Effect of substituents on absorption and fluorescence properties of pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines

Article abstract of DOI:10.1139/V08-155

A. convenient route was successfully developed for the synthesis of novel heterocycles such as pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines (PPP) from pyrazolo[3,4-b]pyridines in good yield. The PPP derivatives synthesized were further studied for their photophysical properties, and it was observed that absorption and emission λ_max changed, owing to the substituent effect at 4 positions. These compounds were obtained from highly reactive starting materials, 5-aminopyrazoles and α-acetyl γ-butyrolactone.

Full text of DOI:10.1139/V08-155

1070
Effect of substituents on absorption and
fluorescence properties of pyrazolo[3,4-
b]pyrrolo[2,3-d]pyridines
Bhausaheb K. Ghotekar, Muddassar A. Kazi, Madhukar N. Jachak, and
Raghunath B. Toche
Abstract: A convenient route was successfully developed for the synthesis of novel heterocycles such as pyrazolo[3,4-
b]pyrrolo[2,3-d]pyridines (PPP) from pyrazolo[3,4-b]pyridines in good yield. The PPP derivatives synthesized were fur-
ther studied for their photophysical properties, and it was observed that absorption and emission λmax changed, owing
to the substituent effect at 4 positions. These compounds were obtained from highly reactive starting materials, 5-
aminopyrazoles and α-acetyl γ-butyrolactone.
Key words: α-acetyl γ-butyrolactone, pyrazolo[3,4-b]pyrrolo[2,3-d] pyridine, absorption, emission, fluorescence.
Résumé : On a mis au point une voie réactionnelle qui permet de réaliser la synthèse de nouveaux hétérocycles, telles
les pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines (PPP) à partir de pyrazolo[3,4-b]pyridines, avec de bons rendements. On a
soumis les dérivés PPP synthétisés à des études de leurs propriétés photophysiques et on a observé que les valeurs des
λmax de leurs absorptions et de leurs émissions varient sous l’effet d’un changement dans la nature des substituants en
position 4. On a obtenu ces composés à partir de produits de départ très réactifs, soit les 5-aminopyrazoles et l’α-acé-
tyl-γ-butyrolactone.
Mots-clés : l’α-acétyl-γ-butyrolactone, pyrazolo[3,4-b]pyrrolo[2,3-d]pyridine, absorption, émission, fluorescence.
[Traduit par la Rédaction] Ghotekar et al. 1076
Introduction
sively studied (7–9). The 4-N,N-dimethylaminophenyl
derivatives of bis-pyrazolo[3,4-b;4′,3′-e]pyridine (DMA-
DMPP) and pyrazolo[3,4-b]quinoline (DMA-DPPQ) (Fig. 1)
are further representatives of bulky π-electron donor–accep-
tor compounds and were recently investigated in some detail
both experimentally and semi-empirically (10–16). It has
been shown that pyrazoloquinoline derivatives are efficient
blue emitters with absorption in the near-ultraviolet region
and with a fluorescence quantum yield near to unity. Substi-
tution with a dimethylanilino donor group results in strongly
solvent-polarity-dependent fluorescence properties (11, 12).
Derivatives of 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-
b;4′,3′-e]-pyridine (DMA-DMPP) with different substituents
in position 4 are compounds showing intense fluorescence in
the blue–green region and are considered for application as
fluorescence standards and luminophores in organic light-
emitting diodes (17–20). The derivatives with phenyl and 4-
methoxyphenyl substituent (H-DMPP and CH3O-DMPP,
respectively) are characterized by a very intense, solvent-
independent fluorescence. The nitro derivatives (NO2-
DMPP) show an unexpected photophysical behaviour.
Similarly, The pyrazolopyrrolopyridines with different sub-
stituents in position 4 are compounds that show intense
fluorescence in the blue–green region with a fluorescence
quantum yield near to unity (Fig. 2).
Photoinduced intramolecular electron transfer plays a key
role in the photophysics of electron-donor (D)-electron-
acceptor (A) pi/π conjugated systems connected formally by
a single bond. The molecular structure of their charge-
transfer excited states has been the object of numerous dis-
cussions for more than twenty years. One of the best known
and also widely accepted proposals for such a system is the
so-called TICT (twisted intramolecular charge transfer) state
model (1–3). The perpendicular geometry was questioned
recently as an inherent precondition to obtain intramolecular
charge transfer in such D–A systems (4, 5). For bulky π do-
nor and acceptor systems (e.g., 9-(4-dimethylaminophenyl)-
anthracene or 9,9′-bianthryl), the D–A conformation in the
fluorescing charge-transfer excited state was postulated to
become more coplanar than in the ground state (6). Recently,
several donor and (or) acceptor substituted compounds re-
lated to carbazoles, acridines, and so forth were synthesized,
and their photoinduced charge separation has been inten-
Received 26 May 2008. Accepted 17 September 2008.
Published on the NRC Research Press Web site at
canjchem.nrc.ca on 29 October 2008.
B.K. Ghotekar, M.A. Kazi, M.N. Jachak, and R.B. Toche.1
Organic Chemistry Research Center, Department of
Chemistry, K.R.T. Arts B.H. Commerce and A.M. Science
College, Gangapur Road, Nashik 422002, (M.S.), India.
Apart from this, the pyrazolo[3,4-b]pyridines and
pyrazolo[3,4-b]pyrrolo[2,3-d]pyridine are examples of con-
densed heterocyclic compounds with notable pharmacologi-
cal and biological activities, such as hypotensive (21),
cytotoxic (22), and antibacterial activity (23), and found to
1Corresponding author (e-mail:
raghunath_toche@rediffmail.com).
Can. J. Chem. 86: 1070–1076 (2008)
doi:10.1139/V08-155
© 2008 NRC Canada

Ghotekar et al.
1071
Fig. 1. X-diphenylpyrazoloquinolines.
compounds 4. Thus, the syntheses of pyrazolo[3,4-
b]pyrrolo[2,3-d]pyridines 4 were achieved by the reaction of
compounds 3 with primary aliphatic or aromatic amines con-
taining catalytic amount of triethylamine at 120–150 °C to
furnish the desired compound 4. All crude products that
were dissolved in methanol and collected by suction filtra-
tion, washed with ethanol, and purified by using suitable
solvents, afforded the title compounds 4 in about 70%–80%
yield. These compounds showed the absorption and fluores-
cence near ultraviolet region.
1
The structure of compounds 3 was confirmed by IR, H
NMR, 13C NMR, mass spectroscopy, and formulae confirm
1
by elemental analysis. For example, the H NMR spectrum
of 3a showed a singlet at δ 2.81 for methyl protons, two trip-
lets at δ 3.39 and 3.75 corresponding to –CH2 and –CH2Cl
protons, respectively, and aromatic protons showed expected
chemical shifts and splitting patterns. The mass spectrum of
3a revealed a molecular ion peak m/z at 416. The 13C NMR
spectrum of this compound is in agreement with the struc-
ture proposed. Further, this structure was confirmed by NOE
experiment; thus, on irradiation of methyl signal at δ 2.81,
the compounds showed no NOE enhancement of any aro-
matic protons. This indicated that the methyl group present
at 6 positions and not at 4, which supported the proposed
structure 3. Similarly, the structure of 4 was confirmed by
spectroscopic and analytical methods, which is given in the
Experimental section. The absorption and fluorescence data
listed in Tables 1 and 2 showed that the incorporation of
constituent on to the pyrazolo[3,4-b]pyridine and
pyrazolo[3,4-b]pyrrolo[2,3-d]pyridine nucleus at 4 position
has profound influence on the absorption and emission prop-
erties. It was observed that substitution at 4 position showed
considerable increase in the emission wavelength, as the
π-electron-donating effect of amino group increases
(Scheme 1).
It was interesting to note that the pyrazolo[3,4-b]pyridines
showed the lower absorption and emission maxima than
pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines. In the pyrazolo[3,4-
b]pyridines 3, this might be due to a chloro group at 4 posi-
tion with a weak and small electron-donor unit in para
position, and in pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines 4,
the substituted amino group at 4 position with a strong and
bulky electron-donor unit at para position showed higher ab-
sorption and emission maxima.
Fig. 2. R,X-pyrazolopyrrolopyridines.
be potential purine antagonists (24). Analogous to bis-
pyrazolo-[3,4-b;4′,3′-e]-pyridines and pyrazolo-[3,4-b]-
quinolines, the substituent effect of 4 position on the fluores-
cence of pyrazolo[3,4-b]pyrrolo[2,3-d]pyridine has attracted
us to synthesize this particular family of compounds. As a
part of our ongoing interest in this area (25–28), we have
reported the synthesis of pyrazolo[3,4-b]pyridines, pyrazolo-
[3,4-b]quinolines, pyrazolonaphthyridines, and pyrazolo-
pyridopyrimidines by Friedlander condensation of reactive
methylene compounds with 5-aminopyrazole and synthesis
of fused pyrimidines. In our recent communications (29, 30),
we reported the synthesis and study of fluorescence proper-
ties of benzo[h]quinolines and dipyrazolo[3,4-b:3,4-d]pyri-
dines.
In the present article, we report the study of the
photophysical properties of newly synthesized pyrazolo[3,4-
b]pyridines with 4-chloro-5-chloroethyl side chain (31, 32)
and pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines. All these com-
pounds, having large non-coplanar π-electron donor and ac-
ceptor subsystems because of sterical hindrance, are suitable
objects to check the validity of the photophysical properties
of these compounds.
It is clearly indicated that the fluorescence property of
organic compounds is the substituent-dependent property, and it
is changed with changing the nature of different substituent
(e.g., in compounds 3 and 4, the nature of substituent
changed the photophysical properties of these compounds).
It is enhanced by substituting the strong π-electron donor
group at 4 position. Compounds 3a and 4a showed good ab-
sorption and emission spectra as shown in Fig. 3.
Photophysical properties
From the results obtained (Tables 1 and 2), it has been
found that both pyrazolo[3,4-b]pyridines 3 and pyrazolo[3,4-
b]pyrrolo[2,3-d]pyridines 4 show florescence properties. The
compounds 4f, 4g, 4k, and 4l exhibit remarkable fluores-
cence characters with high relative quantum yields. It is con-
cluded that the attachment of an electron-donating group to
phenyl function oriented at the 4 position of pyrazolo[3,4-
Results and discussion
The syntheses of pyrazolo[3,4-b]pyridines
3
were
achieved from highly reactive starting materials, 5-
aminopyrazoles 1 and α-acetyl-γ-butyrolactone 2. Com-
pounds 3 were used as precursors for the syntheses of
© 2008 NRC Canada

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Can. J. Chem. Vol. 86, 2008
Table 1. The photophysical data for electronic absorption (abs) and fluorescence (flu) of
pyrazolo[3,4-b]pyridines 3 in CHCl3.
Compound
Ar
λabs (CHCl3)
λflu (CHCl3)
ΦF (CHCl3)
3a
3b
3c
p-ClC6H4
p-BrC6H4
p-MeC6H4
362.00
366.00
361.00
394.00
401.00
392.00
0.120
0.126
0.123
Table 2. The photophysical data for electronic absorption (abs) and fluorescence (flu) of
pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines 4 in CHCl3.
Compd.
Ar
R
λabs (CHCl3)
λflu (CHCl3)
ΦF (CHCl3)
4a
4b
4c
4d
4e
4f
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-BrC6H4
p-BrC6H4
p-BrC6H4
p-BrC6H4
p-BrC6H4
p-MeC6H4
p-MeC6H4
C6H5CH2–
C6H5
392.00
393.00
392.00
395.00
397.00
399.00
380.00
395.00
390.00
373.00
396.00
398.00
377.00
396.00
416.00
419.00
417.00
421.00
422.00
425.00
429.00
420.00
412.00
421.00
425.00
427.00
406.00
418.00
0.141
0.145
0.138
0.136
0.129
0.151
0.146
0.119
0.124
0.127
0.148
0.153
0.133
0.131
m-ClC6H4
p-ClC6H4
p-BrC6H4
o-MeC6H4
p-MeC6H4
m-ClC6H4
p-ClC6H4
p-BrC6H4
o-MeC6H4
p-MeC6H4
C6H5CH2–
C6H5
4g
4h
4i
4j
4k
4l
4m
4n
Scheme 1. Synthesis of pyrazolopyridines 3 and pyrazolopyrrolopyridines 4.
b]pyrrolo[2,3-d]pyridine enhances the fluorescence proper-
ties and in turn gives high relative quantum yield.
Pyrazolo[3,4-b]pyrrolo[2,3-d]pyridines showed higher fluo-
rescent properties than the pyrazolo[3,4-b]pyridines; it might
be due to the steric hindrance of bulky phenyl rings and
large π-electron donor group at 4 position.
Conclusion
The reactions reported here represent the synthesis of a
new class of fluorescent compounds in which the 4 position
of pyrazolopyridine ring is substituted by the chlorine or ni-
trogen atom with different phenyl rings. These compounds
Experimental
General
Melting points were determined on a Gallenkamp melt-
ing-point apparatus, Model MFB595, in open capillary tubes
showed considerable absorption and emission λmax
.
© 2008 NRC Canada

Ghotekar et al.
1073
Fig. 3. The comparative (a, b) absorption and (c, d) emission
spectra of compounds 3a and 4a.
2.81 (s, 3H, CH3), 3.39 (t, J = 7.2 Hz, 2H, CH2), 3.75 (t, J =
7.2 Hz, 2H, CH2), 7.44–7.54 (m, 5H, Ar–H), 7.69 (d, J =
8.2, Hz, 2H, Ar–H), 8.26 (d, J = 8.2, Hz, 2H, Ar–H). 13C
NMR (75 MHz, CDCl3) δ: 22.3, 27.5, 55.4, 115.4, 121.2,
124.6, 126.5, 128.2, 129.6, 130.5, 131.1, 134.3, 139.5,
144.8, 154.2, 156.1, 159.2. MS (70 eV) m/z (%): 422 (20)
[M + 6], 420 (60) [M + 4], 418 (92) [M + 2], 416 (90) [M+],
366 (10), 255 (10), 111 (15), 85 (40), 71 (60), 57 (100), 55
(30). Anal. calcd. for C21H16Cl3N3 (416.73): C, 60.52; H,
3.87; N, 10.08. Found: C, 61.02; H, 3.18; N, 10.18.
3-(4-Bromophenyl)-4-chloro-5-(2-chloroethyl)-6-methyl-1-
phenyl-1H-pyrazolo[3,4-b] pyridine (3b)
Yield: 3.55 g (77%), recrystallized from acetonitrile/etha-
nol (8:2) to afford colourless needles; mp 175–176 °C. IR
(KBr): 2911m, 1591m, 1500s, 1259m, 1149m, 1093w,
1018w, 910w, 762m, 686w cm–1. 1H NMR (300 MHz,
CDCl3) δ: 2.82 (s, 3H, CH3), 3.40 (t, J = 7.2 Hz, 2H, CH2),
3.70 (t, J = 7.2 Hz, 2H, CH2), 7.29–7.48 (m, 5H, Ar–H),
7.53 (d, J = 8.2 Hz, 2H, Ar–H), 8.14 (d, J = 8.2 Hz, 2H, Ar–
H). 13C NMR (75 MHz, CDCl3) δ: 21.1, 29.8, 53.9, 118.2,
122.3, 125.8, 128.1, 129.2, 130.6, 131.5, 132.1, 134.1,
139.9, 144.8, 155.3, 157.2, 163.2. MS (70 eV) m/z (%): 466
(20) [M + 6], 464 (60) [M + 4], 462 (92) [M + 2], 460 (90)
[M+], 437 (10), 425 (100), 404 (10), 378 (15), 362 (40), 338
(60), 313 (10), 299 (30), 182 (30), 127 (15), 117 (40), 77
(80), 65 (60). Anal. calcd. for C21H16BrCl2N3 (461.18): C,
54.69; H, 3.50; N, 9.11. Found: C, 54.98; H, 3.78; N, 9.21.
and are uncorrected. 1H and 13C NMR spectra were recorded
on a Varian XL-300 spectrometer (300 and 75 MHz). Chem-
ical shifts are reported in ppm from internal
tetramethylsilane standard and are given in δ units. The sol-
vents for NMR spectra was duterio-chloroform unless other-
wise stated. Infrared spectra were taken on a Shimadzu IR-
408 (a Shimadzu FTIR instrument) in potassium bromide
pellets unless otherwise stated. UV spectra were recorded on
a Shimadzu UV-1601 UV–vis Spectrophotometer. Com-
pounds for UV scan were dissolved in methanol. Fluores-
cence spectra were recorded using RF-5301 PC
spectrofluorophotometer. Compounds for fluorescence mea-
surements were dissolved in chloroform. UV and fluores-
cence scans were recorded from 200 to 600 nm. Elemental
analyses were performed on a Hosli CH-Analyzer and the
results are within 0.4 of the theoretical percentage. HR-
MS were obtained with a Mat 112 Varian Mat Bremen
(70 eV) mass spectrometer. Solutions were concentrated in a
rotary evaporator under reduced pressure. All reactions were
monitored by TLC, carried out on 0.2 mm silica gel 60 F
254 (Merck) plates, using UV light (254 and 366 nm) for
detection. Common reagent-grade chemicals are either com-
mercially available and were used without further purifica-
tion or prepared by standard literature procedures.
4-Chloro-5-(2-chloroethyl)-6-methyl-3-(4-methylphenyl)-1-
phenyl-1H-pyrazolo[3,4-b] pyridine (3c)
Yield: 2.57 g (65%), recrystallized from acetonitrile/etha-
nol (8:2) to afford colourless needles; mp 145–146 °C. IR
(KBr): 2919m, 1592m, 1500s, 1255m, 1149m, 1093w,
1020w, 915w, 760m, 688w cm–1. 1H NMR (300 MHz,
CDCl3) δ: 2.39 (s, 3H, Ar–CH3), 2.80 (s, 3H, CH3), 3.38 (t,
J = 7.3 Hz, 2H, CH2), 3.68 (t, J = 7.3 Hz, 2H, CH2), 7.30–
7.56 (m, 5H, Ar–H), 7.60 (d, J = 8.3 Hz, 2H, ArH), 8.18 (d,
J = 8.3 Hz, 2H, ArH). 13C NMR (75 MHz, CDCl3) δ: 18.1,
24.3, 26.8, 41.3, 106.7, 120.2, 126.3, 127.4, 129.4,
129.6 130.1, 138.4, 138.9, 139.7, 140.6, 145.9, 150.5, 155.3.
Anal. calcd. for C22H19Cl2N3 (396.31): C, 66.67; H, 4.83; N,
10.60. Found: C, 66.90; H, 4.38; N, 10.83.
General procedure for the 4-chloro-5-(2-chloroethyl)-3-
(4-aryl)-6-methyl-1-phenyl-1H-pyrazolo[3,4-b]pyridine
(3a–3c)
General procedure for the synthesis of 1-phenyl-8-(4-
aryl)-4-methyl-6-phenyl-1,2,3,6-tetrahydropyrazolo[3,4-
b]pyrrolo[2,3-d]pyridine (4a–4n)
A mixture of 3 (0.01 mol) and primary aliphatic and aro-
matic amines (0.04 mol) was heated at 110–120 °C for about
2 h until TLC showed no more starting material. Then, the
mixture was cooled at 20 °C. After cooling, methanol
(20 ml) was added, and the resulting solid was filtered by
suction, washed with methanol, dried, and recrystallized
from the proper solvent to afford 4 in good yield.
A mixture of 5-aminopyrazole 1 (0.01 mol) and α-acetyl-
γ-butyrolactone 2 (1.282 g, 1.078 mL 0.01 mol) was
refluxed in phosphorous oxychloride (20 mL) until the end
of the exothermic reaction, which usually starts about 80–
90 °C. The mixture was then refluxed for further 4 h. Excess
POCl3 was removed under vacuum, and the oily residue so-
lidified upon treatment with ice-water, neutralization with
sodium carbonate, and stirring overnight. The separated
product was then filtered, dried, and recrystallized to afford
compound 3 in good yield.
1-Benzyl-8-(4-chlorophenyl)-4-methyl-6-phenyl-1,2,3,6-
tetrahydropyrazolo[3,4-b]pyrrolo [2,3-d]pyridine (4a)
Yield: 4.01 g (89%), recrystallized from ethanol to afford
colourless prisms; mp 150–151 °C. IR (KBr): 2909m,
1744m, 1598s, 1500s, 1257m, 1147m, 1093m, 1018m,
4-Chloro-5-(2-chloroethyl)-3-(4-chlorophenyl)-6-methyl-1-
phenyl-1H-pyrazolo[3,4-b]pyridine (3a)
Yield: 3.25 g (78%), recrystallized from acetonitrile/ethanol
(8:2) to afford colourless prisms; mp 163–164 °C. IR (KBr):
2918m, 1595m, 1500s, 1257m, 1147m, 1093w, 1018w,
906w, 756m, 686w cm–1. H NMR (300 MHz, CDCl3) δ:
906m, 756w, 686m cm–1. H NMR (300 MHz, CDCl3) δ:
1
1
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Can. J. Chem. Vol. 86, 2008
2.50 (s, 3H, CH3), 3.06 (t, J = 7.3 Hz, 2H, CH2), 3.50 (t, J =
7.3 Hz, 2H, CH2), 4.14(s, 2H, Ar–CH2), 6.98–7.06 (m, 5H,
Ar–H), 7.24 (d, J = 8.2 Hz, 2H, ArH), 7.43–7.49 (m, 5H,
ArH), 8.29 (d, J = 8.2 Hz, 2H, ArH). 13C NMR (75 MHz,
CDCl3) δ: 21, 26, 53, 56, 101, 112, 115, 122, 124, 126, 126,
127, 128, 128, 129, 130, 133, 134, 136, 143, 154 156. MS
m/z (%): 452 (90) [M + 2], 450 (100) [M+], 447 (20), 359
(25), 324 (35), 186 (20), 167 (20), 139 (10), 123 (30), 111
(15), 91 (90), 77 (80), 65 (60). Anal. calcd. for C28H23ClN4
(450.96): C, 74.57; H, 5.14; N, 12.42. Found: C, 74.67; H,
4.88; N, 12.69.
1-(4-Bromophenyl)-8-(4-chlorophenyl)-4-methyl-6-phenyl-
1,2,3,6-tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4e)
Yield: 4.07 g (79%), recrystallized from ethanol/DMF
(6:4) to afford yellow prisms; mp 231–232 °C. IR (KBr):
2918m, 1743m, 1595s, 1500s, 1257m, 1147, 1093m, 1018m,
1
906m, 756w, 686m cm–1. H NMR (300 MHz, CDCl3) δ:
2.58 (s, 3H CH3), 3.28 (t, J = 7.1 Hz, 2H, CH2), 4.87 (t, J =
7.1 Hz, 2H, CH2), 6.22–7.55 (m, 9H, Ar–H), 8.20 (d, J =
7.8 Hz, 2H, Ar–H), 8.33 (d, J = 7.8 Hz, 2H, Ar–H). 13C
NMR (75 MHz, CDCl3) δ: 17.9, 23.2, 52.5, 103.2, 111.5,
115.7, 120.2, 124.7, 126.3, 128.9, 129.4, 131.2, 132.5,
134.3, 139.7, 138.2, 145.9, 148.4, 149.1, 154.3, 156.2. Anal.
calcd. for C27H20BrClN4 (515.83): C, 62.87; H, 3.91; N,
10.86. Found: C, 62.63; H, 4.16; N, 11.09.
8-(4-Chlorophenyl)-4-methyl-1,6-diphenyl-1,2,3,6-
tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4b)
Yield: 3.58 g (82%), recrystallized from ethanol/DMF
(8:2) to afford light green prisms; mp 212–213 °C. IR
(KBr): 2910m, 1738m, 1588s, 1503s, 1251m, 1144m,
1099m, 1017m, 906m, 756w, 686m cm–1. 1H NMR
(300 MHz, CDCl3) δ: 2.56 (s, 3H, CH3), 3.21 (t, J = 7.1 Hz,
2H, CH2), 4.20 (t, J = 7.1 Hz, 2H, CH2), 6.74–6.90 (m, 5H,
Ar–H), 7.24–7.45 (m, 5H, Ar–H), 7.48 (d, J = 7.5 Hz, 2H,
Ar–H), 8.31 (d, J = 7.5 Hz, 2H, Ar–H). 13C NMR (75 MHz,
CDCl3) δ: 19.3, 22.8, 51.5, 104.6, 113.5, 117.2, 120.2,
124.7, 126.8, 128.9, 129.4, 129.6 131.1, 134.3, 139.7, 145.9,
149.1, 154.3, 156.3. MS m/z (%): 438 (80) [M + 2], 436
(100) [M+], 419 (20), 399 (25), 361 (35), 298 (20), 255 (20),
218 (10), 200 (30), 192 (15), 179 (30), 152 (35), 111 (10),
91 (10), 77 (60), 65 (15). Anal. calcd. for C27H21ClN4
(436.93): C, 74.22; H, 4.84; N, 12.82. Found: C, 73.87; H,
5.05; N, 12.68.
8-(4-Chlorophenyl)-1-(2-methylphenyl)-4-methyl-6-phenyl-
1,2,3,6-tetrahydropyrazolo-[3,4-b]pyrrolo[2,3-d]pyridine
(4f)
Yield: 3.96 g (88%), recrystallized from ethanol/DMF
(8:2) to afford light green prisms; mp 220–222 °C. IR
(KBr): 2918m, 1743m, 1595s, 1500s, 1257m, 1147, 1093m,
1018m, 906m, 756w, 686m cm–1. 1H NMR (300 MHz,
CDCl3) δ: 2.30 (s, 3H, Ar–CH3), 2.61 (s, 3H, CH3), 3.25 (t,
J = 7.2 Hz, 2H, CH2), 4.22 (t, J = 7.2 Hz, 2H, CH2), 7.24–
7.45 (m, 9H, Ar–H), 7.45 (d, J = 8.2 Hz, 2H, Ar–H), 8.35
(d, J = 8.2 Hz, 2H, Ar–H). 13C NMR (75 MHz, CDCl3) δ:
15.8, 18.6, 21.3, 51.8, 105.6, 113.3, 117.1, 120.2, 124.7,
126.2, 126.3, 126.6, 128.9, 129.4, 129.9, 131.2, 134.3,
139.7, 145.9, 149.1, 154.9, 157.4. Anal. calcd. for
C28H23ClN4 (450.96): C, 74.57; H, 5.14; N, 12.42. Found: C,
74.78; H, 4.96; N, 12.18.
1-(3-Chlorophenyl)-8-(4-chlorophenyl)-4-methyl-6-phenyl-
1,2,3,6-tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4c)
Yield: 3.67 g (78%), recrystallized from ethanol/DMF
(6:4) to afford light green prisms; mp 217–218 °C. IR
(KBr): 2918m, 1743m, 1595s, 1500s, 1257m, 1147m,
1093m, 1018m, 906m, 756w, 686m cm–1. 1H NMR
(300 MHz, CDCl3) δ: 2.57 (s, 3H, CH3), 3.26 (t, J = 7.2 Hz,
2H, CH2), 4.86 (t, J = 7.2 Hz, 2H, CH2), 6.23–7.51 (m, 9H
Ar–H), 8.20 (d, J = 7.8 Hz, 2H, Ar–H), 8.34 (d, J = 7.8 Hz,
2H, Ar–H). 13C NMR (75 MHz, CDCl3) δ: 19.4, 22.5, 53.2,
106.2, 113.6, 114.1, 116.1, 121.3, 122.8, 123.5, 128.2,
129.1, 130.5, 131.6, 133.3, 134.1, 135.4, 140.7, 144.9,
148.3, 151.2, 154.6, 158.6. Anal. calcd. for C27H20Cl2N4
(471.38): C, 68.80; H, 4.28; N, 11.89. Found: C, 69.09; H,
4.60; N 12.10.
8-(4-Chlorophenyl)-1-(4-methylphenyl)-4-methyl-6-phenyl-
1,2,3,6-tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4g)
Yield: 3.74 g (83%), recrystallized from ethanol/DMF
(8:2) to afford light green prisms; mp 229–230 °C. IR
(KBr): 2918m, 1743m, 1595s, 1500s, 1257m, 1147, 1093m,
1018m, 906m, 756w, 686m cm–1. 1H NMR (300 MHz,
CDCl3) δ: 2.30 (s, 3H, Ar–CH3), 2.60 (s, 3H, CH3), 3.25 (t,
J = 7.2 Hz, 2H, CH2), 4.22 (t, J = 7.2 Hz, 2H, CH2), 7.24–
7.45 (m, 9H, Ar–H), 7.45 (d, J = 8.2 Hz, 2H, Ar–H), 8.35
(d, J = 8.2 Hz, 2H, Ar–H). 13C NMR (75 MHz, CDCl3) δ:
18.6, 21.5, 24.3, 51.5, 103.6, 113.4, 120.2, 124.7, 126.3,
126.8, 128.9, 129.4, 129.9, 131.2, 134.9, 139.2, 145.9,
146.4, 149.9, 154.9, 157.4. Anal. calcd. for C28H23ClN4
(450.96): C, 74.57; H, 5.14; N, 12.42. Found: C, 74.69; H,
5.32; N 12.61.
1,8-Bis(4-chlorophenyl)-4-methyl-6-phenyl-1,2,3,6-
tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4d)
Yield: 3.39 g (72%), recrystallized from ethanol/DMF
(6:4) to afford light yellow prisms; mp 235–237 °C. IR
(KBr): 2918m, 1743m, 1595s, 1500s, 1257m, 1147m,
1093m, 1018m, 906m, 756w, 686m cm–1. 1H NMR
(300 MHz, CDCl3) δ: 2.57 (s, 3H, CH3), 3.26 (t, J = 7.2 Hz,
2H, CH2), 4.86 (t, J = 7.2 Hz, 2H, CH2), 6.23–7.51 (m, 9H,
Ar–H), 8.20 (d, J = 7.8 Hz, 2H, Ar–H), 8.34 (d, J = 7.8 Hz,
2H, Ar–H). 13C NMR (75 MHz, CDCl3) δ: 18.4, 21.6, 52.5,
108.6, 115.3, 119.2, 121.2, 124.7, 126.3, 128.8, 129.1,
129.9, 132.3, 135.4, 138.2, 144.4, 147.5, 149.1, 154.3,
155.3. Anal. calcd. for C27H20Cl2N4 (471.38): C, 68.80; H,
4.28; N, 11.89. Found: C, 68.67; H, 4.10; N, 12.02.
8-(4-Bromophenyl)-1-(3-chlorophenyl)-4-methyl-6-phenyl-
1,2,3,6-tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4h)
Yield: 3.92 g (76%), recrystallized from ethanol/DMF
(6:4) to afford light brown prisms; mp 226–227 °C. IR
(KBr): 2918m, 1743m, 1595s, 1500s, 1257m, 1147, 1093m,
1018m, 906m, 756w, 686m cm–1. 1H NMR (300 MHz,
CDCl3) δ: 2.59 (s, 3H CH3), 3.28 (t, J = 7.2 Hz, 2H, CH2),
4.87 (t, J = 7.2 Hz, 2H, CH2), 6.22–7.55 (m, 9H, Ar–H),
8.20 (d, J = 8.2 Hz, 2H, ArH), 8.33 (d, J = 8.2 Hz, 2H,
ArH). 13C NMR (75 MHz, CDCl3) δ: 18.7, 21.3, 51.5,
104.6, 111.2, 113.8, 117.3, 120.8, 123.1, 124.9, 126.2,
129.1, 129.5, 131.6, 132.2, 132.1, 135.4, 139.7, 145.9,
149.3, 150.2, 154.6, 157.6. Anal. calcd. for C27H20BrClN4
© 2008 NRC Canada

Ghotekar et al.
1075
(515.83): C, 62.87; H, 3.91; N, 10.86. Found: C, 63.09; H,
4.06; N 10.66.
126.8, 127.9, 129.4, 129.9, 131.2, 134.9, 140.2, 142.9,
146.4, 147.9, 151.9, 156.2. Anal. calcd. for C28H23BrN4
(495.41): C, 67.88; H, 4.68; N, 11.31. Found: C, 68.06; H,
4.98; N 11.52.
8-(4-Bromophenyl)-1-(4-chlorophenyl)-4-methyl-6-phenyl-
1,2,3,6-tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4i)
Yield: 3.81 g (74%), recrystallized from ethanol/DMF
(6:4) to afford light green prisms; mp 240–241 °C. IR
(KBr): 2918m, 1743m, 1595s, 1500s, 1257m, 1147, 1093m,
1018m, 906m, 756w, 686m cm–1. 1H NMR (300 MHz,
CDCl3) δ: 2.58 (s, 3H, CH3), 3.28 (t, J = 7.2 Hz, 2H, CH2),
4.87 (t, J = 7.2 Hz, 2H, CH2), 6.22–7.55 (m, 9H, Ar–H),
8.20 (d, J = 7.8 Hz, 2H, Ar–H), 8.33 (d, J = 7.8 Hz, 2H, Ar–
H). 13C NMR (75 MHz, CDCl3) δ: 16.4, 23.6, 53.5, 107.6,
116.3, 119.2, 123.2, 124.7, 126.3, 127.8, 129.8, 130.9,
131.3, 135.4, 140.2, 145.4, 148.5, 150.1, 154.3, 159.3. Anal.
calcd. for C27H20BrClN4 (515.83): C, 62.87; H, 3.91; N,
10.86. Found: C, 63.13; H, 4.21; N 10.61.
1-Benzyl-4-methyl-8-(4-methylphenyl)-6-phenyl-1,2,3,6-
tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4m)
Yield: 3.48 g (81%), recrystallized from ethanol to afford
colourless prisms; mp 166–67 °C. IR (KBr): 2918m, 1743m,
1595s, 1500s, 1257m, 1147, 1093m, 1018m, 906m, 756w,
686m cm–1. 1H NMR (300 MHz, CDCl3) δ: 2.39 (s, 3H, Ar–
CH3), 2.80 (s, 3H, CH3), 3.38 (t, J = 7.1 Hz, 2H, CH2), 3.68
(t, J = 7.1 Hz, 2H, CH2), 4.14(s, 2H, Ar–CH2), 6.98–7.06
(m, 5H, Ar–H), 7.24 (d, J = 8.2 Hz, 2H, Ar–H), 7.43–7.49
(m, 5H, Ar–H), 8.29 (d, J = 8.2 Hz, 2H, Ar–H). 13C NMR
(75 MHz, CDCl3) δ: 17.6, 21.7, 24.3, 53.4, 104.9, 120.2,
124.7, 126.3, 127.1, 127.4, 128.6, 128.0, 129.4, 129.6,
130.1, 136.5, 138.4, 139.7, 145.9, 149.9, 154.9, 158.8. Anal.
calcd. for C29H26N4 (430.54): C, 80.90; H, 6.09; N, 13.01.
Found: C, 80.66; H, 6.41; N 13.32.
1,8-Bis(4-bromophenyl)-4-methyl-6-phenyl-1,2,3,6-
tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4j)
Yield: 4.31 g (77%), recrystallized from ethanol/DMF
(6:4) to afford light green prisms; mp 243–244 °C. IR
(KBr): 2918m, 1743m, 1595s, 1500s, 1257m, 1147, 1093m,
1018m, 906m, 756w, 686m cm–1. 1H NMR (300 MHz,
CDCl3) δ: 2.58 (s, 3H, CH3), 3.28 (t, J = 7.2 Hz, 2H, CH2),
4.87 (t, J = 7.2 Hz, 2H, CH2), 6.22–7.55 (m, 9H, Ar–H),
8.20 (d, J = 7.8 Hz, 2H, Ar–H), 8.33 (d, J = 7.8 Hz, 2H, Ar–
H). 13C NMR (75 MHz, CDCl3) δ: 19.9, 24.2, 50.5, 107.2,
114.5, 118.7, 121.2, 125.7, 127.3, 128.1, 129.6, 130.1,
131.0, 134.6, 139.0, 138.2, 143.9, 147.4, 148.1, 151.3,
160.2. Anal. calcd. for C27H20Br2N4 (560.28): C, 57.88; H,
3.60; N, 10.00. Found: C, 57.63; H, 3.21; N, 10.27.
1,6-Diphenyl-4-methyl-8-(4-methylphenyl)-1,2,3,6-
tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine (4n)
Yield: 3.49 g (84%), recrystallized from ethanol/DMF
(8:2) to afford colourless prisms; mp 208–209 °C. IR (KBr):
2918m, 1743m, 1595s, 1500s, 1257m, 1147, 1093m, 1018m,
1
906m, 756w, 686m cm–1. H NMR (300 MHz, CDCl3) δ:
2.40 (s, 3H, ArCH3), 2.82 (s, 3H, CH3), 3.21 (t, J = 7.1 Hz,
2H, CH2), 4.20 (t, J = 7.1 Hz, 2H, CH2), 6.74–6.90 (m, 5H,
Ar–H), 7.24–7.45 (m, 5H, Ar–H), 7.48 (d, J = 7.5 Hz, 2H,
Ar–H), 8.31 (d, J = 7.5 Hz, 2H, Ar–H). 13C NMR (75 MHz,
CDCl3) δ: 18.6, 21.3, 24.3, 51.5, 103.6, 113.5, 117.2, 120.2,
124.7, 126.3, 127.4, 129.6, 130.1, 138.4, 139.7, 145.9,
149.9, 154.9, 157.3. Anal. calcd. for C28H24N4 (416.51): C,
80.74; H, 5.81; N, 13.45. Found: C, 80.54; H, 5.98; N 13.52.
8-(4-Bromophenyl)-1-(2-methylphenyl)-4-methyl-6-phenyl-
1,2,3,6-tetrahydropyrazolo[3,4-b]pyrrolo[2,3-d]pyridine
(4k)
Yield: 4.35 g (88%), recrystallized from ethanol/DMF
(8:2) to afford light green prisms; mp 230–232 °C. IR
(KBr): 2918m, 1743m, 1595s, 1500s, 1257m, 1147, 1093m,
1018m, 906m, 756w, 686m cm–1. 1H NMR (300 MHz,
CDCl3) δ: 2.32 (s, 3H, Ar–CH3), 2.61 (s, 3H, CH3), 3.23 (t,
J = 7.2 Hz, 2H, CH2), 4.24 (t, J = 7.2 Hz, 2H, CH2), 7.25–
7.46 (m, 9H, ArH), 7.46 (d, J = 8.1 Hz, 2H, Ar–H), 8.36 (d,
J = 8.1 Hz, 2H, Ar–H). 13C NMR (75 MHz, CDCl3) δ: 16.8,
19.3, 25.3, 53.8, 109.6, 111.3, 118.1, 121.2, 123.7, 125.2,
126.3, 126.6, 128.9, 129.3, 129.3, 131.0, 134.1, 139.2,
145.1, 149.3, 154.9, 157.9. Anal. calcd. for C28H23BrN4
(495.41): C, 67.88; H, 4.68; N, 11.31. Found: C, 67.63; H,
4.51; N, 11.49.
Acknowledgements
The authors thank UGC, New Delhi, India, for financial
support of this research project. We thank Professor D.D.
Dhavale, Department of Chemistry, University of Pune, In-
dia, for his valuable cooperation for the measurements of
fluorescence properties.
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