First Total Synthesis of a Cytotoxic Derivative of the Natural Product Aaptamine

Article abstract of DOI:10.1055/s-0036-1588752

A synthetic sequence to the benzonaphthyridinone framework is described. The key step is a one-pot, base-catalyzed vicarious nucleophilic substitution followed by ring closure. Additionally, the synthesis represents the application of a vicarious nucleoph

Full text of DOI:10.1055/s-0036-1588752

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2017, 49, A–G
paper
A
S. Puvvala et al.
Paper
Synthesis
First Total Synthesis of a Cytotoxic Derivative of the Natural
Product Aaptamine
Srinu Puvvalaa,b
Vinod D. Jadhav*a
O
Umesh C. Narkhedea
M. V. V. S. R. N. Anji Karuna
Ch. Venkata Ramana Reddyb
H2N
Vicarious nucleophilic
substitution
N
O
N
a Chemistry Services, GVK Biosciences Pvt. Ltd., IDA Nacharam,
Hyderabad 500076, India
O
Intramolecular cyclization
Classical C–H activation
&
&
vinod.jadhav@gvkbio.com
vinodjadhav1977@gmail.com
b Department of Chemistry, Jawaharlal Nehru Technological
University, Kukatpally, Hyderabad 500085, India
Riley oxidation
Krapcho decarboxyaltion
Received: 02.01.2017
Accepted after revision: 24.02.2017
Published online: 17.03.2017
Among recently isolated aaptamines6 1, ii, iiia–d and
iva–c, compound ii possesses a piperidinyl group fused to a
aaptamine moiety, whereas compounds iiia–d share an im-
idazole-fused 1H-benzo[de][1,6]naphthyridin-2(4H)-one
skeleton (Figure 2).
To the best of our knowledge, there are no reports de-
scribing the synthesis of any of these compounds to date in
the literature.7 The presence of a novel isoquinoline skele-
ton and its potent cytotoxic activity encouraged us to look
for an efficient synthetic pathway toward the synthesis of
derivative 1.
DOI: 10.1055/s-0036-1588752; Art ID: ss-2017-t0004-op
Abstract A synthetic sequence to the benzonaphthyridinone frame-
work is described. The key step is a one-pot, base-catalyzed vicarious
nucleophilic substitution followed by ring closure. Additionally, the syn-
thesis represents the application of a vicarious nucleophilic substitution
in the total synthesis of a cytotoxic aaptamine derivative.
Key words total synthesis, heterocycles, vicarious aromatic substitu-
tion, cyclization, alkylation
Looking for a rapid and versatile way to synthesize ben-
zonaphthyridinones, we envisioned a retrosynthetic analy-
sis following different disconnections as shown in (Scheme
1)
Marine sponges have received a lot of attention from re-
searchers, and are known to be a prolific source of novel
chemicals with encouraging therapeutic potentials.1 An an-
cestor aaptamine,2 aaptamine I was isolated for the first
time in 1981 by the Nakamura group from the marine
sponge Aaptos aaptos.3 Over the following years, aaptamine
I and its congeners have been found in many other sponges
(Class: Desmospongiae) from the Indian Ocean and the Red
Sea, off the coasts of Australia, Brazil, Indonesia, and Malay-
sia, from the Caribbean Sea and from a South China Sea
sponge. In particular, the genus Aaptos continues to be an
abundant source of aaptamine alkaloids, which still spurs
interest in finding new bioactive metabolites. Aaptamines
and several members of the class (polycyclic quinones and
hydroquinones) show a broad range of biological activities
including antioxidant,4a enzymatic inhibition,4b antiviral,4c
antimicrobial,4d antifungal,4e cytotoxic,4f and antidepres-
sant.4g Among these, anticancer effects have been the most
frequently reported for aaptamines.4h–m
The crucial steps in our strategy are a vicarious nucleo-
philic substitution (VNS) followed by an intramolecular es-
ter–amine coupling to construct the cyclic diketone inter-
mediate 23. The key substrate 20 for VNS cyclization can be
obtained from chloroisoquinoline 14, which in turn can be
prepared from amide 7. Amide 7 can be synthesized from
functionalized 4-nitrobenzamide 5. Thus, our first aim was
to synthesize amide 5, which can be easily prepared from
commercially available 4-bromo-3-methylmethoxyben-
zene (2) (Scheme 2).
HN
HN
C
C
H3CO
H3CO
H3CO
H3CO
N
N
A
A
B
B
The reported syntheses of aaptamine5 use either the
isoquinoline (AB) or quinoline (AC) components of the ben-
zo[de][1,6]naphthyridine ring as a platform to build the
third ring (Figure 1).
Figure 1 Reported syntheses to construct the AB and AC ring systems
of aaptamine
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

B
S. Puvvala et al.
Paper
Synthesis
H2O2/Na2CO3 to give amide 5. Treatment of 5 with dimeth-
ylformamide–dimethyl acetal (DMF–DMA) gave amidine 6
in good yield. However, the base-catalyzed cyclization of 6
to afford 7 was not encouraging due to the very low conver-
sion and poor yields. Attempted optimization of the reac-
tion by screening a number of strong bases, viz. NaOMe,
NaH, LiHMDS and KOt-Bu proved unsuccessful.
O
O
O
O
O
N
H2N
N
HN
O
N
N
O
N
N
NH
ii
O
R1
2
iii
Quickly, we changed our synthetic approach as shown
in Scheme 3. The commercially available nitrobenzoic acid
8 on nitration gave dinitro compound 9. The selective dis-
placement of the para-nitro group with MeOH under basic
conditions afforded compound 10, which was further con-
verted into a methyl ester using SOCl2/MeOH. The obtained
ester 118 on treatment with 1-tert-butoxy-N,N,N′,N′-te-
tramethylmethanediamine afforded enamine 12. The sub-
sequent treatment of enamine 12 with electron-rich 2,4-di-
methoxybenzylamine afforded the cyclized product 13. The
TFA-mediated debenzylation of intermediate 13 afforded
the required intermediate 7, which was further treated
with phosphorous(V) oxychloride to give the key chloroiso-
quinoline 14 in good yield.9,10
iiia R1
iiib R1
=
=
O
iva R2
ivb R2
=
=
N
H
O
N
N
H
N
iiic R1
iiid R1
=
=
R2
iii
ivc R2
=
O
N
H
O
O
Figure 2 Structures of recently isolated aaptamine derivatives6
4-Bromo-3-methylmethoxybenzene (2) underwent ni-
tration in the presence of concentrated HNO3 and Ac2O to
give nitrobromobenzene 3 in quantitative yield. The cop-
per-catalyzed SNAr displacement of bromine with CuCN af-
forded cyanide 4, which was subjected to oxidation using
Initially, a straightforward synthesis of compound 18
was envisioned via the reaction sequence shown in path A
(Scheme 4). The Suzuki coupling of 14 with methyl boronic
O
O
O
O
O
one-pot VNS, cyclization,
Riley oxidation
HN
O
N
O
N
N
reduction
N
selective O-alkylation
O
O2N
N
H2N
O
O2N
O
O2N
O
N
O
1
24
23
20
C–C coupling,
Krapcho
decarboxylation
O
O
Cl
N
O2N
O2N
O
cyclization
chlorination
OH
NH
O2N
O
8
7
14
Scheme 1 Retrosynthetic analysis
O
Br
O2N
O
Br
O2N
O
CN
O2N
H2O2
NH2
CuCN
Ac2O
DMF, 140 °C
58%
Na2CO3
62%
O
98% HNO3
80%
O
2
3
4
5
85% DMF–DMA
O
O
N
O2N
O
NH
KOtBu
O2N
O
N
6
7
Scheme 2 Attempted synthesis of precursor 7
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

C
S. Puvvala et al.
Paper
Synthesis
N
O
O2N
O
CO2Me
O
O
O2N
O
CO2Me
N
O
SOCl2
MeOH
O2N
O2N
O
KOH
OH
OH
OH
H2SO4, HNO3
89%
MeOH
O2N
93%
96%
81%
O2N
N
8
9
10
11
12
H2N
OMe
O
OMe
O
Cl
O2N
O2N
O2N
POCl3
TFA
94%
OMe
95%
N
NH
N
90–110 °C
74%
O
OMe
O
O
13
7
14
Scheme 3 Synthesis of precursor 14
acid yielded compound 15, which underwent allylic bromi-
nation and displacement of the resulting bromo intermedi-
ate 16 using sodium cyanide gave cyano intermediate
17.11,12 Surprisingly, hydrolysis of cyano intermediate 17 to
give acid 18 was not effective under both acidic (HCl, THF,
50 °C) and basic (KOH, EtOH/H2O) conditions. Alternatively,
access to intermediate ester 20 via an initial Stille coupling
starting from chloroisoquinoline 14 (path B, Scheme 4) to
give compound 19 was quite low yielding, and further at-
tempted oxidations (NaIO4, RuCl3·3H2O in MeCN/CCl4/H2O
at r.t. to 60 °C) to give acid intermediate 18 were not prom-
ising. The synthetic challenge to access key intermediate 20
was finally overcome by employing a C–H activation route
(path C, Scheme 4). For the classical condensation13a of di-
ethyl malonate (DEM), different bases such as Na/EtOH,
K2CO3 in DMF, KOtBu in THF, and NaH in DMSO were
screened. Among these, Cs2CO3 in anhydrous DMF turned
out to be best yielding compound 21 in excellent yield. Fur-
thermore, Krapcho decarboxylation,13b using LiCl and
DMSO at 130 °C afforded the key ester 20 in 81% yield. Ester
20 was observed as a mixture with its exocyclic isomer (an
enamine), as was confirmed by variable temperature (VT)
1H NMR spectroscopy.14
The key one-pot conversion into compound 22 was
achieved successfully via a VNS reaction using trimethyl-
hydrazinium iodide followed by base-catalyzed intramolec-
ular cyclization. It was observed that this one-pot conver-
sion led directly to compound 22 and not the expected in-
termediate 22a, as was confirmed from HSQC and HMBC
analysis.
Riley oxidation of intermediate 22 using SeO2 in 1,4-di-
oxane15a at 90 °C for two hours gave compound 23 with low
conversion. However, moderate yields were obtained by
treatment with benzeneseleninic acid anhydride15b in dry
COOH
CN
Br
NaCN
DMF
O2N
O2N
O2N
O
MeB(OH)2
Pd(PPh3)4
O2N
O
NBS
N
N
N
N
O
O
18
17
path A
16
15
O
N
O
N
allyl tributyltin
allyl BF3K
OH
OEt
O2N
O2N
O
O2N
O
14
N
path B
O
19
18
20
Na, EtOH
DEM, 90 °C
O
O
O
O
O
I–
N+ NH2
Cs2CO3
DMF, 100 °C,16 h
LiCl
DMSO–H2O
O
O
O
HN
HN
O2N
O
O2N
O2N
O2N
O
KOtBu, DMSO
41%
130 °C
81%
N
N
N
NH
path C
92%
O
O
21
22a
22
20
Scheme 4 Synthesis of 1H-benzo[de][1,6]naphthyridin-2(4H)-one (22)
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

D
S. Puvvala et al.
Paper
Synthesis
O
OH
O
O
O
N
O
N
O
N
HN
N
N
N
EtI
Ag2CO3
benzeneseleninic
acid anhydride
Pd/C, H2 atm
EtOAc, r.t.
50%
O2N
O
NO2
O
NH2
O
NO2
O
NH
THF, 50 °C
33%
THF, r.t.
30%
22
23
24
1
Scheme 5 Synthesis of natural product 1
THF (Scheme 5).15c–e The selective O-alkylation over N-al-
kylation of compound 23 was achieved by using Ag2CO3/
ethyl iodide16 to give intermediate 24 in 30% yield. Finally,
reduction of the nitro group in intermediate 24 using Pd/C
in EtOAc under hydrogen balloon pressure gave compound
16 h. The mixture was poured into ice-cold H2O and the resulting sol-
id was collected by filtration and dried to provide 2-methyl-4,5-dini-
trobenzoic acid (9) (10 g, 89%) as an off-white solid.
IR (KBr): 3431, 3005, 1693, 1538, 1363, 1275, 1260, 764, 750 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 14.06 (br s, 1 H), 8.50 (s, 1 H), 8.22
(s, 1 H), 2.67 (s, 3 H).
13C NMR (101 MHz, DMSO-d6): δ = 165.6, 147.6, 143.0, 138.7, 135.2,
128.0, 127.0, 20.9.
1
1 in a reasonable 50% yield. H NMR and 13C NMR analyses
were identical with reported isolated natural product data.6
In conclusion, a synthetic sequence to the benzonaph-
thyridinone skeleton has been developed, which could po-
tentially open a synthetic pathway for the preparation of
variously substituted compounds with a benzonaphthyridi-
none skeleton. The key steps involved in our strategy in-
clude a vicarious nucleophilic substitution followed by a
ring closure. This approach is modular and rapid and may
potentially be extended to synthesize biologically active
natural product analogues or other useful building blocks.
MS (ESI+): m/z = 225.10 [M – H]+.
4-Methoxy-2-methyl-5-nitrobenzoic Acid (10)
2-Methyl-4,5-dinitrobenzoic acid (9) (5.0 g, 22.12 mmol) was added
to KOH (6.206 g, 110.60 mmol) in MeOH (120 mL) and the resulting
mixture was heated to 70 °C for 1.5 h. The reaction mixture was acid-
ified with 2 M HCl and the solid was collected by filtration and dried
to provide 4-methoxy-2-methyl-5-nitrobenzoic acid (10) (3.8 g, 81%)
as a pale yellow solid; mp 249–251 °C.
IR (KBr): 3430, 3005, 1683, 1610, 1275, 1260, 764, 750 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 13.15 (br s, 1 H), 8.37 (s, 1 H), 7.32
(s, 1 H), 3.62 (s, 3 H), 2.64 (s, 3 H).
13C NMR (101 MHz, DMSO-d6): δ = 166.3, 154.3, 148.1, 136.1, 127.9,
121.9, 117.0, 57.0, 21.9.
All commercially available reagents used in this study were pur-
chased from local suppliers, Sigma-Aldrich and Alfa Aesar and were
used as such. Thin-layer chromatography was performed on pre-coat-
ed TLC silica gel 60 F254 aluminum plates (Merck). TLC plates were
made visual under UV light (254 and 360 nm) or with iodine. Column
chromatography was performed using Merck silica gel (230–400
mesh). Automated purifications were accomplished using Teledyne
ISCO Combiflash companion and Grace reversal unless otherwise not-
ed; all reactions were carried out under an atmosphere of argon (un-
less otherwise stated) in dried glassware using standard techniques.
THF was distilled from Na/benzophenone. FA = formic acid. Analytical
grade solvents were used without further drying/purification. Melt-
ing points were recorded using a Buchi melting point apparatus and
are uncorrected. IR spectra (KBr pellets) were obtained using a Perkin
Elmer Spectrum 2000 FT-IR spectrometer. 1H and 13C NMR spectra
were recorded on Bruker 500 MHz, Varian 400 MHz, and Varian 300
MHz spectrometers. Chemical shifts are reported in ppm with TMS as
reference. Data are reported as follows: chemical shift, multiplicity, (s
= singlet, d = doublet, t = triplet, q = quartet, br = broad, m = multi-
plet), coupling constant(s), multiplicity. HRMS data was recorded us-
ing a Waters Q-Tof micro spectrometer under electrospray ionization
mode.
MS (ESI+): m/z = 212.16 [M + H]+.
Methyl 4-Methoxy-2-methyl-5-nitrobenzoate (11)
To a solution of 4-methoxy-2-methyl-5-nitrobenzoic acid (10) (5.0 g,
23.69 mmol) in MeOH (50 mL) was added SOCl2 (1.71 mL, 23.69
mmol) dropwise. The resulting mixture was heated to reflux for 5 h
and then allowed to cool. The solvent was removed under reduced
pressure and the residue was partitioned between CH2Cl2 (60 mL) and
sat. NaHCO3 (60 mL). The organic layer was dried over Na2SO4, con-
centrated under reduced pressure and the residue was purified by
chromatography on SiO2 (15–25% EtOAc in hexanes) to provide meth-
yl 4-methoxy-2-methyl-5-nitrobenzoate (11) (5.0 g, 93%) as a pale
yellow solid; mp 130–133 °C.
IR (KBr): 3444, 2986, 2956, 1722, 1625, 1565, 1525, 1351, 1310, 1288,
1114, 986, 757 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 8.37 (s, 1 H), 7.35 (s, 1 H), 4.00 (s, 3
H), 3.83 (s, 3 H), 2.63 (s, 3 H).
13C NMR (101 MHz, DMSO-d6): δ = 165.0, 154.5, 148.1, 136.1, 127.8,
120.6, 117.1, 57.0, 52.0, 21.6.
2-Methyl-4,5-dinitrobenzoic Acid (9)
To concd H2SO4 (50 mL) at 0 °C was added concd HNO3 (50 mL) drop-
wise and the mixture was allowed to stir at 0 °C for 10 min. Next, 2-
methyl-4-nitrobenzoic acid (8) (9.0 g, 49.71 mmol) was added por-
tionwise. The resulting reaction mixture was allowed to stir at r.t. for
HRMS (ESI+): m/z [M + H]+ calcd for C10H12NO5: 226.0715; found:
226.0717.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

E
S. Puvvala et al.
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Synthesis
Methyl (E)-2-[2-(Dimethylamino)vinyl]-4-methoxy-5-nitrobenzo-
13C NMR (101 MHz, DMSO-d6): δ = 160.8, 154.4, 143.0, 138.1, 133.1,
ate (12)
124.9, 118.5, 109.2, 103.7, 57.0.
HRMS (ESI+): m/z [M + H]+ calcd for C10H9N2O4: 221.0562; found:
221.0567.
A mixture of methyl 4-methoxy-2-methyl-5-nitrobenzoate (11) (3.0
g, 13.32 mmol) and 1-tert-butoxy-N,N,N′,N′-tetramethylmethanedi-
amine (3.3 mL, 15.99 mmol) was heated at 115 °C (no solvent) for 3.5
h. After cooling to r.t., the mixture was triturated with EtOAc/hexanes
(1:6). The solid compound was collected by filtration and dried to
provide of methyl (E)-2-[2-(dimethylamino)vinyl]-4-methoxy-5-ni-
trobenzoate (12) (3.6 g (96%) as an orange colored solid; mp 160–
163 °C.
1-Chloro-6-methoxy-7-nitroisoquinoline (14)
A solution of 6-methoxy-7-nitroisoquinolin-1(2H)-one (7) (3.0 g,
13.63 mmol) in POCl3 (40 mL) was stirred at 90–100 °C for 3 h. The
reaction mixture was cooled to r.t. and the excess POCl3 removed un-
der reduced pressure. The crude product was dissolved in CH2Cl2 (50
mL) and washed with sat. NaHCO3 (2 × 30 mL). The organic layer was
dried over Na2SO4 and concentrated under reduced pressure. The resi-
due was purified by chromatography on SiO2 (30–50% EtOAc in hex-
anes) to provide 1-chloro-6-methoxy-7-nitroisoquinoline (14) (2.4 g,
74%) as a yellow solid; mp 198–201 °C.
IR (KBr): 3444, 2950, 1708, 1630, 1590, 1544, 1315, 1260, 1109, 764,
749 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 8.40 (s, 1 H), 7.72 (d, J = 13.2 Hz, 1
H), 7.17 (s, 1 H), 6.28 (d, J = 13.2 Hz, 1 H), 3.98 (s, 3 H), 3.79 (s, 3 H),
2.98 (s, 6 H).
13C NMR (101 MHz, DMSO-d6): δ = 165.7, 155.3, 150.0, 148.3, 131.1,
130.7, 113.8, 104.2, 92.4, 56.5, 51.7.
IR (KBr): 3429, 2927, 1629, 1521, 1377, 1350, 1303, 1204, 1084, 955,
864, 722 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 8.75 (s, 1 H), 8.39 (d, J = 5.9 Hz, 1
HRMS (ESI+): m/z [M + H]+ calcd for C13H17N2O5: 281.1137; found:
H), 7.85–7.92 (m, 2 H), 4.07 (s, 3 H).
281.1156.
13C NMR (101 MHz, DMSO-d6): δ = 152.3, 150.8, 144.3, 141.7, 140.1,
2-(2,4-Dimethoxybenzyl)-6-methoxy-7-nitroisoquinolin-1(2H)-
122.9, 120.2, 119.6, 108.9, 57.3.
one (13)
HRMS (ESI+): m/z [M + H]+ calcd for C10H8N2O3Cl: 239.0223; found:
To a solution of methyl (E)-2-[2-(dimethylamino)vinyl]-4-methoxy-
5-nitrobenzoate (12) (3.0 g, 10.71 mmol) in toluene (20 mL) was add-
ed 2,4-dimethoxybenzylamine (2.90 mL, 14.72 mmol). The resulting
reaction mixture was stirred at 125 °C for 3.5 h during which the col-
or changed from deep red to yellow. After cooling to r.t., the mixture
was triturated with EtOAc/hexanes (1:2). The precipitate was collect-
ed by filtration to provide 2-(2,4-dimethoxybenzyl)-6-methoxy-7-ni-
troisoquinolin-1(2H)-one (13) (3.8 g, 95%) as a yellow solid; mp 225–
228 °C.
239.0235.
Diethyl 2-(6-Methoxy-7-nitroisoquinolin-1-yl)malonate (21)
A mixture of 1-chloro-6-methoxy-7-nitroisoquinoline (14) (2.0 g,
8.40 mmol), diethyl malonate (DEM) (2.54 mL, 16.80 mmol) and
Cs2CO3 (5.45 g, 16.80 mmol) in DMF (20 mL) was stirred at 100 °C for
6 h. After cooling to r.t., the reaction mixture was filtered, diluted
with H2O (50 mL) and extracted with EtOAc (2 × 50 mL). The com-
bined organic layer was dried over Na2SO4 and concentrated under re-
duced pressure. The residue was purified by chromatography on SiO2
(20–50% EtOAc in hexanes) to provide diethyl 2-(6-methoxy-7-ni-
troisoquinolin-1-yl)malonate (21) (2.8 g, 92%) as an off-white solid;
mp 98–101 °C.
IR (KBr): 3445, 2956, 2848, 1722, 1626, 1604, 1507, 1261, 1275, 764,
750 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 8.60 (s, 1 H), 7.57 (d, J = 7.3 Hz, 1
H), 7.49 (s, 1 H), 7.05 (d, J = 8.3 Hz, 1 H), 6.64 (d, J = 7.3 Hz, 1 H), 6.59
(d, J = 2.0 Hz, 1 H), 6.48 (dd, J = 8.3, 2.4 Hz, 1 H), 5.01 (s, 2 H), 4.01 (s, 3
H), 3.81 (s, 3 H), 3.74 (s, 3 H).
IR (KBr): 3448, 2984, 2940, 1732, 1633, 1532, 1372, 1277, 1259, 1202,
1045, 866, 764, 750 cm–1
.
13C NMR (101 MHz, DMSO-d6): δ = 160.3, 159.9, 158.0, 154.3, 141.9,
138.3, 137.0, 130.2, 125.2, 117.9, 116.5, 109.1, 104.6, 103.7, 98.4, 57.0,
55.5, 55.2, 46.6.
HRMS (ESI+): m/z [M + H]+ calcd for C19H19N2O6: 371.1243; found:
371.1233.
1H NMR (400 MHz, DMSO-d6): δ = 8.68 (s, 1 H), 8.53 (d, J = 5.9 Hz, 1
H), 7.84 (d, J = 5.5 Hz, 1 H), 7.81 (s, 1 H), 6.07 (s, 1 H), 4.20 (q, J = 7.1
Hz, 4 H), 4.06 (s, 3 H), 1.18 (t, J = 7.0 Hz, 6 H).
13C NMR (75 MHz, DMSO-d6): δ = 166.9, 154.6, 151.3, 144.2, 141.3,
138.8, 122.2, 120.3, 120.0, 108.7, 61.4, 57.0, 13.8.
HRMS (ESI+): m/z [M + H]+ calcd for C17H19N2O7: 363.1192; found:
363.2824.
6-Methoxy-7-nitroisoquinolin-1(2H)-one (7)
A solution of 2-(2,4-dimethoxybenzyl)-6-methoxy-7-nitroisoquino-
lin-1(2H)-one (13) (2.5 g, 6.75 mmol) in TFA (40 mL) was stirred at
85 °C for 2.5 h. The reaction mixture was cooled to r.t. and the TFA
was removed under reduced pressure. The crude product was rinsed
once with MeOH (40 mL) and dried under high vacuum. The resulting
solid was triturated with EtOAc (30 mL) and collected by filtration to
provide 6-methoxy-7-nitroisoquinolin-1(2H)-one (7) (1.4 g, 94%) as a
yellow solid; mp 305–308 °C.
Ethyl 2-(6-Methoxy-7-nitroisoquinolin-1-yl)acetate (20)
A solution of diethyl 2-(6-methoxy-7-nitroisoquinolin-1-yl)malonate
(21) (2.0 g, 5.52 mmol) in DMSO (20 mL), LiCl (0.245 g, 5.85 mmol)
and H2O (0.17 mL, 9.83 mmol) was stirred at 130 °C for 6 h. After
cooling to r.t., the mixture was diluted with H2O (40 mL) and extract-
ed with EtOAc (2 × 50 mL). The combined organic layer was dried over
Na2SO4 and concentrated under reduced pressure. The residue was
purified by chromatography on SiO2 (20–30% EtOAc in hexanes) to
provide ethyl 2-(6-methoxy-7-nitroisoquinolin-1-yl)acetate (20) (1.3
g, 81%) as orange solid; mp 170–173 °C.
IR (KBr): 3304, 3170, 3050, 2932, 1821, 1655, 1637, 1564, 1518, 1359,
1309, 1295, 1046, 869, 688 cm–1
.
1H NMR (400 MHz, DMSO-d6): δ = 11.47 (br s, 1 H), 8.57 (s, 1 H), 7.50
(s, 1 H), 7.36 (t, J = 6.4 Hz, 1 H), 6.58 (d, J = 6.8 Hz, 1 H), 4.02 (s, 3 H).
IR (KBr): 3445, 2989, 1629, 1609, 1275, 1260, 764, 750 cm–1
.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

F
S. Puvvala et al.
Paper
Synthesis
1H NMR (400 MHz, DMSO-d6): δ (mixture with the exocyclic isomer) =
12.11 (d, J = 3.9 Hz, 1 H), 8.79 (s, 1 H), 8.58 (s, 1 H), 8.48 (d, J = 5.9 Hz,
1 H), 7.70–7.83 (m, 2 H), 7.46 (t, J = 6.4 Hz, 1 H), 6.42 (d, J = 6.8 Hz, 1
H), 5.48 (s, 1 H), 4.38 (s, 2 H), 4.10 (qd, J = 7.1, 1.2 Hz, 6 H), 3.98 (s, 3
H), 1.05–1.28 (m, 6 H).
HRMS (ESI+): m/z [M + H]+ calcd for C12H8N3O5: 274.0464; found:
274.0466.
2-Ethoxy-8-methoxy-9-nitro-3H-benzo[de][1,6]naphthyridin-
3-one (24)
To a stirred solution of 8-methoxy-9-nitro-3H-benzo[de][1,6]naph-
thyridine-2,3-dione (23) (0.25 g, 0.91 mmol) in THF (10 mL) was add-
ed silver carbonate (0.75 g, 2.74 mmol) and ethyl iodide (0.22 mL,
2.74 mmol) at r.t. The resulting mixture was stirred at r.t. for 6 h and
then diluted with H2O (50 mL) and extracted with EtOAc (2 × 25 mL).
The combined organic layer was dried over Na2SO4, filtered and con-
centrated under reduced pressure. The residue was purified by flash
column chromatography on silica gel (100–200 mesh, 40–60% EtOAc
in hexanes) to afford compound 24 (0.083 g, 30%) as a pale yellow sol-
id; mp 134–136 °C.
1H NMR at 90 °C
1H NMR (400 MHz, DMSO-d6): δ = 8.70 (s, 1 H), 8.46 (d, J = 5.9 Hz, 1
H), 7.73 (d, J = 5.9 Hz, 1 H), 7.70 (s, 1 H), 4.34 (s, 2 H), 4.11 (q, J = 6.8
Hz, 2 H), 4.05 (s, 3 H), 1.16 (t, J = 7.1 Hz, 3 H).
13C NMR (101 MHz, DMSO-d6): δ (mixture with the exocyclic isomer) =
169.8, 169.5, 156.1, 153.3, 151.2, 150.1, 144.5, 141.1, 139.7, 138.6,
138.5, 132.6, 123.2, 121.9, 120.6, 119.2, 116.5, 109.2, 108.3, 104.1,
75.6, 60.5, 58.1, 56.9, 56.8, 41.0, 14.5, 13.9.
HRMS (ESI+): m/z [M + H]+ calcd for C14H15N2O5: 291.0981; found:
291.0989.
IR (KBr): 3446, 2981, 2932, 1851, 1738, 1698, 1620, 1602, 1541, 1464,
1385, 1284, 1192, 1087, 869 cm–1
.
1H NMR (500 MHz, DMSO-d6): δ = 9.06 (d, J = 5.3 Hz, 1 H), 7.93 (d, J =
5.3 Hz, 1 H), 7.18 (s, 1 H), 4.59 (q, J = 7.0 Hz, 2 H), 4.09 (s, 3 H), 1.50 (t,
J = 7.2 Hz, 3 H).
13C NMR (125 MHz, DMSO-d6): δ = 172.5, 158.6, 153.3, 148.5, 145.8,
138.9, 136.8, 131.3, 124.8, 117.6, 103.8, 65.2, 57.0, 13.8.
8-Methoxy-9-nitro-1H-benzo[de][1,6]naphthyridin-2(4H)-one
(22)
To a solution of ethyl 2-(6-methoxy-7-nitroisoquinolin-1-yl)acetate
(20) (1.5 g, 5.17 mmol) in dry DMSO (30 mL) was added trimethylhy-
drazinium iodide (TMHI) (1.25 g, 6.20 mmol) and the mixture was
stirred for 5 min. KOt-Bu (1.45 g, 12.92 mmol) was added portionwise
and the resulting mixture was poured onto an ice-cold solution of
50% AcOH and extracted with 10% MeOH in EtOAc (2 × 100 mL). The
combined organic layer was dried over Na2SO4 and concentrated un-
der reduced pressure. The residue was purified by chromatography
on C-18 (30–50% MeCN/0.1 M FA) to provide 8-methoxy-9-nitro-1H-
benzo[de][1,6]naphthyridin-2(4H)-one (22) (0.55 g, 41%) as a yellow
solid; mp 302–306 °C.
HRMS (ESI+): m/z [M + H]+ calcd for C14H12N3O5: 302.0777; found:
302.0787.
9-Amino-2-ethoxy-8-methoxy-3H-benzo[de][1,6]naphthyridin-
3-one (1)
To stirred solution of 2-ethoxy-8-methoxy-9-nitro-3H-ben-
zo[de][1,6]naphthyridin-3-one (24) (0.02 g, 0.0062 mmol) in EtOAc (5
mL) was added 10% Pd/C (5% w/w). The reaction mixture was stirred
under a H2 atm (20 psi) for 6 h (until the starting material had been
consumed). The reaction mixture was filtered through a pad of Celite,
the filtrate was dried over Na2SO4 and concentrated under reduced
pressure. The residue was purified by reverse-phase chromatography
on C-18 using a Grace reversal instrument (20–30% MeCN/0.01 M FA)
to afford 9-amino-2-ethoxy-8-methoxy-3H-benzo[de][1,6]naphthyri-
din-3-one (1) (9 mg, 50%) as a red solid; mp 150–153 °C.
IR (KBr): 3446, 3005, 1627, 1275, 1260, 764, 750 cm–1
.
1H NMR (500 MHz, DMSO-d6): δ = 10.39 (br s, 1 H), 8.44 (br s, 1 H),
7.29 (d, J = 6.0 Hz, 1 H), 6.61 (s, 1 H), 6.19 (s, 1 H), 5.34 (s, 1 H), 3.91 (s,
3 H).
13C NMR (125 MHz, DMSO-d6): δ = 161.6, 156.3, 148.4, 140.4, 136.6,
135.6, 121.6, 107.4, 103.9, 97.1, 87.0, 56.8.
HRMS (ESI+): m/z [M + H]+ calcd for C12H10N3O4: 260.0671; found:
260.0680.
IR (KBr): 3383, 2924, 2853, 2347, 1600, 1482, 1362, 1266, 1096, 789
cm–1
.
1H NMR (500 MHz, DMSO-d6): δ = 8.73 (d, J = 4.5 Hz, 1 H), 7.93 (d, J =
4.5 Hz, 1 H), 7.31 (s, 1 H), 7.26 (br s, 2 H), 4.51 (q, J = 7.0 Hz, 2 H), 4.04
(s, 3 H), 1.39 (t, J = 7.0 Hz, 3 H).
2-Hydroxy-8-methoxy-9-nitro-3H-benzo[de][1,6]naphthyridin-
3-one (23)
To a stirred solution of benzeneseleninic acid anhydride (2.77 g, 7.72
mmol) in dry THF (40 mL) at 50 °C was slowly added a solution of 8-
13C NMR (125 MHz, DMSO-d6): δ = 171.0, 155.1, 152.3, 144.5, 141.8,
141.7, 132.0, 124.2, 118.2, 113.8, 103.5, 62.0, 56.4, 14.2.
methoxy-9-nitro-1H-benzo[de][1,6]naphthyridin-2(4H)-one
(22)
HRMS (ESI+): m/z [M + H]+ calcd for C14H14N3O3: 272.1035; found:
272.1042.
(1.0 g, 3.86 mmol) in THF (10 mL) dropwise. The resulting mixture
was stirred at the same temperature for 1 h (until the starting materi-
al had been consumed). The mixture was cooled to r.t., diluted with
EtOAc (100 mL) and washed with H2O (3 × 50 mL). The organic layer
was dried over Na2SO4 and concentrated under reduced pressure. The
residue was purified by reverse-phase chromatography on C-18 using
a Grace reversal instrument (40–50% MeCN/0.01 M FA) to provide 8-
Acknowledgment
The authors are thankful to GVK Biosciences Private Limited for fi-
nancial support. Support from the Analytical Department, GVK Bio-
sciences Private Limited is also acknowledged.
methoxy-9-nitro-3H-benzo[de][1,6]naphthyridine-2,3-dione
(23)
(0.350 g, 33%) as an orange solid; mp 241–273 °C.
IR (KBr): 3225, 3087, 2923, 1703, 1629, 1541, 1446, 1378, 1346, 1310,
1086, 874 cm–1
1H NMR (500 MHz, DMSO-d6): δ = 11.91 (br s, 1 H), 8.85 (d, J = 5.3 Hz,
1 H), 8.06 (d, J = 5.5 Hz, 1 H), 7.36 (s, 1 H), 4.02 (s, 3 H).
.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1588752.
13C NMR (125 MHz, DMSO-d6): δ = 176.2, 157.5, 152.2, 147.7, 146.8,
S
u
p
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u
p
p
o
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137.1, 128.8, 128.1, 124.1, 115.1, 99.2, 57.1.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

G
S. Puvvala et al.
Paper
Synthesis
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G