Synthesis of new pyrrolo-[3,4-c]isoxazole, pyrrolo[2,3-d]-[1,2,3]triazole, triazolo[4,5-c]-pyridazine, and dipyrrolo-[3,2-b:3′,4′-d]pyran derivatives

Article abstract of DOI:10.1007/s10593-007-0241-4

New pyrrolo[3,4-c]isoxazole derivatives were synthesized from the key intermediates 4-cyanopyrrolidin-3-ones in two steps. Pyrrolo[2,3-d][1,2,3] triazoles and triazolo[4,5-c]pyridazine were obtained from 2-arylhydrazono-4- cyano-1-(4′-methoxyphenyl)-3-oxo

Full text of DOI:10.1007/s10593-007-0241-4

Chemistry of Heterocyclic Compounds, Vol. 43, No. 12, 2007
SYNTHESIS OF NEW PYRROLO-
[3,4-c]ISOXAZOLE, PYRROLO[2,3-d]-
[1,2,3]TRIAZOLE, TRIAZOLO[4,5-c]-
PYRIDAZINE, AND DIPYRROLO-
[3,2-b:3',4'-d]PYRAN DERIVATIVES
F. A. Amer1, M. Hammouda1, A.-A. S. El-Ahl1, B. F. Abdel-Wahab2
New pyrrolo[3,4-c]isoxazole derivatives were synthesized from the key intermediates 4-cyano-
pyrrolidin-3-ones in two steps. Pyrrolo[2,3-d][1,2,3]triazoles and triazolo[4,5-c]pyridazine were
obtained from 2-arylhydrazono-4-cyano–1-(4'-methoxyphenyl)-3-oxopyrrolidines by refluxing with
phenylhydrazine in either ethanol or glacial acetic acid. Aldol self-condensation of 1-aryl-4-cyano-
pyrrolidin-3-ones afforded dipyrrolo-[3,2-b:3',4'-d]pyran derivatives.
Keywords: oxime, β-oxo nitriles, pyrrolidin-3-ones, aldol condensation
N-Arylpyrrolidin-3-one is a key starting material for the synthesis of a number of fused heterocycles, among
them certain analogs of the anticancer drug methotrexate [1, 2]. Pyrrolidine derivatives which bear an alkyloxime
substituent in position 4 and an aminoalkyl substituent in position 3 of the pyrrolidine ring were used to produce a
series of fluoroquinolone antibacterials (e.g., gemifloxacin, norfloxacin) and also used as bax inhibitors [3–6]. Some
of the 3-pyrrolidinone derivatives were used as inhibitors of HIV-1 replication [7]. We are reporting here the
synthesis of some new fused heterocycles having a pyrrolidine moiety, such as pyrrolo[3,4-c]isoxazoles,
pyrrolo[2,3-d][1,2,3]triazoles, triazolo[4,5-c]pyridazine, and dipyrrolo[3,2-b:3',4'-d]pyran derivatives.
The starting pyrrolidin-3-ones (considered as heterocyclic β-oxonitriles) 1a,b were prepared from
primary aromatic amines, acrylonitrile, and ethyl bromoacetate according to the methods reported [8–10]. We
investigated their reaction with NH2OH·HCl in refluxing ethanol containing sodium acetate. The reaction
furnished the expected oxime derivatives [11, 12] 2a,b (Scheme 1), but all attempts to cyclize these oximes into
the pyrroloisoxazole derivatives 3a,b were unsuccessful. We assumed that oximes 2a,b were present completely
in the Z-form relative to the pyrrolidine ring, in which the OH group was oriented away from the cyano group.
The Z-form of 2a,b was stable under thermal, acidic, or basic conditions, therefore, the transformation of 2a,b
into 3a,b proved to be difficult. We envisioned that if we introduce a bulky substituent in position 2 of the
3-pyrrolidinone 1a,b, the geometry of the resulted oxime would be changed. Accordingly, we investigated the
reaction of 1a with aryldiazonium salts under mild basic conditions (ethanol and sodium acetate); the reaction
__________________________________________________________________________________________
1Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt. 2Applied
Chemistry Department, National Research Center, Dokki, Giza, Egypt; e-mail: Beko_bfa@yahoo.com.
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1841-1848, December, 2007. Original
article submitted February 08, 2007.
0009-3122/07/4312-1559©2007 Springer Science+Business Media, Inc.
1559

furnished the arylazopyrrolidine derivatives, 4a−c. The treatment of compounds 4a,c with hydroxylamine
hydrochloride in ethanolic sodium ethoxide afforded the pyrroloisoxazole derivatives 5a,b. The formation of the
pyrroloisoxazoles 5a,b from the reaction of 4b,c with hydroxylamine hydrochloride proceeded via the oxime
intermediate A which was presumably present in the E-form due to the steric interaction of the hydroxyl group
with the bulky arylazo group in position 2 of the pyrroline ring (Scheme 1).
Scheme 1
..
O
H2N
N
NC
N
OH
.
NH2OH
HCl
N
N
Ar
3
NaOAc/EtOH
Ar
NC
O
2a,b
NC
NC
N
O
OH
Ar
1a,b
ArN2Cl
NNHAr
N=NAr
N
N
NaOAc/EtOH
OMe
OMe
4a-c
.
NH2OH
HCl
NaOEt/EtOH
OH
O
H2N
N
NC
N
..
H
N
H
N
N
N Ar
N
N
Ar
OMe
OMe
A
5a,b
1,2 a Ar = 4-MeOC6H4, b Ar = 4-MeC6H4; 4 a Ar = 4-MeC6H4, b Ar = 4-MeOC6H4,
c Ar = 4-ClC6H4; 5 a Ar = 4-MeOC6H4, b Ar = 3-ClC6H4
The 1H NMR spectrum of 5a showed a broad singlet (2H) at 5.77 ppm attributed to the methylene group
of the pyrrolidine ring. Also two singlets for three protons at 9.02 and 8.19 ppm were present, attributed to NH2
and NH groups, respectively. Physical and spectra data are given in Tables 1-3.
The reaction of compound 4b,c with phenylhydrazine in ethanol afforded the pyrrolotriazole derivatives
6a,b. However, on conducting the reaction in glacial acetic acid under refluxing conditions the arylazo
derivatives 4a-c underwent ring transformation into the triazolopyridazine derivative 7 (Scheme 2).
1560

TABLE 1. Characteristics of the Synthesized Compounds 2 and 4−8
Found, %
——————
Calculated, %
Com-
pound
Empirical
formula
mp, °C
Yield, %
Color
C
H
N
2a
2b
4a
4b
4c
5a
5b
6a
6b
7
C12H13N3O2
C12H13N3O
62.32
62.39
66.96
67.08
68.24
68.42
65.13
65.24
60.93
61.02
62.45
62.56
58.46
58.59
65.69
65.81
61.45
61.39
63.32
63.76
66.65
66.33
71.98
71.78
5.66
5.76
6.09
6.16
5.43
4.62
5.18
5.07
4.26
4.33
5.24
5.32
4.36
4.43
4.93
5.02
4.01
3.96
4.47
4.64
5.59
5.70
6.04
6.22
18.17
18.23
19.52
19.48
16.76
16.75
15.99
16.03
15.79
15.77
19.17
19.07
18.94
18.86
20.16
20.09
19.91
19.79
23.32
23.40
12.96
13.12
13.99
13.83
234-235
247-248
176-177
230-231
267-268
228-229
148-149
264-266
>315
73
72
57
73
77
62
58
49
48
42
56
48
Colorless
Colorless
Red
C19H18N4O2
C19H18N4O3
C18H15ClN4O2
C19H19N5O3
C18H16ClN5O2
C19H17N5O2
C18H14ClN5O
C19H16N6O2
C24H24N4O4
C24H24N4O2
Scarlet red
Orange
Pale yellow
Pale yellow
Colorless
Yellow
115-117
230-232
>310
Colorless
Pale yellow
Pale yellow
8a
8b
Scheme 2
H
H
NC
NC
N
N
Ph
..
N
Ar
N
–PhNH2
N
PhNHNH2
N
N Ar
H
N
N
4b,c
H
EtOH
OMe
OMe
A
6a,b
Me
O
Me
O
N
CHO
CHO
HN
Ph
PhNHNH2
AcOH
HN
Ph
N
N
N
4a-c
N
Ph
N
Ph
N
N
H
N
H
N
7'
7
6 a Ar = 4-MeOC6H4, b Ar = 4-ClC6H4
1561

TABLE 2 Spectroscopic Characteristics of Compounds 4–8
Com-
pound
IR spectrum ν, cm-1 (group)
Mass spectrum, m/z (Irel, %)
4a
1658 (C=O); 2220 (C≡N);
3257 (enolic OH)
335 [M++1] (70); 334 [M+] (100); 305 (15); 290 (56);
157 (10); 108 (8); 77 (45)
4b
4c
5a
5b
6a
6b
7
1634 (C=O); 2221 (C≡N);
3200 (enolic OH)
351 [M++1] (35); 350 [M+] (100); 292 (45); 244 (18);
157 (8); 108 (7); 77 (50)
1642 (C=O); 2222 (C≡N);
3294 (enolic OH)
357 [M++2] (22); 356 [M++1] (73); 355 [M+] (100);
157 (6); 108 (7); 77 (55)
1609 (C=N);
3462-3172 (NH2 and NH)
367 [M++2] (77); 366 [M++1] (100); 335 (13); 150 (6);
135 (5.5)
1606 (C=N);
3408-3172 (NH2 and NH)
372 [M++2] (66); 370 [M+] (100); 337 36); 135(12);
77 (58)
2225 (C≡N); 3278 (NH)
349 [M++2] (24); 348 [M++1] (100); 333 (41); 149 (22);
135 (7); 133 (62)
2227 (C≡N); 3287(NH)
354 [M++2] (24); 353 [M++1] (100); 317 (41); 149 (22);
135 (7); 133 (62)
1647 (amidic C=O);
1735 (aldehydic C=O);
3284 (amidic NH)
360 [M+] (100); 317 (17); 241 (10); 206 (15); 77 (48)
8a
8b
2225 (C≡N);
3258-3102 (NH2, OH)
433 [M++1] (1); 432 [M+] (1.4); 401 (10); 382 (4.2);
370 (3); 217 (0.7); 119 (100); 91 (96)
401 [M++1] (1); 400 [M+] (1.4); 383 (3); 382 (4.2);
366 (3); 217 (0.7); 119 (100); 91 (96)
2225 (C≡N);
3281-3181 (NH2, OH)
1
The H NMR spectrum of compound 6b showed a multiplet at 7.88-6.92 ppm corresponding to eight
aromatic protons overlapped with one proton singlet corresponding to the pyrrole ring hydrogen; the two NH
1
groups were observed as two singlets at 9.37 and 9.28 ppm. The H NMR spectrum of 7 displayed one proton
singlet at 9.58 ppm corresponding to an aldehydic proton and lacked any absorption peak corresponding to the
methoxy group, indicating the removal of p-anisidine part of the molecule (a starting material) during the
reaction. Upon starting with different arylazo derivatives, the same triazolopyridazine was obtained, indicating
the removal of the arylazo group during the reaction. The proposed mechanism for the formation of compound 7
is depicted in Scheme 3. The reaction is thought to proceed via the formation of pyrrolotriazole intermediate B,
which in the further reaction with another molecule of phenylhydrazine, followed by ring opening and
recyclization, afforded the intermediate C. Tautomerization of C would give the intermediate D, which
underwent acetylation by acetic acid to give intermediate E. Hydrolysis of the imine ion E by traces of water in
the reaction medium furnished compound 7 (Scheme 3).
The structures of the triazole derivatives 6a,b and 7 were also confirmed by 13C NMR spectrum and
mass-spectral data which revealed the molecular ion peaks for 6a and 7 at m/z 347 and 360, respectively. Also
the 1H NMR spectrum of compound 7 showed it to be present as a tautomeric mixture (Tables 1-3).
The dimerization of pyrrolidin-3-ones has been well established in the literature [13]. During our study on
pyrrolidin-3-ones 1a,b, we realized that these compounds have a great tendency towards aldol self-condensation in
slightly acidic alcoholic solution. Thus, on being refluxed in ethanol containing a catalytic amount of hydrochloric
acid 1a,b furnished products 8a,b, which are thought to be formed via the aldol self-condensation of intermediate F
(Scheme 4). The structures of 8a,b were based on spectral and analytical data. The 1H NMR spectrum of 8a displayed
a doublet of doublets centered at 2.72 ppm (J = 18 and 8 Hz) assigned to Ha and Hb; an AB system at 3.28 and 3.52
ppm assigned to Hg and Hf (J = 8 Hz); a multiplet at 3.82-3.74 ppm corresponding to two protons (He and Hc). This
multiplet is a doublet of doublets Hc overlapped with another doublet He as a part of another A2B2 system. Another
signal of this A2B2 system was observed at 4.01 ppm (J = 10.1 Hz), which was assigned to Hd.
1562

The aromatic protons of the two p-substituted benzene rings were observed as two indistinguishable A2B2 systems at
7.14, 6.91, 6.78 and 6.43 ppm with a coupling constant (J = 8 Hz). The hydroxy group was observed as a singlet at
3.67 ppm, while the amino group was observed as two broad singlets at 7.94 and 7.11 ppm. Also, two singlets (three
protons each) were observed at 3.73 and 3.65 ppm assigned to the two methoxy groups.
Scheme 3
H
NC
N
NC
N
N
Ph
..
Ph
N
N
–ArNH2
N
PhNHNH2
AcOH
N
N
H
N
4a–c
Ar
OMe
OMe
B
NC
N
H
N
H
N
Ph
N
OMe
N
OMe
N
PhNHNH2
H
H
N
N
N
N
C
HN
Ph
N
N
N
Ph
NH
Ph
N
Ph
..
N
N
Ph N H
N
H
H
OMe
C
N
OMe
Me
O
N
N
OMe
H3O
AcOH
HN
Ph
N
N
H2N
N
N
Ph
N
Ph
N
N
N
Ph
N
H
H
E
D
Me
O
Me
O
N
CHO
CHO
HN
+
N
HN
Ph
N
N
Ph
N
Ph
N
N
N
Ph
N
H
N
H
7
1563

TABLE 3. 1H NMR Spectrum of Compounds 4–8
Com-
pound
δ, ppm (Ј, Hz)
4a
4b
2.51 (3H, s, CH3); 3.62 (3H, s, OCH3); 6.95 (2H, d, J = 7.8, Ar–H);
7.13 (2H, d, J = 7.6, Ar–H); 7.28 (2H, d, J = 7.8, Ar–H); 7.53 (2H, d, J = 7.6, Ar–H);
8.96 (1H, s, OH)
3.68 (3H, s, OCH3); 3.82 (3H, s, OCH3); 6.94 (2H, d, J = 7.8, Ar–H);
7.11 (2H, d, J = 7.6, Ar–H); 7.29 (2H, d, J = 8.0, Ar–H); 7.54 (2H, d, J = 8.0, Ar–H);
8.94 (1H, s, OH)
4c
5a
3.68 (3H, s, OCH3); 6.94 (2H, d, J = 7.8, Ar–H); 7.13 (2H, d, J = 7. 6, Ar–H);
7.28 (2H, d, J = 7.8, Ar–H); 7.53 (2H, d, J = 7.6, Ar–H); 8.94 (1H, s, OH)
3.82 (3H, s, OCH3); 3.98 (3H, s, OCH3); 5.77 (2H, s, CH2); 6.93 (2H, d, J = 9.1, Ar–H);
7.04 (2H, d, J = 9.0, Ar–H); 7.61 (2H, d, J = 2.4, Ar–H); 7.64 (2H, d, J = 2.1, Ar–H);
8.19 (1H, s, NH); 9.02 (2H, s, NH2)
5b
3.88 (3H, s, OCH3); 5.83 (2H, s, CH2); 6.91 (2H, d, J = 9.1, Ar–H);
7.02 (2H, d, J = 9.0, Ar–H); 7.59 (2H, d, J = 2.4, Ar–H); 7.64 (2H, d, J = 2.1, Ar–H);
8.20 (1H, s, NH); 9.03 (2H, s, NH2)
6a
6b
7
3.65 (3H, s, OCH3); 3.76 (3H, s, OCH3); 7.85-6.90 (9H, m, 8Ar–H and H-5);
9.26 (1H, s, NH); 9.37 (1H, s, NH)
3.76 (3H, s, OCH3); 7.88–6.92 (9H, m, 8Ar–H and H-5); 9.28 (1H, s, NH);
9.39 (1H, s, NH)
2.85 (3H, s, CH3); 7.62–6.68 (10H, m, Ar–H); 7.95 (1H, s, NH of pyridazine ring);
8.91 (1H, s, amidic NH); 9.58 (s, 1H, CHO)
8a
2.72 (2H, dd, J = 18 and J = 8, Ha and Hb); 3.28 (1H, d, J = 8.1, Hf);
3.52 (1H, d, J = 8.1, Hg); 3.65 (3H, s, OCH3); 3.67 (1H, s, OH); 3.73 (3H, s, OCH3);
3.82-3.74 (2H, m, He and Hc); 4.01 (1H, d, J = 10.8, Hd); 6.43 (2H, d, J = 7.8, Ar–H);
6.78 (2H, d, J = 7.8, Ar−H); 6.91 (2H, d, J = 7.8, Ar−H); 7.14 (1H, s, NH);
7.16 (2H, d, J = 8.1, Ar−H); 7.94 (1H, s, NH)
8b
1.93 (3H, s, CH3); 1.96 (3H, s, CH3); 2.68 (2H, dd, J = 18 and J = 8, Ha and Hb);
3.26 (1H, d, J = 8.1, Hf); 3.51 (1H, d, J = 8.1, Hg); 3.68 (1H, s, OH);
3.82-3.75 (2H, m, He and Hc); 4.01 (1H, d, J = 10.8, Hd); 6.41 (2H, d, J = 7.8, Ar−H);
6.79 (2H, d, J = 7.8, Ar−H); 6.90 (2H, d, J = 7.8, Ar−H); 7.14 (1H, s, NH);
7.17 (2H, d, J = 8.1, Ar−H); 7.96 (1H, s, NH)
Scheme 4
NC
CN
N
Ar
H
c
H
O
NC
2
H2N
O
O
a
H+, EtOH
H
g
N
C
H
N
∆
b
N
OH
H
OH
H
Ar
f
N
Ar
1a,b
N
d
Ar
H
e
Ar
F
8a,b
1, 8 a Ar = 4-OMeC6H4, b Ar = 4-MeC6H4
The mass spectrum of compound 8b showed the molecular ion peak at m/z 400; this fitted exactly with the
calculated mass. The IR spectra of compound 8a,b showed a weak absorption bands at 2225–2230 cm−1, which
showed the presence of a carbonitrile group. Physical and spectroscopic data are given in Tables 1−3.
1564

EXPERIMENTAL
Melting points were determined on a Gallenkamp apparatus and are uncorrected. IR spectra were
recorded in potassium bromide using Perkin Elmer 1650 and Pye-Unicam SP300 IR spectrophotometers. NMR
spectra were recorded in deutrated DMSO with TMS as an internal standard using a Varian Gemini 300 NMR
spectrometer. Mass spectra were recorded on a GCMS-QP 1000 EX Shimadzu and GCMS 5988-A HP
spectrometers. Elemental analyses were carried out in the Microanalytical Laboratory of Cairo University, Giza,
Egypt. Pyrrolidin-3-one derivatives 1a,b were prepared according to the procedures reported [8-10], and
hydroxylamine⋅HCl and phenylhydrazine were purchased from British Drug Houses (BDH). Acrylonitrile, ethyl
bromoacetate, and primary aromatic amines were purchased from Aldrich Company. Sodium acetate trihydrate,
sodium nitrite, and solvents were purchased from EI-Nasr Pharmaceutical Chemical Co. (ADWIC), Egypt.
1-Aryl-4-cyanopyrrolidine-3 Oximes 2a,b. A mixture of compound 1a or 1b (2 mmol) in ethanol
(20 ml), hydroxylamine⋅HCl (0.14 g, 2 mmol) and fused sodium acetate (0.16 g, 2 mmol) was refluxed for 2 h,
then left to stand at room temperature. The product obtained was filtered off and recrystallized from ethanol.
2-Arylhydrazono-4-cyano-1-(4'-methoxyphenyl)-3-oxopyrrolidines 4a–c. To an ice-cooled mixture of
compound 1a (2.16 g, 1 mmol) and sodium acetate trihydrate (4.1 g, 5 mmol) in ethanol (50 ml) a solution of the
appropriate diazonium salt (1 mmol) was added dropwise with stirring for 15 min. The stirring was continued further
for 30 min, and the reaction mixture was allowed to stand overnight in ice. The solid product was collected by
filtration and recrystallized from ethanol.
6-Amino-4-(4'-methoxyphenyl)-3-phenylhydrazono-3,5-dihydropyrrolo[3,4-c]isoxazoles
5a,b.
Hydroxylamine⋅HCl (0.14 g, 2 mmol) and ethanolic sodium ethoxide (prepared from 0.5 g sodium metal and 15 ml
of ethanol) were added to a suspension of 4b or 4c (2 mmol) in ethanol (20 ml). The reaction mixture was refluxed
for 4 h and then evaporated in vacuo. The remaining solid product was dissolved in water and neutralized with
diluted acetic acid. The resulting solid product was collected by filtration and recrystallized from ethanol.
2-Aryl-6-cyano-4-(4'-methoxyphenyl)-1,3-dihydropyrrolo[2,3-d][1,2,3]triazoles 6a,b. A mixture of 4b or
4c (2 mmol) and phenylhydrazine (0.32 g, 3 mmol) in ethanol (25 ml) was refluxed for 3 h. The crystals obtained
after concentration were filtered off and recrystallized from ethanol.
3-Acetamido-4-formyl-2,6-diphenyl-1,2-dihydro-1,2,3-triazolo[4,5-c]pyridazine (7). A mixture of 4a, 4b
or 4c (2 mmol) and phenylhydrazine (0.27 g, 2.5 mmol) in glacial acetic acid (20 ml) was refluxed for 3 h. The
solvent was evaporated in vacuo to dryness. The resulting solid product was purified with ethanol and then
recrystallized from ethyl acetate. 13C NMR spectrum, δ, ppm: 19.27, 20.67, 38.56, 38.87, 39.18, 39.48, 39.80, 40.11,
40.41, 111.72, 112.10, 118.42, 118.93, 128.73, 129.12, 148.83, 149.43, 169.13, 175.30.
5-Amino-1,7-diaryl-3-cyano-8a-hydroxy-2,3,8,8a-tetrahydro-6H-dipyrrolo[3,2-b:3',4'-d]-4-pyrans
8a,b. A mixture of 1a or 1b (2 mmol) in ethanol (15 ml) and traces of conc. HCl was refluxed for 4 h. Then the
solid product formed during reflux was filtered off and recrystallized from DMF−H2O (3:1). 8a: 13C NMR
spectrum, δ, ppm: 20.77, 38.57, 38.88, 39.50, 39.80, 40.12, 40.42, 45.22, 53.61, 55.40, 60.45, 84.48, 86.10,
112.05, 114.70, 114.83, 122.07, 125.60, 132.13, 141.40, 151.02, 157.12, 162.81, 186.58.
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