Dihydroquinolines as novel n-NOS inhibitors

Article abstract of DOI:10.1016/S0960-894X(02)00481-X

Dihydroquinolines have been synthesized and have been shown to be potent n-NOS inhibitors. Selectivity versus e-NOS was increased to approximately 100-fold through appropriate substitution at the benzene ring.

Full text of DOI:10.1016/S0960-894X(02)00481-X

Bioorganic & Medicinal Chemistry Letters 12 (2002) 2561–2564
Dihydroquinolines as Novel n-NOS Inhibitors
Stefan Jaroch,a,* Peter Holscher,a Hartmut Rehwinkel,a Detlev Sulzle,b
Gerardine Burton,c Margrit Hillmannc and Fiona M. McDonaldc
aDepartment of Medicinal Chemistry, Corporate Research, Schering AG, D-13342-Berlin, Germany
bDepartment of Computational Chemistry, Corporate Research, Schering AG, D-13342-Berlin, Germany
cCNS-Research, Corporate Research, Schering AG, D-13342-Berlin, Germany
Received 17 May 2002; revised 19 June 2002; accepted 21 June 2002
Abstract—Dihydroquinolines have been synthesized and have been shown to be potent n-NOS inhibitors. Selectivity versus e-NOS
was increased to approximately 100-fold through appropriate substitution at the benzene ring. # 2002 Elsevier Science Ltd. All
rights reserved.
Excessive brain levels of nitric oxide (NO) have been
linked to tissue injury in the wake of a cerebral ischemic
event and other neurodegenerative processes.1 Since NO
is formed in central and peripheral nerves through
transformation of arginine into citrulline by constitutive
neuronal nitric oxide synthase (n-NOS),2 suppression of
NO production with an n-NOS inhibitor appears as a
promising neuroprotective treatment for a variety of
disease states, notably stroke. Two further isoforms of
nitric oxide synthase are known, one constitutively
expressed in the endothelial lining of blood vessels (e-
NOS) and another inducible form found in cells of the
immune system (i-NOS). Due to the blood pressure
modulating properties of endothelial NO, it is of para-
mount importance to identify a selective n-NOS inhi-
bitor having minimal interaction with e-NOS.3
Figure 1.
and potency, which was increased 30-fold. The synth-
eses and structure–activity relationship (SAR) in the
dihydroquinoline series are described in this paper.
The compounds were synthesized via two different
routes as outlined in Schemes 1 and 2. The first path
involved a conrotatory8 Nazarov cyclization9 of a
phenyl cycloalkenyl ketone as a key step establishing the
cis-stereochemistry at the stereogenic centers which
eventually would become C-3a and C-9b in the final
product.10 The synthesis was accomplished with a reac-
tion sequence including a Beckmann rearrangement,11 a
thionation process,12 and an ammonolysis step.13
Our compound design started with 3-aminobenzoxazine
1 (Fig. 1) which emerged as a hit from high throughput
screening.4 In an effort to explore the steric demand of
the oxazine moiety, among several structural variations,
the lactate fragment was replaced by the rigid frame-
work of proline leading to dihydroquinoxaline 25 as
prototype. However, this compound proved to be sen-
sitive towards air oxidation, though it still exhibited an
IC50=3.3 mMfor n-NOS. Thus, further optimization
seemed worthwhile and we moved from the oxidation-
prone dihydroquinoxaline core6 on to the less labile dihy-
droquinoline7 template. Fortunately, dihydroquinoline 3
proved to be far superior to 2 both in terms of stability
The second route (Scheme 2) relied on a dissolving
metal reduction of a quinolone,14 which provided pre-
dominantly the trans-dihydroquinolone, followed by the
same endgame as described above. Generally, the qui-
nolones were accessible through enamine addition to
isocyanates followed by sulfuric acid-mediated cycli-
zation.15 However, in the case of electron-deficient phenyl
isocyanates (e.g., R=F, CF3) this process failed to deliver
quinolones. Fortunately, an alternate sequence16 proved
to be successful comprising Suzuki coupling of N-piva-
loylanilide-derived17 boronic acids with triflates II
*Corresponding author. Tel.: +49-30-468-12146; fax: +49-30-469-
92146; e-mail: stefan.jaroch@schering.de
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00481-X

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S. Jaroch et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2561–2564
Scheme 1. (a) SOCl2, reflux; (b) PhH, AlCl3; (c) Me3SiCN, n-BuLi, THF; (d) PhMgCl, THF; aq H2SO4; (e) concd H2SO4; (f) H2NOH x1/2H2SO4,
THF–EtOH–H2O; (g) PPA, 120 ꢀC; (h) Lawesson’s reagent, DME; (i) NH3, MeOH.
Scheme 2. (a) I, CHCl3; (b) concd H2SO4, 100 ꢀC; (c) n-BuLi, THF; B(OMe)3; aq HCl; (d) II, Pd(PPh3)4, Na2CO3, DME-H2O; (e) concd HCl,
reflux; (f) Mg, MeOH; (g) Lawesson’s reagent, DME; (h) NH3, MeOH.
Table 1. Inhibition of NOS isoforms by dihydroquinolines
Compd
X
R
R0
IC50 (mM)a
Selectivity
n-NOS
e-NOS
i-NOS
e/nb
i/nb
3
4
5
6
7
8
CH2
(CH2)2
(CH2)3
O
H
H
H
H
H
H
H
8-Me
8-F
8-Cl
NH2
NH2
NH2
NH2
NHMe
NHOH
Hc
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
0.16
6.80
100
3.3
—
—
0.96
—
—
—
21
2.3
6.2
17
57
34
54
24
6.2
2.3
12
2.7
—
—
4.1
—
—
—
21
—
—
7
17
—
—
32
—
—
—
24
15
41
45
56
60
50
20
6
8
8
6
14
8
48
7
37
0.13
>100
>100
>200
0.50
0.11
0.14
0.31
1.6
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
—
—
—
42
21
44
55
36
136
84
38
33
18
39
17
34
29
99
—
—
9c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
12
1.7
5.7
14
89
15
8-Br
8-CF3
8-NO2
8-CN
8-OMe
7-Me
7-F
7-NO2
7-OMe
6-F
6,7-F2
6-F, 8-Cl
6,8-Cl2
6,7-F2, 8-Cl
0.25
0.64
0.64
0.19
0.13
0.31
0.24
0.17
0.68
0.73
4.1
32
13
1.1
1.1
2.6
1.4
2.3
5.1
35
4.1
5.8
20
72
>200
>200
30
78
2.1
Standards:
l-NAME
l-NNA
1.6
0.08
0.89
1.5
0.32
0.54
8
5.5
1.0
1
4
0.6
5
69
1
l-NMMA
aNOS activity was determined at least three times with recombinant human enzyme according to ref 20.
be/n means IC50(e-NOS)/ IC50(n-NOS) and i/n means IC50(i-NOS)/ IC50(n-NOS).
cThis compound contains an endocyclic CH2–NH group instead of the CH¼N given in the generic structural formula on top of the table (i.e., 2,3,3a,4,5,9b-
hexahydro-1H-cyclopenta[c]quinoline (cf. ref 21)).

S. Jaroch et al. / Bioorg. Med. Chem. Lett. 12 (2002) 2561–2564
2563
followed by depivaloylation and cyclization. Further
introduction of substituents into the benzene ring is
feasible at the dihydroquinolone stage through classical
aromatic substitution reactions.18
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field, see: Cheshire, D. R. IDrugs 2001, 4, 795. Lowe, J. A., III
IDrugs 2000, 3, 63. Some recent literature reports on n-NOS
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W.; Volkmann, R. A.; Heck, S.; Nowakowski, J.; Nelson, R.;
Nolan, C.; Liston, D.; Ward, K.; Zorn, S.; Johnson, C.;
Vanase, M.; Faraci, W. S.; Verdries, K. A.; Baxter, J.; Doran,
S.; Sanders, M.; Ashton, M.; Whittle, P.; Stefaniak, M. Bioorg.
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Shearer, B. G.; Lee, S.; Oplinger, J. A.; Frick, L. W.; Garvey,
E. P.; Furfine, E. S. J. Med. Chem. 1997, 40, 1901.
5. The compound was synthesized by adding proline to 1-
fluoro-2-nitrobenzene (cf. Abou-Gharbia, M.; Freed, M. E.;
McCaully, R. J.; Silver, P. J.; Wendt, R. L. J. Med. Chem.
1984, 27, 1743) followed by zinc reduction and amidine for-
mation as described in Scheme 1.
6. A well-known property which was used for a 2-aminoqui-
noxaline synthesis by Pfister, K., III; Sullivan, A. P., Jr.;
Weijlard, J.; Tishler, M. J. Am. Chem. Soc. 1951, 73, 4955. A
new example is given in Maidwell, N. L.; Rezai, M. R.; Roes-
chlaub, C. A.; Sammes, P. G. J. Chem. Soc., Perkin Trans. 1
2000, 1541.
7. Iminopiperidines are described as i-NOS inhibitors in Moore,
W. M.; Webber, R. K.; Fok, K. F.; Jerome, G. M.; Connor, J. R.;
Manning, P. T.; Wyatt, P. S.; Misko, T. P.; Tjoeng, F. S.; Currie,
M . G.J. Med. Chem. 1996, 39, 669. Webber, R. K.; Metz, S.;
Moore, W. M.; Connor, J. R.; Currie, M. G.; Fok, K. F.;
Hagen, T. J.; Hansen, D. W., Jr.; Jerome, G. M.; Manning,
P. T.; Pitzele, B. S.; Toth, M. V.; Trivedi, M.; Zupec, M. E.;
Tjoeng, F. S. J. Med. Chem. 1998, 41, 96.
8. Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. 1969,
8, 781. Santelli-Rouvier, C.; Santelli, M. Synthesis 1983, 429.
Jones, T. K.; Denmark, S. E. Helv. Chim. Acta 1983, 66, 2397.
9. Baker, W.; Jones, P. G. J. Chem. Soc. 1951, 787. Ohta, S.;
Yamashita, M.; Arita, K.; Kajiura, T.; Kawasaki, I.; Noda,
K.; Izumi, M. Chem. Pharm. Bull. 1995, 43, 1294.
We started to explore the SAR of the dihydroquinolines
by determining the optimal ring size of the annulated
ring. As is apparent from Table 1 (3–5) both cyclohex-
ane and cycloheptane annulation led to a dramatic loss
in potency. The cyclopentane could be replaced by a
tetrahydrofuran ring at the expense of diminished
selectivity versus e-NOS (6). Substitution at or removal
of the 4-amino group was detrimental for activity (7–9).
Modification of the benzene substitution pattern
allowed us to improve the selectivity versus e-NOS.
Whereas substitution at C-7 was broadly accepted irre-
spective of the electronic nature of the substituent and
had only minor effects on the selectivity against e-NOS
(18–21), introduction of residues at position 8 had a
more severe impact (10–17). The 8-chloro derivative 12
proved to be more potent than both the methyl or
methoxy analogue 10 and 17. In the 8-halogen series,
increasing the size of the substituent correlated with a
moderate loss in potency (11–14) with the maximum
selectivity against e-NOS found for the bromo deriva-
tive 13. A more than 100-fold selectivity and a fair
potency was observed for the 8-nitro derivative 15,
while the high selectivity of 8-cyanoquinoline 16 was
compromised by a drop in potency. The 6-fluoro deri-
vative 22 showed a moderately improved selectivity
compared to 3. Combination with a chloro substituent
into 8-chloro-6-fluoroquinoline 24 led to an increased
selectivity but a 5-fold loss in potency, a profile com-
parable to that of 8-cyanoquinoline 16. Further di- and
trisubstitution resulted in poorly active n-NOS inhibi-
tors (23, 25, and 26). Taken together, compounds dis-
playing reasonable potency and fair selectivity were 8-
chloro-, 8-bromo-, 8-nitro-, and 6-fluoroquinoline (12,
13, 15, 22); these seem to be clearly superior to arginine-
derived standards19 especially in terms of selectivity
against e-NOS.
In summary, novel, potent, and selective dihydroquino-
line-based n-NOS inhibitors have been identified, and
two synthetic routes have been described. The SAR
reported herein sets the stage for further medicinal chem-
istry optimization and for an extensive pharmacological
characterization.
10. The stereochemical assignment was proven by X-ray
crystallography on the thiolactam stage.
11. Hino, K.; Nagai, Y.; Uno, H. Chem. Pharm. Bull. 1988,
36, 2386.
12. Thomson, I.; Claussen, K.; Scheibye, S.; Lawesson, S.-O.
Org. Syn., Coll. VII 1990, 372.
13. A new application of a well-known reaction (cf. Gautier,
J.-A.; Miocque, M.; Farnoux, C. C. In The Chemistry of
Amidines and Imidates; Patai, S., Ed.; John Wiley & Sons:
London; 1975, p 283) is given in Papandreou, G.; Tong, M. K.;
Ganem, B. J. Am. Chem. Soc. 1993, 115, 11682. For the N-
hydroxyimino derivative see: Behringer, H.; Meier, H. Liebigs
Ann. Chem. 1957, 607, 67.
14. Blount, W. H.; Perkin, W. H., Jr.; Plant, S. G. P. J. Chem.
Soc. 1929 1975; 1983. Brettle, R.; Shibib, S. M. J. Chem. Soc.,
Perkin Trans. 1 1981, 2912.
Acknowledgements
The dedicated and skilfull technical assistence by Mrs.
Barbel Bennua-Skalmowski and Mr. Detlev Schmidt is
gratefully acknowledged.
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