An extended planar C5 conformation and a 310-helical structure of peptide foldamer composed of diverse alpha-ethylated alpha,alpha-disubstituted alpha-amino acids.

Article abstract of DOI:10.1002/chem.200204476

Optically active peptide foldamers Tfa-[(S)-(alphaEt)Leu]-[(S)-(alphaEt)Nva]-Deg-[(S)-(alphaEt)Nle]-OEt (10) and Tfa-[(S)-(alphaEt)Val]-[(S)-(alphaEt)Leu]-[(S)-(alphaEt)Nva]-Deg-[(S)-(alphaEt)Nle]-OEt (11) composed of diverse alpha-ethylated alpha,alpha-disubstituted alpha-amino acids were synthesized. The dominant conformation of these peptides in solution was an unusual, fully extended planar conformation, and that in the crystal state was both right-handed (P) and left-handed (M) 3(10)-helical structures in 10 and a P 3(10)-helical structure in 11, respectively. The preferred planar C(5) conformation of the peptides prepared from chiral alpha-ethylated alpha,alpha-disubstituted alpha-amino acids was drastically different from the 3(10)-helical structure of the peptides prepared from chiral alpha-methylated alpha,alpha-disubstituted alpha-amino acids.

Full text of DOI:10.1002/chem.200204476

FULL PAPER
An Extended Planar C5 Conformation and a 310-Helical Structure
of Peptide Foldamer Composed of Diverse a-Ethylated a,a-Disubstituted
a-Amino Acids
Masakazu Tanaka,*[a] Shin Nishimura,[a] Makoto Oba,[a] Yosuke Demizu,[a]
Masaaki Kurihara,[b] and Hiroshi Suemune*[a]
Abstract: Optically active peptide foldamers Tfa-[(S)-(aEt)Leu]-[(S)-(aEt)Nva]-
Deg-[(S)-(aEt)Nle]-OEt (10) and Tfa-[(S)-(aEt)Val]-[(S)-(aEt)Leu]-[(S)-(aEt)-
Nva]-Deg-[(S)-(aEt)Nle]-OEt (11) composed of diverse a-ethylated a,a-disubsti-
tuted a-amino acids were synthesized. The dominant conformation of these peptides
Keywords: amino acids ¥ conforma-
in solution was an unusual, fully extended planar conformation, and that in the
tion analysis ¥ helical structures ¥
crystal state was bothright-handed ( P) and left-handed (M) 310-helical structures in
peptidomimetics
10 and a P 310-helical structure in 11, respectively. The preferred planar C5
conformation of the peptides prepared from chiral a-ethylated a,a-disubstituted a-
amino acids was drastically different from the 310-helical structure of the peptides
prepared from chiral a-methylated a,a-disubstituted a-amino acids.
cules except for glycine. They reported that the homo- and
heteropeptides prepared from chiral a-methylated a,a-dis-
ubstituted a-amino acids [(aMe)AAs] formed the 310-helical
structures in the crystal state and in solution, and the screw
sense of helicity, right-handed (P) or left-handed (M) helicity,
depended on the chiral center of the quaternary carbon of
(aMe)AAs.[3e,f, 6, 7] On the other hand, we reported that the
conformation of homopeptides prepared from a chiral a-
ethylated a,a-disubstituted a-amino acid [(aEt)AA]; (S)-
butylethylglycine [(S)-Beg,a-ethylnorleucine, (S)-(aEt)Nle]
was the fully planar C5 conformation bothin the crystal state
and in solution.[8] The fully extended conformation was
formed in the case of unusual homopeptides prepared from
glycine,[9] Deg, Dpg, or (S)-(aEt)Nle (Beg), and also was
observed in the case of unusual heteropentapeptides contain-
ing one chiral a-amino acid as a guest molecule in the
sequences of Deg residues.[10, 11] We herein describe for the
first time the synthesis of heteropentapeptide Tfa-[(S)-
(aEt)Val]-[(S)-(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-
OEt (11), in whicheachof the amino acid residues is a
different a-ethylated a,a-disubstituted a-amino acid, and also
report its 310-helical and planar C5 conformation in the crystal
state and in solution.
Introduction
Foldamers, which were named by Gellman,[1] are oligomers
having well-defined secondary structural preferences. Within
the past decade, many unnatural oligomers bearing interest-
ing conformational properties have been reported, because
control of the folding pattern leads to new types of molecules
withuseful properties. In particular, peptide-foldamers such
as b-peptides,[2] which are made from b-amino acids, and the
peptides prepared from a,a-disubstituted a-amino acids[3]
have been focused on by organic, peptide, and medicinal
chemists.
It has been well known that the homopeptides prepared
from achiral 2-aminoisobutyric acid (Aib) form a 310-helical
structure,[4] whereas those from diethylglycine [Deg: 2-ethyl-
2-aminobutyric
acid
((aEt)Abu)],[5a]
dipropylglycine
(Dpg),[5b,c] and diphenylglycine form a fully extended planar
C5 conformation.[5] Recently, the Toniolo and the Seebach
groups concentrated on the conformation of oligopeptides
prepared from optically active a,a-disubstituted a-amino
acids, because proteinogenic a-amino acids are chiral mole-
[a] Prof. M. Tanaka, Prof. H. Suemune, S. Nishimura, M. Oba, Y. Demizu
Graduate School of Pharmaceutical Sciences
Kyushu University, Fukuoka 812-8582 (Japan)
Fax : (‡81)92-642-6545
E-mail: mtanaka@phar.kyushu-u.ac.jp
Results and Discussion
suemune@phar.kyushu-u.ac.jp
[b] Dr. M. Kurihara
Design of heteropentapeptide: As an (aEt)AA heteropep-
tide, we designed pentapeptide Tfa-[(S)-(aEt)Val]-[(S)-
Division of Organic Chemistry
National Institute of HealthSciences
Tokyo 158-8501 (Japan)
(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt
(11),
3082
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200204476
Chem. Eur. J. 2003, 9, 3082 3090

3082 3090
which has different (aEt)AAs
as the individual amino acid
residues, and the structure is
very different from those of the
Deg and (S)-(aEt)Nle homo-
peptides,[8] which preferentially
form the planar C5 conforma-
tion.
Asymmetric synthesis of (S)-a-
ethylated a,a-disubstituted a-
amino acids: We synthesized
the optically active (aEt)AAs
by an asymmetric alkylation of
the b-keto ester by using (R,R)-
cyclohexane-1,2-diol as a chiral
Scheme 1. Synthesis of the optically active (aEt)AAs.
auxiliary,
and
subsequent
Schmidt rearrangement, as
shown in Scheme 1.[12, 13] That
is to say, chiral 1, which consists
of (R,R)-cyclohexane-1,2-diol
and ethyl 2-ethylacetoacetate,
was alkylated withLDA
(5 equiv), Pr-I (5 equiv), and
HMPA (5 equiv) in THF at
À78 to À408C or room temper-
ature to give enol ethers 2a and
2b in 83 and 70% yield, respec-
tively. The cyclohexane-1,2-diol
moiety in 2 was removed by
treatment withBF 3 ¥ OEt2 in
EtOH/H2O to afford b-keto
esters 3a (83%) and 3b
(70%). The optical purities
(>95% op) and absolute con-
figurations of 3a and 3b were
determined by comparison with
the reported specific rota-
tions.[13] The obtained b-keto
esters 3a and 3b could be con-
verted into the a,a-disubstitut-
ed a-amino acids 4a in 48%
and 4b in 40% yield by Schmidt
rearrangement. The protecting
group in 4 was removed by
hydrolysis with concentrated
HCl, and then the N terminus
was protected as a trifluoroace-
tyl group to produce Tfa-[(S)-
(aEt)Nva]-OH 5a in 55% and
Tfa-[(S)-(aEt)Val]-OH 5b in
40% yield, respectively.
Scheme 2. Preparation of heteropentapeptide 11. Yields are based on recovered materials.
(Scheme 2). At first, the dipeptide 8 was prepared in 74%
yield by the coupling of H-(S)-(aEt)Nle-OEt (6) and Tfa-
Deg-OH (7) by using 1-ethyl-3-[3-(dimethylamino)propyl]-
carbodiimide hydrochloride (EDC). Removal of the trifluoro-
acetyl group in 8 by NaBH4 reduction followed by coupling
withTfa-[( S)-(aEt)Nva]-OH (5a) by treatment withEDC in
refluxing MeCN gave tripeptide 9 in 50% yield based on the
Preparation of heteropentapeptide Tfa-[(S)-(aEt)Val]-[(S)-
(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt:
We
prepared the heteropentapeptide Tfa-[(S)-(aEt)Val]-[(S)-
(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt (11) by
the solution-phase methods, employing an ethyl ester as the
C terminus and a trifluoroacetyl group as the N terminus
Chem. Eur. J. 2003, 9, 3082 3090
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M. Tanaka, H. Suemune, et al.
FULL PAPER
Table 2. Intra- and intermolecular hydrogen bond parameters for the peptides 9,
10, and 11.[a]
recovered material. Tetra- and pentapeptides 10 (55%) and
11 (31%) were synthesized in a manner similar to that
described for 9. The spectroscopic data of all compounds
supported their structures.
Donor
Acceptor
A
Distance [ä]
D ¥¥¥ A
Angle [8]
Symmetry
operations
À
D H
À
D H ¥¥¥ A
Tfa-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt (9)
À
N1
À
N3
N2
H
H
O1
O3
O2'
2.54
2.60
2.93
110
108
173
x,y,z
x,y,z
Crystal-state conformational analysis: We determined the
molecular and crystal structures of the three terminally
protected tri-, tetra-, and pentapeptides 9, 10, and 11 by
X-ray crystallographic analysis.[14] Crystals of good quality for
X-ray analysis were obtained by slow evaporation of an EtOH
or EtOH/CHCl3 solution at room temperature. The molecular
structures of 9, 10, and 11 withatomic-numbering schemes are
given in Figures 1 4. Relevant backbone and side-chain
torsion angles are summarized in Table 1. The intra- and
intermolecular hydrogen-bond parameters are listed in Ta-
ble 2.
1
1
À
H
À x ‡ ³2, À y, z ‡ ³
2
Tfa-[(S)-(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt (10)
A (M)
À
N3a
À
N4a
H
H
O0a
O1a
3.18
3.06
164
166
x,y,z
x,y,z
B (P)
À
N3b
H
O0b
O1b
3.01
3.00
161
150
x,y,z
x,y,z
À
N4b
H
À
N1a
H
O4b'
O5b'
O4a'
O5a'
2.92
172
135
179
141
x,y,z
x,y,z
N2a
H
3.34[b]
2.87
À
À
N1b
N2b
H
À
H
x À 1,y À 1,z À 1
3.25[b]
x À 1,y À 1,z À 1
The structure of tripeptide 9 was solved in the space group
P212121. Two intramolecular hydrogen bonds are observed,
that is to say, intramolecularly hydrogen-bonded C5 confor-
mations of the residues (S)-(aEt)Nva1 and (S)-(aEt)Nle3 are
formed in the crystal state. The set of torsion angles f,y for
the residue are ‡177.6, À179.88 for (S)-(aEt)Nva1 and
‡177.0, ‡177.58 for (S)-(aEt)Nle3. The N1 ¥¥¥ O1 distance is
2.54 ä and the N3 ¥¥¥ O3 is 2.60 ä. The torsion angles of Deg2
are À59.5, À42.88. In the packing mode, one intermolecular
Tfa-[(S)-(aEt)Val]-[(S)-(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt (11)
À
N3
H
H
H
H
H
O0
O1
O2
O4'
O5'
3.10
3.12
3.13
2.91
3.47[b]
168
162
179
172
158
x,y,z
x,y,z
x,y,z
À
N4
À
N5
À
N1
N2
x,y,z À 1
À
x,y,z À 1
[a] The number of the amino-acid residues begins at the N terminus of the peptide
chain. [b] The distance of D ¥¥¥ A is somewhat long for a hydrogen bond.
À
hydrogen bond is shown between the H N2 peptide donor
crystal state is a bent planar C5 conformation, which is very
similar to that of homotripeptide Tfa-(Deg)3-OEt prepared
from diethylglycine[5d] and that of homotripeptide Tfa-[(S)-
(aEt)Nle]3-OEt prepared from (S)-butylethylglycine (Fig-
ure 1).[8]
ˆ
and the C2' O2' carbonyl oxygen atom of the peptide of a
1
1
symmetry-related molecule (Àx ‡ ³2, Ày, z ‡ ³2), withan
N2 ¥¥¥ O2' distance of 2.93 ä. The conformation of 9 in the
Table 1. Selected torsion angles w, f, y, and c[a] [8] for the peptides 9, 10,
and 11 as determined by X-ray crystallographic analysis.
Torsion Tripeptide 9
Tetrapeptide 10
Pentapeptide 11
angle
molecule A (M) molecule B (P)
w0
f1
y1
w1
f2
y2
w2
f3
y3
w3
f4
y4
w4
f5
y5
w5
À 173.1
177.6
176.5
53.7
37.1
169.6
56.0
26.0
177.8
53.7
33.6
179.0
À 48.6
À 53.0
À 175.1
À 174.6
À 58.2
À 34.9
À 178.4
À 48.4
À 34.5
À 175.8
À 52.0
À 39.7
À 177.2
46.9
À 170.6
À 58.3
À 37.5
À 173.9
À 57.4
À 19.1
176.3
À 48.7
À 32.1
À 175.1
À 52.8
À 37.5
À 178.9
47.2
À 179.8
À 172.8
À 59.5
À 42.8
À 171.2
177.0
177.5
À 176.7
56.2
174.0
52.7
174.9
c1e
c1a
c2e
c2a
c3e
c3a
c4e
c4a
c5e
c5a
À 53.2
52.3
65.7
À 177.1
À 58.5
57.7
À 176.1
À 66.5
176.3
À 61.4
À 177.1
À 49.5
65.2
58.7
179.6
64.7
30.3
À 71.0[b]
66.2
81.0
63.3
179.2
60.1
179.6
À 174.9
À 65.3
Figure 1. ORTEP drawing of the molecular structure of Tfa-[(S)-(aEt)-
Nva]-Deg-[(S)-(aEt)Nle]-OEt (9) withatom numbering (ellipsoids at 50%
probability).
À 178.4
60.8
177.5
À 175.1
Tetrapeptide 10 crystallizes in the triclinic space group P1.
Two crystallographically independent molecules A and B
exist in the asymmetric unit of 10. Bothmolecules A and B are
folded into the 310-helical structure: molecule A has a left-
handed (M) structure and molecule B is right-handed (P), as
shown in Figure 2. The corresponding f,y torsion angles
À 175.8
À 66.9
[a] The superscripts e and a refer to the ethyl and the alkyl side chains,
respectively. [b] The angle ca1 ˆ 165.88 also exists because the substituent of
the side chain is an isopropyl group.
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Chem. Eur. J. 2003, 9, 3082 3090

310-Helical Peptide Foldamer
3082 3090
The structure of heteropentapeptide 11 was solved in the
monoclinic space group P21. Only one 310-helical structure
exists in the asymmetric unit (Figure 3). The screw sense of
helicity is a right-handed (P) helix, but a flip of the torsion
Figure 3. ORTEP Drawing of the molecular structure of Tfa-[(S)-(aEt)-
Val]-[(S)-(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt (11) with
atom numbering (ellipsoids at 50% probability).
Figure 2. ORTEP drawing of the molecular structure of Tfa-[(S)-(aEt)-
Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt (10) withatom numbering
(ellipsoids at 50% probability).
angles at the C terminus occurs, that is, the signs of the f and
y torsion angles (‡47.2, ‡52.78) of th e (S)-(aEt)Nle5 residue
are opposite to those of the preceding residues (S)-(aEt)Val1,
(S)-(aEt)Leu2, (S)-(aEt)Nva3 and Deg4 (negative signs),
corresponding to a change (M) in the handedness of the helix
at the C terminus. The mean values of the f and y torsion
angles for the sequence (S)-(aEt)Val1 to Deg4 are f ˆ À54.3
and y ˆ À31.68, close to the ideal right-handed (P) 310-helix
(À49 and À268).[3d] Figure 4 shows the ORTEP drawing of the
between two molecules of opposite helicity are different by
sign, but the absolute values are similar. The relationship of
the two molecules A and B is not enantiomeric but diaster-
eomeric because the chiral centers of amino acid residues in
bothmolecules A and B are the same S configuration. In
molecule A, the signs of the f and y torsion angles (À48.6,
À53.08) of th e (S)-(aEt)Nle4a residue at the C terminus are
opposite to those of the preceding residues (S)-(aEt)Leu1a,
(S)-(aEt)Nva2a and Deg3a (positive signs); also, in molecule B,
the signs of the torsion angles (‡46.9, ‡56.28) of th e S()-
(aEt)Nle4b residue are opposite to those of the preceding
residues (negative signs); this phenomenon is frequently
observed in the 310-helical peptides of Aib.[4] Molecule A
shows two intramolecular hydrogen bonds between the
À
ˆ
H N3a and C0a O0a oxygen atom of the trifluoroacetyl
group withan N3a ¥¥¥ O0a distance of 3.18 ä and between
À
ˆ
H N4a and C1a O1a (N4a ¥¥¥ O1a ˆ 3.06 ä), and molecule B
similarly shows two intramolecular hydrogen bonds between
À
ˆ
H N3b and C0b O0b (N3b ¥¥¥ O0b ˆ 3.01 ä) and between
À
ˆ
H N4b and C1b O1b (N4b ¥¥¥ O1b ˆ 3.00 ä). In the packing
mode, two intermolecular hydrogen bonds are formed
À
ˆ
between the H N1a peptide donor and C4b' O4b' O atom
À
ˆ
(N1a ¥¥¥ O4b' ˆ 2.92 ä), and between H N1b and C4a' O4a'
of a symmetry-related molecule (x À 1, y À 1, z À 1) (N1b ¥¥¥
O4a' ˆ 2.87 ä). Moreover, two weak intermolecular hydrogen
Figure 4. ORTEP Drawing of 11 as view along the helix axis (ellipsoids at
50% probability).
À
ˆ
bonds are observed between H N2a and C5b' O5b' (N2a ¥¥¥
À
ˆ
O5b' ˆ 3.34 ä) and also between H N2b and C5a' O5a' of a
symmetry-related molecule (x À 1, y À 1, z À 1) (N1b ¥¥¥
O5a' ˆ 3.25 ä). The chains of intermolecularly hydrogen-
bonded molecules are formed in a head-to-tail alignment of
P 310-helix (molecule B) and M 310-helix (molecule A), that is,
¥ ¥¥P¥¥¥M¥¥¥P¥¥¥M¥¥¥ chains of the 310-helical molecules A
and B are formed.
P 310-helical structure along the helix axis. There are three
À
intramolecular hydrogen bonds between H N3 and the
ˆ
C0 O0 oxygen atom of the trifluoroacetyl group with an
À
ˆ
N3 ¥¥¥ O0 distance of 3.10 ä, between H N4 and C1 O1
(N4 ¥¥¥ O1 ˆ 3.12 ä), and between H N5 and C2 O2 (N5 ¥¥¥
À
ˆ
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M. Tanaka, H. Suemune, et al.
FULL PAPER
O2 ˆ 3.13 ä). In the packing mode, one intermolecular
N- and C-terminal NH signals of dipeptide 8. The precise
assignments of the two remaining internal NH protons in 10
and three NH protons in 11 cannot be made, and these signals
appear in a narrow region of d ˆ 7.35 7.49 ppm. The chemical
shifts of all NH protons in 10 were essentially independent of
the concentration at the examined range of 1.0 10.0mm in 10.
The additional effects of the strong hydrogen-bonding accept-
or solvent, DMSO or the paramagnetic free radical 2,2,6,6-
tetramethyl-1-piperidyloxyl (TEMPO), on the chemical shifts
of NH signals were measured for the tetrapeptide 10 and
pentapeptide 11. Figure 6 shows the results that all NH signals
bothin 10 and 11 are almost insensitive to the addition of the
two perturbing agents DMSO (0 10% (v/v)) and TEMPO
(0 5 Â 10À2 % (w/v)); this means that no solvent-exposed NH
protons exist in the peptides. It has been known that two NH
protons forming intermolecular hydrogen bonds of the 310-
helical structures are sensitive to addition of the perturbing
agents, but all NH protons of the fully planar C5 conformation
are insensitive.[6, 10] These 1H NMR experiments of the
heteropeptides 10 and 11 are the same as those of the Deg
and (S)-(aEt)Nle homopeptides, which form the fully planar
C5 conformation in solution.[5d, 8a]
À
ˆ
hydrogen bond is observed between H N1 and the C4' O4'
O atom of a symmetry-related molecule (x, y, z À 1) (N1 ¥¥¥
O4' ˆ 2.91 ä), and the N2 ¥¥¥ O5' distance of 3.47 ä is some-
À
ˆ
what long for a hydrogen bond between H N2 and C5' O5'
carbonyl oxygen atom. As a result, the chains of intermolec-
ularly hydrogen-bonded 310-helices are formed in the
¥ ¥¥P¥¥¥P¥¥¥P¥¥¥P¥¥¥ mode of the head-to-tail alignment,
along the c direction.
Solution conformational analysis: At first, the preferred
conformation of the heteropeptides in CDCl3 solution was
studied by FT-IR spectroscopy. The IR spectra of tetra- and
pentapeptides 10 and 11 remain essentially unchanged at the
concentration range of 1.0 10.0mm. These results mean that
the concentration of the peptide does not affect the strength
of the intermolecular hydrogen bonds. Figure 5 shows the IR
Figure 5. FT-IR Absorption spectra (3500 3250 cmÀ1 region) of the
heteropeptides 8, 9, 10, and 11 in CDCl3 solution. Peptide concentration
1.0 mm.
absorption of the di- to pentapeptides 8 11 in the 3250
3500 cmÀ1 region at a peptide concentration of 1.0mm. Th e
band at 3380 3420 cmÀ1 is assigned to amide NH groups with
À
ˆ
a relatively strong C F ¥¥¥ H(N) ¥¥¥ O C intramolecular hydro-
gen bond, and that at 3340 3360 cmÀ1 to peptide NH groups
À
ˆ
withN H ¥¥¥ O C intramolecular hydrogen bonds of different
strength. With increasing the peptide main-chain length, the
relative intensity of the absorption band at 3340 3360 cmÀ1
region increases, and also the absorption observed at
3340 cmÀ1 in the dipeptide 8 shifts to higher wave numbers
(3360 cmÀ1 in the pentapeptide 11). These IR spectra are very
similar to those of the Deg and (S)-(aEt)Nle homopeptides
that form the extended planar C5 conformation in solu-
tion,[5d, 8a] but very different from those of Aib homopeptides
and heteropeptides which form the 310-helical structure.[4, 8a, 10]
Next, we measured the 1H NMR spectra of the tetrapeptide
10 and the pentapeptide 11 under various conditions. In the
1H NMR spectra of 10 and 11 in CDCl3, the signals of the
trifluoroacetamide NH at the N terminus are unambiguously
determined by their high-field position at d ˆ 6.78 ppm (brs,
1H) bothin 10 and 11, and those of the amide NH at the C
terminus are assigned by their low-field position to d ˆ 8.10
(brs, 1H) in 10 and 8.17 ppm (brs, 1H) in 11, on analogy of the
Figure 6. a) Plots of NH chemical shifts in the 1H NMR spectra of Tfa-
[(S)-(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt (10; 1.0mm),
and b) of Tfa-[(S)-(aEt)Val]-[(S)-(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-
(aEt)Nle]-OEt (11; 1.0 mm) as a function of increasing percentages of
DMSO (v/v) added to the CDCl3 solution; c) plots of the bandwidth of the
NH protons of 10 (1.0mm), and d) of 11 (1.0mm) as a function of increasing
percentages of TEMPO (w/v) added to the CDCl3 solution.
We also measured the CD spectra of the heteropeptides 8,
9, 10, and 11 in 2,2,2-trifluoroethanol (CF3CH2OH). It is
known that the negative and positive maxima and intensity of
two bands at 222 nm and 208 nm, and a band at 192 nm in the
CD spectra, indicates the screw sense of helicity and also a 310-
or a-helical structure of peptides that contain chiral a-
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Chem. Eur. J. 2003, 9, 3082 3090

310-Helical Peptide Foldamer
3082 3090
Table 3. Selected calculated (MacroModel and MMFF) torsion angles w, f, y, and c[a] [8] for the
heteropentapeptides 11.
methylated a,a-disubstituted a-
amino acids.[15] The CD spectra
of 10 and 11 are quite different
from those of the 310-helical
peptides. This may be attribut-
ed to the fact that the peptides
10 and 11 form the fully planar
C5 conformation in solution,
albeit the main-chain length of
peptide is too short for the
precise analysis of conforma-
tion by the CD spectra (not
shown).
AMBER*
B[c]
MMFF
E[c]
A[b]
C[d]
D[e]
F[d]
G[d]
energy [kcalmolÀ1
]
0
À 179.5
À 51.3
À 25.5
À 174.4
À 51.0
À 20.1
172.6
À 42.8
À 36.8
À 178.6
À 54.1
À 23.0
À 177.7
À 45.0
À 35.9
178.3
‡ 1.90
178.1
51.5
‡ 25.4
À 178.8
À 177.2
159.4
177.7
173.6
À 173.3
À 173.1
179.1
À 172.6
À177.0
178.7
À 179.2
À 177.1
179.7
‡ 1.80
À 167.4
À 57.7
À 28.1
À 177.2
À 49.9
À 28.4
À 170.9
À 61.7
À 35.9
À 178.8
68.6
‡ 2.27
167.0
50.9
31.8
174.7
54.1
32.8
172.3
60.0
21.8
176.0
62.4
‡ 1.08
À 178.8
À 177.7
171.9
0
À 178.8
À 177.7
171.9
w0
f1
y1
w1
f2
y2
w2
f3
y3
w3
f4
y4
w4
f5
y5
w5
c1e
ci1p
ci1p
c2e
ci2b
c3e
c3p
c4e
c4e
28.8
À 175.4
177.0
177.0
45.7
À 174.2
À 172.1
À 173.9
À 172.9
176.8
À 174.2
À 171.8
À 174.1
À 172.6
176.3
33.4
À 178.9
54.2
18.7
À 169.9
178.2
176.8
178.1
176.9
44.7
Computational analysis:[16] The
conformational searchcalcula-
tion withMacroModel was ap-
plied to the heteropentapeptide
11. AMBER* and MMFF were
used as a force field. The calcu-
lated torsion angles are sum-
marized in Table 3. The calcu-
lation by AMBER* produced
the P 310-helical structure (con-
formation A) as a global mini-
41.7
175.2
46.7
À 45.9
À 176.0
179.7
32.8
À 178.4
À 179.1
À 179.0
À 179.9
179.6
À 177.3
À 177.5
À 178.3
À 1.0
À 179.7
À 42.4
52.9
- 179.8
À 54.5
77.3
178.5
À 62.9
À 19.4
À 175.5
174.9
À 59.9
64.9
176.2
À 43.0
175.2
À 59.2
173.8
À 57.4
62.0
51.9
À 179.8
À 6.5
179.9
À 46.9
À 37.7
À 161.0
À 48.4
À 49.7
68.8
À 30.7
À 56.6
178.9
176.8
À 58.5
179.9
À 176.0
31.1
À 52.4
49.6
173.7
63.5
59.8
58.9
À 42.3
À 174.4
À 173.9
60.7
À 65.9
À 81.5
59.4
À 176.7
À 65.0
À 60.6
À 56.1
54.8
52.7
48.1
À 179.9
À 54.3
77.3
45.8
64.3
61.3
À 46.9
À 49.1
À 49.1
À 172.4
À 68.9
33.1
46.6
54.3
54.0
52.1
À 57.1
53.9
À 56.7
53.8
À 53.0
À 60.3
62.0
mum-energy
conformation.
c5e
c5b
52.1
67.4
À 61.5
À 61.0
The conformational search
starting from the extended
structure as an initial confor-
mation by the Monte Carlo
method[17] did not afford the
66.1
62.9
62.6
[a] The superscripts e, b, p, ip, and ib refer to the ethyl, butyl, propyl, isopropyl, and isobutyl side chains,
respectively. [b] P 310-helix. [c] M 310-helix. [d] Planar. [e] P distorted 310-helix.
‡2.27 kcalmolÀ1) with the flip of torsion angles at the C
terminus as the local minimum-energy conformations, re-
spectively.
Conformation A is similar to that determined by the X-ray
crystallographic analysis, except for the C-terminal structure.
Figure 7 shows the pentapeptide 11 as determined by X-ray
M 310-helical structure or the planar C5 conformation. There-
fore, the calculation was performed starting from the typical
M 310-helix (f ˆ 60, y ˆ 308), and the M 310-helical structure
(conformation B) obtained as a local minimum-energy con-
formation which exhibits an energy of ‡1.90 kcalmolÀ1. Th e
energy of the planar C5 conformation (conformation C) in
which the torsion angles of the peptide main-chain were
constrained as the planar conformation, was estimated to be
‡25.4 kcalmolÀ1 by AMBER*.
In the conformational search starting from the extended
structure by the Monte Carlo method, the calculation by
MMFF produced the planar C5 conformation (conforma-
tion G), in which the torsion angles (f ˆ À178.3, y ˆ À1.08)
at the (S)-(aEt)Nle5 residue were planar, but not withthe C5
conformation as the global minimum-energy conformation.
The fully extended C5 planar structure (conformation F) for
which five consecutive C5 conformations were formed, was
obtained as
a
local minimum-energy conformation
(‡1.08 kcalmolÀ1). The 310-helical structure was not given
by MMFF when the extended structure was used as an initial
structure of the conformational search. Therefore, the con-
formations A and B, which were produced as the minimum-
energy conformations by AMBER*, were used as the initial
structure of the conformational search by the Monte Carlo
method and MMFF. The MMFF calculation afforded the
Figure 7. a), b) Superimposition of the conformation determined by X-ray
analysis (in dark) and of the calculated (MacroModel, AMBER*)
minimum-energy conformation A (in light) of the heteropentapeptide 11;
c) the calculated (MacroModel, MMFF) minimum-energy conformation F
of 11; d) the calculated (MacroModel, MMFF) minimum-energy confor-
mation G of 11.
P 310-helical structure (conformation D, ‡1.80 kcalmolÀ1)
4
withdistorted torsion angles at Deg
and (S)-(aEt)Nle5
residues, and the (M) 310-helical structure (conformation E,
Chem. Eur. J. 2003, 9, 3082 3090
www.chemeurj.org
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3087

M. Tanaka, H. Suemune, et al.
FULL PAPER
crystallographic analysis, superimposed on the minimum-
energy conformation A calculated by AMBER*, and the
conformations F and G as the minimum-energy conformation
calculated by MMFF of MacroModel, which may preferen-
tially be formed in solution.
conformation built of (aEt)AAs will be used as a novel
structure for the design of molecular devices and catalysts.[18]
Experimental Section
General: Ethyl (2RS)-3,3-[(1R,2R)-cyclohexane-1,2-dioxy]-2-ethylbu-
tanoate (1), (S)-(aEt)Leu (5c), (S)-(aEt)Nle (6), and Deg 11 were
prepared according to our previous reports[5d, 8, 10, 13]. Optical rotations
[a]D were measured witha Jasco DIP-316 polarimeter with1.0 dm cell.
Circular dichroism spectra (CD) were measured with a Jasco J-720W
spectropolarimeter witha 1.0 mm pathlengthcell. Infrared spectra (IR)
were recorded on a Nicolet Avatar-320 spectrometer for conventional
measurement (KBr), and the solution (CDCl3) method used an NaCl cell
witha 0.1 mm pathlength. 1H NMR spectra were determined at 270 MHz
(Jeol GX-270). FABMS spectra were taken on a Jeol JMS 610H or Jeol
JMS-SX 102 spectrometer. Elemental analyses were performed at the
Analytical Center of the Faculty of Sciences, Kyushu University.
Conclusion
We have synthesized chiral a-ethylated amino acids, (S)-
(aEt)Val and (S)-(aEt)Nva, by using (R,R)-cyclohexane-1,2-
diol as a chiral auxiliary, and we have also prepared the
heteropeptides 10 and 11 composed of diverse chiral
(aEt)AAs. The overall yield of pentapeptide was not sat-
isfactory because of the steric hindrance of (aEt)AAs, but for
the first time the heteropentapeptide containing different
(aEt)AAs was prepared. The X-ray crystallographic analysis
revealed that the preferred conformation of the tetrapeptide
10, which has three chiral centers, in the crystal state was both
P and M 310-helical structures in a 1:1 ratio, and that of the
pentapeptide 11, which has four chiral centers of S config-
uration, was the right-handed P 310-helical structure. This may
be attributed to the fact that three chiral quaternary carbons
of (aEt)AAs are too weak to govern the screw sense of the
310-helical structure, and four chiral centers of S configuration
may regulate the screw sense of helicity to the right-handed
(P) helix, or perhaps the P 310-helical structure of 11
crystallized out by chance. The relationship between the
screw sense of helicity and the chiral center of the (aEt)AAs
seems to be that the S configuration of amino acid induces the
right-handed (P) helix, as natural l-a-amino acids (S config-
uration) induce the right-handed (P) a-helix. The calculation
by MacroModel also suggested that the P 310-helical structure
of 11 was more stable than the M helical structure. In solution,
the dominant conformation of 10 and 11 was not the 310-
helical structure shown in the crystal state, but the fully planar
C5 conformation; similarly the Deg homopeptide and the Deg
heteropeptide with an (S)-(aEt)Nle residue have different
conformations in the crystal state and in solution.[5d, 10] We
speculate that two intermolecular hydrogen bonds exist in the
310-helical structure, but no intermolecular hydrogen bonds in
the planar C5 conformation. Therefore, the intermolecular
hydrogen bonds in a minor 310-helical structure existing in
solution affect the nucleation events, and the 310-helical
structure was preferentially induced in the crystal state. The
conformation of (S)-(aEt)Nle homotetrapeptide was the fully
planar C5 conformation bothin solution and in the crystal
state,[8a] but those of 10, 11, and the Deg homopeptide have
different conformations in solution and in the crystal state. It
is not clear why the different conformations in the crystal state
were formed from the similar dominant planar conformations
of 10, 11, and (S)-(aEt)Nle homopeptide in solution. The
Toniolo and the Seebach groups independently reported that
the homo- and heteropeptides prepared from chiral
(aMe)AAs would form the 310-helical structure.[6, 7] However,
the results presented here and previously by us[8] establishfor
the first time that the homopeptides and heteropeptides 10
and 11 prepared from chiral (aEt)AAs preferentially form the
fully extended planar C5 conformation. The fully planar
Ethyl
(2R)-2-ethyl-2-propyl-3-[(1R,2R)-2-hydroxycycloheyloxy]-3-bu-
tenoate (2a):[13] nBuLi (3.1 mL, 47.8 mmol, 1.5m in hexane) was added
dropwise to a stirred solution of diisopropylamine (6.8 mL, 47.8 mmol) in
THF (40 mL) at À788C; the solution was warmed to 08C and then stirred
for 30 min at 08C. The solution was cooled to À788C, HMPA (8.3 mL,
47.8 mmol) was added, and then 1 (2.48 g, 9.56 mmol) in THF (5 mL) was
added dropwise. The solution was stirred at À788C for 30 min, and then
1-iodopropane (4.63 mL, 47.8 mmol) was added dropwise to the stirred
solution. The solution was stirred at À788C for 3 h, À408C for 2 h, and
diluted withsaturated aqueous NH 4Cl. This was then extracted with
EtOAc, and dried over MgSO4. Removal of the solvent afforded an oily
residue, which was purified by column chromatography on silica gel. The
fraction eluted with 10% EtOAc in hexane gave enol ether 2a (2.37 g,
83%) as a colorless oil: [a]2D3 À 60.5 (c ˆ 1.00 in CHCl3).
Ethyl (2R)-2-ethyl-2-isopropyl-3-[(1R,2R)-2-hydroxycycloheyloxy]-3-bu-
tenoate (2b):[13] Compound 2b was prepared from 1 in a manner similar to
that described for the preparation of 2a: 70%; a colorless oil; [a]2D8 À 43.1
(c ˆ 1.10, CHCl3).
Ethyl (2R)-2-ethyl-2-propylacetoacetate (3a):[13] BF3 ¥ OEt2 (10 mL,
83.9 mmol) was added dropwise to a stirred solution of 2a (2.50 g,
8.39 mmol) in EtOH (125 mL) and H2O (50 mL) at room temperature.
After being stirred for 1 h, the solution was diluted with brine, extracted
withEtOAc, and dried over MgSO 4. Removal of the solvent afforded an
oily residue, which was purified by column chromatography on silica gel
(10% EtOAc in hexane) to give b-keto ester 3a (1.4 g, 83%) as a colorless
oil: [a]2D7 À 1.1 (c ˆ 1.40 in CHCl3).
Ethyl (2R)-2-ethyl-2-isopropylacetoacetate (3b):[13] Compound 3b was
prepared from 2b in a manner similar to that described for the preparation
of 3a: 70%; a colorless oil; [a]2D8 ‡ 13.8 (c ˆ 1.08 in CHCl3).
(S)-N-Acetyl-a-ethylnorvaline ethyl ester (4a):[13] Methansulfonic acid
(5.0 mL, 70.0 mmol) was added dropwise to a stirred solution of b-keto
ester 3a (1.40 g, 6.97 mmol) in CHCl3 (35 mL) at 08C; NaN3 (1.81 g,
27.8 mmol) was then added. After refluxing for 6 h, the reaction mixture
was cooled to room temperature, diluted withH 2O, neutralized withdiluted
aqueous NH3, extracted with diethyl ether, and dried over MgSO4.
Removal of the solvent afforded an oily residue, which was purified by
column chromatography on silica gel (50% EtOAc in hexane) to give 4a
(719 mg, 48%): colorless crystals: m.p. 49 508C (recryst. from CHCl3);
[a]3D0 ‡ 12.0 (c ˆ 0.99 in CHCl3).
(S)-N-Acetyl-a-ethylvaline ethyl ester (4b):[13] Compound 4b was prepared
from 3b in a manner similar to that described for the preparation of 4a:
40%; a colorless oil; [a]3D0 ‡ 12.2 (c ˆ 1.92 in CHCl3).
(S)-N-Trifluoroacetyl-a-ethylnorvaline (Tfa-[(S)-(aEt)Nva]-OH; 5a): A
mixture of 4a (1.08 g, 5.06 mmol) in concentrated aqueous HCl (5 mL) was
refluxed for 48 h, and then the solution was evaporated. The residue was
dissolved in (Tfa)2O (2.0 mL), and the solution was stirred at room
temperature for 24 h. The mixture was poured into 5% aqueous NaHCO3,
and the solution washed with Et2O and then acidified with citric acid. The
solution was extracted withEtOAc, and dried over MgSO 4. Removal of the
solvent afforded 5a (676 mg, 55%). The acid 5a was used in the next
reaction without purification: colorless crystals. M.p. 75 768C; [a]2D7 ‡ 11.6
3088
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2003, 9, 3082 3090

310-Helical Peptide Foldamer
3082 3090
1
(c ˆ 0.91 in CHCl3); H NMR (270 MHz, CDCl3): d ˆ 7.26 (brs, 1H), 2.80
29H); IR (KBr): nÄ ˆ 3399, 3348 (br), 3312, 1728, 1679, 1660, 1492 cmÀ1
;
‡
‡
(br, 1H), 2.41 2.58 (m, 2H), 1.78 1.95 (m, 2H), 0.96 1.32 (m, 2H), 0.92
MS (FAB): m/z: 687 [M‡Na] , 665 [M‡H] ; elemental analysis calcd (%)
(t, J ˆ 7.2 Hz, 3H), 0.81 ppm (t, J ˆ 7.3 Hz, 3H); IR (KBr): nÄ ˆ 3349, 3120
for C33H59F3N4O6: C 59.62, H 8.94, N 8.43; found C 59.62, H 8.94, N 8.34.
(br), 1740, 1708, 1543 cmÀ1; MS (FAB): m/z: 242 [M‡H] .
‡
Ethyl trifluoroacetyl-(S)-a-ethylvalyl-(S)-a-ethylleucyl-(S) a,a-ethylnor-
valyl-diethylglycyl-(S)- a-ethylnorleucinate (Tfa-[(S)-(aEt)Val]-[(S)-
(aEt)Leu]-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt; 11): Compound 11
was prepared from 10 and 5b in a manner similar to that described for the
preparation of 9: 5% (31% based on recovered material); colorless
crystals; m.p. 152 1538C (recryst. from CHCl3/EtOH); [a]2D2 ‡ 11.7 (c ˆ
0.33 in CHCl3); IR (KBr): nÄ ˆ 3336 (br), 3228, 1725, 1709, 1681, 1666, 1644,
1529 cmÀ1; 1H NMR (270 MHz, CDCl3): d ˆ 8.17 (brs, 1H), 7.49 (brs, 1H),
7.41 (brs, 1H), 7.35 (brs, 1H), 6.78 (brs, 1H), 4.27 (q, J ˆ 7.2 Hz, 2H), 2.35
2.90 (m, 11H), 1.55 1.95 (m, 8H), 1.32 (t, J ˆ 7.2 Hz, 3H), 1.15 1.35 (m,
5H), 1.10 (d, J ˆ 6.9 Hz, 3H), 0.97 (d, J ˆ 6.9 Hz, 3H), 0.73 1.00 ppm (m,
(S)-N-Trifluoroacetyl-a-ethylvaline (Tfa-[(S)-(aEt)Val]-OH 5b): Com-
pound 5b was prepared from 4b in a manner similar to that described
for the preparation of 5a: 40% yield; colorless crystals; m.p. 99 1018C;
[a]2D2 ‡ 12.1 (c ˆ 0.98 in CHCl3); 1H NMR (270 MHz, CDCl3): d ˆ 7.29 (brs,
1H), 4.00 (br, 1H), 2.61 2.71 (m, 2H), 2.09 (m, 1H), 1.07 (d, J ˆ 6.9 Hz,
3H), 0.97 (d, J ˆ 6.9 Hz, 3H), 0.82 ppm (t, J ˆ 7.3 Hz, 3H); IR (KBr): nÄ ˆ
3339, 3118, 1736, 1714, 1548 cmÀ1; MS (FAB): m/z: 242 [M‡H] .
‡
Ethyl trifluoroacetyldiethylglycyl-(S)-a-ethylnorleucinate (Tfa-Deg-[(S)-
(aEt)Nle]-OEt; 8): A solution of 6 (500 mg, 2.66 mmol), 7 (402 mg,
2.21 mmol), and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydro-
chloride (EDC; 508 mg, 2.66 mmol) in MeCN (12 mL) was refluxed for
24 h, and the solution was then evaporated. The residue was diluted with
CHCl3, washed with 3% HCl, 5% aqueous NaHCO3 and brine, and dried
over MgSO4. After removal of the solvent, the residue was purified by
column chromatography on silica gel (10% EtOAc in hexane) to afford 8
(645 mg, 74%). Colorless crystals: m.p. 69 708C (recryst. from EtOH);
[a]2D9 ‡ 5.48 (c ˆ 1.12 in CHCl3); 1H NMR (270 MHz, CDCl3): d ˆ 7.95 (brs,
1H), 6.80 (brs, 1H), 4.28 (q, J ˆ 7.3 Hz, 2H), 2.59 2.75 (m, 2H), 2.36 2.52
(m, 2H), 1.61 1.90 (m, 4H), 1.32 (t, J ˆ 7.3 Hz, 3H), 1.14 1.26 (m, 2H),
0.88 1.05 (m, 2H), 0.72 0.88 ppm (m, 12H); IR (KBr): nÄ ˆ 3350, 3322,
‡
32H); MS (HR-FAB(‡)): m/z calcd for C40H73O7N5F3 [M‡H] : 792.5462;
found 792.5545.
X-raycrystal structure determination : The crystals of 9 and 10 were grown
from EtOH, and 11 from CHCl3/EtOH. Data collection was performed on
a Rigaku-RAXIS-RAPID Imaging Plate diffractometer, equipped with
graphite-monochromated MoKa radiation. Indexing was performed from
1 oscillation which was exposed for 10 min. The camera radius was
127.4 mm. Readout was performed in the 0.10 mm pixel mode. A total of
44 images, corresponding to 2208 oscillation angles, were collected withtwo
different goniometer settings. Exposure time was 4.0 mindegÀ1 for 9, and
5.0 mindegÀ1 for 10 and 11. Data were processed by the PROCESS-AUTO
program package. Crystal and collection parameters are listed in Table 4.
All crystals remained stable during the X-ray data collection. The
structures were solved by direct methods[19] and expanded by Fourier
techniques.[20] All non-H atoms were given anisotropic thermal parameters,
some H atoms were refined isotropically, and the remaining H atoms
included in calculated positions given isotropic thermal parameters. The
final cycle of full-matrix least-squares refinement of 9 gave an R factor of
0.139 (Rw ˆ 0.184) based on 3875 [I >À 10.00s(I)] reflections and an R1
factor of 0.063 based on 971 [I > 2.0s(I)] reflections, and the largest peak
and hole in the final difference Fourier map were 0.29 and À0.22 eäÀ3. Th e
R factor of 10 was 0.176 (Rw ˆ 0.240) based on 8924 [I >À 10.00s(I)]
reflections and an R1 factor of 0.106 based on 4803 [I > 2.0s(I)] reflections,
and the largest peak and hole in the final difference Fourier map were 0.71
and À0.30 eäÀ3. Th eR factor of 11 was 0.099 (Rw ˆ 0.154) based on 5289
[I >À 10.00s(I)] reflections and an R1 factor of 0.062 based on 2822 [I >
2.0s(I)] reflections, and the largest peak and hole in the final difference
1718, 1663, 1518 cmÀ1; MS (FAB): m/z: 419 [M‡Na] , 397 [M‡H] ;
elemental analysis calcd (%) for C18H31F3N2O4: C 54.53, H 7.88, N, 7.07;
found C 54.25, H 7.79, N 6.84.
‡
‡
Ethyl trifluoroacetyl-(S)-a-ethylnorvalyldiethylglycyl-(S)-a-ethylnorleuci-
nate (Tfa-[(S)-(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt; 9): NaBH4 (400 mg,
10.6 mmol) was added portionwise to a stirred solution of 8 (800 mg,
2.02 mmol) in EtOH (50 mL) at room temperature. After being refluxed
for 2 h, the mixture was poured into 1% aqueous HCl (50 mL), and then
EtOH was evaporated. The residue was diluted with 5% aqueous
NaHCO3, extracted withEtOAc, and dried over MgSO 4. After removal
of the solvent, the residue was purified by column choromatography on
silica gel (2% MeOH in CHCl3 to give H-Deg-[(S)-(aEt)Nle]-OEt
(280 mg, 46%, 79% based on recovered material). The solution of
H-Deg-[(S)-(aEt)Nle]-OEt (325 mg, 1.08 mmol), 5a (217 mg, 0.903 mmol),
and EDC (206 mg, 1.08 mmol) in MeCN (17 mL) was refluxed for 24 h.
After evaporation, the residue was diluted with CHCl3, washed with 3%
aqueous HCl, and 5% aqueous NaHCO3, and dried over MgSO4. Removal
of the solvent afforded the white solid,
which was purified by column choromatog-
Table 4. Crystal and diffraction parameters of the peptides 9, 10, and 11.
raphy on silica gel. The fraction eluted with
50% EtOAc in hexane gave 9 (356 mg,
63%). Colorless crystals: m.p. 162 1638C
(recryst. from EtOH); [a]2D6 ‡ 15.7 (c ˆ 0.93
in CHCl3); 1H NMR (270 MHz, CDCl3):
d ˆ 8.00 (brs, 1H), 7.39 (brs, 1H), 6.78 (brs,
1H), 4.28 (q, J ˆ 7.3 Hz, 2H), 2.34 2.72 (m,
6H), 1.58 1.90 (m, 6H), 1.31 (t, J ˆ 7.3 Hz,
3H), 1.00 1.27 (m, 4H), 0.85 1.00 (m,
2H), 0.73 0.92 ppm (m, 18H); IR (KBr):
nÄ ˆ 3390, 3328, 3295, 1731, 1655 cmÀ1; MS
(HR-FAB(‡)): m/z calcd for C25H44O5N3F3
9
10
11
formula
Mr
crystal dimensions [mm]
T [8C]
crystal system
a [ä]
b [ä]
c [ä]
C25H44O5N3F3
C33H59O6N4F3 Â 2
664.8 Â 2
0.50 Â 0.30 Â 0.15
23
triclinic
10.852
19.179
10.564
101.40
113.53
89.25
1970.9
P1
1
1.120
720
0.86
55
8924
830
0.176/0.240
4803
0.106
2.07
EtOH
C40H72O7N5F3
792.0
523.6
0.40 Â 0.10 Â 0.10
23
orthorhombic
15.490
18.264
10.759
90
0.30 Â 0.30 Â 0.30
23
monoclinic
11.637
18.071
12.222
90
115.76
90
2314.8
P21
a [8]
b [8]
g [8]
90
90
‡
[M‡H] : 524.3311; found 524.3297.
Ethyl trifluoroacetyl-(S)-a-ethylleucyl-(S)
a,a-ethylnorvalyldiethylglycyl-(S)-a-ethyl-
V [ä3]
space group
Z
3043.9
P212121
4
1.143
1128
0.91
54.9
3875
327
0.139/0.184
971
0.063
0.90
norleucinate
(Tfa-[(S)-(aEt)Leu]-[(S)-
2
(aEt)Nva]-Deg-[(S)-(aEt)Nle]-OEt; 10):
Compound 10 was prepared from 9 and
5c in a manner similar to that described for
the preparation of 9: 31% (55% based on
recovered material); colorless crystals; m.p.
136 1378C (recryst. from CHCl3/EtOH);
[a]2D4 ‡ 14.5 (c ˆ 1.48 in CHCl3); 1H NMR
(270 MHz, CDCl3): d ˆ 8.10 (brs, 1H), 7.42
(brs, 1H), 7.36 (brs, 1H), 6.78 (brs, 1H),
4.27 (q, J ˆ 7.2 Hz, 2H), 2.43 2.68 (m, 8H),
1.59 1.84 (m, 8H), 1.32 (t, J ˆ 7.2 Hz, 3H),
1.00 1.34 (m, 5H), 0.73 1.00 ppm (m,
1calcd [gcmÀ3
F(000)
]
1.136
860
0.85
55
5289
518
0.099/0.154
2822
0.062
1.32
CHCl3/EtOH
m (MoKa) [cmÀ1
2qmax [8]
]
observed reflections [I >À 10.0s(I)]
parameters
R/Rw
reflections used for R1 [I > 2.0s(I)]
R1
GOF
solvent
EtOH
Chem. Eur. J. 2003, 9, 3082 3090
www.chemeurj.org
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3089

M. Tanaka, H. Suemune, et al.
FULL PAPER
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Molecular mechanics calculations: Conformational searchcalculations
were performed withthe package of MacroModel Ver. 6.5 [17] on an SGI O2
workstation. The parameters used were as follows: conformational search,
Monte Carlo method; force field, AMBER* or MMFF; more than 15000
structures were minimized; solvent: water for AMBER* and CHCl3 for
MMFF. The fully extended conformation of 11 was used as the initial
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minimum-energy conformation. The energy of the conformation C (planar
C5 conformation) was estimated to be ‡25.4 kcalmolÀ1 by the AMBER*
calculation. The calculation by MMFF afforded the conformations F
(‡1.08 kcalmolÀ1
; planar conformation) and G ; planar
(0 kcalmolÀ1
conformation) as the global minimum-energy conformations of 11 within
3.0 kcalmolÀ1, but not 310-helical structures. By using the conformations A
and B as the initial conformations, the calculation by MMFF produced the
conformation D (‡1.80 kcalmolÀ1; P distorted 310-helix) and the confor-
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Acknowledgement
[14] CCDC-194811, CCDC-194812 and CCDC-194813 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB21EZ, UK; fax: (‡44)1223-336-033; or depos-
it@ccdc.cam.uk).
This work was partly supported by a Grant-in-Aid for Scientific Research
(C) from the Japan Society for the Promotion of Science, and also
supported by the grant from the Takeda Science Foundation. M.O. was a
recipient of a Research Fellowship of the Japan Society for the Promotion
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Received: October 4, 2002
Revised: January 24, 2003 [F4476]
3090
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2003, 9, 3082 3090