Preparation of novel heteroisoindoles from nitropyridines and nitropyridones

Article abstract of DOI:10.3987/com-02-s(m)22

The reaction of nitropyridine derivatives with ethyl isocyanoacetate in the presence of 1,8-diazabicyclo[5.4.0]undecene proceeded tandem cyclization to give polycyclic pyrrolopyridines or imidazopyridines. On the other hand, N-protected 3-nitro- and 5-nitropyridones and N,N-diprotected 5-nitrouracil gave corresponding bicyclic pyrroles in good yields under the similar conditions.

Full text of DOI:10.3987/com-02-s(m)22

HETEROCYCLES, Vol. 58, 2002, pp. 301-310, Received, 18th March, 2002
PREPARATION
OF
NOVEL
HETEROISOINDOLES
FROM
NITROPYRIDINES AND NITROPYRIDONES
Takashi Murashima,*a Keiji Nishi,a Ken-ichi Nakamoto,a Atsushi Kato,a Ryuji
Tamai,a Hidemitsu Uno,b and Noboru Ono*a
a Department of Chemistry, Faculty of Science, Ehime University, Bunkyo-cho
2-5, Matsuyama 790-8577, Japan
b
Advanced Instrumentation Center for Chemical Analysis, Ehime University,
Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
Dedicated to Professor A. I. Meyers on the occasion of his 70th birthday.
Abstract– The reaction of nitropyridine derivatives with ethyl isocyanoacetate
in the presence of 1,8-diazabicyclo[5.4.0]undecene proceeded tandem
cyclization to give polycyclic pyrrolopyridines or imidazopyridines. On the
other hand, N-protected 3-nitro- and 5-nitropyridones and N,N-diprotected 5-
nitrouracil gave corresponding bicyclic pyrroles in good yields under the similar
conditions.
Since a large number of medicinally important natural products include azaarene skeletons in their
structure,1 these heteroarenes such as quinolines, indoles, isoindoles and azaisoindoles have generated
interest as challenging targets for synthesis. There were various methods reported to prepare quinolines,2
indoles3 and isoindoles.3 On the contrary, relatively little attention is given to heteroarene fused pyrroles
such as pyrrolopyridinones,4 pyrrolopyrimidines,5 and pyrrolopyrroles6 because of difficulties arose in
the synthesis. Most of studies on the preparation of this class of molecules dealt with pyrrole fused with
pyridine on its [1,2-x] and [2,3-x] position.7 The synthesis of pyrrolo[3,4-x]pyridines described by

Armarego8 is the only reported procedure, but the final products have protecting groups on the pyrrole
ring nitrogen.
In a recent report, we have described various facets of the reaction of unsaturated nitro compounds with
ethyl isocyanoacetate and discussed the formation of fused pyrroles,9a,c 1-hydroxypyrazoles10 and fused
pyrimidine N-oxides.9b Herein, we wish to report the reaction of simple nitroheteroarenes with ethyl
isocyanoacetate.
According to our interest in the coordination ability of fused ring nitrogen, we have synthesized 1,10-
phenanthrolinopyrrole, thiadiazolopyrrole, quinolinopyrroles and quinoxalinopyrroles9 by the improved
Barton-Zard methodology in moderate to good yields.11 However, as Lash et al. reported, 3- and 4-
nitropyridines were not suitable as a precursor for the pyridopyrrole.12 The reaction of 4-nitropyridine
with ethyl isocyanoacetate gave a complicated product, diethyl imidazo[3,4-a]imidazo[3’,4’:1,2]pyrido-
[3,4-e]pyrimidine-1,10-dicarboxylate (2a) in quite low yield. This compound was diagnosed using
NMR, MS and IR spectroscopies. The proton NMR spectrum for this product showed the presence of
two non-equivalent ethyl esters and two doublets at 7.72 and 8.86 ppm. Proposed mechanism for the
formation of the tetracyclic molecule is illustrated in Scheme 1.
N
N
NC
-
N
EtO2C
EtO2C
N
N
N
CNCH2CO 2Et
DBU
CNCH2CO 2Et
DBU
EtO2C
H
H
H
EtO2C
N
NO2
NO2
NO2
N
+
O
O
C
-
-
1a
N
N
N
N
EtO2C
EtO2C
EtO2C
EtO2C
DBUH+
N
N
N
N
N
N
-H2O
H
H
H
H
H
H
EtO2C
EtO2C
EtO2C
EtO2C
N
N
H
N
H
N
H
N
N
OH
H+
O
O
O
H+
OH
H
N
N
N
N
H+
N
N
N
N
N
N
EtO2C
EtO2C
EtO2C
EtO2C
EtO2C
EtO2C
Decarboxylation EtO2C
EtO2C
-H2O
CNCH2CO 2Et
DBU
aromatization
-
N
N
N
H
N
N
N
N
N
EtO2C
N
C
EtO2C
EtO2C
H
2a
Scheme 1. Proposed Mechanism for the Formation of Tetracyclic Heteroarene (2a)
When 3-nitropyridine (1b) was reacted with two equivalents of carbanion derived from ethyl
isocyanoacetate and DBU, tandem cyclization occurred to give the tricyclic compound, diethyl 8H-

imidazo[1,5-a]pyrrolo[3,4-c]pyridine-1,7-dicarboxylate (2b).12 With the exception of recovered
substrate, the compound (2b) was the only isolated product. The use of equimolar amounts of ethyl
isocyanoacetate and 1b also led to the production of 2b and no mono cyclization product was obtained.
Therefore, the second cyclization was faster than the first one.
Since the key step of this reaction is the nucleophilic attack of carbanions to substrates, the existence of
nitro group is essential for the progress of each reaction. Thus, the initial product may be not
pyrrolo[3,4-b]pyridine (3) but imidazo[1,5-a]pyridine (4). Once imidazo[1,5-a]pyridine (4) was formed,
it was readily reacted with another carbanion and the nitro moiety was removed at the final stage of
reaction to give the tricyclic compound (2b). (Scheme 2) The structures of compounds (2b) and other
pyrrolic compounds (2c,e,f) were confirmed in comparison with the reported data.12
NO2
NO2
NC
NO2
_
CO2Et
N
N
N
N
N
CO2Et
N
4
EtO2C
NC
EtO2C
NC
NO2
NH
NO2
_
N
_
CO2Et
3
CO2Et
N
N
EtO2C
NH
EtO2C
NH
CO2Et
N
N
2b
Scheme 2. Reaction Course of the Formation of Pyrrolopyridine (2b)
To improve the yields of pyrrolic compounds such as 2b, some 3-nitropyridine derivatives were used as
starting materials. Nitropyridines were prepared by the nitration of substituted pyridines with modified
kyodai nitration13 which was the improved Bakke method.14
The reaction of 2-methyl-5-nitropyridine (1c) with ethyl isocyanoacetate in the presence of DBU
resulted in the production of similar tricyclic heteroarene, diethyl 5-methyl-8H-imidazo[1,5-
a]pyrrolo[3,4-c]pyridine-1,7-dicarboxylate (2c) in 9% yield. Since these cyclization reactions were
accelerated in the presence of an electron withdrawing group, it would be expected to facilitate the

production of pyrrolic compound by the use of 2-cyano-5-nitropyridine (1d) as the starting material.
This, however, was not the case, the product formed being the imidazole type compound, diethyl 5-
cyanoimidazo[3,4-a]imidazo[1',5':1,2]pyrido[3,4-d]pyrimidine-1,11-dicarboxylate (2d) derived from 1d
in 6% yield. The proton NMR spectral feature was analogous to that of 2a except that one of the two
doublets (7.72 ppm) was disappeared and another doublet (8.86 ppm) was changed into singlet. The
existence of cyano group was confirmed by IR spectrum.
Table 1. Products and Yields of the Reaction of Nitropyridines
Substrate
N
Product
Yield/%
Substrate
N
Product
Yield/%
N
N
N
N
NC
NC
CO2Et
EtO2C
6a (18c)
4a (12c)
2d
N
N
NO2
2a
N
N
NO2
1d
CO2Et
1a
N
N
EtO2C
N
N
N
54a
N
N
CO2Et
O2N
15b
2e
2f
2b
NO2
N
1e
H
CO2Et
CO2Et
1b
N
H
EtO2C
N
N
N
N
N
CO2Et
N
12a (24c)
9b
2c
EtO2C
NO2
NO2
N
H
1c
N
H
1f
EtO2C
a Reagents: CNCH2CO2Et/DBU 1.0 eq. b Reagents: CNCH2CO2Et/DBU 2.0 eq.
c Yields based on isocyanoacetate.
In spite of the use of only one equivalent of ethyl isocyanoacetate and DBU, 4-nitroisoquinoline (1f)
was also subject to tandem cyclization to give tetracyclic compound, diethyl 2H-imidazo[4,3-
a]pyrrolo[3,4-c]isoquinoline-3,7-dicarboxylate (2f) in 12% yield.
On the contrary, 3-nitroquinoline (1e) was reacted with equimolar amount of reagent giving single
cyclization product, ethyl 2H-pyrrolo[3,4-c]quinoline-1-carboxylate (2e) in the yield of 54%. When an
excess amount of reagent was used for this reaction, we have observed no tandem cyclization product
such as 2b, 2c and 2f. These results suggest that the initial nucleophilic attack occurred on the 4-position
of 3-nitroquinoline (1e) and a pyrrole ring was constructed instead of an imidazole ring or a pyrimidine

ring as observed in the cases of 1a-d and 1f. The product (2e) was no longer subject to nucleophilic
attack, thus the subsequent cyclization could not proceed.
The reactions of nitropyridine derivatives, except 3-nitroquinoline, did not give expected pyrrolic
compounds in good yields. Thus, to improve the yields of heteroarene annulated pyrroles, we have tried
the similar reactions with nitropyridones and nitrouracil. The commercially available nitro-2-pyridones
and nitrouracil were reacted with p-methoxybenzyl chloride to give N-protected starting materials. The
reactions of 3-nitro-N-p-methoxybenzyl-2-pyridone (1g), 5-nitro-N-p-methoxybenzyl-2-pyridone (1h)
and 5-nitro-N,N-di-p-methoxybenzyluracil (1i) with ethyl isocyanoacetate in the presence of DBU
resulted in the production of expected bicyclic pyrroles (2g-i) in the yields of 65, 50 and 88%,
respectively.
Table 2. Products and Yields of the Reaction of Nitropyridones and Nitrouracil
Substrate
Product
O
Yield/%
PMB
BMP
N
O
N
65
2g
NH
1g
1h
NO
2
2
CO Et
2
PMB
N
PMB
N
CO Et
2
O
O
50
88
NH
2h
NO
PMB
N
PMB
N
CO Et
2
O
O
1i
NH
2i
N
N
BMP
NO
BMP
2
O
O
Reagents: CNCH CO Et/DBU 1.0 eq.
2
2
The nitroalkene moiety of nitropyridone derivatives was less aromatic than that of nitropyridines, thus
compounds (1g-i) were more reactive toward nucleophilic attack. Another advantage is that once
nucleophilic attack occurred, the resulting compounds (2g-i) have no reaction points because their
reactive N-positions were protected with p-methoxybenzyl groups. Therefore, N-protected
nitropyridones and nitrouracil were good precursors for the preparation of heteroarene annulated
pyrroles.
In summary, we have described the preparation of heteroarene annulated pyrroles starting from
nitropyridine derivatives and nitropyridone derivatives. Some nitropyridines gave expected

azaisoindoles, but others yielded imidazopyridines. These types of polycyclic heteroarenes are known to
have extremely high fluorescence quantum efficiencies in the visible spectra, therefore these organic
materials are ideal for electro luminescence materials. Investigations are in progress to apply these
azaheterocycles to EL materials. On the other hand, N-protected nitropyridones and nitrouracil were
better precursors for azaisoindoles. When the N-protected groups of these compounds are removed, the
resulting NH protons could form hydrogen bonds to adjacent carbonyl groups of neighboring molecules
to give dimers.
EXPERIMENTAL
Melting points were measured by a Yanaco hot stage apparatus and are uncorrected. NMR spectra
were recorded on a JEOL-JNM-GSX 270 or JNM-EX 400 spectrometer at ambient temperature by
using CDCl3 or DMSO-d6 as a solvent and tetramethylsilane as an internal standard for 1H and 13C and
J values are given in Hz. IR and UV-VIS spectra were obtained with a Hitachi 270-30 and Shimadzu
UV-2200 spectrophotometer, respectively. MS spectra were measured with a Hitachi M80B-LCAPI
spectrometer under the following ionizing conditions: electron impact, 20 eV, high boiling PFK as a
standard. THF was freshly distilled from sodium benzophenone ketyl. Acetonitrile was distilled from
P2O5. Ethyl isocyanoacetate was prepared from ethyl N-formylglycinate using POCl3 and
triethylamine.15 Unless otherwise specified, all nitro aromatics which were not commercially available
were prepared by kyodai-nitration methods of heteroaromatics using NO2/O3 and sodium hydrogen
sulfite.
Nitropyridine derivatives (1b-f)
Into a solution of liquid nitrogen dioxide (1 mL) in dry CH2Cl2 (50 mL) ozonized oxygen was slowly
introduced for 1.5 h at –78 °C. Pyridines (10 mmol) were added in one portion to the solution and the
resulting suspension was stirred for 1 h at rt, then the mixture was poured into 5% sodium hydrogen
sulfite. The reaction mixture was worked up as usual, and the residue was purified by recrystallization
from methanol.
Diethyl Imidazo[3,4-a]imidazo[3',4':1,2]pyrido[3,4-e]pyrimidine-1,10-dicarboxylate (2a)

DBU (1.52 g, 10 mmol) was added dropwise to a solution of 4-nitropyridine (1a) (1.24 g, 10 mmol) and
ethyl isocyanoacetate (1.13 g, 10 mmol) in acetonitrile (50 mL) at 0 °C. The resulting mixture was
stirred at ambient temperature for 96 h. After 10% hydrochloric acid (50 mL) was added, the crude
mixture was extracted with ethyl acetate (3 x 20 mL). The organic phase was washed with 3% sodium
hydrogen carbonate, water and brine, dried over Na2SO4. The solvent was removed and the residue was
washed with ethanol to give pale yellow precipitate. This solid product was purified by recrystallization
with chloroform and ethanol, giving the title compound (2a) (0.141 g, 3%), mp 268 °C (decomp); 1H-
NMR (CDCl3) δ 1.50 (t, 3H, J 7.32), 1.52 (t, 3H, J 7.32), 4.54 (q, 2H, J 7.32), 4.56 (q, 2H, J 7.32), 7.72
(d, 1H, J 5.37), 8.40 (s, 1H), 8.86 (d, 1H, J 5.37), 10.00 (s, 1H) and 10.86 (s, 1H); 13C-NMR (CDCl3) δ
14.3, 30.9, 61.9, 62.0, 108.1, 111.0, 119.0, 124.5, 127.1, 127.2, 129.6, 132.9, 136.3, 150.7, 151.7, 162.1
and 163.1; m/z 353 (M+, 65%), 308 (M+-EtO) and 281 (100); HRMS Calcd for C17H15N5O4 353.1124;
Found: 353.1122.
Diethyl 8H-Imidazo[1,5-a]pyrrolo[3,4-c]pyridine-1,7-dicarboxylate (2b)
To a solution of 3-nitropyridine (1b) (1.24 g, 10 mmol) in THF (50 mL) was added dropwise ethyl
isocyanoacetate (2.25 g, 20 mmol) then DBU (3.03 g, 20 mmol) at 0 °C. The resulting mixture was
stirred at ambient temperature for 24 h, diluted with chloroform, and washed with 50 mL of water. The
water phase was extracted with chloroform (50 mL). The combined organic phase was washed with 5%
sodium hydrogen carbonate, water and brine, dried over sodium sulfate. The solvent was removed and
the residue was purified by recrystallization with methanol to give the title compound (2b) (0.452 g,
15%), mp 242-243 °C; 1H-NMR (CDCl3) δ 1.47 (t, 3H, J 7.32), 1.48 (t, 3H, J 7.32), 4.47 (q, 2H, J 7.32),
4.50 (q, 2H, J 7.32), 7.26 (d, 1H, J 9.27), 7.33 (d, 1H, J 3.42), 7.86 (d, 1H, J 9.77), 9.50 (s, 1H) and 9.68
(br s, 1H); 13C-NMR (CDCl3) δ 14.4, 60.3, 113.7, 120.2, 120.6, 121.9, 124.5, 125.9, 126.9, 127.6, 129.3,
144.3, 148.0 and 160.3; m/z 301 (M+, 100%), 255 (M+-EtOH, 70), 229 (27), 210 (27) and 183 (46);
HRMS Calcd for C15H15N3O4 301.1063; Found: 301.1055.
The polyheterocyclic arenes (2c-f) were prepared by similar procedures to that described for the
preparation of 2b, under the conditions described in Table 1.
Diethyl 5-Methyl-8H-imidazo[1,5-a]pyrrolo[3,4-c]pyridine-1,7-dicarboxylate (2c)

Yield 9%; mp >250 °C; 1H-NMR (CDCl3) δ 1.47 (t, 3H, J 7.33), 1.49 (t, 3H, J 7.32), 2.66 (s, 3H), 4.46
(q, 2H, J 7.33), 4.50 (q, 2H, J 7.32), 7.27 (d, 1H, J 2.75), 8.02 (s, 1H), 8.57 (d, 1H, J 2.74) and 10.08 (br
s, 1H); 13C-NMR (CDCl3) δ 14.4, 14.5, 56.0, 59.6, 60.0, 106.5, 110.8, 113.3, 121.3, 121.8, 122.0, 130.0,
130.1, 130.3, 160.4 and 163.4; m/z 315 (M+, 81%), 269 (M+-EtOH, 100), 243 (11), 224 (37) and 197
(76); HRMS Calcd for C16H17N3O4 315.1219; Found: 315.1222.
Diethyl 5-Cyanoimidazo[3,4-a]imidazo[1',5':1,2]pyrido[3,4-d]pyrimidine-1,11-dicarboxylate (2d)
Yield 6%; mp >250 °C; 1H-NMR (CDCl3) δ 1.51 (t, 3H, J 7.02), 1.54 (t, 3H, J 7.02), 4.55 (q, 2H, J
7.02), 4.58 (q, 2H, J 7.02), 8.53 (s, 1H), 9.34 (s, 1H) and 10.11 (s, 2H); m/z 378 (M+, 38%), 269 (M+-
EtO, 12) and 306 (100); IR (nujol mull) 1295 (C-O), 1297 (C-O), 1714 (C=O), 1716 (C=O) and 2227
(CN); HRMS Calcd for C18H14N6O4 378.1077; Found: 378.1069.
Ethyl 2H-Pyrrolo[3,4-c]quinoline-1-carboxylate (2e)
Yield 54%; mp 233-234 °C; 1H-NMR (CDCl3) δ 1.48 (t, 3H, J 7.02), 4.52 (q, 2H, J 7.02), 7.59-7.71 (m,
3H), 8.11 (dd, 1H, J 1.83, 7.62), 9.15 (s, 1H), 9.55 (d, 1H, J 1.83) and 9.58 (br s, 1H); 13C-NMR
(CDCl3) δ 14.4, 60.3, 113.7, 120.2, 120.6, 121.9, 124.5, 125.9, 126.9, 127.6, 129.3, 144.3, 148.0 and
160.3; m/z 240 (M+, 79%) and 194 (M+-EtOH, 100); Anal. Calcd for C14H12N2O2: C, 69.99; H, 5.03; N,
11.66. Found: C, 69.69; H, 5.05; N, 11.60.
Diethyl 2H-Imidazo[4,3-a]pyrrolo[3,4-c]isoquinoline-3,7-dicarboxylate (2f)
Yield 12%; mp 184-186 °C; 1H-NMR (CDCl3) δ 1.47 (m, 6H), 4.47 (q, 2H, J 7.02), 4.55 (q, 2H, J 7.02),
7.50-7.54 (m, 2H), 7.58 (d, 1H, J 3.66), 7.86 (dd, 1H, J 3.66, 7.02), 9.69 (br s, 1H) and 9.71 (s, 2H); 13C-
NMR (CDCl3) δ 14.2, 14.5, 60.7, 61.9, 109.6, 114.1, 115.4, 116.5, 121.7, 124.5, 126.8, 127.7, 127.9,
128.2, 129.0, 131.7, 153.2, 160.0 and 172.0; m/z 351 (M+, 100%), 305 (M+-EtOH, 39), 279 (41), 260
(29) and 233 (55); HRMS Calcd for C19H17N3O4 351.1219; Found: 351.1214
Ethyl 5H-5-p-Methoxybenzyl-4-oxopyrrolo[5,6-c]pyridine-1-carboxylate (2g)
1
Yield 65%; mp 161-162 °C; H-NMR (CDCl3) δ 1.35 (t, 3H, J 7.3), 3.75 (s, 3H), 4.34 (q, 2H, J 7.3),
5.08 (s, 2H), 6.76-6.92 (m, 4H), 7.25 (d, 2H, J 6.8), 7.80 (d, 1H, J 3.4) and 11.30 (br s, 1H); 13C-NMR
(CDCl3) δ 14.4, 50.0, 55.2, 60.4, 101.1, 114.2, 117.5, 122.6, 128.7, 129.2, 129.5, 131.8, 159.2, 160.2,

161.3; m/z 326(M+, 20%) and 121(100); Anal. Calcd for C18H18N2O4+H2O: 62.78; H, 5.85; N, 8.13.
Found: C, 62.64; H, 5.88; N, 8.04.
Ethyl 7H-7-p-Methoxybenzyl-6-oxopyrrolo[6,5-c]pyridine-1-carboxylate (2h)
Yield 50%; mp 152-153 °C; 1H-NMR (CDCl3) δ 1.24 (t, 3H, J 6.84), 3.71 (s, 3H) 4.24 (q, 2H, J 6.84),
6.00 ((s, 2H), 6.41 (d, 1H, J 9.28), 6.75 (d, 2H, J 8.79), 7.01 (d, 1H, J 3.42), 7.13 (d, 2H, J 8.79), 7.54
13
(d, 1H, J 9.28) and 10.27 (br s, 1H); C-NMR (CDCl3) δ 14.3, 48.0, 55.1, 60.7, 105.8, 113.1, 113.5,
117.7, 128.2, 130.5, 132.8, 133.0, 158.2, 159.2 and 164.0; m/z 326 (M+, 34%) and 21 (100); Anal.
Calcd for C18H18N2O4+1/4MeOH: C, 65.69; H, 5.69; N, 8.42. Found: C, 65.69; H, 5.64; N, 8.42.
Ethyl 5H,7H-5,7-Di-p-methoxybenzyl-4,6-dioxopyrrolo[6,5-c]pyrimidine-1-carboxylate (2i)
Yield 88%; mp 169-170 °C; 1H-NMR (CDCl3) δ 1.21 (t, 3H, J 7.33), 3.72 (s, 3H), 3.74 (s, 3H), 4.22 (q,
2H, J 7.33), 5.14 (s, 2H), 5.75 (s, 2H), 6.77 (d, 2H, J 8.79), 6.79 (d, 2H, J 8.79), 7.13 (d, 2H, J 8.79),
13
7.40 (d, 2H, J 8.79), 7.45 (d, 1H, J 3.91) and 10.13 (br s, 1H); C-NMR (CDCl3) δ 14.1, 43.8, 49.5,
55.1, 55.1, 61.1, 106.6, 108.2, 113.6, 113.6, 122.3, 128.2, 129.4, 129.7, 130.1, 132.1, 152.2, 158.5,
158.8 and 159.3; m/z 463 (M+, 90%), 342 (M+-CH2C6H4OMe, 46) and 121(100); Anal. Calcd for
C25H25N3O6+1/2MeOH: C, 63.87; H, 5.68; N, 8.76. Found: C, 63.76; H, 5.46; N, 8.95.
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