Structural Investigation of the Naphthyridone Scaffold: Identification of a 1,6-Naphthyridone Derivative with Potent and Selective Anti-HIV Activity

Article abstract of DOI:10.1002/cmdc.201100073

Building upon a large, previously reported series of anti-HIV 6-desfluoroquinolones endowed with a peculiar mechanism of action, the inhibition of Tat-mediated transcription, replacement of the quinolone nucleus with a naphthyridone core was shown to be very productive. In this work, the naphthyridone scaffold was investigated in depth by synthesizing various analogues. This led to the identification of NM13 as the most selective derivative obtained in MT-4 cells. It is the result of the successful combination of the 1,6-naphthyridone nucleus and the C7 benzothiazolpiperazine group, which, for the first time, not only grants potent anti-HIV activity but displays very high selectivity. Further studies aimed at a more thorough investigation of the anti-HIV profile of this new derivative are in progress. Best in class: The naphthyridone scaffold was investigated in depth by synthesizing various analogues that were tested for their anti-HIV activity. This led to the identification of a very potent and nontoxic compound, NM13, as the most selective anti-HIV derivative ever obtained in the quinolone class of transcription inhibitors. It is the result of a powerful combination of the 1,6-naphthyridone nucleus and the C7 benzothiazolpiperazine group.

Full text of DOI:10.1002/cmdc.201100073

DOI: 10.1002/cmdc.201100073
Structural Investigation of the Naphthyridone Scaffold:
Identification of a 1,6-Naphthyridone Derivative with
Potent and Selective Anti-HIV Activity
Oriana Tabarrini,*[a] Serena Massari,[a] Luca Sancineto,[a] Dirk Daelemans,[b] Stefano Sabatini,[a]
Giuseppe Manfroni,[a] Violetta Cecchetti,[a] and Christophe Pannecouque[b]
Building upon a large, previously reported series of anti-HIV 6-
desfluoroquinolones endowed with a peculiar mechanism of
action, the inhibition of Tat-mediated transcription, replace-
ment of the quinolone nucleus with a naphthyridone core was
shown to be very productive. In this work, the naphthyridone
scaffold was investigated in depth by synthesizing various ana-
logues. This led to the identification of NM13 as the most se-
lective derivative obtained in MT-4 cells. It is the result of the
successful combination of the 1,6-naphthyridone nucleus and
the C7 benzothiazolpiperazine group, which, for the first time,
not only grants potent anti-HIV activity but displays very high
selectivity. Further studies aimed at a more thorough investiga-
tion of the anti-HIV profile of this new derivative are in prog-
ress.
Introduction
Remarkable progress has been made in the treatment of HIV-1
infection/AIDS. However, the emergence of multidrug-resistant
HIV strains and the inability of the current anti-HIV treatment
to completely eradicate the virus in HIV-infected individuals de-
mands new highly potent drugs capable of interfering with
targets other those exploited by the anti-HIV drugs currently
on the market. The 6-desfluoroquinolones (6-DFQs), developed
by our group,[1–9] are innovative compounds that
owe their potent anti-HIV activity to their ability to in-
hibit the HIV-1 Tat-mediated transcription. This crucial
step in HIV replication has not been clinically exploit-
ed in anti-HIV therapy. The molecular target of 6-
DFQs has not yet been fully identified. Indeed, al-
though the lead compound acts by interfering selec-
tively with the Tat–TAR complex formation,[1,10] this is
not strictly correlated to the antiviral activity and
moreover, the same activity was not shown by other
potent quinolone analogues.[9] Some evidence[1–9] (for
example, different activities in various cell lines and
the inability to select for resistance mutations), sug-
gests that a host cellular factor or host cellular
factor–viral component complex, involved in the Tat-
mediated transcription, could be the target. Interfer-
attempting to identify the molecular target(s) that play a major
role in their mechanism of action, our synthetic efforts have
led to the identification of some very interesting compounds.
The most exciting result was recently achieved by replacing
the quinolone ring with the 1,8-naphthyridone core, a classic
bioisosteric replacement also applied in the antibacterial qui-
nolone field.[15,16] Naphthyridone derivative HM13N (Figure 1)
Figure 1. Structures of the compounds synthesized in this study starting from HM13N;
for Ar, see Schemes 2 and 3.
ing with host factors essential for viral replication is
an attractive way of combating HIV-1 and avoiding
the emergence of resistance.[11] Indeed, viral protein–
cofactor interfaces are not only much less tolerant to
[a] Prof. O. Tabarrini, Dr. S. Massari, Dr. L. Sancineto, Dr. S. Sabatini,
mutations than the catalytic site of the highly variable viral
enzyme, but they are particularly attractive targets for develop-
ing selective drugs without compromising the normal process
of transcriptional control at the cellular level of gene expres-
sion.[12–14]
Dr. G. Manfroni, Prof. V. Cecchetti
Dipartimento di Chimica e Tecnologia del Farmaco
Universitꢀ di Perugia, Via del Liceo 1, 06123 Perugia (Italy)
Fax: (+39)075-585-5115
E-mail: oriana.tabarrini@unipg.it
[b] Dr. D. Daelemans, Prof. C. Pannecouque
Rega Institute for Medical Research
Thus, the 6-DFQs possess beneficial properties that make
them particularly attractive for further studies. Whereas we are
Katholieke Universiteit Leuven, 3000 Leuven (Belgium)
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O. Tabarrini et al.
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displayed very potent and selective anti-HIV activity with IC50
values in the low micromolar range in acutely, chronically, and
latently infected cells.[8] The excellent antiviral profile coupled
with the absence of any tendency to select for resistance mu-
tations in vitro[8] increased the value of this type of com-
pounds, driving the design and synthesis of the new ana-
logues reported herein.
Interestingly, the 1,8-naphthyridone scaffold was particularly
beneficial when it was decorated with the benzothiazolpipera-
zine moiety at the C7 position; the other 4-arylpiperazines did
not confer the same favorable profile.[8] Thus, attempting to in-
vestigate this fruitful combination, various naphthyridone de-
rivatives were synthesized. Maintaining the benzothiazolpipera-
zine at the C7 position, the 8-nitrogen atom was moved to the
5 and 6 positions, synthesizing 1,5-naphthyridone 1 and 1,6-
naphthyridone 2, respectively. Then, 1,7-naphthyridone 3 was
prepared by placing the nitrogen at the 7 position while shift-
ing the arylpiperazine to the C8 position. In a more incisive
structural modification, a 1,3-thiazole ring replaced the pyri-
dine ring in derivative 4.
In parallel, the 1,8-naphthyridone scaffold was further inves-
tigated by exploiting the knowledge collected for the large
series of 6-DFQs. In particular, the recent structure–
activity relationship (SAR) acquired for the N1 posi-
tion indicated that a vinyl group yielded compounds
characterized by potent activity coupled with the
ability to completely protect the cells from HIV-1 in-
fection, whereas the presence of an amino group
characterized low toxic compounds.[7] Thus, 1-amino-
derivatives 5 and 6, and 1-vinyl derivatives 7–11,
were synthesized. The 6-aminonaphthyridones 12
and 13, were also prepared, by placing a classic
amino group at the C6 position.[1,2]
Scheme 1. Reagents and conditions: a) SOCl2, 808C, 3 h; b) ethyl 3-(dimeth-
ylamino)-2-propenoate, Et3N, toluene, 808C, 1–3 h; c) MeNH2, Et2O or/and
EtOH, 30 min; d) K2CO3, DMF, 608C, 24 h; e) NaH, THF, 30 min; f) 4% NaOH,
reflux, 30 min; g) 1-(1,3-benzothiazol-2-yl)piperazine, Et3N, DMF, 50–1008C,
1–30 h; h) 1-(1,3-benzothiazol-2-yl)piperazine, N-methyl-2-pyrrolidone,
1058C, 24 h.
Chemistry
The synthesis of the target compounds 1–4 was ac-
Scheme 2. Reagents and conditions: a) NaH, DMF, 0–258C, 24 h; b) 2-chloroethylamine hy-
drochloride, Et2O/EtOH, RT, 24 h; c) Cs2CO3, CH3CN, reflux, 2 h; d) Ar-piperazine, DMF, 70–
808C, 4 h; e) Ar-piperazine, Cs2CO3, CH3CN, reflux, 24 h; f) 4% NaOH, reflux, 2–4 h;
g) NaOEt, EtOH, reflux, 1 h.
complished following a common pathway as shown
in Scheme 1. The reaction of acid chloride of the ap-
propriate dichloroheteroaromatic carboxylic acids
with 3-(dimethylamino)-2-propenoate followed by re-
action with methylamine and successive base-in-
duced cyclization, gave the key esters 23–25. The nucleophilic
reaction of 24, 25, and 26[2] with 1-(1,3-benzothiazol-2-yl)piper-
azine[17] followed by basic hydrolysis gave target acids 2–4.
The hydrolysis step preceded the nucleophilic reaction in the
synthesis of 1,5-naphthyridone 1 starting from synthon 23.
The 1-amino- (5 and 6) and 1-vinyl- (7–11) 1,8-naphthyri-
dones, were synthesized as shown in Scheme 2 by initially pre-
paring the key intermediates 32 and 35. Compound 32 was
prepared by amination of intermediate 31[18] employing freshly
prepared O-p-tolylsulfonylhydroxylamine (TSH)[19] in the pres-
ence of sodium hydride. The 1-vinyl derivative 35, was instead
obtained by a procedure different to that previously report-
ed[20] by reacting acrylate 33[21] with 2-chloroethylamine to
give intermediate 34 followed by cyclization with cesium car-
bonate. The successive nucleophilic reaction of 32 and 35 with
selected 4-arylpiperazines, gave compounds 36–42. The 1-ami-
noderivatives 36 and 37 were then hydrolyzed to the corre-
sponding target acids 5 and 6 by using 4% sodium hydroxide,
whereas employing sodium ethoxide in ethanol at reflux, 1-
chloroethyl esters 38–42 were directly converted into 1-vinyl
derivatives 7–11.
A Gould–Jacob procedure was used to prepare 6-amino-1,8-
naphthyridones 12 and 13 (Scheme 3). Starting from 2,6-di-
chloro-3-nitropyridine 43, a first nucleophilic reaction with 1-
(1,3-benzothiazol-2-yl)piperazine[17] and 1-(2-pyridinyl)pipera-
zine gave intermediates 44 and 45, respectively. They were
then reacted with methylamine under microwave irradiation,
followed by reaction with diethyl 2-(ethoxymethylene)malo-
nate (EMME) and thermal cyclization with polyphosphoric acid
(PPA), to give naphthyridones 48 and 49. Catalytic reduction of
1250
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Naphthyridone Derivatives
potent anti-HIV activity also to the 1,6-naphthyridone scaffold.
But, this is the first example in which its presence did not led
to toxicity.
In contrast, all the other structural modifications made on
the 1,8-naphthyridone scaffold were unproductive. Indeed, the
shifting of the nitrogen atom from the 8 to 5 or 7 positions of
the naphthyridone nucleus gave inactive compounds 1 and 3.
Whereas the replacement of the pyridine ring with a 1,3-thia-
zole one, yielded the more cytotoxic derivative 4.
Certain 1,8-naphthyridone analogues were also synthesized
by modifying the N1 and C6 positions, while placing different
4-arylpiperazines at the C7 position to enlarge the SAR. The re-
placement of the N1 methyl group with an amino group gave
5 which maintained the same low cytotoxicity but was mark-
edly less active, especially against HIV-2. The activity further
decreased with the 7-pyridinylpiperazine analogue 6.
The introduction of a vinyl group at the N1 position proved
much more valuable; it yielded active and selective anti-HIV
compounds (7–11). The highest activity was obtained with 7,
with an EC50 value of 0.02 mm on both HIV-1 and HIV-2, even
slightly lower than those of the lead HM13N; however, it was
50-fold more toxic resulting in lower SI values. More selective
compounds were obtained by introducing the pyridinylpipera-
zine at the C7 position, (derivative 8), and even better when
the m-trifluoromethylphenylpiperazine was used (derivative 9).
Good activity was also obtained with 1-vinyl derivatives, 10
and 11. When comparing 1-vinylnaphthyridones with their qui-
nolone counterparts,[7] it became clear that the 1-vinyl moiety
confers lower toxicity to the former while maintaining the
same high potency.
Scheme 3. Reagents and conditions: a) Ar-piperazine, DMF or toluene, RT, 7–
12 h; b) MeNH2, EtOH, MWI, 758C, 9 min; c) EMME, 1408C, 1 week; d) PPA,
908C, 2 h; e) Raney Ni, H2, EtOH/DMF, RT, 1 h; f) 4% NaOH, reflux, 12 h.
the nitro group and basic hydrolysis finally furnished target
acids 12 and 13.
Biological Activity
The newly synthesized compounds were initially evaluated for
their anti-HIV-1 (IIIB) and anti-HIV-2 (ROD) activity in MT-4 cells
and cytotoxicity of the compounds was determined in parallel.
The results reported in Table 1 clearly indicate that among the
structural modifications made on HM13N, the shifting of the
nitrogen atom from the 8 to the 6 position was particularly
beneficial. Indeed, the 1,6-naphthyridone 2 (NM13) maintained
the same activity of the lead on HIV-1, but it is devoid of any
cytotoxic effect (CC50 >296.58 mm) which made it very selective
with a SI value of ꢀ3707. This is the highest SI value ever ob-
tained in MT-4 cells for the transcription inhibitor quinolone
series of compounds. Thus, the benzothiazolpiperazine is con-
firmed as a very suitable C7 substituent that is able to give
Finally, the introduction of an amino group at the C6 posi-
tion of 1,8-naphthyridone nucleus was required as it character-
izes many of the potent 6-DFQs.[1,2,5] Benzothiazolyl derivative
12 showed good anti-HIV activi-
ty which, however, was not per-
Table 1. Anti-HIV-1 and -HIV-2 activity and cytotoxicity of naphthyridone derivatives in MT-4 cells.
fectly reproducible, whereas 13,
EC50 [mm][a,c]
CC50 [mm][b,c]
SI[d]
bearing the pyridinylpiperazine
Compd
HIV-1(IIIB)
>118.63
ꢀ0.08
>25.57
>2.05
HIV-2(ROD)
IIIB
ROD
as
a C7 substituent, showed
moderate activity coupled with
low cytotoxicity leading to posi-
tive SI values.
1
>118.63
>118.63
>296.58
1
1
1
<1
<1
9
ꢁ11
18
79
154
7
ꢁ18
ꢁ4
20
2 (NM13)
3
4
5
6
7
8
9
>296.58
>25.57
>2.05
ꢀ3707
<1
<1
20
14
22
104
200
24
21
ꢁ12
35
25.57Æ3.02
2.05Æ1.31
23.77Æ3.07
172.99Æ22.90
0.44Æ0.38
16.66Æ12.63
120.27Æ49.91
34.61Æ3.39
18.30Æ9.98
0.34Æ0.32
51.86Æ25.31
26.33Æ6.79
1.18Æ0.18
2.57Æ1.55
ꢀ15.69
12.01Æ3.68
0.02Æ0.00
0.16Æ0.06
0.60Æ0.27
1.46Æ1.14
0.86Æ0.47
ꢀ0.029
Conclusions
0.025Æ0.004
0.21Æ0.015
0.78Æ0.20
4.93Æ0.65
Herein we report the synthesis
of an enlarged series of naph-
thyridone derivatives starting
from 1,8-naphthyridone HM13N.
The structural modifications
confirmed that the 1,8-naph-
thyridone nucleus is very appro-
priate to obtain potent and se-
lective anti-HIV compounds with
the best results obtained when
a vinyl moiety was placed at the
N1 position coupled with a pyri-
dinylpiperazine or a m-trifluoro-
10
11
12
13
ꢀ1.03
ꢀ0.077
2.57Æ0.76
0.04Æ0.00
1.49Æ0.65
0.06Æ0.00
HM13N[e]
439
658
[a] EC50: compound concentration required to achieve 50% protection of MT-4 cells from HIV-induced cytopa-
thogenicity, as determined by the MTT method; ’>’ indicates that no EC50 value (50% inhibition of HIV-in-
duced CPE) was observed at a concentration less than the CC50 value obtained for the compound in MT-4
cells; ’ꢀ’ means that in one separate experiment, performed in triplicate, a reliable EC50 value was obtained,
whereas in the second experiment, although protection against the HIV-1 induced CPE was observed, the level
of 50% protection was not reached (no EC50 value). [b] CC50: compound concentration that decreases the via-
bility of mock-infected cells by 50%, as determined by the MTT method. [c] All data represent mean values
ÆSD for at least two separate experiments. [d] SI: CC50/EC50 ratio. [e] Ref. [8].
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O. Tabarrini et al.
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trate was evaporated to dryness to give a residue which was solu-
bilized in EtOAc and washed with H2O acidified with 2n HCl. The
organic layers were evaporated to dryness to give a residue which
was purified by flash chromatography eluting with MeOH/CH2Cl2
methylphenylpiperazine at the C7 position (compounds 8 and
9).
Although the nitrogen atom must not be moved from the 8
to the 5 or 7 positions, as demonstrated by the inactivity of
derivatives 1 and 3, the shifting to the 6 position is the real
novelty of this study. Indeed, 1,6-naphthyridone NM13 is en-
dowed with potent anti-HIV-1 activity, which combined with
low toxicity, is the most selective derivative in acutely infected
MT-4 cells. It is worth noting that a 1,6-naphthyridone ana-
logue, bearing a pyridinylpiperazine as a C7 substituent, was
previously reported by us as a nontoxic compound but devoid
of any anti-HIV activity at lower concentrations.[2] Thus, the ac-
tivity of NM13 is the result of a very successful combination of
the 1,6-naphthyridone nucleus and the C7 benzothiazolylpiper-
azine, which, for the first time not only confers potent anti-HIV
activity but also resulted in a highly selective compound.
Further elaborating the anti-HIV profile of this new deriva-
tive is clearly warranted. A transcription inhibitor is expected
to inhibit viral production in latently and chronically infected
cells, in which HIV-1 proviral DNA has already been integrated
in the host-cell genome. The evaluation of its antiviral activity
on these cells is currently in progress along with studies to
confirm which step of the viral replicative cycle is inhibited.
Moreover, we are attempting to increase its solubility which is
lower than that of HM13N. Improving this parameter is manda-
tory to enable future in vivo studies.
1
(3%), to give 17 as an oil (1.0 g, 46%); H NMR (CDCl3): d=0.90 (t,
J=7.1 Hz, 3H, CH2CH3), 3.00 and 3.40 (s, each 3H, CH3), 3.90 (q, J=
7.1 Hz, 2H, CH2CH3), 7.75 (d, J=1.8 Hz, 1H, H-4), 7.95 (s, 1H, CH),
8.35 ppm (d, J=1.8 Hz, 1H, H-6).
Ethyl
2-(2,3-dichloroisonicotinoyl)-3-(dimethylamino)-2-prope-
noate (18). Compound 18 was prepared from 2,3-dichloroisonico-
tinic acid 15[22] using Method A (1 h), in 60% yield as an oil after
1
purification by flash chromatography eluting with CH2Cl2; H NMR
([D6]DMSO): d=0.85 (t, 3H, J=7.1 Hz, CH2CH3), 2.95 and 3.40 (s,
each 3H, CH3), 3.85 (q, J=7.1 Hz, 2H, CH2CH3), 7.30 (d, J=4.0 Hz,
1H, H-5), 7.95 (s, 1H, CH), 8.35 ppm (d, J=4.0 Hz, 1H, H-6).
Ethyl
2-[(2,4-dichloro-1,3-thiazol-5-yl)carbonyl]-3-(dimethyl-
amino)-2-propenoate (19). Compound 19 was prepared from 2,4-
dichloro-1,3-thiazole-5-carboxylic acid 16[23] using Method A (2 h),
in 53% yield, after purification by flash chromatography eluting
1
with EtOAc/cyclohexane (50%); mp: 65–668C; H NMR ([D6]DMSO):
d=0.75 (t, J=7.1 Hz, 3H, CH2CH3), 2.20 and 3.10 (s, each 3H, CH3),
3.75 (q, J=7.1 Hz, 2H, CH2CH3), 7.50 ppm (s, 1H, CH).
Preparation of Ethyl 2-[(3,5-dichloro-2-pyridinyl)carbonyl]-3-
(methylamino)-2-propenoate (20, Method B). A mixture of com-
pound 17 (1.0 g, 3.15 mmol) and a 33% solution of MeNH2 in EtOH
(0.44 mL, 3.78 mmol) in a mixture of EtOH/Et2O (1:4, 20 mL), was
stirred at RT for 30 min. The reaction mixture was then concentrat-
ed in vacuo, giving a residue which was purified by treatment with
a mixture of Et2O/petroleum ether, to give 20 (0.65 g, 68%); mp:
138–1398C; 1H NMR (CDCl3): d=1.00 (t, J=7.1 Hz, 3H, CH2CH3),
3.25 (d, J=5.0 Hz, CH3), 4.00 (q, J=7.1 Hz, 2H, CH2CH3), 7.75 (d, J=
1.6 Hz, 1H, H-4), 8.20 (d, J=14.0 Hz, 1H, CH), 8.45 (bs, 1H, H-6),
11.00 ppm (bs, 1H, NH).
Experimental Section
Chemistry
All reactions were routinely checked by thin-layer chromatography
(TLC) on silica gel 60 F254 (Merck) and visualized by UV light or
iodine. Flash column chromatography separations were carried out
on Merck silica gel 60 (mesh 230–400). Melting points were deter-
mined in capillary tubes (Bꢁchi Electrothermal Model 9100) and are
uncorrected. Elemental analyses were performed on a Fisons ele-
mental analyzer, Model EA1108CHN, and the data for C, H, and N
Ethyl
2-(2,3-dichloroisonicotinoyl)-3-(methylamino)-2-prope-
noate (21). Compound 21 was prepared from 18 by Method B in
73% yield, after crystallization by EtOH; mp: 161–1628C; 1H NMR
([D6]DMSO): d=0.80 (t, J=7.1 Hz, 3H, CH2CH3), 3.15 (d, J=5.0 Hz,
3H, CH3), 3.80 (q, J=6.2 Hz, 2H, CH2CH3), 7.20 (d, J=4.8 Hz, 1H, H-
5), 8.10 (d, J=14.0 Hz, 1H, CH), 8.30 (d, J=4.8 Hz, 1H, H-6),
10.70 ppm (bs, 1H, NH).
1
are within Æ0.4% of the theoretical values. H NMR and 13C NMR
spectra were recorded at 200 MHz (Bruker Avance DPX-200) and
400 MHz (Bruker Avance DRX-400) using residual solvents such as
CHCl3 (d=7.26) or DMSO (d=2.48) as an internal standard. Chemi-
cal shifts (d) are given in ppm, and the spectral data are consistent
with the assigned structures. Reagents and solvents were pur-
chased from common commercial suppliers and were used as sup-
plied. After extraction, organic solutions were dried over anhydrous
Na2SO4, filtered, and concentrated with a Bꢁchi rotary evaporator
at reduced pressure. Yields are of purified product and were not
optimized. Microwave irradiated reactions were carried out in an
Initiator 2.0 (Biotage) using 5, 10, and 20 mL Pyrex vials. All starting
materials were commercially available unless otherwise indicated.
Ethyl 2-[(2,4-dichloro-1,3-thiazol-5-yl)carbonyl]-3-(methylamino)-
2-propenoate (22). Compound 22 was prepared from 19 by
Method B using Et2O as solvent, in 73% yield, after treatment with
cyclohexane; mp: 74–758C; 1H NMR ([D6]DMSO): d=1.00 (t, J=
7.1 Hz, 3H, CH2CH3), 3.20 (d, J=4.5 Hz, 3H, CH3), 4.00 (q, J=7.1 Hz,
2H, CH2CH3), 8.00 and 8.15 (d, J=15.0 Hz, each 0.5H, CH), 9.30–
9.45 and 10.40–10.55 ppm (m, each 0.5H, NH).
Ethyl 7-chloro-1-methyl-4-oxo-1,4-dihydro-1,5-naphthyridine-3-
carboxylate (23). A mixture of acrylate 20 (0.60 g, 1.97 mmol) and
K2CO3 (0.82 g, 5.93 mmol) in DMF (8 mL) was heated at 608C for
24 h. After cooling the reaction mixture was poured into ice/H2O
and acidified with 2n HCl (pH 4), to obtain a precipitate which was
filtered and washed with H2O and then with Et2O, to give 23
Preparation of Ethyl 2-[(3,5-dichloro-2-pyridinyl)carbonyl]-3-(di-
methylamino)-2-propenoate (17, Method A). A mixture of 3,5-di-
chloro-2-pyridinecarboxylic acid 14 (1.3 g, 6.77 mmol) and SOCl2
(5 mL) was heated at 808C for ~3 h. The excess SOCl2 was removed
by distillation under reduced pressure, to give a residue which was
solubilized in toluene and added to a mixture of ethyl 3-(dimethy-
lamino)-2-propenoate (1.16 g, 8.12 mmol) and Et3N (0.82 g,
8.12 mmol) in toluene (15 mL). The mixture was heated at 808C for
1 h. After cooling, the precipitate obtained was filtered and the fil-
1
(0.38 g, 72%); mp: 255–2568C; H NMR ([D6]DMSO): d=1.25 (t, J=
7.1 Hz, 3H, CH2CH3), 3.90 (s, 3H, CH3), 4.25 (q, J=7.1 Hz, 2H,
CH2CH3), 8.40, (s, 1H, H-8). 8.70 (s, 1H, H-2), 8.80 ppm (s, 1H, H-6).
Ethyl 8-chloro-1-methyl-4-oxo-1,4-dihydro-1,7-naphthyridine-3-
carboxylate (24). Compound 24 was prepared from acrylate 21
following the procedure used for the synthesis of compound 23 in
71% yield; mp: 129–1308C; 1H NMR ([D6]DMSO): d=1.20 (t, J=
1252
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Naphthyridone Derivatives
7.0 Hz, 3H, CH2CH3), 4.15–4.25 (m, 5H, CH2CH3 and CH3), 8.00 (d,
J=5.0 Hz, 1H, H-5), 8.30 (d, J=5.0 Hz, 1H, H-6), 8.60 ppm (s, 1H,
H-2).
1H, benzothiazole CH), 7.50 and 7.80 (d, J=7.7 Hz, each 1H, ben-
zothiazole CH), 8.35 ppm (s, 1H, H-2).
Ethyl 7-[4-(1,3-benzothiazol-2-yl)-1-piperazinyl]-1-methyl-4-oxo-
1,4-dihydro-1,6-naphthyridine-3-carboxylate (30). Compound 30
was prepared from ethyl 7-chloro-1-methyl-4-oxo-1,4-dihydro-1,6-
naphthyridine-3-carboxylate 26[2] by Method D (1 h) in 29% yield,
after purification by flash chromatography eluting with MeOH/
Ethyl 2-chloro-4-methyl-7-oxo-4,7-dihydro[1,3]thiazolo[4,5-b]pyr-
idine-6-carboxylate (25). A solution of 22 (0.5 g, 1.61 mmol) in
THF (10 mL) was added dropwise to a suspension of NaH (0.04 g,
1.61 mmol) in THF (15 mL). After 30 min the reaction mixture was
poured into ice/H2O, neutralized with 2n HCl, and extracted with
EtOAc. The organic layers were evaporated to dryness to give a
residue which was purified by flash chromatography eluting with
CH2Cl2, to give 25 (0.10 g, 22%); mp: 174–1758C; 1H NMR
([D6]DMSO): d=1.20 (t, J=7.0 Hz, 3H, CH2CH3), 3.95 (s, 3H, CH3),
4.30 (q, J=7.1 Hz, 2H, CH2CH3), 8.65 ppm (s, 1H, H-2); 13C NMR
([D6]DMSO): d=14.5, 60.7, 113.9, 126.2, 129.9, 147.5, 151.4, 158.1,
164.4, 169.06 ppm.
1
CH2Cl2 (3%); mp: 269–2698C; H NMR ([D6]DMSO): d=1.25 (t, J=
7.0 Hz, 3H, CH2CH3), 3.70–3.75 (m, 4H, piperazine CH2), 3.80 (s, 3H,
CH3), 3.95–4.00 (m, 4H, piperazine CH2), 4.20 (q, J=7.0 Hz, 2H,
CH2CH3), 6.68 (s, 1H, H-8), 7.25 and 7.30 (t, J=7.7 Hz, each 1H, ben-
zothiazole CH), 7.50 and 7.80 (d, J=7.5 Hz, each 1H, benzothiazole
CH) 8.55 (s, 1H, H-5), 8.95 ppm (s, 1H, H-2).
7-[4-(1,3-Benzothiazol-2-yl)-1-piperazinyl]-1-methyl-4-oxo-1,4-di-
hydro-1,6-naphthyridine-3-carboxylic acid (2). Compound 2 was
prepared from intermediate 30 by Method C (24 h). After cooling,
the reaction mixture was filtered and the solid obtained was sus-
pended in 2n HCl and stirred for ~1 h. The precipitate was then fil-
tered, washed with Et2O, and crystallized by DMF to give 2 in 92%
yield; mp 359–3608C; 1H NMR ([D6]DMSO): d=3.70–3.80 (m, 4H,
piperazine CH2), 3.95–4.05 (m, 7H, piperazine CH2 and CH3), 6.80 (s,
1H, H-8), 7.15 and 7.35 (t, J=7.5 Hz, each 1H, benzothiazole CH),
7.55 and 7.85 (d, J=8.0 Hz, each 1H, benzothiazole CH), 8.90 (s,
1H, H-5), 9.15 ppm (s, 1H, H-2); 13C NMR ([D6]DMSO): d=36.2, 44.2,
47.8, 89.8, 112.5, 119.1, 121.7, 121.8, 126.4, 130.8, 147.8, 148.7,
151.9, 152.7, 160.7, 166.2, 168.5, 175.3 ppm; Anal. calcd for
C21H19N5O3S: C 59.84, H 4.54, N 16.62, found: C 60.14, H 4.60, N
16.70.
Preparation of 7-Chloro-1-methyl-4-oxo-1,4-dihydro-1,5-naph-
thyridine-3-carboxylic acid (27, method C). A suspension of com-
pound 23 (0.35 g, 1.31 mmol) in 4% NaOH (8 mL) was held at
reflux for 30 min. After cooling, the reaction mixture was acidified
to pH 4 with 2n HCl, obtaining a precipitate which was filtered
and washed with H2O, to give 27 (0.16 g, 51%); mp: 309–3108C;
1H NMR ([D6]DMSO): d=4.00 (s, 3H, CH3), 8.60 (s, 1H, H-6) 8.90 (s,
1H, H-8), 9.00 (s, 1H, H-2), 15.00 ppm (s, 1H, CO2H).
7-[4-(1,3-Benzothiazol-2-yl)-1-piperazinyl]-1-methyl-4-oxo-1,4-di-
hydro-1,5-naphthyridine-3-carboxylic acid (1). A mixture of the
synthon 27 (0.05 g, 0.20 mmol) and 1-(1,3-benzothyazol-2-yl)piper-
azine[17] (0.26 g, 1.2 mmol) in N-methyl-2-pyrrolidone (0.25 mL), was
heated at 1058C for 24 h. After cooling, the reaction mixture was
poured into ice/H2O, obtaining a precipitate which was filtered and
crystallized by EtOH/DMF, to give 1 (0.016 g, 18%); mp: 250–
Sodium
8-[4-(1,3-benzothiazol-2-yl)-1-piperazinyl]-1-methyl-4-
oxo-1,4-dihydro-1,7-naphthyridine-3-carboxylate (3). Compound
3 was prepared from 28 using Method C (15 min). After cooling,
the precipitate obtained was filtered and washed with Et2O to give
1
2518C; H NMR ([D6]DMSO): d=4.00 (s, 3H, CH3), 3.75–3.85 (m, 8H,
piperazine CH2), 7.10 (t, J=7.3 Hz, 1H, benzothiazole CH), 7.25–
7.35 (m, 2H, H-8 and benzothiazole CH), 7.50 and 7.80 (d, J=
8.1 Hz, each 1H, benzothiazole CH), 8.80 (s, 1H, H-6), 8.90 (s, 1H,
H-2), 15.90 ppm (s, 1H, CO2H); 13C NMR ([D6]DMSO): d=41.9, 44.4,
47.9, 111.0, 119.1, 121.7, 121.8, 126.4, 127.1, 130.8, 135.1, 138.9,
139.0, 147.9, 151.1, 166.1, 168.4, 177.2; Anal. calcd (%) for
C21H19N5O3S: C 59.85, H 4.54, N 16.62, found: C 60.13, H 4.47, N
16.58.
1
3 in 74% yield; mp: 101–1028C; H NMR ([D6]DMSO): d=2.90–3.05
(m, 2H, piperazine CH2), 3.45–3.65 (m, 4H, piperazine CH2), 3.90–
4.05 (m, 2H, piperazine CH2), 4.10 (s, 3H, CH3), 7.15 and 7.30 (t, J=
8.0 Hz, each 1H, benzothiazole CH), 7.50 (d, J=8.4 Hz, 1H, benzo-
thiazole CH), 7.70–7.80 (m, 2H, benzothiazole CH and H-5), 8.15 (d,
J=5.1 Hz, 1H, H-6), ppm 8.50 (s, 1H, H-2); 13C NMR ([D6]DMSO):
d=42.8, 47.8, 49.8, 11.4, 113.6, 119.1, 121.6, 121.8, 126.4, 128.4,
130.8, 136.8, 142.1, 152.7, 153.0, 153.9, 164.5, 168.6, 172.0 ppm;
Anal. calcd (%) for C21H18N5NaO3S: C 56.88, H 4.09, N 15.79, found:
C 57.15, H 3.98, N 15.55.
Preparation of Ethyl 8-[4-(1,3-benzothiazol-2-yl)-1-piperazinyl]-1-
methyl-4-oxo-1,4-dihydro-1,7-naphthyridine-3-carboxylate (28,
Method D). A mixture of compound 24 (0.07 g, 0.28 mmol), 1-(1,3-
benzothyazol-2-yl)piperazine[17] (0.18 g, 0.85 mmol), and Et3N
(0.08 g, 0.85 mmol) in DMF (5 mL) was heated at 808C for 30 h.
After cooling, the reaction mixture was poured into ice/H2O giving
a precipitate which was washed with H2O and then with Et2O, to
2-[4-(1,3-Benzothiazol-2-yl)-1-piperazinyl]-4-methyl-7-oxo-4,7-
dihydro[1,3]thiazolo[4,5-b]pyridine-6-carboxylic acid (4). Com-
pound 4 was prepared from 29 using Method C (2 h). After cool-
ing, the precipitate obtained was filtered, suspended in 2n HCl,
and stirred for 30 min. The precipitate was filtered, washed with
H2O, and crystallized by EtOH/DMF, to give 4 in 43% yield; mp:
1
give 28 (0.036 g, 29%); mp: 229–2308C; H NMR ([D6]DMSO): d=
1
1.20 (t, J=7.0 Hz, 3H, CH2CH3), 2.90–3.00 (m, 2H, piperazine CH2),
3.45–3.65 (m, 4H, piperazine CH2), 3.75–3.95 (m, 2H, piperazine
CH2), 4.05 (s, 3H, CH3), 4.20 (q, J=7.1 Hz, 2H, CH2CH3), 7.05 and
7.20 (t, J=7.5 Hz, each 1H, benzothiazole CH), 7.40 (d, J=7.4 Hz,
1H, benzothiazole CH), 7.70 (d, J=5.1 Hz, 1H, H-5), 7.75 (, J=
7.4 Hz, 1H, benzothiazole CH), 8.20 (d, J=5.1 Hz, 1H, H-6),
8.50 ppm (s, 1H, H-2).
352–3538C; H NMR ([D6]DMSO): d=3.80–4.00 (m, 11H, piperazine
CH2 and CH3), 7.15 and 7.35 (t, J=7.5 Hz, each 1H, benzothiazole
CH), 7.55 and 7.85 (d, J=7.5 Hz, each 1H, benzothiazole CH),
8.70 ppm (s, 1H, H-2); 13C NMR ([D6]DMSO): d=19.0, 56.4, 60.2,
113.5, 119.2, 121.7, 121.9, 126.5, 130.9, 145.2, 152.6, 153.3, 164.9,
168.3, 168.4, 171.2 ppm; Anal. calcd (%) for C19H17N5O3S2: C 53.38,
H 4.01, N 16.38, found: C 53.60, H 4.00, N 16.19.
Ethyl 2-[4-(1,3-benzothiazol-2-yl)-1-piperazinyl]-4-methyl-7-oxo-
4,7-dihydro[1,3]thiazolo[4,5-b]pyridine-6-carboxylate (29). Com-
pound 29 was prepared from synthon 25, by Method D (508C, 1 h)
in 72% yield; mp: 268–2698C; H NMR ([D6]DMSO): d=1.25 (t, J=
7.1 Hz, 3H, CH2CH3), 3.75–3.85 (m, 11H, piperazine CH2 and CH3),
4.20 (q, J=7.1 Hz, 2H, CH2CH3), 7.10 and 7.30 (t, J=7.5 Hz, each
Ethyl 1-amino-7-chloro-4-oxo-1,4-dihydro-1,8-naphthyridine-3-
carboxylate (32). NaH (0.06 g, 1.58 mmol) was added portionwise
to a suspension of ethyl 7-chloro-4-oxo-1,4-dihydro-1,8-naphthyri-
dine-3-carboxylate 31[18] (0.4 g, 1.58 mmol) in DMF (5 mL), and the
mixture was maintained at RT for 3 h. The reaction mixture was
then brought to 08C and a solution of freshly prepared O-p-tolyl-
1
ChemMedChem 2011, 6, 1249 – 1257
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemmedchem.org
1253

O. Tabarrini et al.
1-(1,3-benzothyazol-2-yl)piperazine[17]
MED
sulfonylhydroxylamine (TSH)[19] (0.37 g, 1.97 mmol) in CH2Cl2
(10 mL) was added. The mixture was maintained at the same tem-
perature for 10 min and then at RT for a further 21 h. The reaction
mixture was concentrated in vacuo, diluted with CH2Cl2, and
washed with H2O. The organic layers were evaporated to dryness,
giving a solid which was washed with Et2O to give 32 (0.07 g,
0.79 mmol),
(0.17 g,
0.79 mmol), and Cs2CO3 (0.25 g, 0.79 mmol), in CH3CN (5 mL) was
heated at reflux for 24 h. The reaction mixture was then poured
into ice/H2O and extracted with EtOAc. The organic layers were
evaporated to dryness to give a residue which was treated with
Et2O/EtOH, to give 38 (0.22 g, 56%); mp 231–2328C; 1H NMR
([D6]DMSO): d=1.25 (t, J=7.0 Hz, 3H, CH2CH3), 3.70–3.80 and 3.85–
3.95 (m, each 4H, piperazine CH2), 4.05–4.10 (m, 2H, CH2Cl), 4.20
(q, J=7.0 Hz, 2H, CH2CH3), 4.65–4.70 (m, 2H, NCH2), 7.05–7.20 (m,
2H, benzothiazole CH and H-6), 7.30 (t, J=8.0 Hz, 1H, benzothia-
zole CH), 7.45 and 7.80 (d, J=8.0 Hz, each 1H, benzothiazole CH),
8.25 (d, J=8.2 Hz, 1H, H-5), 8.60 ppm (s, 1H, H-2).
1
17%); mp: 163–1658C; H NMR ([D6]DMSO): d=1.30 (t, J=7.0 Hz,
3H, CH2CH3), 4.25 (q, J=7.0 Hz, 2H, CH2CH3), 6.45 (s, 2H, NH2), 7.70
(d, J=8.2 Hz, 1H, H-6), 8.55 (d, J=8.2 Hz, 1H, H-5), 8.70 ppm (s,
1H, H-2).
Ethyl
3-[(2-chloroethyl)amino]-2-[(2,6-dichloro-3-pyridinyl)car-
bonyl]-2-propenoate (34). Compound 34 was prepared from ethyl
2-[(2,6-dichloro-3-pyridinyl)carbonyl]-3-(dimethylamino)-2-prope-
noate 33[21] by Method B (24 h), replacing MeNH2 with 2-chloroe-
thylamine hydrochloride, in 86% yield, after treatment with Et2O;
mp: 123–1248C; 1H NMR (CDCl3): d=1.00 (t, J=7.1 Hz, 3H,
CH2CH3), 3.70–3.80 (m, 4H, CH2CH2), 4.00 (q, J=7.1 Hz, 2H, CH2CH3),
7.30 (d, J=8.0 Hz, 1H, H-5), 7.55 (d, J=8.0 Hz, 1H, H-4), 8.20–8.25
(m, 1H, CH), 9.45–9.55 and 11.00–11.05 ppm (each m, 1H, NH).
A mixture of 38 (0.18 g, 0.36 mmol) and NaOEt (0.17 g, 2.52 mmol)
in EtOH (5 mL) was held at reflux for 1 h. After cooling, the reaction
mixture was concentrated in vacuo, poured into H2O, and 2n HCl
(pH 1) was added to obtain a solid which was filtered, washed with
H2O, then EtOH, and finally crystallized by EtOH/DMF, to give 7
(0.09 g, 57%); mp: 313–3148C; 1H NMR ([D6]DMSO): d=3.70–3.80
and 3.95–4.05 (m, each 4H, piperazine CH2), 5.35 (d, J=8.0 Hz, 1H,
CH=CH2), 5.80 (d, J=15.0 Hz, 1H, CH=CH2), 7.10 (t, J=7.3 Hz, 1H,
benzothiazole CH), 7.20–7.30 (m, 2H, benzothiazole CH and H-6),
7.45 and 7.80 (d, J=7.5 Hz, each 1H, benzothiazole CH), 7.85 (dd,
J=8.0 and 15.0 Hz, 1H, CH=CH2), 8.30 (d, J=8.5 Hz, 1H, H-5), 8.75
(s, 1H, H-2), 15.25 ppm (bs, 1H, COOH); 13C NMR ([D6]DMSO): d=
44.1, 47.8, 108.6, 108.9, 109.2, 110.8, 119.1, 121.7, 121.8, 126.4,
130.8, 132.8, 136.3, 143.7, 149.3, 152.7, 159.1, 166.0, 168.4,
177.3 ppm; Anal. calcd (%) for C22H20ClN5O3S: C 56.23, H 4.29, N
14.90, found: C 56.55, H 4.28, N 14.63
Ethyl 7-chloro-1-(2-chloroethyl)-4-oxo-1,4-dihydro-1,8-naphthyri-
dine-3-carboxylate (35).[20] Cs2CO3 (1.78 g, 5.47 mmol) was added
to a solution of 34 (0.77 g, 2.18 mmol) in CH3CN (10 mL) at reflux.
After 2 h, the reaction mixture was cooled and then poured in ice/
H2O, obtaining a precipitate which was filtered and washed with
H2O and then with Et2O, to give 35 (0.66 g, 96%); mp: 143–1458C
(148–1498C);[20] 1H NMR (CDCl3): d=1.40 (t, J=7.1 Hz, 3H, CH2CH3),
4.00 (t, J=5.5 Hz, 2H, CH2Cl), 4.40 (q, J=7.1 Hz, 2H, CH2CH3), 4.70
(t, J=5.5 Hz, 2H, NCH2), 7.40 (d, J=8.2 Hz, 1H, H-6), 8.60 (s, 1H, H-
2), 8.70 ppm (d, J=8.2 Hz, 1H, H-5).
4-Oxo-7-[4-(2-pyridinyl)-1-piperazinyl]-1-vinyl-1,4-dihydro-1,8-
naphthyridine-3-carboxylic acid (8). Compound 8 was prepared
from synthon 35 by Method E through intermediate 39, replacing
1-(1,3-benzothyazol-2-yl)piperazine with 1-(pyridin-2-yl)piperazine
and adjusting the pH to 7 in the work up of the hydrolysis reac-
tion. After crystallization by DMF, 8 was obtained in 57% overall
1-Amino-7-[4-(1,3-benzothiazol-2-yl)-1-piperazinyl]-4-oxo-1,4-di-
hydro-1,8-naphthyridine-3-carboxylic acid (5). Compound 5 was
prepared from synthon 32 by Method D (708C, 4 h), to afford inter-
mediate 36 in 59% yield, followed by Method C (2 h) in 89% yield,
after crystallization by EtOH/DMF; mp: 320–3218C; 1H NMR
([D6]DMSO): d=3.70–3.80 and 4.05–4.15 (m, each 4H, piperazine
CH2), 6.75 (bs, 2H, NH2), 7.10 (t, J=7.5 Hz, benzothiazole CH), 7.25–
7.35 (m, 2H, benzothiazole CH and H-6), 7.50 and 7.80 (d, J=
7.5 Hz, each 1H, benzothiazole CH), 8.35 (d, J=7.8 Hz, 1H, H-5),
8.70 (s, 1H, H-2), 15.75 ppm (s, 1H, COOH); 13C NMR ([D6]DMSO):
d=47.9, 108.6, 111.3, 119.1, 121.7, 121.8, 126.5, 130.8, 136.3, 146.2,
149.9, 152.7, 159.1, 166.5, 168.4, 175.8 ppm; Anal. calcd (%) for
C20H18N6O3S: C 56.86, H 4.29, N 19.89, found: C 56.98, H 4.31, N
19.93
1
yield; mp: 314–3158C; H NMR ([D6]DMSO): d=3.65–3.70 and 3.80–
3.90 (m, each 4H, piperazine CH2), 5.35 (d, J=8.0 Hz, 1H, CH=CH2),
5.80 (d, J=15.0 Hz, 1H, CH=CH2), 6.70 ( t, J=7.0 Hz, pyridine CH),
6.80 (d, J=8.6 Hz, 1H, H-6), 7.25 (d, J=8.6 Hz, pyridine CH), 7.55 (t,
J=7.5 Hz, pyridine CH) 7.90 (dd, J=8.0 and 15.0 Hz, 1H, CH=CH2),
8.15(d, J=8.0 Hz, 1H, pyridine CH), 8.30 (d, J=9 Hz, 1H, H-5), 8.80
(s, 1H, H-2), 15.25 ppm (bs, 1H, COOH); 13C NMR ([D6]DMSO): d=
44.4, 47.6, 107.6, 108.3, 108.8, 109.3, 110.6, 113.7, 132.8, 136.2,
138.1, 143.9, 148.0, 149.3, 159.0, 159.1, 166.2, 177.2 ppm; Anal.
calcd (%) for C20H19N5O3: C 63.65, H 5.07, N 18.56, found: C 63.98,
H 5.01, N 18.28.
1-Amino-4-oxo-7-[4-(2-pyridinyl)-1-piperazinyl]-1,4-dihydro-1,8-
naphthyridine-3-carboxylic acid (6). Compound 6 was prepared
from synthon 32 by Method D (708C, 4 h), replacing 1-(1,3-benzo-
thyazol-2-yl)piperazine with 1-(pyridin-2-yl)piperazine, to afford in-
termediate 37 in 68% yield, followed by Method C (4 h) 84% yield,
after crystallization by EtOH/DMF; mp: 268–2698C; 1H NMR
([D6]DMSO): d=3.85–3.95 and 4.05–4.15 (m, each 4H, piperazine
CH2), 6.95 (t, J=6.2 Hz, 1H, pyridine CH), 7.25 (d, J=9.1 Hz, H-6),
7.30–7.40 and 7.90–8.00 (m, each 1H, pyridine CH), 8.10 (d, J=
4.7 Hz, 1H, pyridine CH), 8.30 (d, J=9.1 Hz, 1H, H-5), 8.65 (s, 1H, H-
2), 15.90 ppm (bs, 1H, COOH); 13C NMR ([D6]DMSO): d=45.7, 46.5,
107.7, 108.9, 111.7, 113.6, 131.5, 136.9, 138.0, 140.0, 146.7, 151.2,
153.4, 159.4, 159.7, 166.7, 177.2 ppm; Anal. calcd (%) for
C18H18N6O3: C 59.01, H 4.95, N 22.94, found: C 59.32, H 4.79, N
22.78
4-Oxo-7-{4-[3-(trifluoromethyl)phenyl]-1-piperazinyl}-1-vinyl-1,4-
dihydro-1,8-naphthyridine-3-carboxylic acid (9). Compound 9
was prepared from synthon 35 by Method E through intermediate
40, replacing 1-(1,3-benzothyazol-2-yl)piperazine with 1-[3-(trifluor-
omethyl)phenyl]piperazine and adjusting the pH to 4 in the work
up of the hydrolysis reaction. After crystallization by EtOH/DMF, 9
was obtained in 27% overall yield; mp: 224–2258C; 1H NMR
([D6]DMSO): d=3.30–3.40 and 3.90–4.00 (m, each 4H, piperazine
CH2), 5.35 (d, J=8.0 Hz, 1H, CH=CH2), 5.80 (d, J=15.0 Hz, 1H, CH=
CH2), 7.05–7.45 (m, 5H, aromatic CH and N6), 7.95 (dd, J=8.0 and
15.0 Hz, 1H, CH=CH2), 8.30 (d, J=8.1 Hz, 1H, H-5), 8.75 (s, 1H, H-2),
15.25 ppm (bs, 1H, COOH); 13C NMR ([D6]DMSO): d=44.3, 47.5,
108.6, 108.9, 109.3, 110.5, 111.4, 115.2, 119.2, 124.8, 130.2, 130.5,
132.8, 136.1, 143.9, 149.4, 151.1, 159.1, 166.0, 177.3 ppm; Anal.
calcd (%) for C22H19F3N4O3: C 59.46, H 4.31, N 12.61, found: C 59.70,
H 4.58, N 12.60.
Preparation of 7-[4-(1,3-Benzothiazol-2-yl)-1-piperazinyl]-4-oxo-
1-vinyl-1,4-dihydro-1,8-naphthyridine-3-carboxylic acid Hydro-
chloride (7, Method E).
A mixture of synthon 35 (0.25 g,
1254
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ChemMedChem 2011, 6, 1249 – 1257

Naphthyridone Derivatives
4-Oxo-7-[4-(1,3-thiazol-2-yl)-1-piperazinyl]-1-vinyl-1,4-dihydro-
1,8-naphthyridine-3-carboxylic acid (10). Compound 10 was pre-
pared from synthon 35 by Method E through intermediate 41, re-
placing 1-(1,3-benzothyazol-2-yl)piperazine with 1-(2-thiazolyl)pi-
perazine.[24] Compound 10 was obtained in 65% overall yield after
benzothiazole CH), 7.50 and 7.80 (d, J=7.7 Hz, each 1H, benzothia-
zole CH), 8.00–8.10 ppm (m, 2H, NH and H-4).
N-methyl-5-nitro-6-[4-(2-pyridinyl)-1-piperazinyl]-2-pyridinamine
(47). Compound 47 was prepared from 45 following the proce-
dure used for the synthesis of compound 46, in 97% yield; mp:
208–2098C; 1H NMR ([D6]DMSO): d=2.90 (d, J=4.0 Hz, 3H, CH3),
3.50–3.60 and 3.65–3.75 (m, each 4H, piperazine CH2), 6.00 (d, J=
9.0 Hz, 1H, H-5), 6.60–6.70 (m, 1H, pyridine CH), 6.85 (d, J=8.5 Hz,
1H, pyridine CH), 7.50–7.60 (m, 1H, pyridine CH), 7.95 (bs, 1H, NH),
8.05–8.20 ppm (m, 2H, H-4 and pyridine CH).
1
crystallization by EtOH/DMF; mp: 358–3598C; H NMR ([D6]DMSO):
d=3.70–3.80 and 4.00–4.10 (m, each 4H, piperazine CH2), 5.35 (d,
J=8.0 Hz, 1H, CH=CH2), 5.80 (d, J=15.0 Hz, 1H, CH=CH2), 7.10 (d,
J=3.4 Hz, 1H, thiazole CH), 7.30–7.40 (m, 2H, thiazole CH and H-6),
7.95 (dd, J=8.0 and 15.0 Hz, 1H, CH=CH2), 8.35 (d, J=8.9 Hz, 1H,
H-5), 8.75 ppm (s, 1H, H-2); 13C NMR ([D6]DMSO): d=43.8, 48.3,
108.7, 108.8, 109.1, 109.3, 110.7, 132.8, 136.2, 143.8, 149.3, 157.3,
159.0, 166.0, 171.0, 177.4 ppm; Anal. calcd (%) for C18H17N5O3S: C
56.38, H 4.47, N 18.27, found: C 56.70, H 4.33 N 18.01.
Ethyl 7-[4-(1,3-benzothiazol-2-yl)-1-piperazinyl]-1-methyl-6-nitro-
4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylate (48). A mix-
ture of compound 46 (0.82 g, 2.21 mmol) and diethyl 2-(ethoxyme-
thylene)malonate (5 mL) was heated at 1408C for one week. After
cooling the reaction mixture was purified by flash chromatography
eluting with CH2Cl2 to give diethyl 2-{[{6-[4-(1,3-benzothiazol-2-yl)-
1-piperazinyl]-5-nitro-2-pyridinyl}(methyl)amino]methylene}malo-
nate, which was used in the next step without further purification.
A mixture of this compound (0.70 g, 1.29 mmol) and polyphos-
phoric acid (3.5 g) was heated at 908C for 2 h. After cooling, the re-
action mixture was poured into ice/H2O and neutralized with 10%
NaOH, obtaining a precipitate which was filtered and purified by
flash chromatography eluting with MeOH/CHCl3 (2%), to give 48
(0.31 g, 28% overall yield); mp: 256–2578C; 1H NMR (CDCl3): d=
1.25 (t, J=7.0 Hz, 3H, CH2CH3), 3.80–3.95 (m, 11H, piperazine CH2
and CH3), 4.45 (q, J=7.0 Hz, 2H, CH2CH3), 7.10 and 7.30 (t, J=
8.0 Hz, each 1H, benzothiazole CH), 7.55–7.65 (m, 2H, benzothia-
zole CH), 8.50 (s, 1H, H-5), 9.10 ppm (s, 1H, H-2).
7-[4-(1,3-Benzoxazol-2-yl)-1-piperazinyl]-4-oxo-1-vinyl-1,4-dihy-
dro-1,8-naphthyridine-3-carboxylic acid (11). Compound 11 was
prepared from synthon 35 by Method E, through intermediate 42,
replacing 1-(1,3-benzothyazol-2-yl)piperazine with 1-(1,3-benzoxa-
zol-2-yl)piperazine.[24] Compound 11 was obtained in 24% overall
yield after crystallization by EtOH/DMF: mp: 302–3038C; 1H NMR
([D6]DMSO): d=3.75–3.85 and 3.95–4.05 (m, each 4H, piperazine
CH2), 5.40 (dd, J=2.0 and 8.0 Hz, 1H, CH=CH2), 5.80 (dd, J=2.0 and
15.0 Hz, 1H, CH=CH2), 7.00 and 7.20 (t, J=7.5 Hz, each 1H, benzox-
azole CH), 7.30 (d, J=9.1 Hz, 1H, H-6), 7.35 and 7.45 (d, J=7.6 Hz,
each 1H, benzoxazole CH), 7.90 (dd, J=8.0 and 15.0 Hz, 1H, CH=
CH2), 8.30 (d, J=9.1 Hz, 1H, H-5), 8.80 ppm (s, 1H, H-2); 13C NMR
([D6]DMSO): d=44.2, 47.5, 108.6, 108.8, 109.3, 109.7, 110.6, 116.4,
121.0, 124.6, 132.8, 136.2, 142.5, 143.8, 148.2, 149.3, 159.1, 161.7,
166.0, 177.3 ppm; Anal. calcd (%) for C22H19N5O4: C 63.30, H 4.59, N
16.78, found: C 63.10, H 4.36, N 16.72.
Ethyl 1-methyl-6-nitro-4-oxo-7-[4-(2-pyridinyl)-1-piperazinyl]-1,4-
dihydro-1,8-naphthyridine-3-carboxylate (49). Compound 49 was
prepared from 47 following the procedure used for the synthesis
of compound 48 in 10% overall yield, through the intermediate di-
ethyl 2-[(methyl{5-nitro-6-[4-(2-pyridinyl)-1-piperazinyl]-2-pyridiny-
2-[4-(6-Chloro-3-nitro-2-pyridinyl)-1-piperazinyl]-1,3-benzothia-
zole (44). A mixture of 2,6-dichloro-3-nitropyridine 43 (1.0 g,
5.18 mmol),
1-(1,3-benzothyazol-2-yl)piperazine[17]
(1.4 g,
1
6.73 mmol), and Et3N (0.53 g, 6.73 mmol) in DMF (5 mL) was main-
tained at RT for 12 h. The precipitate obtained was filtered and
washed with EtOAc. The filtrate was evaporated to dryness obtain-
ing a residue which was purified by flash chromatography eluting
with Et2O/petroleum ether (20%), to give 44 (0.75 g, 39%); mp:
l}amino)methylene]malonate; mp: 182–1838C; H NMR ([D6]DMSO):
d=1.30 (t, J=7.0 Hz, 3H, CH2CH3), 3.65–3.75 (m, 8H, piperazine
CH2), 3.90 (s, 3H, CH3), 4,20 (q, J=7.0 Hz, 2H, CH2CH3), 6.65–6.70
(m, 1H, pyridine CH), 6.85 (d, J=8.6 Hz, pyridine CH) 7.50–7.60 and
8.15–8.20 (m, each 1H, pyridine CH), 8.75 (s, 1H, H-2), 8.80 ppm (s,
1H, H-5).
1
144–1458C; H NMR (CDCl3): d=3.60–3.70 and 3.75–3.85 (m, each
4H, piperazine CH2), 6.80 (d, J=8.0 Hz, 1H, H-5), 7.15 and 7.30 (t,
J=8.0 Hz, each 1H, benzothiazole CH), 7.65 and 7.70 (d, J=8.0 Hz,
each 1H, benzothiazole CH), 8.20 ppm (d, J=8.0 Hz, 1H, H-4).
Ethyl
6-amino-7-[4-(1,3-benzothiazol-2-yl)-1-piperazinyl]-1-
methyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylate (50).
A stirred solution of compound 48 (0.27 g, 0.54 mmol) in DMF
(30 mL) was hydrogenated over a catalytic amount of Raney nickel
at RT and atmospheric pressure for 1 h. The mixture was then fil-
tered over Celite, and the filtrate was evaporated to dryness to
give a residue which was purified by flash chromatography eluting
with MeOH/CHCl3 (3%), to give 50 (0.08 g, 32%); mp: 301–3028C;
1H NMR ([D6]DMSO): d=1.30 (t, J=7.0 Hz, 3H, CH2CH3), 3.40–3.50
and 3.65–3.75 (m, each 4H, piperazine CH2), 3.85 (s, 3H, CH3), 4.20
(q, J=7.0 Hz, 2H, CH2CH3), 5,30 (bs, 2H, NH2), 7.10 and 7.30 (t, J=
8.0 Hz, each 1H, benzothiazole CH), 7.50 (d, J=7.7 Hz, 1H, benzo-
thiazole CH), 7.65 (s, 1H, H-5), 7.80 (d, J=7.7 Hz, 1H, benzothiazole
CH), 8.55 ppm (s, 1H, H-2).
1-(6-Chloro-3-nitro-2-pyridinyl)-4-(2-pyridinyl)piperazine
(45).
Compound 45 was prepared following the procedure used for the
synthesis of compound 44 (7 h), replacing 1-(1,3-benzothyazol-2-
yl)piperazine with 1-(pyridin-2-yl)piperazine and using toluene as
solvent. Compound 45 was obtained in 80% yield; mp: 128–
1298C; 1H NMR ([D6]DMSO): d=3.45–3.55 and 3.60–3.70 (m, each
4H, piperazine CH2), 6.65–6.70 (m„1H, pyridine CH), 6.80 (d, J=
9.0 Hz, 1H, pyridine CH), 6.95 (d, J=8.4 Hz, 1H, H-5), 7.55–7.60 (m,
1H, pyridine CH) 8.15–8.20 (m, 1H, pyridine CH), 8.30 ppm (d, J=
8.4 Hz, 1H, H-4).
6-[4-(1,3-Benzothiazol-2-yl)-1-piperazinyl]-N-methyl-5-nitro-2-
pyridinamine (46). A mixture of compound 44 (0.10 g, 0.26 mmol)
in EtOH (3 mL) and a 33% solution of MeNH2 in EtOH (0.03 g,
0.39 mmol) were irradiated in a microwave oven at 758C with
three 3 min cycles. The reaction mixture was evaporated to dryness
to give a residue which was treated with Et2O to give 46 (0.098 g,
99%); mp: 180–1818C; H NMR ([D6]DMSO): d=2.90 (d, J=4.0 Hz,
3H, CH3), 3.50–3.55 and 3.60–3.70 (m, each 4H, piperazine CH2),
6.00 (d, J=9.0 Hz, 1H, H-5), 7.05 and 7.30 (t, J=8.0 Hz, each 1H,
Ethyl 6-amino-1-methyl-4-oxo-7-[4-(2-pyridinyl)-1-piperazinyl]-
1,4-dihydro-1,8-naphthyridine-3-carboxylate (51). Compound 51
was prepared from 49 following the procedure used for the syn-
thesis of compound 50 in 81% yield; mp: 276–2778C; 1H NMR
([D6]DMSO): d=1.30 (t, J=7.0 Hz, 3H, CH2CH3), 3.40–3.50 and 3.65–
3.75 (m, each 4H, piperazine CH2), 3.85 (s, 3H, CH3), 4.20 (q, J=
7.0 Hz, 2H, CH2CH3), 5,30 (bs, 2H, NH2), 6.70 (t, J=6.5 Hz, pyridine
CH), 6.80 (d, J=8.8 Hz, pyridine CH) 7.55 (t, J=7.5 Hz, pyridine CH),
1
ChemMedChem 2011, 6, 1249 – 1257
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1255

O. Tabarrini et al.
MED
7.70 (s, 1H, H-5), 8.15–8.20 (m, 1H, pyridine CH), 8.55 ppm (s, 1H,
H-2).
puter-controlled photometer (Safire, Tecan), at two wavelengths
(540 and 690 nm). All data were calculated using the median OD
(optical density) value of three wells. The 50% cytotoxic concentra-
tion (CC50) was defined as the concentration of the test compound
that reduced the absorbance (OD540) of the mock-infected control
sample by 50%. The concentration achieving 50% protection from
the cytopathic effect of the virus in infected cells was defined as
the 50% effective concentration (EC50).
6-Amino-7-[4-(1,3-benzothiazol-2-yl)-1-piperazinyl]-1-methyl-4-
oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylic acid (12). Com-
pound 12 was prepared from intermediate 50 by Method C (12 h).
After cooling, the reaction mixture was filtered, the solid was sus-
pended in 2n HCl, and stirred for ~1 h. The precipitate obtained
was then filtered and washed with Et2O to give 12 in 76% yield;
mp 337–3388C; 1H NMR ([D6]DMSO): d=3.55–3.65 and 3.75–3.85
(m, each 4H, piperazine CH2), 4.00 (s, 3H, NCH3), 5.80 (bs, 2H, NH2),
7.10 and 7.30 (t, J=7.0, each 1H, benzothiazole CH), 7.50 (d, J=
7.5 Hz, 1H, benzothiazole CH), 7.70 (s, 1H, H-5), 7.80 (d, J=7.5 Hz,
1H, benzothiazole CH), 8.90 (s, 1H, H-2), 15.90 ppm (bs, 1H,
COOH); 13C NMR ([D6]DMSO): d=39.4, 46.8, 48.0, 107.3, 114.5,
116.9, 119.0, 121.7, 121.8, 126.5, 130.6, 136.2, 141.5, 146.3, 152.4,
153.4, 167.0, 168.6, 176.9 ppm; Anal. calcd (%) for C21H20N6O3S: C
57.79, H 4.62, N 19.25, found: C 58.09, H 4.48, N 19.07.
Acknowledgements
These studies were supported financially by the MIUR (PRIN 2008,
Rome, Italy) and K.U. Leuven (GOA10/14). S.M. is supported by a
postdoctoral fellowship from the Fondazione Cassa di Risparmio
(Perugia, Italy). We thank Mr. Roberto Bianconi (Department of
Chemistry and Technology of Drugs, Perugia), Ms. Kristien Erven,
Ms. Cindy Heens, and Mr. Kris Uyttersprot (Rega Institute for Med-
ical Research, Leuven, Belgium) for their excellent technical assis-
tance.
6-Amino-1-methyl-4-oxo-7-[4-(2-pyridinyl)-1-piperazinyl]-1,4-di-
hydro-1,8-naphthyridine-3-carboxylic acid (13). Compound 13
was prepared from intermediate 51 by Method C (12 h). After cool-
ing, the reaction mixture was filtered, the solid was suspended in
2n HCl, and stirred for ~1 h. The precipitate obtained was filtered
and washed with Et2O to give 13 in 78% yield after crystallization
by DMF: mp 297–2988C; 1H NMR ([D6]DMSO): d=3.60–3.70 and
3.80–3.90 (m, each 4H, piperazine CH2), 4.00 (s, 3H, CH3), 5.70 (bs,
2H, NH2), 6.90 (t, J=6.4 Hz, pyridine CH), 7.40 (d, J=9.1 Hz, 1H,
pyridine CH), 7.70 (s, 1H, H-5), 7.95 (t, J=6.4 Hz, pyridine CH), 8.05
(d, J=5.5 Hz, 1H, pyridine CH), 8.55 (s, 1H, H-2), 15.90 ppm (bs,
1H, COOH); 13C NMR ([D6]DMSO): d=31.1, 45.4, 46.4, 107.2, 112.4,
113.3, 114.6, 116.8, 135.9, 138.4, 141.5, 143.5, 146.5, 153.2, 159.1,
166.9, 176.9 ppm; Anal. calcd (%) for C19H20N6O3: C 59.99, H 5.30, N
22.09, found: C 60.13, H 5.15, N 22.07.
Keywords: 1,6-naphthyridones · anti-HIV quinolones · antiviral
agents · medicinal chemistry · transcription inhibitors
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1256
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Received: February 8, 2011
Revised: April 1, 2011
Published online on May 12, 2011
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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