Studies on 6-aminoquinolones: Synthesis and antibacterial evaluation of 6-amino-8-ethyl- and 6-amino-8-methoxyquinolones

Article abstract of DOI:10.1016/S0968-0896(99)00207-2

From our quantitative structure-activity relationship (QSAR) study on a large set of 6-aminoquinolones, which indicated that a group larger than methyl could be allocated at C-8 position, we have synthesized two new series of 6-aminoquinolones characteriz

Full text of DOI:10.1016/S0968-0896(99)00207-2

Bioorganic & Medicinal Chemistry 7 (1999) 2465±2471
Studies on 6-Aminoquinolones: Synthesis and Antibacterial
Evaluation of 6-Amino-8-ethyl- and 6-Amino-8-methoxyquinolones
Violetta Cecchetti, a Oriana Tabarrini, a Stefano Sabatini, b Hua Miao, a
Enrica Filipponi a and Arnaldo Fravolini a,*
a
Á
Istituto di Chimica e Tecnologia del Farmaco, Universita di Perugia, 06123 Perugia, Italy
bMediolanum Farmaceutici, via S. G. Cottolengo 31, 20143 Milano, Italy
Received 2 March 1999; accepted 1 June 1999
AbstractÐFrom our quantitative structure±activity relationship (QSAR) study on a large set of 6-aminoquinolones, which indi-
cated that a group larger than methyl could be allocated at C-8 position, we have synthesized two new series of 6-aminoquinolones
characterized by the presence of an ethyl or a methoxy group at C-8 position. The antibacterial evaluation shows that, while the 8-
ethyl derivatives were devoid of any antibacterial activity, the introduction of methoxy group gave compounds with good anti-
bacterial activity, especially against Gram-positive bacteria. A tentative explanation of the di€erent behaviours among the 8-sub-
stituted analogues is given taking into account both the length and electronic properties of the C-8 groups. # 1999 Elsevier Science
Ltd. All rights reserved.
Introduction
to verify whether a larger group could be allocated at C-8,
two new series of 6-aminoquinolones 2 and 3, bearing an
8-ethyl or 8-methoxy group, respectively, were synthesized
and tested (Fig. 1). In addition, all of the new compounds
bear a cyclopropyl group at N-1 position, as well as N-
methylpiperazine, tetrahydroisoquinoline, or piperidine,
as C-7 side chain, as optimal substituents indicated by
QSAR study.
Based on indications from chemometric study,1,2 we
recently developed a new class of antibacterial quino-
lones characterized by an amino group instead of the
typical ¯uorine atom at C-6 position.3,4 In this class of
6-aminoquinolones, compounds 1 (Fig. 1) bearing a
methyl group at C-8 position show the highest anti-
bacterial activity, particularly against Gram-positive
bacteria, including methicillin- and cipro¯oxacin-resistant
Staphylococcus aureus.4,5 In order to optimize this pro-
mising class we carried out a quantitative structure±
activity relationship (QSAR) study on a large set of 6-
aminoquinolones based on principal component analysis
(PCA), projection onto latent structures (PLS), and
response surface (RS).6 The information contained in the
response surface model, gave clear information about N-1
and C-7 substituents, while that for C-8 position was less
clear. In fact, a large group, such as methyl, was indicated
as optimum at C-8, but this information is a biased indi-
cation since the methyl group was the largest substituent
considered in the set of analyzed molecules.6 In fact, the
quadratic model indicated an open range for the C-8 sub-
stituent and perhaps a larger substituent could provide an
even greater increase in antibacterial activity. Therefore,
Figure 1. For R7 substituent see Scheme 2.
Chemistry
To prepare both series of 6-amino-8-substituted quino-
lones the usual intramolecular nucleophilic displacement
cyclization method was used, as illustrated in Scheme 1.
Thus, starting from 2,6-dichloroethylbenzene (5), pre-
pared by standard procedure, or from commercial 2,6-
dichloro¯uorobenzene, the nitroacid derivatives 8 and
Key words: 6-Des¯uoroquinolones; 6-aminoquinolones; anti-
bacterials.
* Corresponding author.
0968-0896/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(99)00207-2

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V. Cecchetti et al. / Bioorg. Med. Chem. 7 (1999) 2465±2471
Biological Assay
The 6-amino-8-substituted quinolone acids prepared for
this study (2a±c and 3a±c) were tested for antibacterial
activity against an assortment of eight Gram-positive
and 10 Gram-negative organisms, including some clin-
ical isolates as well as cipro¯oxacin-resistant Escherichia
coli and Staphylococcus aureus strain and methicillin-
resistant S. aureus (MRSA). In addition, the control
drug cipro¯oxacin (CPX) and 6-amino-8-methyl counter-
parts (1a±c) (Fig. 1), previously reported by us,4 are
included for comparative purposes. The minimun inhibi-
tory concentrations (MICs, mg/mL) were determined by
microdilution technique using nutrient broth, according
to NCCLS;7 the data are presented in Table 1. The geo-
metric means of the MICs were also calculated to facilitate
a comparison of activity.
8-Methoxy-7-tetrahydroisoquinolinyl derivative 3b, as the
most active compound, and corresponding 8-ethyl deriva-
tive 2b, were also tested for their ability to inhibit the
supercoiling of DNA gyrase, using a previously described
protocol.8 This assay measures the conversion of relaxed
closed-circular pUC8 (prepared from E. coli strain
K12JM83) into the supercoiled form by the intervention of
DNA gyrase. The A and B subunits of E. coli DNA gyrase
were puri®ed from over-expression vector strains N4186
and MK47, respectively. The IC50 values were calculated
by the quantitative measurement of the supercoiled DNA
peak in an agarose gel by densitometric assay.
Results and Discussion
Scheme 1. Reagents: (i) Br2, CHCl3, 5ꢀC; (ii) NaBH4, DMSO, 60ꢀC;
(iii) CH3COCl, AlCl3; (iv) 90% HNO3, 98% H2SO4; (v) SOCl2 re¯ux;
(vi) (CH3)2NCHˆCHCO2C2H5, Et3N, toluene, 90ꢀC; (vii) c-PrNH2,
EtOH/Et2O; (viii) KF or K2CO3, DMF, 160ꢀC.
The MIC values reported in Table 1 indicate that the
introduction of ethyl group at C-8 position causes a
marked decrease in antibacterial activity, while with a
methoxy group at C-8 position, the resulting com-
pounds maintain good activity. Compared with the 8-
methyl counterparts 1a±c, the compounds are about one
dilution less potent but have the same trend: the (1-
methyl)piperazinyl derivative 3a is more potent against
Gram-negative bacteria, while the 1,2,3,4-tetra-
hydroisoquinolinyl derivative 3b and piperidinyl deri-
vative 3c are more potent against Gram-positive. It is
noteworthy that, against Gram-positive, compound 3c
is as potent as CPX, and compound 3b is 5 times more
potent than CPX with a MIC value against methicillin-
resistant S. aureus of 0.016 mg/mL.
9, respectively, were prepared via Friedel±Crafts acet-
ylation and oxidative nitration. These intermediates
were converted to the quinolone esters 12 and 13 via the
(dimethylamine)acrylate adducts 10 and 11, corre-
sponding cyclopropyl enamines and ring closure in the
presence of KF and/or K2CO3.
The desired 6-amino-8-ethyl target acids 2a,c were then
synthesized, as illustrated in Scheme 2, by replacing the
C-7 chlorine atom of 12 with N-methylpiperazine or
piperidine followed by catalytic reduction and acid
hydrolysis. To obtain 1,2,3,4-tetrahydroisoquinolinyl
target acid 2b, it was more convenient to carry out the
hydrolysis step before the nucleophilic reaction.
The DNA gyrase inhibitory activity evaluated for all 6-
aminoquinolones having
a
tetrahydroisoquinolinyl
group at C-7 position gave an IC50 value of 65 mg/mL
for 8-ethyl derivative 2b, an IC50 value of 11.3 mg/mL
for the 8-methoxy analogue 3b, and an IC50 value of
3.6 mg/mL for the 8-methyl counterpart 1b (CPX
IC50=0.68 mg/mL) which con®rmed the trend observed
in the MIC values.
Similarly, starting from the 8-¯uoro key intermediate 13,
the desired 6-amino-8-methoxy target acids 3a±c were
obtained, as illustrated in Scheme 2, following four
sequential steps: nucleophilic displacement of C-6
chlorine atom with the requisite side chain, acid hydro-
lysis, additional nucleophilic reaction with MeONa in
MeOH/DMF in order to replace the C-8 ¯uorine atom
with the desired methoxy group, and ®nally catalytic
reduction of the nitro group.
The fact that the presence of 8-methoxy group in the 6-
aminoquinolone class gives compounds with good anti-
bacterial activity, particularly against Gram-positive, is in
agreement with what has already been reported in the

V. Cecchetti et al. / Bioorg. Med. Chem. 7 (1999) 2465±2471
2467
Scheme 2. Reagents: (i) R7H, CH3CN, re¯ux; (ii) H2, Raney-Ni, DMF or CH3OCH2CH2OH; (iii) 6 N HCl, EtOH, re¯ux; (iv) CH3ONa, CH3OH/
DMF, 80ꢀC.
Table 1. Comparative in vitro antibacterial activity for 6-amino-8-ethyl (2) and 6-amino-8-methoxyquinolones (3), prepared in this study, and 8-
methyl analogues (1)
Organismsa
(MICs, mg/mL)
1a
2a
3a
1b
2b
3b
1c
2c
3c
CPXb
Gram-positives
S. au. ATCC 29213
S. au. MPR 5
S. au. M-Rc POMM 6214
S. au. CPX-Rd OBT 687
S. py. OMNFI BI
S. pn. I 043
1
1
4
>16
>16
>16
>16
>16
>16
>16
>16
2
1
4
ꢁ0.016
ꢁ0.016
ꢁ0.016
2
4
4
4
ꢁ0.016
ꢁ0.016
ꢁ0.016
2
0.25
4
0.06
ꢁ0.016
0.125
8
0.5
2
0.5
8
8
16
>16
>16
>16
>16
8
0.06
0.03
0.125
0.5
0.25
1
>16
>16
>16
>16
>16
>16
16
>16
4
8
2
4
8
16
16
8
ꢁ0.016
4
4
4
2
0.5
1
1
0.5
0.25
0.25
E. fe. LEP Br
E. fe. UCMC 39690
0.5
0.125
0.125
0.5
1
Geometric means
3.364
>16
6.169
0.075
6.169
0.164
0.385
12.338
0.908
0.917
Gram-negatives
E. co. ATCC 25922
E. co. 120
0.03
0.06
>16
>16
>16
>16
>16
>16
>16
>16
>16
>16
>16
0.125
0.125
>16
0.5
4
>16
0.08
1
>16
ꢁ0.016
0.125
0.125
>16
0.5
>16
16
0.25
0.5
>16
2
8
>16
0.25
16
8
0.125
0.5
>16
1
4
>16
16
16
0.5
0.5
>16
4
8
>16
0.08
8
>16
ꢁ0.016
ꢁ0.016
8
>16
>16
>16
>16
>16
16
E. co. CPX-Rd OBT 431
E. cl. OMNFI 174
P. mi. OBT 505
P. vu. CNUR 6
K. pn. ATCC 10031
P. ae. ATCC 9027
P. ae. OBT 307
H. in.
>16
>16
>16
>16
4
>16
>16
8
0.125
1
8
0.03
2
8
0.03
ꢁ0.016
0.06
0.5
8
0.03
8
ꢁ0.016
ꢁ0.016
>16
8
4
16
0.06
1
>16
ꢁ0.016
2
0.03
ꢁ0.016
0.03
ꢁ0.016
Geometric means
0.459
>16
0.834
0.998
12.126
1.858
1.005
12.996
1.905
0.083
a
Organisms selected are as follows: S. au., Staphylococcus aureus; S. py., Streptococcus pyogenes; S. pn., Streptococcus pneumoniae; E. fe., Enter-
ococcus faecalis; E. co., Escherichia coli; E. cl., Enterobacter cloacae; P. mi., Proteus mirabilis; P. vu., Proteus vulgaris; K. pn., Klebsiella pneumoniae;
P. ae., Pseudomonas aeruginosa; H. in., Haemophilus in¯uenzae.
CPX=cipro¯oxacin.
M-R=methicillin-resistant.
CPX-R=cipro¯oxacin-resistant.
b
c
d

2468
V. Cecchetti et al. / Bioorg. Med. Chem. 7 (1999) 2465±2471
¯uoroquinolone class. In fact, some 6-¯uoro-8-methoxy
derivatives are in clinical use or preclinical phase (e.g.
gati¯oxacin,9 balo¯oxacin,10 and moxi¯oxacin11).
to a solution of 2,6-dichlorostyrene (5.0 g, 29 mmol) in
CHCl3 (15 mL) maintained at 5ꢀC, and the mixture was
allowed to react at room temperature for 30 min. The
mixture was then washed with a solution of saturated
Na2S2O3 to eliminate the bromine excess, and the organic
layer was dried, and evaporated to dryness. The whitish
semisolid 4 (9.46 g, 98%) was used without further pur-
Turning our attention to 6-aminoquinolones, the sim-
plest explanation for the di€erent behaviour in the three
series (1, 2, and 3) could be due to di€erent steric size of
the substituents at C-8 which can a€ect the conforma-
tional orientation of the groups in the N-1 and C-7
positions. The presence of CH3, C2H5 and OCH3
groups at C-8 position in reality did not cause di€erent
conformational orientations of the substituents at N-1
and C-7 positions. In fact, the graphic analysis of the
molecular structures12 (data not shown) shows how the
width of the atom bonded at C-8 of the aromatic ring is
almost identical for the three considered groups, such
that the conformations of the three series of derivatives
can almost be superimposed. Therefore, the dis-
criminating factor for the activity could be the sub-
stituent length. This hypothesis could well explain the
drastic decrease in activity of the 8-ethyl derivatives (2a±
c) when compared with the shorter methyl (1a±c) but
does not explain the good activity of the 8-methoxy
derivative (3a±c), given that the methoxy group is as
long as the ethyl one. This apparent contradiction could
be explained by considering the di€erent electronic
properties of the ethyl and methoxy groups. If it is true
that the increased length of the substituents causes a
decrease in activity, the presence of an oxygen atom
bonded at C-8 may provide the electronic requirement
that compensates, in part, for the loss of activity. The
oxygen lone pair of the C-8 bonded group may interact
with the p electron of the aromatic ring of quinolone
thereby a€ecting the quinolone staking, as has been
suggested in the various proposed modes of quinolone/
DNA/DNA-gyrase interactions.13,14
1
i®cation in the subsequent reaction: H NMR (CDCl3) d
4.00 (1H, dd, J ˆ 8 and 16 Hz, CH2), 4.72 (1H, t, J ˆ 16
Hz, CHBr), 6.08 (1H, d, J ˆ 8 and 16 Hz, CH2), 7.15±7.45
(3H, m, aromatic H).
1,3-Dichloro-2-ethylbenzene (5). A solution of 4 (5 g, 15
mmol) in dry DMSO (10 mL) was added to a suspen-
sion of NaBH4 (2.3 g, 60 mmol) in dry DMSO (20 mL)
maintained at 60ꢀC and the mixture was allowed to
react for 40 min. After cooling, the reaction mixture was
poured into ice±water and extracted several times with
cyclohexane. The combined organic layers were dried
and evaporated to dryness to give 5 (1.65 g, 63%) as a
1
clear oil: H NMR (CDCl3) d 1.20 (3H, t, J ˆ 7 Hz,
CH2CH3), 2.90 (2H, q, J ˆ 7 Hz, CH2CH3), 6.95 (1H, t,
J ˆ 9 Hz, H-5), 7.20 (2H, d, J ˆ 9 Hz, H-4 and H-6).
1-(2,4-Dichloro-3-ethylphenyl)ethanone (6). Some drops
of acetyl chloride were added to a mixture of 2 (1 g, 5.71
mmol) and AlCl3 (1.52 g, 11.42 mmol) which was heated
to 40ꢀC to trigger the reaction (to generate HCl). After
cooling to room temperature, acetyl chloride (0.45 g,
5.71 mmol) was added dropwise and the reaction mixture
was allowed to react for 2 h. The mixture was poured into
ice±water and acidi®ed with 2 N HCl. The solution was
extracted several times with CH2Cl2 and the combined
organic layers were washed with water, dried, and evapo-
rated to dryness and the residue was puri®ed by column
chromatography eluting with petroleum ether to give 6
1
(0.82 g, 67%) as a yellow oil: H NMR (CDCl3) d 1.20
(3H, t, J ˆ 7 Hz, CH2CH3), 2.60 (3H, s, CH3), 3.00 (2H,
q, J ˆ 7 Hz, CH2CH3), 7.20 (1H, d, J ˆ 9:5 Hz, H-5), 7.35
(1H, d, J ˆ 9:5 Hz, H-6). Anal. (C10H10Cl2O) C, H.
Experimental
Thin layer chromatography (TLC) was performed on
precoated sheets of silica gel 60F254 (Merck) and visua-
lized by using UV. Column chromatography separations
were carried out on Merck silica gel 40 (mesh 70±230).
Melting points were determined in capillary tubes (Buchi
melting point apparatus) and are uncorrected. Elemental
analyses were performed on a Carlo Erba elemental ana-
lyzer, Model 1106, and the data for C, H and N are within
1-(2,4-Dichloro-3-¯uorophenyl)ethanone (7). The title
compound was prepared starting from 1,3-dichloro-2-
¯uorobenzene, using the procedure as described for 6,
except that the reaction mixture was heated at 120ꢀC for
2 h and when the mixture was poured in water, com-
pound 7 was obtained as a white solid in 61% yield: mp
146±147ꢀC; 1H NMR (CDCl3) d 2.60 (3H, s, CH3),
7.20±7.40 (2H, m, H-5 and H-6). Anal. (C8H5Cl2FO) C, H.
1
0.4% of the theoretical values. H NMR spectra were
recorded at 200 MHz (Bruker AC-200) with Me4Si as
internal standard and chemical shifts are given in ppm (d).
The spectral data are consistent with the assigned struc-
tures. Reagents and solvents were purchased from com-
mon commercial suppliers and were used as received.
Organic solutions were dried over anhydrous Na2SO4 and
concentrated with a Buchi rotary evaporator at low pres-
sure. Yields were of puri®ed product and were not opti-
mized. All starting materials were commercially available
unless otherwise indicated.
2,4-Dichloro-3-ethyl-5-nitrobenzoic acid (8). 90% HNO3
(1.22 g, 19.2 mmol) was added to a solution of 6 (1.04 g,
4.8 mmol) in 98.5% H2SO4 (10.5 mL) maintained at
20ꢀC. After heating for 45 min at 50±60ꢀC, the mixture
was poured in ice±water and the resulting precipitate was
®ltered o€, solubilized in diethyl ether and extracted with
5% NaOH solution. After treatment of this solution with
2 N HCl a precipitate was obtained which was ®ltered o€,
washed with water, and dried to give 8 (0.89 g, 70.5%) as a
white solid: mp 136±139ꢀC; 1H NMR (CDCl3) d 1.30 (3H,
t, J ˆ 7 Hz, CH2CH3), 3.20 (2H, q, J ˆ 7 Hz, CH2CH3),
8.25 (1H, s, H-6), 9.10 (1H, bs, COOH). Anal. (C9H7Cl2
NO4) C, H, N.
1,3-Dichloro-2-(1,2-dibromoethyl)benzene (4). Bromine
(5.1 g, 31.9 mmol) in CHCl3 (10 mL) was added dropwise

V. Cecchetti et al. / Bioorg. Med. Chem. 7 (1999) 2465±2471
2469
2,4-Dichloro-3-¯uoro-5-nitrobenzoic acid (9). The title
compound was prepared using the procedure as described
for 8 starting from 7. It was obtained in 70% yield as a
(DMSO-d6) d 1.10±1.35 (7H, m, cyclopropyl CH2 and
CH2CH3), 3.95±4.10 (1H, m, cyclopropyl CH), 4.25
(2H, q, J ˆ 7 Hz, CH2CH3), 8.50 (1H, s, H-5), 8.60 (1H,
s, H-2). Anal. (C15H12ClFN2O5) C, H, N.
white solid: mp 47±49ꢀC; H NMR (CDCl3) d 8.50 (1H,
1
bs, H-5), 10.50 (1H , bs, CO2H). Anal. (C7H2Cl2FNO4),
C, H, N.
6-Amino-1-cyclopropyl-8-ethyl-7-(4-methyl-1-piperazinyl)
-4-oxo-1,4-dihydroquinoline-3-carboxylic acid hydrochlo-
ride (2a). A mixture of 12 (0.30 g, 0.823 mmol), Et3N
(0.332 g, 3.3 mmol) and N-methylpiperazine (0.33 g, 3.3
mmol) in dry CH3CN (5 mL) was re¯uxed for 24 h. The
solvent was then evaporated to dryness and the residue
was triturated with Et2O to give 14a (0.283 g, 80.3%) as a
yellow solid: mp 221±223 C; H NMR (CDCl3) d 0.80±
1.30 (7H, m, cyclopropyl CH2 and CH2CH3), 1.45 (3H, t,
J ˆ 7 Hz, OCH2CH3), 2.40 (3H, s, piperazine CH3), 2.45±
2.70 and 3.10±3.30 (each 4H, m, piperazine CH2), 3.40
(2H, q, J ˆ 7 Hz, CH2CH3), 3.80±3.95 (1H, m, cyclopro-
pyl CH), 4.35 (2H, m, J ˆ 7 Hz, OCH2CH3), 8.50 (1H, s,
H-2), 8.65 (1H, s, H-5).
Ethyl 2-(2,4-dichloro-3-ethyl-5-nitrobenzoyl)-3-(dimethyl-
amino)acrylate (10). A mixture of 8 (0.6 g, 2.3 mmol)
and thionyl chloride (3 mL) was re¯uxed for 3 h. The
excess thionyl chloride was removed by distillation under
reduced pressure to give a mobile oil residue which was
dissolved in dry toluene (10 mL) and added to ethyl 3-
(dimethylamino)acrylate15 (0.427 g, 2.99 mmol) and dry
Et3N (0.35 g, 3.45 mmol). The resulting solution was
heated at 90ꢀC for 2 h. After cooling and ®ltering o€ the
insoluble material, the solvent was evaporated to dryness
and the residue was puri®ed by column chromatography
eluting with EtOAc:petroleum ether (3:7) to give 10 (0.64
g, 72%) as a pale yellow solid: mp 127±129ꢀC; 1H NMR
(CDCl3) d 0.90 (3H, t, J ˆ 7 Hz, CH2CH3), 1.20 (3H, t,
J ˆ 7 Hz, OCH2CH3), 3.00 (3H, s, NCH3), 3.10 (2H, q,
J ˆ 7 Hz, CH2CH3), 3.40 (3H, s, NCH3), 3.90 (2H, q, J ˆ
7 Hz, OCH2CH3), 7.60 (1H, s, vinyl H), 8.00 (1H , s, H-6).
Anal.(C16H18Cl2N2O5) C, H, N.
ꢀ
1
A stirred solution of 14a (0.10 g, 0.23 mmol) in 2-meth-
oxyethanol (30 mL), was hydrogenated over a catalytic
amount of Raney nickel, at room temperature and 2.05
atm pressure for 4 h. The mixture was then ®ltered over
Celite, and the ®ltrate was evaporated to dryness to give
a residue which was solubilized in EtOH:HCl 6 N (1:1)
(4 mL) and re¯uxed for 2 h. After cooling, the crystalline
precipitate was ®ltered o€, dried and crystallized by EtOH
to give 2a (0.076 g, 82%) as white solid: 299±300ꢀC;
1H NMR (DMSO-d6) d 0.80±1.25 (7H, m, cyclopropyl
CH2 and CH2CH3), 2.70 (3H, s, piperazine CH3), 3.10±
3.40 (4H, m, piperazine CH2), 3.50±3.70 (6H, m, piper-
azine CH2 and CH2CH3), 4.05±4.20 (1H, m, cyclopropyl
CH), 7.50 (1H, s, H-5), 8.70 (1H, s, H-2), 10.50 (1H, bs,
Ethyl 2-(2,4-dichloro-3-¯uoro-5-nitrobenzoyl)-3-(dimethyl-
amino)acrylate (11). The title compound was prepared
using the procedure as described for 10 starting from 9. It
was obtained in 65% yield as a yellow solid: mp 76±78ꢀC;
1H NMR (CDCl3) d 1.00 (3H, t, J ˆ 7 Hz, CH2CH3),
3.00 and 3.40 (each 3H, s, NCH3), 4.00 (2H, q, J ˆ 7 Hz,
CH2CH3), 7.80 (1H, d, J ˆ 2 Hz, H-6), 8.00 (1H, s, vinyl
H). Anal. (C14H13Cl2FN2O5) C, H, N.
.
COOH). Anal. (C20H26N4O3 HCl) C, H, N.
Ethyl 1-cyclopropyl-7-chloro-8-ethyl-6-nitro-4-oxo-1,4-di-
hydroquinoline-3-carboxylate (12). A stirred solution of
10 (0.9 g, 2.31 mmol) in EtOH (15 mL) and Et2O
(10 mL) was treated dropwise with cyclopropylamine
(0.211 g, 3.7 mmol). After 30 min at room temperature,
the mixture was evaporated to dryness to give a residue
which was solubilized in dry DMF (10 mL). KF (0.40 g,
6.9 mmol) was added to this solution and the mixture
was heated at 160ꢀC for 4 h. After cooling, the reaction
mixture was poured into ice±water, neutralized with satu-
rated solution of NaHCO3 and extracted with CHCl3. The
combined organic layers were dried and evaporated to
dryness to give a residue which was puri®ed by column
chromatography eluting with EtOAc:cyclohexane (2:8) to
give 12 (0.56 g, 61%) as a yellow solid: mp 187±189ꢀC;
1H NMR (CDCl3) d 0.90±1.00 (2H, m, cyclopropyl CH2),
1.20 (3H, t, J ˆ 7 Hz, CH2CH3), 1.25±1.35 (2H, m,
cyclopropyl CH2), 1.45 (3H, t, J ˆ 7 Hz, OCH2CH3), 3.70
(2H, q, J ˆ 7 Hz, CH2CH3), 3.90±4.05 (1H, m, cyclopro-
pyl CH), 4.40 (2H, q, J ˆ 7 Hz, OCH2CH3), 8.65 (1H, s,
H-2), 8.75 (1H , s, H-5). Anal. (C17H17ClN2O5) C, H, N.
6-Amino-1-cyclopropyl-8-ethyl-7-(1-piperidinyl)-4-oxo-
1,4-dihydroquinoline-3-carboxylic acid hydrochloride (2c).
It was obtained using the same procedure employed for
the preparation of 2a, starting from nitroester 12 and
using piperidine as nucleophile: mp 294±296ꢀC; 1H NMR
(DMSO-d6) d 0.80±0.85 (2H, m, cyclopropyl CH2), 1.05
(3H, t, J ˆ 7 Hz, CH2CH3), 1.15±1.30 (2H, m, cyclopro-
pyl CH2), 1.70±1.85 (6H, m, piperidine CH2), 3.05±3.20
(4H, m, piperidine CH2), 3.40 (2H, q, J ˆ 7 Hz,
CH2CH3), 3.80±3.95 (1H, m, cyclopropyl CH), 5.60 (2H,
bs, NH2), 7.50 (1H, s, H-5), 8.60 (1H, s, H-2). Anal.
.
(C20H25N3O3 HCl) C, H, N.
6-Amino-1-cyclopropyl-8-ethyl-7-(1,2,3,4-tetrahydro-2-
isoquinolinyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic
acid (2b). The mixture of ester 12 (0.123 g, 0.337 mmol)
in EtOH (2 mL) and 6 N HCl (2 mL) was re¯uxed for 3 h.
After cooling, the crystalline precipitate was ®ltered o€,
washed with dry EtOH, and dried to give acid 15 (0.105 g,
93%): mp 228±230ꢀC; 1H NMR (CDCl3) d 0.95±1.05 (2H,
m, cyclopropyl CH2), 1.15 (3H, t, J ˆ 7 Hz, CH2CH3),
1.20±1.30 (2H, m, cyclopropyl CH2), 3.70 (2H, q, J ˆ 7
Hz, CH2CH3), 4.20±4.35 (1H, m, cyclopropyl CH), 8.70
(1H, s, H-2), 8.90 (1H, s, H-5).
Ethyl 7-chloro-1-cycloproyl-8-¯uoro-6-nitro-4-oxo-1,4-di-
hydroquinoline-3-carboxylate (13). The title compound
was prepared using the procedure as described for 12
starting from 11 except that in the cyclization step
K2CO3 was used instead of KF. It was obtained in 65%
A mixture of acid 15 (0.12 g, 0.36 mmol) Et3N (0.216 g,
214 mmol) and 1,2,3,4-tetrahydroisoquinoline (0.24 g,
yield as a yellow solid: mp 150±152ꢀC (dec); H NMR
1

2470
V. Cecchetti et al. / Bioorg. Med. Chem. 7 (1999) 2465±2471
1.78 mmol) in dry Me3CN (5 mL) was re¯uxed for 30 h.
The solvent was then evaporated to dryness and the resi-
due was puri®ed by column chromatography eluting with
CHCl3 to give 16b (0.08 g, 52%): mp 287±291ꢀC;
1H NMR (CDCl3) d 0.90±1.30 (5H, m, cyclopropyl CH2
and CH2CH3), 1.50±1.70 (2H, m, cyclopropyl CH2), 2.90±
3.05 (2H, m, isoquinoline CH2), 3.30±3.50 (4H, m, iso-
quinoline CH2 and CH2CH3), 3.90±4.05 (1H, m, cyclo-
propyl CH), 4.55 (2H, bs, isoquinoline CH2), 7.00±7.30
(4H, m, isoquinoline CH), 8.70 (1H, s, H-2), 8.90 (1H, s,
H-5), 14.20 (1H, bs, COOH).
was evaporated to dryness and the residue was tritu-
rated with a mixture of CHCl3/Et2O to give 3c (0.13 g,
60%): mp 262±263ꢀC; 1H NMR (DMSO-d6) d 0.90±1.15
(4H, m, cyclopropyl CH2), 1.55±1.80 (6H, m, piperidine
CH2), 3.00±3.15 (4H, m, piperidine CH2), 3.60 (3H, s,
OCH3), 4.00±4.15 (1H, m, cyclopropyl CH), 5.40 (2H,
bs, NH2), 7.35 (1H, s, H-5), 8.50 (1H, s, H-2), 15.65
(1H, bs, COOH). Anal. (C19H23N3O4) C, H, N.
6-Amino-1-cyclopropyl-8-methoxy-7-(4-methyl-1-piper-
azinyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (3a).
This compound was synthesized using the same synthetic
procedure employed for the preparation of 3c, starting
from nitro ester 13 and using N-methylpiperazine as
nucleophile: mp 264±266ꢀC; 1H NMR (DMSO-d6) d
0.90±1.20 (4H, m, cyclopropyl CH2), 2.70 (3H, s, piper-
azine CH3), 3.10±3.20 and 3.40±3.65 (each 4H, m, piper-
azine CH2), 3.65 (3H, s, OCH3), 4.05±4.20 (1H, m,
cyclopropyl CH), 5.40 (2H, bs, NH2), 7.40 (1H, s, H-5),
8.55 (1H, s, H-2), 15.60 (1H, bs, COOH). Anal. (C19H24
N4O4) C, H, N.
A stirred solution of 16b (0.080 g, 0.184 mmol) in a
mixture of EtOH:DMF (9:1) (50 mL) was hydrogenated
over catalytic amount of Raney nickel at room tempera-
ture and 2.05 atm pressure for 6 h. The mixture was then
®ltered over Celite, and the ®ltrate was evaporated to
dryness. The solid residue was treated with petroleum
ether to give 2b (0.034 g, 46%) as white solid: mp 287±
291ꢀC; H NMR (DMSO-d6) d 0.90±1.10 (5H, m, cyclo-
1
propyl CH2 and CH2CH3), 1.15±1.30 (2H, m, cyclopropyl
CH2), 2.90±3.05 (2H, m, isoquinoline CH2), 3.35±3.60
(4H, m, isoquinoline CH2 and CH2CH3), 3.90±4.05 (1H,
m, cyclopropyl CH), 4.40±4.55 (2H, m, isoquinoline
CH2), 5.50 (2H, bs, NH2), 7.20±7.50 (4H, m, CH iso-
quinoline), 7.70 (1H, s, H-5), 8.70 (1H, s, H-2), 14.20
(1H, bs, COOH). Anal. (C24H25N3O3) C, H, N.
6-Amino-1-cyclopropyl-8-methoxy-7-(1,2,3,4-tetrahydro-
2-isoquinolinyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic
acid (3b). This compound was synthesized using the
same synthetic procedure employed for the preparation
of 3c, starting from nitro ester 13 and using 1,2,3,4-
tetrahydroisoquinoline as nucleophile: mp 258±260ꢀC;
1H NMR (DMSO-d6) d 0.90±1.20 (4H, m, cyclopropyl
CH2), 2.95±3.05 and 3.40±3.55 (each 2H, m, isoquino-
line CH2), 3.70 (3H, s, OCH3), 4.30 (2H, bs, isoquino-
line CH2), 4.05±4.20 (1H, m, cyclopropyl CH), 5.40
(2H, bs, NH2), 7.05±7.25 (4H, m, isoquinoline aromatic
H), 7.40 (1H, s, H-5), 8.55 (1H, s, H-2), 15.55 (1H, bs,
COOH). Anal. (C23H23N3O4) C, H, N.
6-Amino-1-cyclopropyl-8-methoxy-7-(1-piperidinyl)-4-
oxo - 1,4 - dihydroquinoline - 3 - carboxylic acid (3c). The
mixture of ester 13 (1.59 g, 4.64 mmol) and piperidine
(1.8 mL, 18.5 mmol) in dry MeCN (45 mL) was re¯uxed
for 2 h. After cooling the precipitate solid was ®ltered,
washed with Et2O, suspended in EtOH:HCl 6 N (1:1)
(20 mL) and re¯uxed for 2 h. After cooling, the pre-
cipitate was ®ltered, washed with water, then with Et2O
and dried to give 17c (0.96 g, 55%): mp 237±238ꢀC;
1H NMR (DMSO-d6) d 1.15±1.30 (4H, m, cyclopropyl
CH2), 1.25±1.55 (6H, m, piperidine CH2), 3.10±3.25
(4H, m, piperidine CH2), 4.05±4.20 (1H, m, cyclopropyl
CH), 8.45 (1H, d, J ˆ 2 Hz, H-5), 8.70 (1H, s, H-2),
14.20 (1H , bs, COOH).
Acknowledgment
We thank the Italian funding agency of MURST for
®nancial support.
References and Notes
A solution of MeONa (0.45 g, 8.4 mmol) in dry DMF (8
mL) and dry MeOH (5 mL) was added to a suspension
of 8-¯uoro derivative 17c (0.47 g, 1.2 mmol) in dry
DMF (20 mL). The mixture was heated to 80±90ꢀC for
40 h, then cooled and poured into ice±water. The solu-
tion was made acid with 2 N HCl, and extracted with
EtOAc. The combined organic layers were dried, eva-
porated to dryness and the residue obtained was crys-
tallized by acetone to give 18c (0.2 g, 43.5%) as a white
solid: mp 224±225ꢀC; 1H NMR (CDCl3) d 0.90±1.05
and 1.20±1.35 (each 2H, m, cyclopropyl CH2), 1.70±1.85
(6H, m, piperidine CH2), 3.20±3.35 (4H, m, piperidine
CH2), 3.80 (3H, s, OCH3), 3.95±4.10 (1H, m, cyclopro-
pyl CH), 8.50 (1H, s, H-5), 8.85 (1H, s, H-2), 14.30 (1H,
bs, COOH).
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