Synthesis of a conformationally constrained threonine-valine dipeptide mimetic: Design of a potential inhibitor of plasminogen activator inhibitor-1

Article abstract of DOI:10.1016/S0040-4039(01)02080-9

The stereoselective synthesis of a conformationally restricted threonine-valine dipeptide mimetic and its incorporation into a tetrapeptide is described.

Full text of DOI:10.1016/S0040-4039(01)02080-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 41–43
Synthesis of a conformationally constrained threonine–valine
dipeptide mimetic: design of a potential inhibitor of plasminogen
activator inhibitor-1
Peter R. Guzzo,a,* Michael P. Trova,a Tord Inghardtb and Marcel Linschotenb,†
aAlbany Molecular Research, Inc., 21 Corporate Circle, PO 15098, Albany, NY 12212-5098, USA
bAstraZeneca R&D, Dept. of Medicinal Chemistry, S-431 83 Mo¨lndal, Sweden
Received 2 October 2001; accepted 29 October 2001
Abstract—The stereoselective synthesis of a conformationally restricted threonine–valine dipeptide mimetic and its incorporation
into a tetrapeptide is described. © 2001 Elsevier Science Ltd. All rights reserved.
Peptides and proteins have wide-ranging biological
functions and are important targets for drug discovery.
However, the use of peptides as drugs has generally
been limited due to their metabolic instability and
limited oral bioavailability. One strategy to overcome
the inherent instability of the amide bond in peptides is
to design peptide mimetics that alter and stabilize the
peptide backbone but retain the essential three dimen-
sional pharmacophores necessary to retain receptor
recognition and desired biological activity.1 One com-
mon strategy in peptide mimetic design is to restrict the
conformational freedom available to the peptide and
thus probe the bioactive conformation of the endoge-
nous peptide.2 In many instances, stabilization of a
biologically active conformer can lead to improved
selectivity and potency.
two molecules of the inhibitory reactive-center loop
peptide N-Ac-TVASS-NH2 (amino acid residues 333–
337).4 During the course of a program to develop
therapeutically useful inhibitors of (PAI-1), we found
that the corresponding tetrapeptide, N-Ac-TVAS-NH2,
retained the inhibitory potency of the pentapep-
tide.5 Using the X-ray structural information, we
designed conformationally restricted peptide mimetics
in which the spatial orientation of the side-chains
(particularly those oriented towards the interior of
the protein, i.e. P14Thr, P12Ala and P10Ser) closely
resembled those of the biologically active conforma-
tion.
In this paper, we report the synthesis of a conforma-
tionally constrained threonine–valine dipeptide mimetic
and its incorporation into a tetrapeptide. We chose to
design an asymmetric synthesis of 1, a lactam contain-
ing dipeptide mimetic with an appropriate protection
scheme that would resemble a threonine containing
dipeptide (Fig. 1). Lactams have been shown to be a
useful type of conformational constraint in peptide
mimetics.6 Generic dipeptide mimetic 1 contains three
Plasminogen activator inhibitor-1 (PAI-1) is an endoge-
nous serine protease inhibitor of both tissue-type plas-
minogen activator (t-PA) and urokinase plasminogen
activator and is an important regulator of vascular
fibrinolysis.3 We previously reported an X-ray structure
of a complex of a glucosylated mutant of PAI-1 and
O
O
OH
O
OH
R1
N *
OH
R1
OR2
OR2
H3C
*
HN
OR
H3C
*
HN
H3C
*
HN
N *
*
*
*
H
O
O
SMe
SMe
O
R
O
R
O
R
3
1
2
Figure 1.
Keywords: peptide mimetic; plasminogen activator inhibitor-1.
* Corresponding author. Tel.: 518-464-0279 ext. 2434; fax: 518-464-0289; e-mail: peter@albmolecular.com
† Present address: Medivir AB, S-141 44 Huddinge, Sweden.
0040-4039/02/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02080-9

42
P. R. Guzzo et al. / Tetrahedron Letters 43 (2002) 41–43
stereocenters, one of which is an asymmetric quaternary
carbon, a feature which presents a challenge for its
synthesis.
tide 7 in 56% yield (Scheme 2). Conversion of 7 to the
methylsulfonium salt 8 required for lactamization was
unsuccessful after the standard treatment with methyl
iodide. The formation of sulfonium salts can be
reversible when strong nucleophiles (e.g. I−) are present
in the reaction. However, reaction of 7 with trimethy-
loxonium tetrafluoroborate at room temperature gave
quantitative conversion to sulfonium salt 8.8 Cycliza-
tion of sulfonium salt 8 was induced by treatment with
the potassium salt of N-methylacetamide as base to
give lactams 9 and 10 without any observable epimer-
ization.6c The partial removal of the SEM ether was
unexpected but of no consequence in the synthesis.
Both 9 and 10 were converted to acid 11 by treatment
with TFA.
We envisioned lactam 1 being constructed from dipep-
tide 2 utilizing the lactamization methodology origi-
nally developed by Freidinger.6a Dipeptide 2 could be
prepared from unusual amino acid 3 and presumably
any other suitably protected amino acid using standard
peptide coupling procedures. Seebach’s ‘chiral self
reproduction’ methodology7 appeared to be well suited
for the preparation of the quaternary carbon containing
amino acid 3 needed for our synthesis.
D
-Methionine was converted to oxazolidinone 4 follow-
ing a variation of Seebach’s original method7b (Scheme
1). Seebach’s original method incorporated a ben-
zenamide protecting group for the amino acid nitrogen
atom; however, the Cbz group was found to be more
readily removed in later functional group transfor-
mations. Condensation of the lithium enolate of oxa-
zolidinone 4 with acetaldehyde gave 5 as the only
diastereomer isolated. The stereochemical assignment
of 5 is based upon previous assignments in analogous
systems.7a Notably aldol product 5 has the same stereo-
chemical configuration as natural threonine. Alcohol 5
was protected as a SEM ether to prevent a retro-aldol
reaction in the subsequent base hydrolysis. Unnatural
amino acid 6 was obtained after base hydrolysis and
careful acidification in 63% yield.
Dipeptide mimetic 11 was coupled to dipeptide 12
utilizing EEDQ9 to afford protected tetrapeptide 13
in 67% yield (Scheme 3). Ammonolysis of 13 followed
by hydrogenation and acetylation gave 14, an ana-
log of the lead peptide (TVAS) for biological testing.10
Unfortunately, 14 possessed an order of magni-
tude less affinity for PAI-1 than the lead peptide
(TVAS).
In conclusion, dipeptide mimetic 11 is a versatile con-
formationally constrained intermediate which can be
incorporated into peptides to help define the important
components for binding affinity to a receptor. Impor-
tantly, using the methodology described in this report,
all of the stereocenters in 11 can be controlled by the
choice of the starting amino acids.
Condensation of quaternary amino acid 6 with
L-valine
t-butylester utilizing the BOP reagent provided dipep-
O
SCH3
O
OH
O
SEMO
H3CS
2
3-5
1
O
O
OH
NHCbz
OH
O
N
N
H3CS
H2N
H3CS
Cbz
Cbz
5
4
6
Scheme 1. Reagents and conditions: (1) (i) NaOH, EtOH, rt, evaporate, (ii) pivalaldehyde (1.4 equiv.), CH2Cl2, reflux, 5 h, (iii)
CbzCl, 0°C to rt, 24 h, 37%; (2) (i) LDA, THF, −78°C, (ii) CH3CHO (4 equiv.), −78°C, 1 h, 53%; (3) SEMCl, iPr2NEt, DMAP
(cat.), CH2Cl2, rt, 24 h; (4) NaOH, MeOH, reflux, 24 h; (5) HCl, pH 2, 63%.
O
SEMO
SEMO
O
SEMO
+
O
OtBu
L-ValOtBu
Me3OBF4, CH2Cl2
rt, 16 h
OtBu
OH
NHCbz
N
H
N
H
O
H3CS
H3CS
O
NHCbz
BOP, iPr2NEt
(H3C)2S
NHCbz
CH2Cl2, rt, 18 h
-
BF4
100%
7
6
8
56%
O
O
O
N
HO
SEMO
HO
CH3CONCH3K, THF
0 oC to rt, 1.5 h
60%
TFA, rt, 1 h
100%
OtBu
OtBu
OH
+
N
N
CbzHN
CbzHN
CbzHN
O
O
O
9
10
11
Scheme 2.

P. R. Guzzo et al. / Tetrahedron Letters 43 (2002) 41–43
43
OH
NH2
OH
OH
O
O
HO
O
HO
O
O
H
N
1. NH3, MeOH, 14 h
H
N
EEDQ
O
H2N
O
11 +
N
N
H
N
N
H
Bn
N
H
Bn
AcHN
CbzHN
2. H2, Pd/C, MeOH
3. Ac2O, HOAc,
NaHCO3, 2 days
33%
O
CH3
O
CH2Cl2
EtOAc
16 h
O
CH3
O
CH3
O
13
12
14
67%
Scheme 3.
References
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1
14: H NMR (300 MHz, D2O) l 4.40 (m, 1H), 4.34 (q,
J=7.2 Hz, 1H), 4.24 (d, J=11.1 Hz, 1H), 4.09 (q, J=6.7
Hz, 1H), 3.88 (m, 2H), 3.69 (t, J=9.9 Hz, 1H), 3.58 (dd,
J=7.7, 16.5 Hz, 1H), 2.66 (dd, J=7.5, 14.5 Hz, 1H), 2.2
(m, 3H), 1.96 (s, 3H), 1.39 (d, J=7.2 Hz, 3H), 1.13 (d,
J=6.4 Hz, 3H), 0.97 (d, J=6.2 Hz, 3H), 0.91 (d, J=6.5
Hz, 3H); 13C NMR (75 MHz, CD3OD) l 175.7, 175.4,
172.2, 171.4, 69.7, 66.2, 63.2, 62.2, 56.3, 50.8, 43.2, 27.3,
24.4, 19.2, 19.0, 17.4, 17.1; FABMS m/z=466 [M+Na]+.
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