65.52, H
9.31, N 4.42.
Synthesis and Modification of Homochiral Oligomers.
Ethyl (3aR,7aR)-2-[({(3aR,7aR)-2-[(tert-butoxycarbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-in-
den-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-indene-2-carboxylate (Boc-[(R,R)-Ab5,6¼c]2-
OEt; (R,R)-11). A mixture of (R,R)-5 (100 mg, 0.50 mmol), (R,R)-6 (170 mg, 0.60 mmol), O-
benzotriazol-1-yl-N,N,N’,N’-tetramethyluronium hexafluorophosphate (HBTU; 228 mg, 0.60 mmol),
and EtNiPr2 (DIPEA; 174 ml, 1.00 mmol) in CH2Cl2 (5 ml) was stirred at 408 for 5 h. Then, the soln. was
diluted with AcOEt, washed with 1n aq. HCl, 5% aq. NaHCO3, and brine, and dried (MgSO4). Removal
of the solvent afforded white solids, which were purified by CC (SiO2; 1% MeOH in CHCl3) to give
(R,R)-11 (148 mg, 62%). Colorless crystals. M.p. 209 – 2108. [a]2D2 ¼ þ85.3 (c ¼ 0.38, CHCl3). IR (CDCl3):
3437 (br.), 3024, 2914, 2836, 1718, 1684, 1161. 1H-NMR (400 MHz, CDCl3): 7.26 (br. s, 1 H); 5.60 – 5.67
(m, 4 H); 4.92 (br. s, 1 H); 4.17 (q, J ¼ 7.2, 2 H); 2.80 (m, 1 H); 2.61 (m, 1 H); 2.17 – 2.24 (m, 5 H); 1.30 –
2.05 (m, 13 H); 1.44 (s, 9 H); 1.24 (t, J ¼ 7.2, 3 H). FAB-MS: 473 ([M þ H]þ). Anal. calc. for C27H40N2O5
(472.62): C 68.62, H 8.53, N 5.93; found: C 68.32, H 8.45, N 6.05.
Data of (S,S)-11 Yield: 68%. M.p. 210 – 2118. [a]2D6 ¼ ꢀ79.5 (c ¼ 0.67, CHCl3).
Ethyl (3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[(tert-Butoxycarbonyl)amino]-2,3,3a,4,7,7a-
hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-
2,3,3a,4,7,7a-hexahydro-1H-indene-2-carboxylate (Boc-[(R,R)-Ab5,6¼c]3-OEt; (R,R)-12). A mixture of
(R,R)-11 (156 mg, 0.330 mmol) in conc. aq. HCl (2 ml) and AcOEt (6 ml) was stirred at r.t. for 5 h. The
soln. was neutralized with 5% aq. NaHCO3, extracted with AcOEt, and dried (MgSO4). After removal of
the solvent, the residue was filtered through a short SiO2 column (10% MeOH in CHCl3) to leave an
amine (114 mg). A soln. of the amine (114 mg, 307 mmol), (R,R)-6 (103 mg, 0.367 mmol), HBTU
(140 mg, 0.370 mmol), and DIPEA (0.10 ml, 0.60 mmol) in CH2Cl2 (10 ml) was stirred at 408 for 48 h.
Then, the soln. was diluted with CHCl3, washed with 1n aq. HCl, 5% aq. NaHCO3, and brine, and dried
(MgSO4). After removal of the solvent, the residue was purified by CC (SiO2; 5% MeOH in CHCl3) to
give (R,R)-12 (165 mg, 85%). Colorless crystals. M.p. 238 – 2398 (recryst. from CHCl3). [a]2D4 ¼ þ70.6
(c ¼ 0.85, CHCl3). IR (CDCl3): 3433 (br.), 3365 (br.), 3024, 2913, 2835, 1720, 1706, 1521, 1271, 1160.
1H-NMR (400 MHz, CDCl3): 7.66 (br. s, 1 H); 6.59 (br. s, 1 H); 5.63 – 5.66 (m, 6 H); 5.07 (br. s, 1 H);
4.10 – 4.17 (m, 2 H); 2.72 – 2.87 (m, 3 H); 2.20 – 2.38 (m, 7 H); 1.50 – 2.17 (m, 17 H); 1.47 (s, 9 H); 1.20 –
1.31 (m, 6 H). HR-FAB-MS: 636.4017 ([M þ H]þ, C37H54N3O6þ ; calc. 636.4013).
Data of (S,S)-12: 68%. M.p. 237 – 2388 (recryst. from CHCl3/EtOH). [a]2D4 ¼ ꢀ81.0 (c ¼ 0.85,
CHCl3).
(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[(tert-Butoxycarbonyl)amino]-2,3,3a,4,7,7a-hexahy-
dro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,
4,7,7a-hexahydro-1H-indene-2-carboxylic Acid (Boc-[(R,R)-Ab5,6¼c]3-OH; (R,R)-13). A soln. of (R,R)-
12 (35 mg, 55 mmol) in 1n aq. NaOH (2.0 ml, 2.0 mmol) and MeOH (6 ml) was stirred at 608 for 12 h.
Then, the soln. was neutralized with 1n aq. HCl, and MeOH was evaporated. The aq. residue was
extracted with AcOEt and dried (Na2SO4). Removal of the solvent gave crude (R,R)-13, which was used
for the next reaction without purification. M.p. 148 – 1498. [a]2D3 ¼ þ112.6 (c ¼ 0.88, CHCl3). IR (KBr):
1
3323 (br.), 3021, 2907, 1684 (br.), 1520, 1167. H-NMR (400 MHz, CDCl3): 11.0 (br., 1 H); 7.77 (br. s,
1 H); 6.88 (br. s, 1 H); 5.63 – 5.66 (m, 6 H); 5.31 (br. s, 1 H); 2.80 – 2.93 (m, 3 H); 2.46 (m, 1 H); 2.21 –
2.29 (m, 7 H); 1.60 – 2.09 (m, 15 H); 1.46 (s, 9 H); 1.10 – 1.30 (m, 4 H). HR-FAB-MS: 608.3693 ([M þ
H]þ, C35H50N3Oþ6 ; calc. 608.3700).
Ethyl (3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,
7aR)-2-[(tert-Butoxycarbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,
4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)ami-
no]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}car-
bonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-indene-2-carboxylate (Boc-[(R,R)-Ab5,6¼c]6-OEt; (R,R)-15).
Aq. conc. HCl (0.7 ml) was added dropwise to the stirred soln. of (R,R)-12 (30 mg, 47 mmol) in AcOEt
(2 ml) at 08. After stirring at r.t. for 5 h, the soln. was neutralized with 5% aq. NaHCO3, extracted with
AcOEt, and dried (MgSO4). Removal of the solvent afforded a residue, which was purified by short CC
(SiO2; 10% MeOH in CHCl3) to give an N-terminal-free tripeptide (R,R)-14. A mixture of (R,R)-13

Helvetica Chimica Acta – Vol. 95 (2012)
1711
(33 mg, 55 mmol), HBTU (21 mg, 55 mmol), and DIPEA (16 ml, 94 mmol) in CH2Cl2 (2 ml) was added to
the stirred soln. of the N-terminal-free tripeptide (R,R)-14 in CH2Cl2 (3 ml) at r.t. After stirring at 408 for
48 h, the soln. was diluted with CHCl3, washed with 1n aq. HCl, 5% aq. NaHCO3, and brine, and dried
(MgSO4). Removal of the solvent afforded white solids, which were purified by CC (SiO2; 5% MeOH in
CHCl3) to give (R,R)-15 (27 mg, 50%). Colorless crystals. M.p. 2408 (dec.). [a]2D4 ¼ þ215.3 (c ¼ 0.64,
CHCl3). IR (CDCl3): 3426 (br.), 3332 (br.), 2913, 2835, 1725, 1698, 1525, 1280, 1158. 1H-NMR
(500 MHz, CDCl3): 8.16 (br. s, 1 H); 7.91 (br. s, 1 H); 7.65 (br. s, 1 H); 7.57 (br. s, 1 H); 6.90 (br. s, 1 H);
5.60 – 5.70 (m, 12 H); 5.32 (br. s, 1 H); 4.11 – 4.18 (m, 2 H); 3.10 – 3.23 (m, 4 H); 2.85 (m, 1 H); 2.48 – 2.70
(m, 3 H); 2.05 – 2.30 (m, 13 H); 1.56 – 1.95 (m, 33 H); 1.48 (s, 9 H); 1.20 – 1.42 (m, 9 H). FAB-MS: 1126
([M þ H]þ). HR-FAB-MS: 1147.6840 ([M þ Na]þ, C67H92N6NaO9þ ; calc. 1147.6823.
Data of (S,S)-15 Yield: 29%. M.p. 2408 (dec.). [a]2D5 ¼ ꢀ202.0 (c ¼ 0.395, CHCl3).
Ethyl (3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,
7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[(tert-Butoxycarbonyl)amino]-2,3,3a,4,7,7a-
hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-
2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)-
amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}-
carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-
inden-2-yl}carbonyl)amino]-2,3,3a,4,7,7a-hexahydro-1H-indene-2-carboxylate (Boc-[(R,R)-Ab5,6¼c]9-
OEt; (R,R)-16). Conc. aq. HCl (2 ml) was added to the stirred soln. of (R,R)-15 (90 mg, 80 mmol) in
AcOEt (5 ml) at 08. After stirring at r.t. for 12 h, the soln. was neutralized with 5% aq. NaHCO3,
extracted with CHCl3, and dried (MgSO4). After removal of the solvent, the residue was purified by
short CC (SiO2; 10% MeOH in CHCl3) to leave an N-terminal-free hexapeptide (45 mg, 44 mmol). A
soln. of the hexapeptide amine (45 mg, 44 mmol), (R,R)-13 (33 mg, 54 mmol), HBTU (21 mg, 54 mmol),
and DIPEA (15 ml, 86 mmol) in CH2Cl2 (5 ml) was stirred at 408 for 48 h. The soln. was diluted with
CHCl3, washed with 1n aq. HCl, 5% aq. NaHCO3, and brine, and dried (MgSO4). After removal of the
solvent, the residue was purified by CC (SiO2). The fraction eluted with 5% MeOH in CHCl3 afforded
(R,R)-16 (42 mg, 32%). White powder. M.p. 2508 (dec.). [a]2D6 ¼ þ247.1 (c ¼ 0.20, CHCl3). IR (CDCl3):
1
3424 (br.), 3314 (br.), 3023, 2912, 2890, 1724, 1697, 1650, 1528, 1281. H-NMR (400 MHz, CDCl3): 8.29
(br. s, 1 H); 8.17 (br. s, 1 H); 8.12 (br. s, 1 H); 8.09 (br. s, 1 H); 7.96 (br. s, 1 H); 7.68 (br. s, 1 H); 7.64 (br. s,
1 H); 6.97 (br. s, 1 H); 5.34 (br. s, 1 H); 5.60 – 5.67 (m, 18 H); 4.10 – 4.18 (m, 2 H); 3.09 – 3.24 (m, 8 H);
2.57 – 2.92 (m, 8 H); 2.05 – 2.25 (m, 20 H); 1.52 – 1.94 (m, 45 H); 1.48 (s, 9 H); 1.26 – 1.35 (m, 12 H). FAB-
MS: 1638 ([M þ Na]þ).
Ethyl (3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,7aR)-2-[({(3aR,
7aR)-2-[(tert-Butoxycarbonyl)amino]octahydro-1H-inden-2-yl}carbonyl)amino]octahydro-1H-inden-2-
yl}carbonyl)amino]octahydro-1H-inden-2-yl}carbonyl)amino]octahydro-1H-inden-2-yl}carbonyl)ami-
no]octahydro-1H-inden-2-yl}carbonyl)amino]octahydro-1H-indene-2-carboxylate (Boc-[(R,R)-Ab5,6c]6-
OEt; (R,R)-17). A suspension of (R,R)-15 (50 mg, 0.044 mmol) and 20% Pd(OH)2/C (50 mg) in MeOH
(9 ml) and THF (1 ml) was stirred under H2 for 24 h. Then, the catalyst was filtered off, and the filtrate
was evaporated. The residue was purified by CC (SiO2; 5% MeOH in CHCl3) to give (R,R)-17 (36 mg,
70%). Colorless crystals. M.p. 272 – 2738. [a]D26 ¼ þ132.1 (c ¼ 0.315, CHCl3). IR (CDCl3): 3426 (br.), 3336
(br.), 2925, 2853, 1723, 1699, 1661, 1522, 1448, 1287. 1H-NMR (500 MHz, CDCl3): 8.12 (br. s, 1 H); 7.83
(br. s, 1 H); 7.61 (br. s, 1 H); 7.51 (br. s, 1 H); 6.86 (br. s, 1 H); 5.29 (br. s, 1 H); 4.09 – 4.19 (m, 2 H); 2.97 –
3.10 (m, 4 H); 2.78 (m, 1 H); 2.34 – 2.58 (m, 3 H); 1.62 – 1.86 (m, 33 H); 1.51 (s, 9 H); 0.80 – 1.45 (m,
46 H). FAB-MS: 1138 ([M þ H]þ). HR-FAB-MS: 1159.7736 ([M þ Na]þ, C67H104O9N6Naþ; calc.
1159.7762).
Crystal Data for (S,S)-12 and (R,R)-15. (S,S)-12: 4(C37H53N3O6); Mr 2543.3; Triclinic; P1, a ¼
11.586 ꢅ, b ¼ 12.450 ꢅ, c ¼ 25.295 ꢅ; a ¼ 92.828, b ¼ 102.858, g ¼ 90.508; V¼ 3552.0 ꢅ3; Z ¼ 4; Dcalc
¼
1.189 g/cm3; m(MoKa) ¼ 0.80 cmꢀ1; No. of observations (I > 2s(I)) ¼ 14441; No. of variables, 1657;
R1 ¼ 0.0685, and Rw ¼ 0.1466.
(R,R)-15: 4(C67H92N6O9) · 2 EtOH; Mr 4594.0; Triclinic; P1, a ¼ 15.765 ꢅ, b ¼ 16.535 ꢅ, c ¼ 26.90 ꢅ;
a ¼ 74.228, b ¼ 82.328, g ¼ 75.398; V¼ 6514 ꢅ3; Z ¼ 4; Dcalc ¼ 1.171 g/cm3; m(MoKa) ¼ 0.78 cmꢀ1; No. of
observations (I > 2s(I)) ¼ 29059; No. of variables, 3045; R1 ¼ 0.0589, and Rw ¼ 0.1373. CCDC-892764 for
12 and CCDC-284623 for 15 contain the supplementary crystallographic data for this paper. These data

1712
Helvetica Chimica Acta – Vol. 95 (2012)
can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (þ44)1223-336-033; or
deposit@ccdc.cam.ac.uk).
This work was supported in part by a Grant-in-Aid for Scientific Research (B) (22390022) from JSPS,
by a Grant-in-Aid for Scientific Research on Innovative Areas (24105525) from MEXT, and by a Grant
from the Naito Foundation.
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Received July 31, 2012