Synthesis and evaluation of the β-turn properties of 4-amino-1,2,4,5-tetrahydro-2-benzazepin-3-ones and of their spirocyclic derivative

Article abstract of DOI:10.1002/ejoc.200500996

A series of 4-amino-tetrahydro-2-benzazepin-3-one derivatives (Ac-Aba-Xxx-NHMe) were prepared as tetrapeptide mimetics. Considering the structural resemblance with the so-called Freidinger lactams, their propensity to adopt a β-turn conformation was inves

Full text of DOI:10.1002/ejoc.200500996

FULL PAPER
DOI: 10.1002/ejoc.200500996
Synthesis and Evaluation of the β-Turn Properties of 4-Amino-1,2,4,5-
tetrahydro-2-benzazepin-3-ones and of Their Spirocyclic Derivative
Karolien Van Rompaey,[a] Steven Ballet,[a] Csaba Tömböly,[a] Rien De Wachter,[a]
Kenno Vanommeslaeghe,[a] Monique Biesemans,[b] Rudolph Willem,[b] and Dirk Tourwé*[a]
Keywords: Lactams / Asymmetric synthesis / Conformation analysis / Molecular modeling
A series of 4-amino-tetrahydro-2-benzazepin-3-one deriva-
tives (Ac–Aba–Xxx–NHMe) were prepared as tetrapeptide
mimetics. Considering the structural resemblance with the
so-called Freidinger lactams, their propensity to adopt a β-
turn conformation was investigated by NMR spectroscopy
(solvent and temperature dependence) and molecular model-
ing. Interestingly, most of these lactams adopt extended con-
formations, only the spiro-benzazepinone 9 has a strong pref-
erence for the formation of a β-turn.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
The three-dimensional structure of a protein is charac- membered ring, preserving the amino acid side-chain orien-
terized by an arrangement of α-helices, β-sheets, tight turns, tations. In external mimetics, the central dipeptide unit of
bulges, and random coil structures. Tight turns, which cause the β-turn is replaced by a dipeptide isosteric skeleton. In
the polypeptide chain to reverse its overall direction, play a this way, the conformational flexibility of the peptide is re-
key role in diverse aspects of the protein: without them the duced by a rigidified skeleton lying outside the pseudo-ten-
protein is not capable of adopting a compact globular membered ring, orienting the surrounding peptide chain in
structure, and their location on the surface of the protein a turn conformation.[2]
allows them to function as molecular recognition ele-
ments.[1] They are involved in interactions between peptides
and their receptors, antibodies and antigens, and regulatory
enzymes and their corresponding substrates.[2] Tight turns
provide important information allowing the design of new
potent and selective drugs, pesticides and antigens,[1] and
consequently much effort has been done to prepare small
mimetics of turn structures.[2–6]
β-Turns belong, together with δ-, γ-, α- and π-turns, to
the collection of tight turns.[1] In a β-turn, the reversal of
the peptide backbone occurs over four amino acid residues
in such a way that the carbonyl oxygen atom of the first
residue (i) and the amide NH proton of the fourth residue
(i+3) come close in space, and in most cases are involved
in an intramolecular hydrogen bridge forming a pseudo-
ten-membered ring.[7]
Examples of external β-turn mimetics are the Freidinger
lactams. In the 1980’s, a five-membered γ-lactam (1) was
incorporated in luteinizing hormone-releasing hormone,
fixing residues five to eight in a type IIЈ β-turn (Figure 1).[8]
This analog was more active than the parent hormone. The
six- and seven-membered δ- and ε- Freidinger lactams 2
and 3[9] were reported to stabilize type II β-turns. Dehydro-
Freidinger lactams 4 were also claimed in literature to be
β-turn mimics.[10;11]
The
4-amino-1,2,4,5-tetrahydro-2-benzazepin-3-one
(Aba) skeleton has been applied successfully to constrain a
Phe or Tyr side chain in biologically active peptides.[12–20]
Considering their structural resemblance with dehydro-
Freidinger lactams 4, we have investigated their potential
for adopting turn conformations.
N-acetyl dipeptide NЈ-methylamides 5, 6, 7, 8 and 9 were
prepared as mimics for the tetrapeptides[21] and their con-
formational properties were examined by NMR and molec-
ular modeling to evaluate their β-turn mimicry. Analogs
containing Gly as the i+2 residue (7), N-methylated Aba 8,
heterochiral (5) and homochiral sequences (6) were studied.
Several spirolactams have been reported to be very ef-
ficient β-turn inducers.[22–25] We now also report the prepa-
ration of the spiro derivative 9, and its conformational
analysis.
β-Turn mimetics can be categorized in two distinct
classes: internal and external mimetics. Internal mimetics
are constructed upon frameworks within the pseudo-ten-
[a] Organic Chemistry Department, Vrije Universiteit Brussel,
Pleinlaan 2, 1050 Brussels, Belgium
[b] High Resolution NMR Centre, Vrije Universiteit Brussel,
Pleinlaan 2, 1050 Brussels, Belgium
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D. Tourwé et al.
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ium bromide, resulting in the alkylated compound 12 (Fig-
ure 2). Hydrolysis of the benzophenone imine 12 using
aqueous citric acid provides (S)-o-CN–Phe–OtBu (13).
Figure 2. Synthesis of (S)-o-cyano–Phe. a) CsOH.H2O, CH2Cl2, O-
allyl-N-(9-anthracenylmethyl) cinchonidinium bromide, –78 °C,
27 h; b) 15% citric acid, THF, o.n., room temp., 90% yield (a+b);
c) 6  HCl, CH2Cl2, room temp., o.n., 97%.
At first a liquid/liquid phase-transfer alkylation of 10
(50% aq. KOH/toluene) at room temperature was exam-
ined,[27] which resulted in a low stereoinduction (62% ee).
Excellent results were, however, obtained with solid/liquid
PTC (CsOH·H2O, CH2Cl2) at –78 °C:[28] 96% enantiomeric
excess and 90% yield were achieved. In order to prevent the
decomposition of the catalyst,[29] the base was suspended in
CH2Cl2, the mixture was cooled, and then the catalyst and
the Gly derivative were added.
A homogeneous alkylation of 10 using the phosphazene
base BEMP was also evaluated,[30] but this resulted in lower
yields and ee values (83% yield, 86% ee).
The tert-butyl ester of 13 was removed by acidolysis with
6  HCl/CH2Cl2, 1:1 resulting in (S)-o-CN–Phe·HCl
(14).[31] Other acidolytic conditions (TFA/H2O, 9:1[32] and
TFA/anisole, 3:1[33]) provided insufficient scavenging of the
tBu cation and caused tert-butylation of the aromatic ring
of o-CN–Phe.
The stereoinduction of the alkylation reaction was deter-
mined by HPLC analysis of the FDAA derivative of 14,[34]
as the FDAA derivative of tert-butyl ester 13 and its optical
antipode eluted at the same retention time.
(S)-o-CN–Phe·HCl 14 was phthaloyl-protected by means
of methyl 2-[(succinimidooxy)carbonyl]benzoate (MSB)[35]
and reduction of the nitrile 15 with H2/Raney-Ni provided
the aldehyde 16 (Figure 3).[36] This aldehyde was used in a
reductive amination with (R)-Ala–OBn·pTosOH followed
by ring closure, according to a procedure reported ear-
Figure 1. Freidinger lactams and benzazepinones as β-turn mi-
metics.
Results and Discussion
Synthesis of Tetrapeptide Mimetics
The starting material for Ac–(S)–Aba–(R)–Ala–NHMe lier.[37] Hydrogenolysis of Phth–(S)-Aba–(R)-Ala–OBn (17)
(5), (S)-o-CN–Phe·HCl (14), was prepared through an provided the carboxylic acid 18. Phthaloyl deprotection by
asymmetric phase-transfer-catalysed alkylation of tert-butyl hydrazinolysis led to (S)-Aba–(R)-Ala–OH (19).[18] Acety-
N-(diphenylmethylene)glycinate (10)[26] with o-cyanobenzyl lation with acetic anhydride in water provided compound
bromide (11) using the commercially available and inexpen- 20,[38] which was used in a coupling reaction with methyl-
sive catalyst O-allyl-N-(9-anthracenylmethyl)cinchonidin- amine using TBTU activation. During this coupling 30%
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Figure 3. a) MSB, CH3CN, H2O, Na2CO3, 81 %; b) Raney-Ni, H2 (50 psi), H2O/AcOH/Py, 1:1:1, 50 °C, 38 h, 78 %; c) 1) (R)-Ala-
OBn·pTosOH, NaCNBH3, CH2Cl2, MgSO4, NMM, 2) DCC, Py, AcN, 39% (2 steps); d) H2 (50 psi), 10% Pd/C, dioxane/H2O 3:2; e)
NH2NH2, EtOH, reflux, 1.5 h, 84%; f) Ac2O, NEt3, water, 60%; g) CH3NH2·HCl, TBTU, NEt3, CH2Cl2, room temp., o.n.
racemisation occurred, resulting in Ac–(S)-Aba–(S)-Ala–
NHMe (6), beside the correct compound Ac–(S)-Aba–(R)-
Ala–NHMe (5). Both compounds were isolated by prepara-
tive HPLC and were studied separately by NMR spec-
troscopy.
Likewise, (S)-Aba–Gly–OH[18] (21) was acetylated and
converted into tetrapeptide mimic Ac–(S)-Aba–Gly–NHMe
(7) (Figure 4).
Figure 4. a) Ac2O, NEt3, water, 45 %; b) CH3NH2·HCl, TBTU,
NEt3, CH2Cl2, room temp., 2 h, 55%.
N-Methylation of Boc–(S)-Aba–Gly–OH[18] (23) with so-
dium hydride and iodomethane[39] followed by Boc-depro-
tection, acetylation and coupling with methylamine pro-
vided the N-methylated tetrapeptide analog N-Ac,N-Me-
(S)-Aba–Gly–NHMe (8) (Figure 5).
For the spiro derivative 9, (R,S)-Boc-α-(o-cyanobenzyl)-
proline (27) was reduced to the corresponding aldehyde 28
with H2/Raney-Ni in aqueous pyridinium acetate (Fig-
ure 6).[36] After a few hours the reduction stopped, and re-
petitive addition of the catalyst was required to obtain com-
CH2Cl2, 1:1, room temp., o.n., 57%; c) Ac2O, Et3N, water, 45%;
plete conversion of the starting material. A problem associ- d) CH3NH2·HCl, TBTU, NEt3, CH2Cl2, room temp., 2 h, 47%.
Figure 5. a) NaH, MeI, THF, 24 h, room temp., 79%; b) 6  HCl/
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Figure 6. a) Raney-Ni, H2 (50 psi), H2O/AcOH/Py, 1:1:1, 50 °C, 56 h, 32%; b) 1) Gly-OBn·pTosOH, NaCNBH3, CH2Cl2, MgSO4, NMM,
2) DCC, Py, AcN, 21% (2 steps); c) CH3NH2, MeOH, o.n., room temp., 93%; d) TFA, CH2Cl2, 0 °C, 2 h, 92%; e) Ac2O, NEt3, room
temp., 2 h, 91%.
ated with this reaction that took 48 h, was the formation of mylbenzyl)proline (34) was used to prepare a benzazepi-
side product, which was identified as 2,3,5,10-tetrahydro- none 35 with glycine benzyl ester (32% yield). Clearly, the
1H-pyrrolo[1,2-b]isoquinoline-10a-carboxylic acid (29). Moc protection is more suitable for this synthetic strategy
This compound is obtained by in situ removal of the Boc than the Boc protection.
group and intramolecular reductive amination. A similar
side reaction has been reported before for the reduction of
Molecular Modeling Results
Boc–o-CN–Phe and bis-Boc–o-CN–Phe resulting in Boc–
Tic.[37]
In order to evaluate the intrinsic β-turn-inducing potency
of this class of lactams, conformational searches were per-
formed on Ac–(S)-Aba–Gly–NHMe (7), Ac–(S)-Aba–(R
and S)-Ala–NHMe 5 and 6 and Ac–(S)-spiro-Aba–Gly–
NHMe (9) using Macromodel 5.0.[40] Briefly, the procedure
consisted of a MM3*[41] low-mode search[42] on the formyl–
Xxx–NMe rings (Xxx = Aba, spiro-Aba) in vacuo (Fig-
ure 8).
The aldehyde 28 was used in a reductive amination with
Gly–OBn·pTosOH. Ring closure provided Boc-spiro-(R,S)-
Aba–Gly–OBn (30) (21% yield). Aminolysis of the benzyl
ester 30 with a methanolic solution of methylamine fol-
lowed by Boc deprotection and acetylation provided a fifth
tetrapeptide analog 9 in racemic form.
In order to avoid the side reaction during the reduction
of nitrile 27, the Moc-protecting group was studied as an
alternative nitrogen protection for α-(o-cyanobenzyl)proline
(Figure 7). As for the Boc-protected compound, repetitive
addition of catalyst was required in order to achieve com-
plete conversion of the starting material 33 into the alde-
hyde 34. In this case the side product 29 was not observed,
and a much higher yield (69%) was obtained compared to
the Boc derivative (32%). The crude (R,S)-Moc-α-(o-for-
Figure 8. Formyl–Xxx–NMe (Xxx = Aba, spiro-Aba) structures.
Subsequently, the exocyclic functions were added to ob-
tain the desired tetrapeptide mimics. Then, the peptide
backbone was submitted to a systematic unbounded mul-
tiple minimum search (SUMM)[43] in water, using the GB/
SA solvation model.[44] All conformations within 50 kJ/mol
of the global minimum were clustered into families based
on geometrical similarity.
The β-turn mimicry of the different tetrapeptide mi-
metics was evaluated by means of different criteria (Fig-
ure 9):
Figure 7. a) Raney-Ni, H2 (50 psi), H2O/AcOH/Py, 1:1:1, 50 °C,
35 h, 69%; b) 1) Gly-OBn·pTosOH, NaCNBH3, CH2Cl2, MgSO4,
NMM, 2) DCC, Py, AcN, 32% (2 steps).
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Studies on Tetrapeptide Mimetics
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(5) and Ac–(S)-Aba–(S)-Ala–NHMe (6): none of the low-
energy structures has a turn character.
Figure 9. Criteria used to evaluate β-turn mimicry.
(1) The distance between the N-acetyl carbonyl oxygen
atom and the methyl carboxamide proton should be smaller
than 2.5 Å and the associated NH–O angle larger than
120°. This criterion reflects the presence of a hydrogen bond
in type I and II β-turns.[45]
(2) The Cα1–Cα4 interatomic distance should be less than
7 Å.
(3) The virtual torsion angle β, defined by C1, Cα2, Cα3
and N4 of the tetrapeptide model[46] should lie between
specified limits (Table 1).
Figure 10. 3D representation of the lowest-energy conformers.
In contrast, the lowest-energy conformer of Ac–(S)-
spiro-Aba–Gly–NHMe (9) displays a β-turn of type IIЈ, in-
dicating that spiro-Aba has β-turn inducing properties.
Moreover, within the 16.74 kJ/mol range, several more turn
structures were found.
Interestingly, when the ring conformation of the tetra-
peptide mimetic adopts a trans 1 conformation, which is
boat-like, a β-turn of type IIЈ can be induced. In contrast,
none of the chair-like trans 2 ring conformations have β-
turn properties.
Table 1. Virtual torsion angle β defining β-turn classes.
Type of β-turn
Virtual torsion angle β
I
IЈ
10 Ͻ β Ͻ 80
–79 Ͻ β Ͻ –10
20 Ͻ β Ͻ 40
II
–69 Ͻ β Ͻ –60
–49 Ͻ β Ͻ –40
–29 Ͻ β Ͻ 40
–59 Ͻ β Ͻ –50
–29 Ͻ β Ͻ 30
IIЈ
(4) The φ- and ψ angles of the central residues of the β-
turn, which are characteristic for the turn type.[1]
NMR Analysis
In Table 2 and Figure 10, the lowest-energy conformers
The constitutional structure of all synthesized com-
1
are given, together with the values for the different criteria. pounds was evidenced by H and 13C NMR, the chemical
1
The lowest-energy conformer of Ac–(S)-Aba–Gly–NHMe shift assignments being supported by 2D H homonuclear
(7) is not a β-turn, and the lowest-energy conformation that DQF COSY[47] and 2D heteronuclear 1H-13C HMQC[48]
does adopt a turn is 14.85 kJ/mol above the minimum. Sim- and HMBC[49] NMR spectroscopy. The conformation of
ilar results were obtained for Ac–(S)-Aba–(R)-Ala–NHMe the benzazepinone ring was investigated by 2D 1H NOESY
Table 2. The lowest-energy conformers and values for the different criteria.
Structure
Ac–(S)-Aba–Gly–
NHMe (7)
Ac–(S)-Aba–(R)-Ala–
NHMe (5)
Ac–(S)-Aba–(S)-Ala–
NHMe (6)
Ac–(S)-spiro-Aba–Gly–
NHMe (9)
Ring conformation
Turn
d (NH–CO) [Å]
NHO angle [°]
d (Cα1–Cα4) [Å]
β [°]
trans 2[a]
/
6.5
149
9.3
trans 2[a]
/
6.0
153
8.7
trans 2[a]
/
6.7
137
9.7
trans 1[a]
IIЈ
2.0
164
5.4
–11
58
139
162
–61
162
149
161
φ2 [°]
ψ2 [°]
φ3 [°]
ψ3 [°]
–162
105
–26
–163
–124
34
–164
126
–43
–123
–98
17
E of first turn (kJ/mol)
14.85
g(+)1
type I
24.94
g(+)1
type I
13.39
g(+)1
type I
0
[a] The ring conformations are primarily classified by the χ1 torsion angle of the Aba amino acid, which can either be gauche(+) or
trans.[18] However, for each of these possible orientations of χ1, two ring conformations are possible (e.g. trans 1 and trans 2). In the trans
1 conformation, one of the Cβ-H is in close proximity with one of the Cε-H, whereas in the trans 2 conformation the Cα2-H is in close
proximity with one of the Cε-H. (see also Figure 10).
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D. Tourwé et al.
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NMR.[50] In every compound, the benzazepinone ring was
The intramolecular hydrogen bond between the carbonyl
shown to adopt a trans χ1 conformation around the Cα–Cβ oxygen atom of the first residue and the amide NH proton
bond (Figure 10).[18] This was demonstrated by the presence of the fourth residue, which is present in most of the β-
of NOE cross peaks as well as by the value of coupling turns, was also studied by NMR spectroscopy. Indeed, un-
constants (Table 3). For 5, the small coupling constant and like intermolecular hydrogen bonds, intramolecular hydro-
a strong NOE between Hα and Hβ, and the opposite for gen bonds are very little influenced by external factors such
Hα and HβЈ demonstrates the presence of the trans rotamer as temperature and solvent.
1H NMR spectra of the benzazepine compounds were
measured in CDCl3 and [D6]DMSO, to evaluate the effect
of switching from a non-hydrogen bond-forming solvent to
a strong hydrogen bond-forming solvent on the chemical
shift of the NH–Me. In a second experiment, samples in
[D6]DMSO were heated, to examine the effect of the tem-
perature increase on the chemical shift of NH–Me.
In compounds 5, 6 and 7 (Table 4), the chemical shift in
CDCl3 of NH–Me is smaller than the chemical shift of Ac–
NH. This is in contrast with the expectation for the benza-
zepinones in the β-turn conformation. In that case the NH–
Me would be involved in an intramolecular hydrogen bond,
meaning that the proton is deshielded and that it should
appear at higher chemical shift than the Ac–NH.[51]
(see Figure 11 for atom labeling). Further evidence is given
by the NOE cross peaks between the β-hydrogen atoms and
NH–Ac, and the strong one between only one β-hydrogen
(Hβ) and the aromatic H6. A similar reasoning based on
the NOE’s only for spiro derivative 9 also indicates a trans
χ1 conformation.
Table 3. NMR spectroscopic data determining the conformation of
the benzazepinone core. Compound 5 is chosen as a representative
example of compounds 5, 6, 7 and 8 (solvent [D6]DMSO).
Compound 5
Compound 9
3J(Hα,Hβ)
4.1 Hz
13.1 Hz
strong
weak
–
–
3J(Hα,HβЈ)
NOE Hα/Hβ
NOE Hα/HβЈ
NOE H3/Hβ
NOE H3/HβЈ
strong
absent
NOE NHAc/Hβ strong
NOE NHAc/HβЈ strong
Switching from CDCl3 to [D6]DMSO resulted in a large
increase in chemical shift for both Ac–NH and NH–Me in
5, 6 and 7. When samples of 5, 6 and 7 were heated in
[D6]DMSO (temperature range between 298 K and 343 K,
temperature increments of 5 K), both Ac–NH and NH–Me
showed a large fluctuation of the chemical shift with the
temperature. The resonances of the amide protons shifted
NOE Hβ/H6
NOE Hα/Hε
NOE HβЈ/HεЈ
NOE Hε/H9
NOE HεЈ/H9
strong
strong
absent
absent
strong
NOE Hβ/H6
strong
NOE HβЈ/HεЈ strong
NOE Hε/H9
NOE HεЈ/H9
strong
absent
Interestingly, the CH2 ε protons show a different pattern linearly, resulting in temperature coefficients around –6 to
in 9, compared to 5, 6, 7 and 8. Indeed, in 5 a strong NOE –7 ppb/K. Only coefficients between 0 and –4 ppb/K are
is observed between Hα and Hε, and between HεЈ and H9, accepted for solvent-shielded amide protons involved in in-
which corresponds to the calculated trans 2 conformation. tramolecular hydrogen bonds.[21] These data indicate that 5,
In 9, a strong NOE between HβЈ and HεЈ, and between Hε 6, 7 and 8 do not adopt a β-turn conformation. Further-
and the aromatic H9 indicates a preferred trans 1 conforma- more, the expected NOE correlations for these compounds
tion (Figure 10).
in a β-turn conformation were not observed.
Figure 11. Atom labeling as used in the NMR interpretation.
Table 4. Solvent and temperature effect on amide protons in the benzazepinone compounds.
δ
δ
∆δ
∆δ/∆T
CDCl3
[D6]DMSO
CDCl3 Ǟ [D6]DMSO
Ac–(S)-Aba–(R)-Ala–NHMe (5)
Ac–(S)-Aba–(S)-Ala–NHMe (6)
Ac–(S)-Aba–Gly–NHMe (7)
Ac–NH
NH–Me
Ac–NH
NH–Me
Ac–NH
NH–Me
NH–Me
6.99
6.41
6.92
5.45
6.93
6.57
7.51
7.81
8.08
7.86
8.12
7.55
8.12
7.87
7.83
7.71
1.09
1.45
1.20
2.10
1.19
1.30
0.32
–0.10
–6.6
–5.7
–6.2
–6.9
–6.9
–6.6
–5.8
–4.3
N-Ac,N-Me–Aba-Gly-NHMe (8)
Ac-(R,S)-spiro-Aba-Gly-NHMe (9) NH–Me
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Studies on Tetrapeptide Mimetics
FULL PAPER
mum were discarded. The generated structures were clustered in
families with Xcluster 1.7. A RMSD value of 0.2 Å was used.
For the spiro derivative 9, however, the amide resonance
is only very weakly influenced by switching from CDCl3 to
[D6]DMSO. Moreover, the temperature coefficient is ap-
General Methods: (R,S)-Boc-α-(o-cyanobenzyl)proline (27) and
proximately –4 ppb/K, indicating the presence of an intra- (R,S)-Moc-α-(o-cyanobenzyl)proline (33) were kindly provided by
BioQuadrant Inc. RP-HPLC was performed using a RP C-18 col-
umn (Supelco Discovery®BIO Wide Pore C18, l = 25 cm, d =
0.45 cm, PS = 5 µm). The mobile phase (water/acetonitrile) con-
tained 0.1% TFA. Products were eluted using the following gradi-
ent: t = 0 min, 3% CH3CN, t = 30 min, 80% CH3CN, t = 40 min,
100% CH3CN. Flow rate: 1.0 mL min–1, λ = 215 nm. Preparative
HPLC was performed using a RP C-18 column (Supelco Discov-
ery®BIO Wide Pore C18, l = 25 cm, d = 2.12 cm, PS = 10 µm).
The above mentioned gradient was used with a flow rate of
20.0 mL min–1. FDAA analysis was performed as described ear-
molecular hydrogen bridge for this compound. Further evi-
dence of the intramolecular hydrogen bond was given by
the NOE correlations within the molecule, indicating a rigid
conformation for all the backbone atoms, which are in-
volved in the pseudo-ten-membered ring. A strong NOE
cross peak between the resonances of the acetyl methyl pro-
tons and the hydrogen atoms at C-5 of the spiro ring (see
Figure 11 for atom labeling), combined with the presence
1
of only one set of signals in the H NMR spectrum, indi-
cates the presence of exclusively the trans rotamer for the lier.[37] TLC analysis was performed with a plastic sheet precoated
with silica gel 60F254 (Merck). Silicagel 60 (0.040–0.063 mm) from
Merck was used for flash column chromatography (w/w, 60:1).
Melting points were measured with a Büchi B 540 melting point
apparatus, with a temperature increment of 1 °C·min–1. Optical ro-
tations were measured at room temperature using an optical ac-
tivity type AA-5 polarimeter. 1H and 13C NMR spectra were re-
corded at 250.13 and 62.90 MHz, respectively, with a Bruker Av-
ance DRX250 spectrometer, or at 500.13 and 125.75 MHz with a
Bruker AMX500 spectrometer, using TMS or the residual solvent
signal as internal reference. For the advanced NMR analysis of 5,
6, 7, 8 and 9, samples were measured in CDCl3 and [D6]DMSO
with the Bruker AMX500 spectrometer, using pulse sequences of
the Bruker program library. The temperature study was performed
in [D6]DMSO, with a temperature increment of 5 K between 298 K
and 343 K. For the atom labelling in the NMR spectra: see Fig-
ure 11. Mass spectra were recorded on a VG Quattro II spectrome-
ter using electrospray ionisation (positive ion mode).
acetyl-spiro amide bond. Additionally, the existence of an
NOE effect between Hε (not HεЈ) and only one of the α-
hydrogen atoms of Gly indicates that there is no rotational
freedom around the N2–CαGly bond. The NMR spectro-
scopic data are therefore in complete agreement with the
lowest energy conformer (Figure 10), which was obtained
in the molecular modeling.
Conclusions
Although structurally similar seven-membered dehydro-
Freidinger lactams were reported in literature as β-turn
mimics, this finding could not be extended to the 4-amino-
1,2,4,5-tetrahydro-2-benzazepin-3-ones 5, 6, 7 and 8, as we
have demonstrated by NMR analysis and molecular model-
ing.
tert-Butyl (S)-o-Cyanophenylalaninate (13): CsOH·H2O (34.10 g,
203 mmol) was put in a 250 mL flask and dried overnight at 130 °C
under vacuum. After cooling to room temperature the system was
put under Ar, and the CsOH was suspended in CH2Cl2 (analytical
grade, 60 mL). The mixture was further cooled to –78 °C, and after
15 min stirring tert-butyl N-(diphenylmethylene)glycinate (10,
6.00 g, 20.3 mmol) and the phase-transfer catalyst O-allyl-N-(9-an-
thracenylmethyl)cinchonidinium bromide (1.23 g, 2.03 mmol) were
added. The mixture was stirred for 30 min, followed by the addition
of o-cyanobenzyl bromide (11, 7.96 g, 40.6 mmol, addition in 4
portions with 3 min intervals). After a 27-h reaction time at –78 °C,
the mixture was diluted with Et2O (600 mL), washed with H2O
(3×150 mL) and brine (1×150 mL). The organic layer was dried
(MgSO4), filtered and evaporated and the so obtained crude N-
(diphenylmethylene)-(o-cyanophenyl)alanine tert-butyl ester (12)
was immediately hydrolysed in a mixture of THF/15% aqueous
citric acid solution (2:1, 180 mL). After stirring overnight, the THF
was evaporated and the residue was taken up in H2O (400 mL).
The aqueous layer was washed with Et2O (3×150 mL), and the pH
was adjusted with 20% K2CO3 until pH 10. The basic aqueous
phase was extracted with EtOAc (4×120 mL). Drying (MgSO4),
filtration and evaporation of the organic layer afforded the crude
(S)-o-cyanophenylalanine tert-butyl ester (13). The product was
purified by flash column chromatography (EtOAc). A light brown
paste was obtained. The enantiomeric purity was determined by
FDAA analysis after removal of the tBu ester [see (S)-o-cyanophe-
nylalanine hydrochloride (14)], 96% ee was observed. Yield 4.51 g
(90%). HPLC tret = 15.5 min; Rf = 0.49 (EtOAc/MeOH, 2:1).
HRMS calcd. 247.1446, found 247.1453. [α]2D0 = +22.9 (c = 1.7,
In contrast, these NMR and molecular modeling data
for the spiro derivative 9 are consistent with a β-turn con-
formation. It was also observed that the conformation of
the seven-membered lactam ring was different in the spiro-
lactam 9 and in benzazepinones 5, 6, 7 and 8, suggesting
that this may contribute to the β-turn mimicry of 9.
Experimental Section
Molecular Modeling: The calculations were carried out using
Macromodel 5.0[40] with Maestro 8.0 as a graphic interface. The
MM3* force field[41] was used in vacuo for the energy minimiza-
tions on the formyl–Xxx–NMe rings (Xxx = Aba, spiro-Aba). The
conformational analyses of the rings were carried out with the Pure
Low-Mode search.[42] 1000 Structures were generated and mini-
mized by means of the Polak–Ribière conjugate gradient method
as implemented in Macromodel, using a gradient convergence crite-
rion of 0.02 kJ/mol·Å. For the tetrapeptide mimetics, the MM3*
force field was used as well, but this time in combination with the
GB/SA solvation model of Still et al.,[44] using Macromodel’s de-
fault parameters for an aqueous medium. The conformational
space was sampled by a systematic unbounded multiple mimimum
search[43] in which 3 internal coordinates (φ2, φ3 and ψ3) were var-
ied. Here the generation of 2000 structures was conducted and the
conformers were minimized to an energy convergence of 0.1 kJ/
mol·Å by use of the Polak–Ribière conjugate gradient method. Af-
ter this search the found conformations were again minimized to an
energy convergence of 0.01 kJ/mol·Å. In both strategies, duplicate
structures and those greater than 50 kJ/mol above the global mini-
1
EtOAc). H NMR (CDCl3, 250 MHz): δ = 1.42 (s, 9 H, tBu), 1.7
(br. s, 2 H, NH2), 3.01 [dd, 2J(Hβ,HβЈ) = 13.8 Hz, 3J(Hα,Hβ) =
Eur. J. Org. Chem. 2006, 2899–2911
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2905

D. Tourwé et al.
FULL PAPER
8.4 Hz, 1 H, Hβ], 3.26 [dd, 2J(Hβ,HβЈ) = 13.9 Hz, 3J(Hα,HβЈ) =
arom.), 10.15 (s, 1 H, H aldehyde) ppm. 13C NMR (CDCl3
63 MHz): δ = 32.58 (CH2 β), 51.75 (CH α), 123.50 (2 CH Phth),
5.8 Hz, 1 H, HβЈ], 3.70 [dd, 3J(Hα,Hβ) = 8.4 Hz, 3J(Hα,HβЈ) =
5.8 Hz, 1 H, Hα], 7.28–7.65 (m, 4 H, H arom.) ppm. 13C NMR 127.77, 132.08, 133.66, 135.27 (CH arom.), 134.13 (2 CH Phth),
(CDCl3, 63 MHz): δ = 28.36 (CH3 tBu), 40.17 (CH2 β), 56.41 (CH
α), 82.03 (Cq tBu), 113.74 (Cq arom. next to CϵN), 118.48 (CϵN),
127.61, 130.93, 133.08, 133.28 (CH arom.), 142.53 (Cq arom. next
to CH2 β), 174.09 (C=O) ppm.
131.42, 138.55 (Cq arom.), 167.33, 173.66 (C=O), 193.44 (C=O al-
dehyde) ppm.
Benzyl (2R)-[(4S)-4-(Phthalimido)-3-oxo-1,3,4,5-tetrahydro-benz[c]-
azepin-2-yl]propionate (17) [Phth-(S)-Aba-(R)-Ala-OBn]: To a solu-
tion of the aldehyde 16 (983 mg, 3.04 mmol) in CH2Cl2 (analytical
grade, 60 mL) (R)-Ala–OBn·pTosOH (1.12 g, 3.19 mmol) was
added. The pH was adjusted to 6 with N-methylmorpholine, fol-
lowed by the addition of MgSO4 (197 mg, 20 wt.-%) and
NaCNBH3 (478 mg, 7.60 mmol). When the reaction was complete
(after 2 hours) the solvent was removed under reduced pressure.
The residue was redissolved in acetonitrile (200 mL), and pyridine
(492 µL, 6.08 mmol) was added. The mixture was cooled in an ice
bath for 10 minutes, and DCC (690 mg, 3.34 mmol) was added.
After 1 hour stirring at 0 °C the reaction was continued overnight
at room temperature. Oxalic acid (5  solution in DMF, 6.4 mL)
was added and after 30 minutes stirring the abundant precipitate
was filtered. The filtrate was evaporated and redissolved in EtOAc
(400 mL). After washing with 1  HCl and saturated aqueous
NaHCO3 (3×100 mL), the organic layer was dried (MgSO4), fil-
tered and the solvents evaporated. The small particles of DCU in
the obtained paste could be removed by filtration after dissolving
the paste in toluene. The product was further purified by flash col-
umn chromatography (eluent EtOAc/hexane, 12:20), a colourless
paste was obtained. Yield 552 mg (39%). HPLC tret = 26.5 min; Rf
(S)-o-Cyanophenylalanine Hydrochloride (14): (S)-o-Cyanophenyl-
alanine tert-butyl ester (13, 4.51 g, 18.3 mmol) was dissolved in
CH2Cl2/6  HCl (1:1, 520 mL). After stirring overnight, the solvent
was removed in vacuo. A white powder was obtained after re-dis-
solving the residue in CH3CN/H2O (1:1) and lyophilisation. Yield
4.03 g (97%). M.p. (dec.) 225 °C; HPLC tret = 9.6 min; Rf = 0.61
(EtOAc/BuOH/AcOH/H2O, 1:1:1:1). HRMS calcd. 191.0820,
found 191.0829. [α]2D0 = +11.2 (c = 1, H2O). 1H NMR (D2O,
250 MHz): δ = 3.26 [dd, 2J(Hβ,HβЈ) = 14.6 Hz, 3J(Hα,Hβ) =
7.7 Hz, 1 H, Hβ], 3.41 [dd, 2J(Hβ,HβЈ) = 14.6 Hz, 3J(Hα,HβЈ) =
7.2 Hz, 1 H, HβЈ], 4.17 [pseudo t, 3J(Hα,Hβ) Ϸ 3J(Hα,HβЈ) =
7.4 Hz, 1 H, Hα], 7.34–7.69 (m, 4 H, H arom.) ppm. 13C NMR (20
wt.-% DCl in D2O, 63 MHz): δ = 32.24 (CH2 β), 51.19 (CH α),
109.88 (Cq arom. next to CϵN), 116.11 (CϵN), 126.79, 128.70,
131.89, 132.23 (CH arom.), 135.50 (Cq arom. next to CH2 β),
167.91 (C=O) ppm.
(S)-N,N-Phthaloyl-o-cyanophenylalanine (15): (S)-o-Cyanophenyl-
alanine hydrochloride (14, 1.19 g, 5.25 mmol) was dissolved in
CH3CN/H2O (65:40, 105 mL), to which Na2CO3·H2O (3.26 g,
26.3 mmol) was added, as well as methyl 2-[(succinimidooxy)car-
bonyl]benzoate (MSB) (1.46 g, 5.26 mmol). After stirring at room
temperature for 8 h complete conversion was obtained, and the re-
action mixture was acidified to pH 2 (2  HCl). The mixture was
diluted with EtOAc (200 mL), and the organic layer was washed
with 1  HCl (2×100 mL) and water (2×100 mL). A light yellow
solid was obtained after drying (MgSO4), filtration and evapora-
tion. The product was used without further purification. Yield
1.36 g (81%). M.p. 198.8–200.7 °C; HPLC tret = 19.9 min; Rf =
0.46 (EtOAc/BuOH/AcOH/H2O, 16:1:1:1). HRMS calcd. 321.0875,
found 321.0869. [α]2D0 = –302.9 (c = 1.24, AcN/H2O, 14:1). 1H
NMR (CD3OD, 250 MHz): δ = 3.65 [dd, 2J(Hβ,HβЈ) = 14.4 Hz,
=
0.36 (EtOAc/hexane, 1:1). HRMS calcd. 469.1763, found
469.1771. [α]2D0 = +47.8 (c = 0.993, CHCl3). 1H NMR (CDCl3,
250 MHz): δ = 1.43 (d, 3J = 7.3 Hz, 3 H, CH3 Ala), 3.17 [dd,
2J(Hβ,HβЈ) = 16.2 Hz, 3J(Hα,Hβ) = 4.2 Hz, 1 H, Hβ], 4.26 [dd,
2J(Hβ,HβЈ) = 16.1 Hz, J(Hα,HβЈ) = 13.3 Hz, 1 H, HβЈ], 4.37 [d,
3
2J(Hε,HεЈ) = 16.4 Hz, 1 H, Hε], 4.81 [d, J(Hε,HεЈ) = 16.4 Hz, 1
2
3
H, HεЈ], 5.14 (s, 2 H, CH2 Bn ester), 5.40 (q, J = 7.3 Hz, 1 H, Hα
Ala), 5.58 [dd, 3J(Hα,Hβ) = 4.4 Hz, 3J(Hα,HβЈ) = 13.2 Hz, 1 H,
Hα benzazepine], 7.11–7.33 (m, 9 H, H arom. benzazepine, Bn es-
ter), 7.71–7.89 (m, 4 H, Phth) ppm. 13C NMR (CDCl3, 63 MHz):
δ = 15.68 (CH3 Ala), 34.50 (CH2 β), 48.65 (CH2 ε), 51.96 (CH α
benzazepine), 53.96 (CH α Ala), 67.92 (CH2 Bn ester), 123.91,
127.36, 128.48, 128.70, 128.76, 128.99, 130.50, 134.50 (CH arom.),
132.42, 135.20, 135.84, 136.42 (Cq arom.), 168.35, 169.84, 171.90
(C=O) ppm.
2
3J(Hα,Hβ) = 11.4 Hz, 1 H, Hβ], 3.81 [dd, J(Hβ,HβЈ) = 14.4 Hz,
3J(Hα,HβЈ) = 4.6 Hz, 1 H, HβЈ], 5.20 [dd, 3J(Hα,Hβ) = 11.4 Hz,
3J(Hα,HβЈ) = 4.6 Hz, 1 H, Hα], 7.28–7.62 (m, 4 H, H arom.), 7.73–
7.85 (m, 4 H, H arom. Phth) ppm. 13C NMR (CD3OD, 63 MHz):
δ = 34.60 (CH2 β), 53.37 (CH α), 113.82 (Cq arom. next to CϵN),
118.49 (CϵN), 124.42 (2 CH Phth), 128.86, 131.64, 134.10 and
134.25 (CH arom.), 135.75 (2 CH Phth), 132.72 (Cq arom. Phth),
142.55 (Cq arom. next to CH2 β), 168.72, 171.24 (C=O) ppm.
(2R)-[(4S)-4-(Phthalimido)-3-oxo-1,3,4,5-tetrahydro-benz[c]azepin-
2-yl]propionic Acid (18) [Phth–(S)-Aba–(R)-Ala–OH]: Phth–(S)-
Aba–(R)-Ala–OBn (17, 284 mg, 0.606 mmol) was dissolved in diox-
ane/H2O (3:2, 30 mL). After the addition of 10% Pd/C (20wt.-%,
(S)-N,N-Phthaloyl-o-formylphenylalanine (16): Wet Raney-Ni 60 mg) the suspension was hydrogenated during 5 h (50 psi) in a
(1.5 g, 50 µpore, Aldrich) was washed with milliQ water
(15×10 mL) and suspended with (S)-Phth–o-cyano–Phe (15,
350 mg, 1.09 mmol, 1 equiv.) in Py/AcOH/H2O (1:1:1, 30 mL). The
suspension was hydrogenated in a Parr apparatus (50 psi, 50 °C,
38 h). The mixture was filtered through dicalite and rinsed with
water. After evaporation of the solvent, the residue was redissolved
Parr hydrogenation apparatus. The mixture was filtered through
dicalite, and a white solid was obtained after evaporation of the
filtrate followed by lyophilisation from CH3CN/H2O. Yield 225 mg
(98 %). M.p. 170.0–179.2 °C; HPLC tret = 19.1 min; Rf = 0.48
(EtOAc/MeOH, 2:1). HRMS calcd. 379.1294, found 379.1285.
[α]2D0 = +28.6 (c = 0.46, CH2Cl2). 1H NMR (CDCl3, 250 MHz):
in EtOAc (100 mL) and washed with 1  HCl (3×50 mL). The δ = 1.34 (m, 3 H, CH3 Ala), 3.15 [dd, 2J(Hβ,HβЈ) = 16.2 Hz,
organic layer was dried (MgSO4), filtered and the solvents evapo-
rated. A white sticky foam was obtained. Yield 275 g (78%). HPLC
3J(Hα,Hβ) = 3.1 Hz, 1 H, Hβ], 4.17–4.35 (m, 2 H, HβЈ, Hε), 4.87
2
[d, J(Hε,HεЈ) = 16.3 Hz, 1 H, HεЈ], 5.11 (m, 1 H, Hα Ala), 5.60
tret = 19.7 min; Rf = 0.42 (EtOAc/BuOH/AcOH/H2O, 16:1:1:1). (m, 1 H, Hα benzazepine), 7.10–7.29 (4 H, H arom. benzazepine),
HRMS calcd. 324.0872, found 324.0877. [α]2D0 = –253 (c = 0.664,
7.63–7.79 (m, 4 H, Phth) ppm. 13C NMR (CDCl3, 63 MHz): δ =
15.78 (CH3 Ala), 34.27 (CH2 β), 48.92 (CH2 ε), 51.97 (CH α benza-
zepine, CH α Ala), 123.88 (2 CH Phth), 127.26, 128.47, 128.94,
130.39 (CH arom.), 134.46 (2 CH Phth), 132.27, 135.20, 136.02 (Cq
arom.), 168.54, 170.30, 176.69 (C=O) ppm.
2
EtOAc). 1H NMR (CDCl3, 250 MHz): δ = 3.65 [dd, J(Hβ,HβЈ) =
3
2
13.6 Hz, J(Hα,Hβ) = 11.3 Hz, 1 H, Hβ], 4.24 [dd, J(Hβ,HβЈ) =
13.6 Hz, 3J(Hα,HβЈ) = 4.7 Hz, 1 H, HβЈ], 5.43 [dd, 3J(Hα,Hβ) =
11.1 Hz, 3J(Hα,HβЈ) = 4.7 Hz, 1 H, Hα], 7.01–8.07 (m, 8 H, H
2906
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2899–2911

Studies on Tetrapeptide Mimetics
FULL PAPER
(2R)-[(4S)-4-Amino-3-oxo-1,3,4,5-tetrahydro-benz[c]azepin-2-yl]pro-
304.1667. [α]2D0 = +32.5 (c = 0.6, EtOH). 1H NMR (CDCl3,
3
pionic Acid (19) [(S)-Aba–(R)-Ala–OH]: Phth–(S)-Aba–(R)-Ala– 500 MHz, 298 K): δ = 1.26 (d, J = 7.1 Hz, 3 H, CH3 Ala), 2.10 (s,
OH (18, 169 mg, 0.448 mmol) was dissolved in EtOH (5 mL). After 3 H, CH3 Ac), 2.83 (d, 3J = 4.8 Hz, 3 H, NH–CH3), 2.95 [dd,
3
the addition of NH2NH2·H2O (130 µL, 2.68 mmol) the mixture
was refluxed for 1.5 h. The solvent was then removed under re-
duced pressure and the residue was suspended in H2O (2.5 mL).
The pH was adjusted with AcOH until pH 5. After 1 h stirring
at room temperature the precipitate was filtered. The filtrate was
evaporated and the product was further purified by preparative
HPLC. A white powder that rapidly became a transparent paste
2J(Hβ,HβЈ) = 16.9 Hz, J(Hα,HβЈ) = 13.0 Hz, 1 H, HβЈ], 3.45 [dd,
2J(Hβ,HβЈ) = 16.9 Hz, 3J(Hα,Hβ) = 4.2 Hz, 1 H, Hβ], 4.18 [dЈ,
2
2J(Hε,HεЈ) = 17.2 Hz, 1 H, Hε], 4.97 [d, J(Hε,HεЈ) = 17.2 Hz, 1
H, Hε], 5.16 (q, 3J = 7.1 Hz, 1 H, Hα Ala), 5.45 (m, 1 H, Hα
3
3
benzazepine), 6.41 (q, J = 4.8 Hz, 1 H, NH–CH3), 6.99 (d, J =
6.3 Hz, 1 H, NH–Ac), 7.08–7.22 (m, 4 H, H arom.) ppm. 1H NMR
([D6]DMSO, 500 MHz, 298 K): δ = 1.10 (d, 3J = 7.4 Hz, 3 H, CH3
3
was obtained. Yield 93 mg (84%). HPLC tret = 11.4 min; Rf = 0.65 Ala), 1.94 (s, 3 H, CH3 Ac), 2.61 (d, J = 4.5 Hz, 3 H, NH–CH3),
3
(EtOAc/BuOH/AcOH/H2O, 1:1:1:1). HRMS calcd. 249.1239,
found 249.1243. [α]2D0 = +40.7 (c = 0.786, dioxane/H2O, 9:1). 1H
NMR (CD3OD, 250 MHz): δ = 1.35 (d, 3J = 7.4 Hz, 3 H, CH3
2.85 [dd, 2J(Hβ,HβЈ) = 17.3 Hz, J(Hα,HβЈ) = 13.1 Hz, 1 H, HβЈ],
3.17 [dd, 2J(Hβ,HβЈ) = 17.3 Hz, 3J(Hα,Hβ) = 4.1 Hz, 1 H, Hβ],
2
3
4.16 [d, J(Hε,HεЈ) = 17.5 Hz, 1 H, HεЈ], 5.02 (q, J = 7.4 Hz, 1
2
2
Ala), 3.18–3.44 (m, 2 H, 2 Hβ), 4.24 [d, J(Hε,HεЈ) = 17.5 Hz, 1
H, Hα Ala), 5.06 [d, J(Hε,HεЈ) = 17.5 Hz, 1 H, Hε], 5.40 (m, 1
H, Hε], 5.07–5.23 (m, 3 H, HεЈ, Hα Ala, Hα benzazepine), 7.17–
7.32 (m, 4 H, H arom.) ppm. 13C NMR (CD3OD, 63 MHz): δ =
20.11 (CH3 Ala), 39.26 (CH2 β), 53.35 (CH2 ε), 55.29 (CH α benza-
zepine, CH α Ala), 132.54, 132.65, 134.27, 135.99 (CH arom.),
138.84, 139.88 (Cq arom.), 174.91, 178.87 (C=O) ppm.
H, Hα benzazepine), 7.12–7.27 (m, 4 H, H arom.), 7.86 (q, 3J =
3
4.5 Hz, 1 H, NH–CH3), 8.09 (d, J = 7.0 Hz, 1 H, NH–Ac) ppm.
13C NMR (CDCl3, 126 MHz, 298 K): δ = 14.4 (CH3 Ala), 22.8
(CH3 Ac), 26.3 (NH–CH3), 36.1 (CH2 β), 47.5 (CH2 ε), 48.6 (CH
α benzazepine), 52.7 (CH α Ala), 126.4, 127.9, 128.2, 130.6 (CH
arom.), 133.2, 134.7 (Cq arom.), 170.1 (C=O Ac), 170.9 (C=O
benzazepine), 172.0 (C=O Ala) ppm. 13C NMR ([D6]DMSO,
126 MHz, 298 K): δ = 15.7 (CH3 Ala), 22.5 (CH3 Ac), 25.8 (NH–
CH3), 36.1 (CH2 β), 47.0 (CH2 ε), 47.9 (CH α benzazepine), 52.3
(CH α Ala), 126.1, 127.3, 128.4, 130.4 (CH arom.), 135.1, 135.4
(Cq arom.), 168.5 (C=O Ac), 170.7 (C=O Ala), 171.6 (C=O benz-
azepine) ppm.
(2R)-[(4S)-4-Acetylamino-3-oxo-1,3,4,5-tetrahydro-benz[c]azepin-2-
yl]propionic Acid (20) [Ac–(S)-Aba–(R)-Ala–OH]: (S)-Aba–(R)-
Ala–OH (19, 70 mg, 0.282 mmol) was dissolved in water (3 mL).
The pH was adjusted to 6 with NEt3, and Ac2O (133 µL,
1.41 mmol) was added in three portions. Meanwhile the pH was
kept at 6 with NEt3. After 2 h stirring at room temperature HPLC
analysis demonstrated completion of the reaction. The reaction
mixture was evaporated, and the product was further purified by
preparative HPLC. A white powder that rapidly became a trans-
Characterisation of 6: HPLC tret = 12.0 min; Rf = 0.70 (EtOAc/
BuOH/AcOH/H2O, 1:1:1:1). HRMS calcd. 304.1661, found
1
304.1656. H NMR (CDCl3, 500 MHz, 298 K): δ = 1.41 (d, 3J =
parent paste was obtained. Yield 49 mg (60 %). HPLC tret
=
7.0 Hz, 3 H, CH3 Ala), 2.09 (s, 3 H, CH3 Ac), 2.33 (d, 3J = 5.0 Hz,
3 H, NH–CH3), 2.93 [dd, 2J(Hβ,HβЈ) = 17.0 Hz, 3J(Hα,HβЈ) =
13.8 min; Rf = 0.24 (EtOAc/MeOH, 2:1). HRMS calcd. 291.1345,
1
found 291.1348. [α]2D0 = +65.4 (c = 1, MeOH). H NMR (CDCl3,
2
3
3
12.7 Hz, 1 H, HβЈ], 3.56 [dd, J(Hβ,HβЈ) = 17.0 Hz, J(Hα,Hβ) =
250 MHz): δ = 1.34 (d, J = 7.4 Hz, 3 H, CH3 Ala), 2.14 (s, 3 H,
2
3
CH3 Ac), 2.96 [dd, 2J(Hβ,HβЈ) = 16.8 Hz, J(Hα,Hβ) = 12.8 Hz, 1
5.0 Hz, 1 H, Hβ], 4.12 [dЈ, J(Hε,HεЈ) = 16.7 Hz, 1 H, Hε], 4.92
2
3
H, Hβ], 3.49 [dd, 2J(Hβ,HβЈ) = 16.9 Hz, 3J(Hα,HβЈ) = 4.0 Hz, 1
H, HβЈ], 4.00 [d, 2J(Hε,HεЈ) = 17.3 Hz, 1 H, Hε], 5.13 [d,
2J(Hε,HεЈ) = 17.1 Hz, 1 H, HεЈ], 5.30 (q, 3J = 7.3 Hz, 1 H, Hα
Ala), 5.52 (m, 1 H, Hα benzazepine), 5.80 (br. s, 1 H, NH amide),
7.04–7.30 (m, 4 H, H arom.) ppm. 13C NMR (CDCl3, 63 MHz): δ
= 15.73 (CH3 Ala), 23.26 (CH3 Ac), 36.58 (CH2 β), 49.29 (CH2 ε),
53.32 (CH α benzazepine, CH α Ala), 127.05, 128.60, 128.66,
131.33 (CH arom.), 133.76, 135.45 (Cq arom.), 171.71, 172.44,
174.93 (C=O) ppm.
[d, J(Hε,HεЈ) = 16.7 Hz, 1 H, Hε], 5.16 (q, J = 7.0 Hz, 1 H, Hα
Ala), 5.40 (m, 1 H, Hα benzazepine), 5.45 (q, 3J = 5.0 Hz, 1 H,
3
NH–CH3), 6.92 (d, J = 6.4 Hz, 1 H, NH–Ac), 7.05–7.21 (m, 4 H,
H arom.) ppm. H NMR ([D6]DMSO, 500 MHz, 298 K): δ = 1.20
1
3
3
(d, J = 7.0 Hz, 3 H, CH3 Ala), 1.92 (s, 3 H, CH3 Ac), 2.29 (d, J
= 4.5 Hz, 3 H, NH–CH3), 2.99 [dd, 2J(Hβ,HβЈ) = 17.0 Hz,
3J(Hα,HβЈ) = 13.2 Hz, 1 H, HβЈ], 3.11 [dd, J(Hβ,HβЈ) = 17.3 Hz,
2
3J(Hα,Hβ) = 4.3 Hz, 1 H, Hβ], 4.18 [d, 2J(Hε,HεЈ) = 16.7 Hz, 1
3
2
H, HεЈ], 4.80 (q, J = 7.0 Hz, 1 H, Hα Ala), 4.95 [d, J(Hε,HεЈ) =
16.7 Hz, 1 H, Hε], 5.19 (m, 1 H, Hα benzazepine), 7.05–7.18 (m,
3
3
(2R)-[(4S)-4-Acetylamino-3-oxo-1,3,4,5-tetrahydro-benz[c]azepin-2-
yl]-N-methylpropionamide (5) [Ac–(S)-Aba–(R)-Ala–NHMe]: Ac–
(S)-Aba–(R)-Ala–OH (20, 104 mg, 0.358 mmol) was dissolved in
CH2Cl2 (6.5 mL). NEt3 (50 µL, 0.358 mmol) and TBTU (138 mg,
0.430 mmol) were added, and the reaction mixture was stirred for
5 min, followed by the addition of CH3NH2·HCl (26.6 mg,
0.394 mmol) and NEt3 (150 µL, 1.07 mmol). The coupling was con-
tinued overnight. HPLC analysis indicated that the starting mate-
rial had been completely consumed. The reaction mixture was di-
luted with CH2Cl2 (50 mL) and washed with 1  HCl (1×25 mL)
in order to remove most of the HOBt. The product was dried
(MgSO4), filtered and the solvents evaporated. LC/MS demon-
strated that 30% racemisation had occurred with formation of Ac–
(S)-Aba–(S)-Ala–NHMe (6). The product was further purified by
preparative HPLC (both diastereomers were collected) until suf-
ficient material for the NMR study was obtained (21 mg of 5, 9 mg
of 6).
4 H, H arom.), 7.55 (q, J = 4.5 Hz, 1 H, NH–CH3), 8.12 (d, J =
7.0 Hz, 1 H, NH–Ac) ppm. 13C NMR (CDCl3, 126 MHz, 298 K):
δ = 13.7 (CH3 Ala), 23.0 (CH3 Ac), 25.7 (NH-CH3), 36.1 (CH2 β),
47.0 (CH2 ε), 48.8 (CH α benzazepine), 52.4 (CH α Ala), 126.1,
127.9, 128.9, 130.4 (CH arom.), 132.7, 134.4 (Cq arom.), 169.5
(C=O Ac), 170.0 (C=O Ala), 171.7 (C=O benzazepine) ppm. 13C
NMR ([D6]DMSO, 126 MHz, 298 K): δ = 14.8 (CH3 Ala), 22.6
(CH3 Ac), 25.5 (NH-CH3), 35.8 (CH2 β), 47.4 (CH2 ε), 48.4 (CH
α benzazepine), 53.3 (CH α Ala), 125.6, 127.2, 128.7, 130.2 (CH
arom.), 134.7, 135.6 (Cq arom.), 168.8 (C=O Ac), 170.1 (C=O Ala),
171.0 (C=O benzazepine) ppm.
[(4S)-4-Acetylamino-3-oxo-1,3,4,5-tetrahydro-benz[c]azepin-2-yl]-
acetic Acid (22) [Ac–(S)-Aba–Gly–OH]: (S)-Aba–Gly–OH (21,
300 mg, 1.11 mmol) was dissolved in water (15 mL). The pH was
adjusted to 6 with NEt3, and Ac2O (1.1 mL, 11.5 mmol) was
added. Meanwhile the pH was kept at 6 with NEt3. After overnight
stirring at room temperature HPLC analysis demonstrated comple-
tion of the reaction. The reaction mixture was acidified with 1 
Characterisation of 5: HPLC tret = 12.9 min; Rf = 0.64 (EtOAc/
BuOH/AcOH/H2O, 1:1:1:1). HRMS calcd. 304.1661, found HCl to pH 2 and extracted with EtOAc (4×10 mL). The organic
Eur. J. Org. Chem. 2006, 2899–2911
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2907

D. Tourwé et al.
FULL PAPER
layer was dried (MgSO4), filtered and evaporated, and the product
was further purified by preparative HPLC. A white solid was ob-
rated and the residue was divided between water (20 mL) and ether
(10 mL). The aqueous phase was extracted with diethyl ether
(2×10 mL), and the combined organic layer was extracted with
saturated aqueous NaHCO3 (15 mL). The combined aqueous layer
was acidified with 10% KHSO4 until pH 3, followed by extraction
with EtOAc (3×20 mL). The organic layer was dried (MgSO4), fil-
tered and the solvents evaporated. Yield 825 mg (79%). M.p. 110–
127 °C. HPLC tret = 20.5 min; Rf = 0.53 (EtOAc/MeOH, 2:1).
tained. Yield 198 mg (65 %). M.p. 287–289 °C; HPLC tret
=
10.4 min; Rf = 0.53 (EtOAc/BuOH/AcOH/H2O, 1:1:1:1). HRMS
calcd. 277.1188, found 277.1195. [α]2D0 = 123.1 (c = 1, DMF). 1H
NMR ([D6]DMSO, 250 MHz): δ = 1.92 (s, 3 H, CH3 Ac), 2.90 [dd,
3
2J(Hβ,HβЈ) = 16.8 Hz, J(Hα,Hβ) = 13.0 Hz, 1 H, Hβ], 3.16 [dd,
2J(Hβ,HβЈ) = 17.0 Hz, 3J(Hα,HβЈ) = 4.0 Hz, 1 H, HβЈ], 3.94 [d,
2J(Hα,HαЈ) = 17.3 Hz, 1 H, Hα Gly], 4.16 [d, 2J(Hε,HεЈ) = HRMS calcd. 349.1763, found 349.1760. [α]2D0 = –1.8 (c = 1,
2
1
17.0 Hz, 1 H, Hε], 4.25 [d, J(Hα,HαЈ) = 17.3 Hz, 1 H, HαЈ Gly],
MeOH). H NMR (CDCl3, 250 MHz): δ = 1.43 (s, 9 H, tBu), 2.97
2
5.21 [dЈ, J(Hε,HεЈ) = 16.9 Hz, 1 H, Hε], 5.37 (m, 1 H, Hα benz- (br. s, 4 H, N–Me, Hβ), 3.43 (m, 1 H, HβЈ), 4.26 (m, 3 H, Hα Gly,
azepine), 7.10–7.24 (m, 4 H, H arom.), 8.12 [d, 3J(NH–Hα) =
2 Hε), 4.83 (m, 1 H, HαЈ Gly), 5.15 (m, 1 H, Hα), 7.10–7.41 (m, 4
7.5 Hz, 1 H, NH amide] ppm. 13C NMR ([D6]DMSO, 63 MHz): δ H, H arom.) ppm. 13C NMR (CDCl3, 63 MHz): δ = 28.37 (CH3
= 22.9 (CH3 Ac), 36.3 (CH2 β), 48.0 (CH2 ε), 49.5 (CH2 Gly), 52.3 Boc), 31.75 (N–Me), 34.35 (CH2 β), 44.68 (CH2 ε), 53.33 (CH2
(CH α benzazepine), 126.2, 127.8, 129.2, 130.7 (CH arom.), 134.5,
135.9 (Cq arom.), 169.1, 170.8, 172.2 (C=O) ppm.
Gly), 55.64 (CH α), 80.38 (Cq Boc), 126.64, 128.13, 128.66, 130.13
(CH arom.), 134.55, 135.68 (Cq arom.), 156.46 (C=O Boc), 172.92
(2 C=O) ppm.
2-[(4S)-4-Acetylamino-3-oxo-1,3,4,5-tetrahydro-benz[c]azepin-2-yl]-
N-methylacetamide (7) [Ac–(S)-Aba–Gly–NHMe]: Ac–(S)-Aba–
Gly–OH (22, 60 mg, 0.22 mmol) was dissolved in CH2Cl2 (12 mL).
Et3N (30 µL, 0.22 mmol) and TBTU (85 mg, 0.264 mmol) were
added, and the reaction mixture was stirred for 5 min, followed
by the addition of CH3NH2·HCl (16.3 mg, 0.242 mmol) and Et3N
(90 µL, 0.66 mmol). After a reaction time of 2 h complete conver-
sion to 7 was observed by HPLC analysis. The solvent was removed
from the reaction mixture, and the crude product was purified by
preparative HPLC. Yield 35 mg (55%); HPLC tret = 10.55 min; Rf
= 0.65 (EtOAc/BuOH/AcOH/H2O, 1:1:1:1). HRMS 290.1504,
found 290.1508. [α]2D0 = +163 (c = 0.83, CHCl3). 1H NMR (CDCl3,
500 MHz): δ = 2.06 (s, 3 H, CH3 Ac), 2.74 [d 3J(CH3–NH) =
4.7 Hz, 3 H, NH–CH3], 2.91 [dd, 2J(Hβ,HβЈ) = 17.0 Hz, 3J(Hα,Hβ)
[(4S)-4-Methylamino-3-oxo-1,3,4,5-tetrahydro-benz[c]azepin-2-yl]-
acetic Acid Hydrochloride (25) [N-Me–(S)-Aba–Gly–OH·HCl]: N-
Boc,N-Me–(S)-Aba–Gly–OH (24, 0.5 g, 1.45 mmol) was dissolved
in a 6  HCl/CH2Cl2 mixture (1:1, 20 mL) and stirred at room tem-
perature overnight after which the solvent was removed in vacuo.
Purification was performed by flash column chromatography
(MeOH/CH2Cl2, 1:9). The desired product 25 was obtained as a
white solid. Yield 233 mg (57%). M.p. 128–130 °C. HPLC tret
=
8.4 min; Rf = 0.47 (EtOAc/BuOH/AcOH/H2O, 1:1:1:1). HRMS
calcd. 249.1239, found 249.1244. [α]2D0 = +0.46 (c = 1, MeOH). H
1
NMR ([D6]DMSO, 250 MHz): δ = 2.67 (s, 3 H, NMe), 3.04 [dd,
3
2J(Hβ,HβЈ) = 16.3 Hz, J(Hα,Hβ) = 13.2 Hz, 1 H, Hβ], 3.48 [dd,
2J(Hβ,HβЈ) = 16.3 Hz, 3J(Hα,HβЈ) = 4.0 Hz, 1 H, HβЈ], 4.05 [d,
2J(Hε,HεЈ) = 17.4 Hz, 1 H, Hε], 4.25 (m, 2 H, 2 Hα Gly), 5.20 (m,
2 H, HεЈ, Hα Aba), 7.14–7.30 (m, 4 H, H arom.), 9.14 (br. s, 1 H,
NH) ppm. 13C NMR ([D6]DMSO, 63 MHz): δ = 31.6 (CH3N), 49.6
(CH2 β), 51.9 (CH2 ε), 52.3 (CH2 α Gly), 56.4 (CH α benzazepine),
126.8, 128.2, 129.4, 130.9 (CH arom.), 133.7, 133.9 (Cq arom.),
169.3, 170.2 (C=O) ppm.
2
3
= 13.5 Hz, 1 H, Hβ], 3.48 [dd, J(Hβ,HβЈ) = 17.0 Hz, J(Hα,HβЈ)
= 4.7 Hz, 1 H, HβЈ], 3.77 [d, 2J(Hα,HαЈ) = 16.1 Hz, 1 H, Hα Gly],
3.98 [d, 2J(Hε,HεЈ) = 16.8 Hz, 1 H, Hε], 4.44 [d, 2J(Hα,HαЈ) =
2
16.1 Hz, 1 H, HαЈ Gly], 5.22 [d, J(Hε,HεЈ) = 16.8 Hz, 1 H, HεЈ],
5.43 (m, 1 H, Hα benzazepine), 7.10–7.23 (m, 4 H, H arom.), 6.57
(br. s, 1 H, NH–CH3), 6.93 [d, 3J(NH–Hα) = 6.3 Hz, 1 H, NH–Ac]
ppm. 1H NMR ([D6]DMSO, 500 MHz): δ = 1.92 (s, 3 H, CH3 Ac),
2.59 [d, 3J(CH3–NH) = 4.5 Hz, 3 H, NH–CH3], 2.85 [dd,
{(4S)-4-[Acetyl(methyl)amino]-3-oxo-1,3,4,5-tetrahydro-benz[c]-
azepin-2-yl}acetic Acid (26) [N-Ac,N-Me–(S)-Aba–Gly–OH]: N-
Me–(S)-Aba–Gly–OH·HCl (25, 200 mg, 0.69 mmol) was dissolved
in water (10 mL). The pH was adjusted to 6 with NEt3, and Ac2O
(0.8 mL, 6.4 mmol) was added. Meanwhile the pH was kept at 6
with NEt3. After 2 h stirring at room temperature HPLC analysis
demonstrated completion of the reaction. The solvent was removed
in vacuo, and the product was further purified by preparative
HPLC. A white solid was obtained. Yield 90 mg (45%). M.p. 86–
88 °C. HPLC tret = 12.1 min; Rf = 0.66 (EtOAc/BuOH/AcOH/H2O,
1:1:1:1). HRMS 291.1345, found 291.1340. [α]2D0 = +1.2 (c = 1,
3
2J(Hβ,HβЈ) = 17.1 Hz, J(Hα,Hβ) = 13.1 Hz, 1 H, Hβ], 3.18 [dd,
2J(Hβ,HβЈ) = 17.1 Hz, 3J(Hα,HβЈ) = 4.8 Hz, 1 H, HβЈ], 3.61 [d,
2J(Hα,HαЈ) = 16.4 Hz, 1 H, Hα Gly], 4.07 [d, 2J(Hε,HεЈ) =
2
16.7 Hz, 1 H, Hε], 4.23 [d, J(Hα,HαЈ) = 16.4 Hz, 1 H, HαЈ Gly],
5.19 [d, 2J(Hε,HεЈ) = 16.7 Hz, 1 H, HεЈ], 5.37 (m, 1 H, Hα benza-
zepine), 7.12–7.21 (m, 4 H, H arom.), 7.87 [q, 3J(NH–CH3) =
3
4.5 Hz, 1 H, NH–CH3], 8.12 [d, J(NH–Hα) = 7.1 Hz, 1 H, NH–
Ac] ppm. 13C NMR (CDCl3, 126 MHz): δ = 22.9 (CH3 Ac), 26.1
(NH–CH3), 36.1 (CH2 β), 48.6 (CH α benzazepine), 51.5 (CH2
Gly), 53.0 (CH2 ε), 126.4, 128.3, 128.8, 130.7 (CH arom.), 132.1,
134.8 (Cq arom.), 168.8 (C=O Gly), 170.1 (C=O Ac), 172.1 (C=O
benzazepine) ppm. 13C NMR ([D6]DMSO, 126 MHz): δ = 22.3
(CH3 Ac), 25.2 (NH–CH3), 35.7 (CH2 β), 47.6 (CH α benzazepine),
49.9 (CH2 Gly), 51.5 (CH2 ε), 125.6, 127.2, 128.6, 130.1 (CH
arom.), 133.7, 135.2 (Cq arom.), 167.5 (C=O Gly), 168.1 (C=O Ac),
172.15 (C=O benzazepine) ppm.
1
DMF). H NMR ([D6]DMSO, 250 MHz): δ = 2.02 and 2.05 (2s, 3
H, CH3 Ac), 2.84 and 2.99 (2s, 3 H, CH3N), 2.95 and 3.19 [dd,
2J(Hβ,HβЈ) = 16.2 Hz, 3J(Hα,Hβ) = 3.9 Hz, 1 H, Hβ], 3.41 and
2
3.46 (m, 1 H, HβЈ), 4.04 [d, J(Hα,HαЈ) = 17.6 Hz, 1 H, Hα Gly],
4.15 [d, 2J(Hα,HαЈ) = 17.6 Hz, 1 H, HαЈ Gly] 4.43 and 4.47 [2d,
2
2J(Hε,HεЈ) = 16.3 Hz, 1 H, Hε], 4.84 and 5.01 [2d, J(Hε,HεЈ) =
16.5 Hz, 1 H, HεЈ], 5.21 and 5.61 [2 dd, 3J(Hα,HβЈ) = 12.0 Hz,
3J(Hα,Hβ) = 3.9 Hz, 1 H, Hα Aba], 7.10–7.30 (m, 4 H, H arom.)
ppm. 13C NMR ([D6]DMSO, 63 MHz): δ = 22.1 (CH3 Ac), 29.2
and 32. 9 (CH3N), 33.6 and 34.3 (CH2 β), 50.5 and 52.3 (CH2 ε),
53.6 (CH2 Gly), 57.3 (CH α benzazepine), 126.7, 128.2, 128.9, 130.2
(CH arom.), 135.6 and 135.7 (Cq arom.), 136.2 and 136.6 (Cq
arom.), 170.5 (C=O Gly), 170.9 (C=O Ac), 171.6 and 171.9 (C=O
benzazepine) ppm.
[(4S)-4-[tert-Butoxycarbonyl(methyl)amino]-3-oxo-1,3,4,5-tetra-
hydro-benz[c]azepin-2-yl]acetic Acid (24) [N-Boc,N-Me–(S)-Aba–
Gly–OH]: Boc–(S)-Aba–Gly–OH (23, 1.00 g, 3.0 mmol) was dis-
solved in THF (20 mL), followed by the addition of CH3I (1.5 mL,
24 mmol). The reaction mixture was cooled for 20 min in an ice
bath, after which NaH (55–65% dispersion in oil, 393 mg, 9 mmol)
was added. After removal of the ice bath the mixture was stirred
for 24 h at room temperature. EtOAc (25 mL) was added, followed
by the dropwise addition of water (10 mL). The solvent was evapo-
2-[(4S)-4-[Acetyl(methyl)amino]-3-oxo-1,3,4,5-tetrahydro-benz[c]-
azepin-2-yl]-N-methylacetamide (8) [N-Ac,N-Me–(S)-Aba–Gly–
2908
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2899–2911

Studies on Tetrapeptide Mimetics
FULL PAPER
2
NHMe]: N-Ac,N-Me–(S)-Aba–Gly–OH (26, 40 mg, 0.14 mmol)
was dissolved in CH2Cl2 (8 mL). Et3N (20 µL, 0.15 mmol) and
TBTU (54 mg, 0.168 mmol) were added and the reaction mixture
was stirred for 5 minutes, followed by the addition of CH3NH2·HCl
(10 mg, 0.154 mmol) and Et3N (60 µL, 0.45 mmol). After a reac-
tion time of 2 h complete conversion to 8 was observed by HPLC
analysis. The solvent was removed from the reaction mixture and
the crude product was purified by preparative HPLC. Yield 20 mg
(47%); M.p. 170–172 °C. HPLC tret = 11.0 min; Rf = 0.54 (EtOAc/
BuOH/AcOH/H2O, 1:1:1:1). HRMS calcd. 304.1661, found
304.1668. [α]2D0 = +1.2 (c = 1, CHCl3). 1H NMR (CDCl3,
500 MHz): δ = 2.16 (s, 3 H, CH3 Ac), 2.82 [d, 3J(CH3–NH) =
4.6 Hz, 3 H, NH–CH3], 3.27 (s, 3 H, CH3N), 2.89 [dd, 2J(Hβ,HβЈ)
(m, 1 H, H1Ј), 3.07 (m, 1 H, H3), 3.22 [d, J(H10,H10Ј) = 15.4 Hz,
1 H, H10], 3.40 [d, J(H10,H10Ј) = 15.4 Hz, 1 H, H10Ј], 3.78 (m, 1
2
H, H3Ј), 4.28 [d, 2J(H5,H5Ј) = 14.0 Hz, 1 H, H5], 4.52 [dЈ, 2J(H5,H5Ј)
= 14.0 Hz, 1 H, H5], 7.35 (m, 4 H, H arom.) ppm. 13C NMR ([D6]-
DMSO, 63 MHz): δ = 22.8 (CH2 2), 34.6 (CH2 10), 35.6 (CH2 1),
51.4 (CH2 5), 55.7 (CH2 3), 73.7 (Cq 10a), 127.7, 127.8, 128.3, 129.2
(CH arom.), 130.2, 133.7 (Cq arom.), 172.4 (C=O) ppm.
[Boc-spiro-(R,S)-Aba–Gly–OBn] (30): To a solution of aldehyde 28
(700 mg, 2.1 mmol) in CH2Cl2 (analytical grade, 35 mL) Gly–
OBn·pTosOH (742 mg, 2.2 mmol) was added. The pH was adjusted
to 6 with N-methylmorpholine, followed by the addition of MgSO4
(140 mg, 20 wt.-%) and NaCNBH3 (331 mg, 5.25 mmol). After
18 h stirring at room temperature the solvent was removed under
reduced pressure. The residue was redissolved in acetonitrile
(130 mL) and pyridine (336 µL, 4.2 mmol) was added. The mixture
was cooled in an ice bath for 10 minutes and DCC (474 mg,
2.3 mmol) was added. After 1 hour stirring at 0 °C the reaction was
continued overnight at room temperature. Oxalic acid (5  solution
in DMF, 5 mL) was added, and after 30 minutes stirring the abun-
dant precipitate was filtered. The filtrate was evaporated and the
residue was redissolved in EtOAc (200 mL). After washing with 1 
HCl and satd. aq. NaHCO3 (3 × 50 mL), the organic layer was
dried (MgSO4), filtered and the solvents evaporated. The small par-
ticles of DCU in the obtained paste could be removed by filtration
after dissolving the paste in toluene. The product was further puri-
fied by flash column chromatography (EtOAc/petroleum ether,
1:3). Yield 200 mg (21%). HPLC tret = 26.4 min; Rf = 0.53 (EtOAc/
petroleum ether, 1:2); Rf = 0.60 (EtOAc/cHex, 1:1). HRMS calcd.
3
2
= 14.3 Hz, J(Hα,Hβ) = 5.3 Hz, 1 H, Hβ], 3.62 [dd, J(Hβ,HβЈ) =
14.3 Hz, 3J(Hα,HβЈ) = 13.0 Hz, 1 H, HβЈ], 3.72 [1 H, broadened
d, Hα Gly, 2J(Hα,HαЈ) = 16.4 Hz], 4.07 [1 H, broadened d, Hε,
2J(Hε,HεЈ) = 15.1 Hz], 4.73 [d, 2J(Hα,HαЈ) = 16.6 Hz, 1 H, HαЈ
2
Gly], 5.05 [d,Ј, J(Hε,HεЈ) = 15.1 Hz 1 H, Hε], 4.24 (dd, 1 H, Hα
benzazepine, partially hidden by Hα Gly), 7.21–7.30 (m, 4 H, H
arom.), 7.51 (br. s, 1 H, NH–CH3) ppm. 1H NMR ([D6]DMSO,
500 MHz): δ = 2.05 (s, 3 H, CH3 Ac), 2.59 [d, 3J(CH3–NH) =
4.5 Hz, 3 H, NH–CH3], 2.85 and 3.05 (s, 3 H, CH3N), 2.96 and
3.19 [dd, 2J(Hβ,HβЈ) = 15.3 Hz, 3J(Hα,Hβ) = 4.7 Hz, 1 H, Hβ],
3.41 and 3.45 [dd, 2J(Hβ,HβЈ) = 14.8 Hz, J(Hα,HβЈ) = 12.9 Hz, 1
3
H, HβЈ], 3.82 and 3.95 [d, 2J(Hα,HαЈ) = 16.2 Hz, 1 H, Hα Gly],
4.01 and 4.17 [d, 2J(Hα,HαЈ) = 16.4 Hz, 1 H, HαЈ Gly] 4.54 and
4.30 [2d, 2J(Hε,HεЈ) = 16.1 Hz, 1 H, Hε], 4.66 and 5.00 [2d,
2J(Hε,HεЈ,1 H, HεЈ) = 16.2 Hz], 5.14 and 5.23 [2dd, J(Hα,HβЈ) =
3
12.4 Hz, 3J(Hα,Hβ) = 4.4 Hz, 1 H, Hα benzazepine], 7.20–7.30 (m,
4 H, H arom.), 7.75 and 7.83 [q, 3J(NH–CH3) = 4.5 Hz, 1 H, NH–
CH3] ppm. 13C NMR (CDCl3, 126 MHz): δ = 21.9 (CH3 Ac), 26.4
(CH3NH), 28.2 (CH3N), 33.2 (CH2 β), 51.8 (CH2 ε), 52.3 (CH α
benzazepine), 53.4 (CH2 Gly), 127.7, 128.2, 128.6, 129.5 (CH
arom.), 136.3, 136.5 (Cq arom.), 169.9 (C=O Gly), 170.2 (C=O Ac),
171.8 (C=O Aba) ppm. 13C NMR ([D6]DMSO, 126 MHz): δ = 21.7
(CH3 Ac), 25.4 (CH3NH) 28.8 and 34.3 (CH3N), 33.0 and 34.00
(CH2 β), 51.4 and 52.1 (CH2 ε), 51.2 and 51.6 (CH2 Gly), 55.9 and
56.8 (CH α benzazepine), 127.6, 128.0, 128.8, 129.3 (CH arom.),
135.3, 135.4 (Cq arom.), 168.3 (C=O Gly), 170.2 (C=O Ac), 170.5
(C=O Aba) ppm.
1
465.2389, found 465.2381. H NMR (CDCl3, 250 MHz): δ = 1.46
and 1.51 (2s, 9 H, tBu), 1.91 (m, 4 H, CH2 3 and CH2 4), 2.56 and
2.64 [2d, 2J(Hβ,HβЈ) = 14.2 Hz, 1 H, Hβ], 3.55 and 3.72 (2m, 2 H,
CH2 5), 3.89 and 3.94 [2d, 2J(Hε,HεЈ) = 14.6 Hz, 1 H, Hε], 3.99
2
and 4.11 [2d, J(Hα,HαЈ) = 17.0 Hz, 1 H, Hα Gly], 4.09 and 4.23
[2d, 2J(Hβ,HβЈ) = 14.2 Hz, 1 H, HβЈ], 4.55 and 4.72 [2d,
2J(Hα,HαЈ) = 17.0 Hz, 1 H, HαЈ Gly], 5.09 and 5.14 (2s, 2 H, CH2
Bn ester), 5.09 and 5.31 [2d, 2J(Hε,HεЈ) = 14.6 Hz, 1 H, HεЈ], 7.27
(m, 9 H, H arom. benzazepine, Bn ester) ppm. 13C NMR (CDCl3,
63 MHz): δ = 22.3 and 23.2 (CH2 4), 28.5 (CH3 tBu), 37.9 and 39.7
(CH2 3), 39.9 and 40.9 (CH2 β), 48.6 and 48.7 (CH2 5), 52.9 and
53.2, 53.4 and 53.5 (CH2 ε, CH2 α Gly ), 66.8 and 66.9 (CH2 Bn
ester), 69.0 and 69.3 (Cq α benzazepine), 80.0 and 81.1 (Cq tBu),
127.2, 127.4, 128.1, 128.2, 128.3, 128.4, 128.5, 128.7, 128.8, 129.8,
130.0 (CH arom.), 136.6, 137.2, 137.6 (Cq arom.), 154.0 and 154.4
(C=O Boc), 169.0 and 169.1 (C=O Gly), 175.4 and 175.7 (C=O
benzazepine) ppm.
(R,S)-1-(tert-Butoxycarbonyl)-2-(2-formylbenzyl)pyrrolidine-2-car-
boxylic Acid (28): Wet Raney-Ni (50 µpore, Aldrich 2800, 1.00 g)
was washed with milliQ water (15×4 mL) and suspended with the
nitrile 27 (1.00 g, 3.03 mmol) in Py/AcOH/H2O (1:1:1, 50 mL). The
suspension was hydrogenated in a Parr apparatus (50 psi, 50 °C).
After 4 h, 8 h, 24 h, 32 h and 48 h fresh catalyst (1 g) was added.
After a total reaction time of 56 h the mixture was filtered through
dicalite and rinsed with water. After evaporation of the solvent, the
residue was redissolved in EtOAc (200 mL) and washed with 1 
HCl (3×100 mL). The organic layer was dried (MgSO4), filtered
and the solvents evaporated. A brown oil was obtained (320 mg,
[Boc-spiro-(R,S)-Aba–Gly–NHMe] (31): Benzyl ester 30 (96 mg,
0.20 mmol) was dissolved in a 33 % solution of methylamine in
methanol (25 mL, 200 mmol), and the reaction mixture was stirred
overnight at room temperature The solvent was evaporated and
the residue was redissolved in abs. EtOH. After evaporation of the
solvent, the process of dissolving in EtOH and evaporation was
32 %); HPLC tret = 16.0 min, HRMS calcd. 334.1654, found repeated twice. The crude product was purified by preparative
334.1661. The side product 2,3,5,10-tetrahydro-1H-pyrrolo[1,2-b]-
isoquinoline-10a-carboxylic acid (29) could be isolated as follows:
The aqueous washing phase described above was evaporated and
the residue was redissolved in water (500 mL). The pH was ad-
justed to 8 with an ammonia solution. The NiII salts were precipi-
tated by the addition of a saturated ethanolic solution of dimeth-
ylglyoxime to the warmed solution. The red precipitate was re-
moved by filtration, and the filtrate was evaporated. The residue
was further purified by preparative HPLC. HPLC tret = 8.7 min,
HRMS calcd. 218.1181, found 218.1172. 1H NMR ([D6]DMSO,
250 MHz): δ = 1.75 (m, 1 H, H2), 1.99 (m, 2 H, H2Ј and H1), 2.42
HPLC. Yield 74 mg (93 %). HPLC tret = 22.3 min; Rf = 0.83
(CH3CN/MeOH/H2O, 4:1:2); Rf = 0.30 (EtOAc/cHex, 2:1). HRMS
calcd. 388.2236, found 388.2246. 1H NMR ([D6]DMSO,
250 MHz): δ = 1.40 and 1.44 (2s, 9 H, tBu), 1.86 (m, 4 H, CH2 3
3
and CH2 4), 2.58 and 2.66 [2d, J(CH3–NH) = 4.5 Hz, 3 H, NH–
2
CH3], 2.69 [d, J(Hβ,HβЈ) = 13.9 Hz, 1 H, Hβ], 3.46 (m, 2 H, CH2
5), 3.69 [d, 2J(Hα,HαЈ) = 16.6 Hz, 1 H, Hα Gly], 3.84 and 3.93 [2d,
2
2J(Hβ,HβЈ) = 13.9 H,1 H, HβЈz], 4.15 and 4.18 [2d, J(Hε,HεЈ) =
2
14.5 Hz, 1 H, Hε], 4.46 [d J(Hα,HαЈ) = 16.6 Hz, 1 H, HαЈ Gly,],
2
4.89 and 5.04 [2d, J(Hε,HεЈ) = 14.5 Hz, 1 H, HεЈ], 7.32 (m, 4 H,
H arom.), 7.51 and 7.72 (2q, 3J = 4.5 Hz, 1 H, NH–CH3) ppm. 13
C
Eur. J. Org. Chem. 2006, 2899–2911
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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D. Tourwé et al.
FULL PAPER
NMR ([D6]DMSO, 63 MHz): δ = 22.2 and 23.1 (CH2 4), 25.7 and
in 11 portions of 1.00 g over 35 h. After work-up a brown oil was
26.0 (NH–CH3), 28.4 and 28.5 (CH3 tBu), 378.0 (CH2 3), 38.8 obtained. Yield 2.10 g (69%); HPLC tret = 17.6 min (gradient 16%
(CH2 β), 48.5 and 48.8 (CH2 5), 51.3 and 52.4 (CH2 ε), 53.6 and
54.5 (CH2 α Gly ), 68.7 and 69.3 (Cq α benzazepine), 79.5 and 80.2
(Cq tBu), 127.1 and 127.4, 128.2 and 128.4, 128.5 and 128.7, 130.0
and 130.2 (CH arom.), 137.3 and 138.0, 138.3 and 138.4 (Cq
arom.), 154.8 (C=O Boc), 168.7 and 169.0 (C=O Gly), 174.0 and
174.5 (C=O benzazepine) ppm.
CH3CN Ǟ 40 % CH3CN + 0.1 % TFA in 30 min); Rf = 0.65
(CH2Cl2/MeOH/AcOH, 90:8:2). HRMS calcd. 292.1185, found
292.1180. 1H NMR ([D6]DMSO, 250 MHz): δ = 0.64 (m, 1 H, H4),
1.54 (m, 1 H, H4Ј), 2.02 (m, 2 H, CH2 3), 2.85 (m, 1 H, H5), 3.29 (m,
1 H, H5Ј), 3.55 and 3.65 (2s, 3 H, CH3 Moc), 3.72 [d, 2J(Hβ,HβЈ) =
13.9 Hz, 1 H, Hβ], 3.85 [d, 2J(Hβ,HβЈ) = 13.9 Hz, 1 H, HβЈ], 7.35–
7.83 (m, 4 H, H arom.), 10.13 and 10.15 (2s, 1 H, CHO) ppm. 13C
NMR ([D6]DMSO, 63 MHz): δ = 22.2 and 22.8 (CH2 4), 33.1 and
34.5 (CH2 β), 35.1 and 36.8 (CH2 3), 48.0 and 48.9 (CH2 5), 52.3
and 52.6 (CH3 Moc), 68.6 and 69.0 (Cq α), 129.6, 130.0, 127.7,
133.5, 133.6, 133.7 (CH arom.), 135.9 and 136.0, 136.0 and 140.0
(Cq arom.), 154.9 and 155.1 (C=O Moc), 175.1 and 175.4 (C=O
COOH), 192.6 and 192.8 (C=O ald.) ppm.
[Spiro-(R,S)-Aba–Gly–NHMe] (32): Boc-spiro-Aba–Gly–NHMe
(31, 45 mg, 0.116 mmol) was dissolved in cold CH2Cl2 (5 mL,
0 °C). To this solution cold TFA was added (10 mL, 0 °C). After
2 h stirring at 0 °C, the solvent was removed in vacuo. The crude
product was purified by preparative HPLC. Yield 43 mg TFA salt
(92 %). HPLC tret = 9.7 min; Rf = 0.50 (CH3CN/MeOH/H2O,
4:1:1). HRMS 288.1712, found 288.1718. 1H NMR ([D6]DMSO,
3
[Moc-spiro-(R,S)-Aba-Gly-OBn] (35): See compound 30 for the
procedure. Yield 980 mg, 32% (starting from 2.10 g 34); HPLC tret
= 24.6 min; Rf = 0.35 (EtOAc/cHex, 1:1). HRMS calcd. 423.1920,
250 MHz): δ = 2.03 (m, 4 H, CH2 3 and CH2 4), 2.62 [d, J(CH3–
2
NH) = 4.5 Hz, 3 H, NH–CH3], 3.24 [d, J(Hβ,HβЈ) = 15.2 Hz, 1
2
H, Hβ], 3.40 (br. s, 2 H, CH2 5), 3.56 [d, J(Hβ,HβЈ) = 15.2 Hz, 1
1
H, HβЈ], 4.08 [d, 2J(Hα,HαЈ) = 16.1 Hz, 1 H, Hα Gly], 4.17 [d,
2J(Hα,HαЈ) = 16.1 Hz, 1 H, HαЈ Gly], 4.66 (s, 2 H, 2 Hε), 7.30 (m,
4 H, H arom.), 7.96 (q, 3J = 4.5 Hz, 1 H, NH–CH3), 8.94 and 9.47
(2s, 1 H, NH Pro) ppm. 13C NMR ([D6]DMSO, 63 MHz): δ = 23.6
(CH2 4), 25.8 (NH–CH3), 35.6 (CH2 3), 37.1 (CH2 β), 45.5 (CH2
5), 52.8 (CH2 ε), 53.7 (CH2 α Gly ), 71.1 (Cq α benzazepine), 127.7,
128.6, 129.0, 130.6 (CH arom.), 135.0 and 136.2 (Cq arom.), 168.2
(C=O Gly), 171.3 (C=O benzazepine) ppm.
found 423.1925. H NMR ([D6]DMSO, 250 MHz): δ = 1.65 (m, 2
2
H, CH2 3), 1.88 (m, 1 H, CH2 4), 2.63 and 2.73 [2d, J(Hβ,HβЈ) =
14.0 Hz, 1 H, Hβ], 3.49 (m, 2 H, CH2 5), 3.56 and 3.59 (2s, 3 H,
CH3 Moc), 3.88 and 4.05 [2d, 2J(Hβ,HβЈ) = 14.0 Hz, 1 H, HβЈ],
2
2
4.14 [d, J(Hα,HαЈ) = 16.8 Hz, 1 H, Hα Gly], 4.33 [d, J(Hε,HεЈ)
= 14.7 Hz, 1 H, Hε], 4.45 [d, 2J(Hα,HαЈ) = 16.8 Hz, 1 H, HαЈ Gly],
2
5.05 [d, J(Hε,HεЈ) = 14.7 Hz, 1 H, HεЈ], 5.06 and 5.14 (2s, 2 H,
CH2 Bn), 7.19–7.37 (m, 9 H, H arom.) ppm. 13C NMR ([D6]-
DMSO, 63 MHz): δ = 22.4 and 23.1 (CH2 4), 37.8 and 39.0 (CH2
3), 39.2 and 40.3 (CH2 β), 48.2 and 48.8 (CH2 5), 51.8–52.8 (CH2
ε, CH2 α Gly, CH3 Moc), 65.9 and 66.2 (CH2 Bn), 69.0 and 69.4
(Cq α), 127.2–130.2 (CH arom.), 136.2, 137.8, 138.4 (Cq arom.),
154.5 and 154.6 (C=O Moc), 169.3 and 169.5 (C=O Gly), 174.4
and 177.7 (C=O benzazepine) ppm.
[Ac-spiro-(R,S)-Aba–Gly–NHMe] (9): Spiro-Aba–Gly–
NHMe·TFA (31, 43 mg, 0.107 mmol) was dissolved in a mixture
of Ac2O and NEt3 (5:1, 12 mL). After 2 h stirring at room tempera-
ture, the solvent was evaporated and the crude product was purified
by preparative HPLC. Yield 32 mg (91%). HPLC tret = 22.2 min;
Rf = 0.65 (CH3CN/MeOH/H2O, 4:1:1). HRMS calcd. 330.1817,
found 330.1827. 1H NMR (CDCl3, 500 MHz, 298 K): δ = 1.86–
2.16 (m, 4 H, CH2 3 and CH2 4), 2.13 (s, 3 H, CH3 Ac), 2.56 [d,
Acknowledgments
3
2J(Hβ,HβЈ) = 14.0 Hz, 1 H, Hβ], 2.86 (d, J = 4.5 Hz, 3 H, NH–
We are grateful to the Institution for the Promotion of Innovation
by Science and Technology in Flanders (IWT) and the Fund for
Scientific Research-Flanders (Belgium) for financial support (grant
no G.0036.04).
2
CH3), 3.55 [d, J(Hα,HαЈ) = 16.8 Hz, 1 H, Hα Gly], 3.74 (m, 2 H,
CH2 5), 3.88 [d, 2J(Hε,HεЈ) = 14.5 Hz, 1 H, Hε], 4.04 [d,
2
2J(Hβ,HβЈ) = 14.0 Hz, 1 H, HβЈ], 5.00 [d, J(Hα,HαЈ) = 16.8 Hz,
1 H, HαЈ Gly], 5.28 [d, 2J(Hε,HεЈ) = 14.5 Hz, 1 H, HεЈ], 7.23–7.33
3
(m, 4 H, H arom.), 7.81 [q, J(NH–CH3) = 4.5 Hz, 1 H, NH–CH3]
1
ppm. H NMR ([D6]DMSO, 500 MHz, 298 K): δ = 1.80 (m, 2 H, [1] K.-C. Chou, Anal. Biochem. 2000, 286 1–16.
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2
3 H), 3.66 (m, 2 H, CH2 5), 3.67 [d, J(Hα,HαЈ) = 16.5 Hz, 1 H,
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ppm. 13C NMR (CDCl3, 126 MHz): δ = 23.2 (CH3 Ac), 23.9 (CH2
4), 26.5 (NH–CH3), 36.4 (CH2 3), 38.6 (CH2 β), 49.7 (CH2 5), 52.3
(CH2 ε), 54.8 (CH2 Gly), 70.3 (Cq α benzazepine), 127.7, 127.8,
128.9, 130.0 (CH arom.), 136.5, 137.6 (Cq arom.), 170.0 (C=O Gly),
170.4 (C=O Ac), 173.8 (C=O benzazepine) ppm. 13C NMR ([D6]
DMSO, 126 MHz): δ = 22.7 (CH3 Ac), 23.1 (CH2 4), 25.4 (NH–
CH3), 35.9 (CH2 3), 37.3 (CH2 β), 48.7 (CH2 5), 50.7 (CH2 ε), 53.9
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arom.), 136.7, 138.0 (Cq arom.), 168.0 (C=O Gly), 168.8 (C=O Ac),
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2910
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2899–2911

Studies on Tetrapeptide Mimetics
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Received: December 21, 2005
Published Online: April 19, 2006
Eur. J. Org. Chem. 2006, 2899–2911
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2911