Synthesis of a conformationally restricted dipeptide analogue and its evaluation as a β-turn mimic

Article abstract of DOI:10.1016/S0040-4039(01)01080-2

Dichloropyrazinone 3 was converted into a conformationally restricted dipeptide analogue 8 by means of a Diels-Alder strategy. The β-turn characteristics of molecule 8 were examined by molecular modeling and NMR spectroscopy.

Full text of DOI:10.1016/S0040-4039(01)01080-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 5693–5695
Synthesis of a conformationally restricted dipeptide analogue and
its evaluation as a b-turn mimic
Wim M. De Borggraeve, Frederik J. R. Rombouts, Erik V. Van der Eycken, Suzanne M. Toppet
and Georges J. Hoornaert*
Laboratorium voor Organische Synthese, K.U. Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
Received 1 June 2001; revised 18 June 2001; accepted 19 June 2001
Abstract—Dichloropyrazinone 3 was converted into a conformationally restricted dipeptide analogue 8 by means of a Diels–Alder
strategy. The b-turn characteristics of molecule 8 were examined by molecular modeling and NMR spectroscopy. © 2001 Elsevier
Science Ltd. All rights reserved.
The b-turn is one of the three major motifs of peptide
and protein secondary structure. It plays a key role in
many molecular recognition events including interac-
tions between antigens and antibodies, peptide hor-
mones and their receptors, and enzymes and their
corresponding substrates.1 Due to the often poor
bioavailability and low stability of natural peptide ther-
apeutics, much effort has been devoted to the synthesis
of peptide mimics to overcome the aforementioned
problems.
tain a 3,6-disubstituted 2-piperidinone ring bearing a
cis-oriented C-3 amine group and C-6 carboxyl group
as core structure.
Our approach to the 3,6-disubstituted 2-piperidinone is
based on the pyrazinone chemistry previously devel-
oped in our laboratory4 (Scheme 1). The starting
pyrazinone 3 is synthesized from an aminonitril and
oxalyl chloride.4a The chlorine at the 3-position is sub-
stituted by a methyl group by reacting 3 with 1.1 equiv.
of Me4Sn in toluene at 110°C using Pd(PPh3)4 as a
catalyst.4b Product 4 was purified by column chro-
matography (silica gel, gradient hexane/dichlorometh-
ane 15/85“dichloromethane“ethyl acetate/dichloro-
methane 5/95).
Because of its biological importance, its compact size
and hence synthetic accessibility, the b-turn is an inter-
esting motif to mimick.1 A b-turn mimic usually con-
sists of a conformationally restrained dipeptide. Based
on known b-turn mimics such as piperidinone deriva-
tive 1 and the bicyclic lactam 2, respectively described
by Kemp2 and Germanas3 (Fig. 1), we developed a
method for the synthesis of the structurally related
compound 8. The above-mentioned b-turn mimics con-
The conformational restriction of the system is imposed
by a Diels–Alder reaction of ethene on the substituted
pyrazinone 4 in toluene (steel bomb, 33 atm, 135°C).
The crude imidoyl chloride 5 is converted into 6 by
hydrolysis in CHCl3 exposed to air moisture. Evapora-
tion of this reaction mixture and recrystallization (hex-
ane/dichloromethane) yields the pure bicyclic
compound 6 (overall yield from 4: 79%). The secondary
lactam moiety in 6 is selectively cleaved upon treatment
with an HCl-saturated methanol solution for 12 h.
Under these conditions there is no risk of cleaving the
benzylated amide function of the piperidinone formed.
H
HO2C
N
O
N
O
O
NH2
O
NHBOC
Bn
1
2
Figure 1. Dipeptide b-turn mimics.
In order to prevent recyclization to the bislactam 6
upon neutralization of the reaction mixture, the newly
formed primary amine function is trapped as an acet-
amide by dissolving the evaporated residue from
methanolysis in acetic anhydride and adding Et3N until
Keywords: pyrazinone; Diels–Alder; piperidinone; constrained dipep-
tide; b-turn mimic.
* Corresponding author. Tel.: ++32-16-327409; fax: ++32-16-327990;
e-mail: georges.hoornaert@chem.kuleuven.ac.be
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01080-2

5694
W. M. De Borggrae6e et al. / Tetrahedron Letters 42 (2001) 5693–5695
Bn
N
Bn
N
Bn
N
Bn
N
H
O
H
O
H
O
O
H
O
a
91%
b
c
Cl
N
3
Cl
Cl
N
4
Me
N
H
6
Me
Cl
N
5
Me
79% (b,c)
d,e
82%
Bn
N
Bn
N
H
H
O
O
O
O
f
*
*
Me
NH
92%
*
Me
*
N
Me
O
Me
O
H
HN
O
7
8
Scheme 1. Me4Sn, Pd(PPh3)4, toluene, 110°C; (b) ethene (33 atm), toluene, 135°C; (c) CHCl3, air moisture, rt; (d) HCl/MeOH,
rt; (e) Ac2O/Et3N, rt; (f) MeNH2 (33%)/EtOH, rt.5
no more salts are formed. Removal of the salts by
filtration and evaporation of the reaction mixture yields
product 7, which is further purified by crystallization
from hexane/dichloromethane. Compound 8 is finally
obtained by dissolving 7 in MeNH2 (33%)/ethanol and
stirring this mixture for 12 h at room temperature.
Upon recrystallization (hexane/dichloromethane) pure
8 is obtained.
(Fig. 3) to estimate the interatomic distance aC(1)ꢀaC(4).
The starting conformation was preoptimized by putting
the amide bonds outside the ring in their more stable
trans conformation. Energy minimization of the start-
ing geometry resulted in two possible structures in
which the main difference was the conformation of the
ring system. Based on the axial coupling observed in
the H NMR spectrum of compound 8 for H-6, confor-
mation A (Fig. 3) was found to be the best approxima-
tion of the real geometry of 8. In this model, the
1
The b-turn characteristics of the substituted dipeptide 8
1
were investigated by molecular modeling and H NMR
,
interatomic distance between aC(1)ꢀaC(4) is 5.72 A. This
spectroscopy. A b-turn is defined as a tetrapeptide
,
lies well beneath the 7 A upper limit mentioned in the
sequence in which the interatomic distance aC(1)ꢀaC(4)<
literature. Also, an intramolecular hydrogen bond
between CO(1) and NH(4) can be inferred by the inter-
,
7 A (Fig. 2). A hydrogen bond is often present between
CO(1) and NH(4), although open turns lacking this
,
atomic distance of 2.13 A.
hydrogen bond also exist.1
The presence of a hydrogen bond was further checked
by H NMR spectroscopy on compound 8. According
In order to qualify 8 as a b-turn mimic, the structure
was checked for the above-mentioned criteria. A model-
ing study was performed on the simplified molecule 96
1
to the literature the temperature dependence of the
chemical shift of a hydrogen bonded amide proton is
small (0 to −4 ppb/°C) compared to the temperature
dependence of a solvent exposed proton (<−7 ppb/°C).7
The chemical shifts of the NHCO (singlet at l=8.47
ppm) and NHMe (quartet at l=8.46 ppm) in DMSO-
d6 were recorded at different temperatures. Linear
regression on the collected datapoints provided us with
the following results: the chemical shift dependence of
−3.9 ppb for the NHMe is consistent with the presence
of a hydrogen bond; the NHCO on the other hand is
solvent exposed (shift dependence −7.2 ppb/°C).
A final NMR criterion used was the solvent depend-
ency of the chemical shift of the amide protons upon
changing the solvent from DMSO to CDCl3. The
results are summarized in Table 1. The small shift of
the NHMe also confirms the presence of a hydrogen
bond.
Figure 2.
In summary, we were able to synthesize a conforma-
tionally restricted Ala-Gly analogue 8 in multigram
1
quantities. Both H NMR and modeling studies affirm
the proposed b-turn characteristics of this structure.
Further research will be directed towards the incorpora-
tion of this b-turn mimic in a peptide library.
Figure 3.

W. M. De Borggrae6e et al. / Tetrahedron Letters 42 (2001) 5693–5695
Table 1. Chemical shift dependence of amide protons of 8
5695
lNHMe DMSO (ppm)
lNHMe CDCl3 (ppm) l
DlNHMe (ppm) lNHCO DMSO (ppm)
0.14 8.47
lNHCO CDCl3 (ppm)
DlNHCO (ppm)
2.59
8.46
8.32
5.88
3. Kim, K.; Germanas, P. J. Org. Chem. 1997, 62, 2853–2860
and references cited therein.
Acknowledgements
4. (a) Vekemans, J.; Pollers-Wiee¨rs, C.; Hoornaert, G. J.
Heterocyclic Chem. 1983, 20, 919–923; (b) Buyssens, K. J.;
Vandenberghe, D. M.; Toppet, S. M.; Hoornaert, G. J.
Tetrahedron 1995, 51, 12463–12478; (c) Tahri, A.; Buysens,
K. J.; Van der Eycken, E. V.; Vandenberghe, D. M.;
Hoornaert, G. J. Tetrahedron 1998, 54, 13211–13226; (d)
Tahri, A.; De Borggraeve, W.; Buysens, K.; Van Meervelt,
L.; Compernolle, F.; Hoornaert, G. Tetrahedron 1999, 55,
14675–14684; (e) Rombouts, F. J. R.; Vanraes, D. A. J.;
Wynendaele, J.; Loosen, P. K.; Luyten, I.; Toppet, S.;
Compernolle, F.; Hoornaert, G. J. Tetrahedron 2001, 57,
3209–3220.
We thank the FWO (Fund for Scientific Research,
Flanders, Belgium). We are grateful to R. De Boer for
HRMS measurements and to Professor W. Dehaen for
the use of the Hyperchem package. We thank Professor
D. Tourwe´ for the helpful discussions. W.D.B.
(Research assistant of the FWO, Flanders) thanks the
FWO and F.R. thanks K.U. Leuven for the fellowships
received.
References
5. (a) Relative configuration is indicated; (b) all new com-
pounds exhibit satisfactory spectral and analytical data.
6. Calculations were performed with Hyperchem (BIO+-force
field). Energy minimizations of 8 did not converge, so we
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1
used 9 as a model for 8. H NMR data of 8 are consistent
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