Facile synthesis of 2H-indazole derivatives starting from the Baylis-Hillman adducts of 2-cyclohexen-1-one

Article abstract of DOI:10.1016/j.tetlet.2005.05.149

Facile synthetic method of 2H-indazole derivatives was developed involving DDQ oxidation of pyrazoles, which were prepared starting from the Baylis-Hillman adducts of 2-cyclohexen-1-one.

Full text of DOI:10.1016/j.tetlet.2005.05.149

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5387–5391
Facile synthesis of 2H-indazole derivatives starting from the
Baylis–Hillman adducts of 2-cyclohexen-1-one
Ka Young Lee, Saravanan Gowrisankar and Jae Nyoung Kim*
Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, South Korea
Received 22 April 2005; revised 27 May 2005; accepted 30 May 2005
Abstract—Facile synthetic method of 2H-indazole derivatives was developed involving DDQ oxidation of pyrazoles, which were
prepared starting from the Baylis–Hillman adducts of 2-cyclohexen-1-one.
Ó 2005 Elsevier Ltd. All rights reserved.
The indazole nucleus is a pharmaceutically important
structure and constitutes the key subunit in many drug
substances with a broad range of pharmacological activ-
ities1 including antitumor,1b antimicrobial,1h and
antiplatelet,1d and anti-inflammatory activities.1j How-
ever, there is still a lack of general and efficient method-
ologies for the synthesis of N-substituted indazoles2
although some of the reported methods could be used
effectively.
In order to oxidize the cyclohexane moiety of 2a (entry 1
in Table 1) we tried some reaction conditions including
the use of Pd/C, p-chloranil (tetrachloro-1,4-benzoquin-
one), and DDQ (2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone).4,5 Among the conditions, the DDQ-mediated
oxidations gave the best results. Encouraged by the re-
sults, we synthesized pyrazole derivatives 2 according
to the previous method3,6 and examined the transforma-
tion into 2H-indazole derivatives 3. As shown in Table
1, the pyrazoles 2a–g was prepared in moderate yields
(47–54%). The oxidation reaction of 2a–g with DDQ
(2.0 equiv) in benzene at refluxing temperature gave
the desired 2H-indazole derivatives 3a–g in moderate
to good yields (69–80%).7
Recently, we reported a regioselective synthesis of
1,3,4,5-tetrasubstituted pyrazoles from Baylis–Hillman
adducts.3 A variety of Baylis–Hillman adducts derived
from methyl vinyl ketone, ethyl vinyl ketone, 2-cyclo-
hexen-1-one, and 2-cyclopenten-1-one was converted
into the corresponding pyrazole derivatives.3 If we could
oxidize the cyclohexane moiety of the pyrazole 2, which
was derived from the Baylis–Hillman adduct of 2-
cyclohexen-1-one, into aromatic nucleus we could open
a new route toward the valuable 2H-indazole skeleton as
shown in Scheme 1.
For the pyrazole 2f, elimination of tert-butyl group was
observed and indazole 3f0 was obtained as the major
product (entry 6). During the DDQ oxidation of pyr-
azole 2h (Scheme 2), which was produced from the Bay-
lis–Hillman adduct of 4,4-dimethylcyclohex-2-en-1-one,
we could not find any aromatized compound, which
O
R'
R'
N
N
OH
O
R
H
N
N
R'NHNH2.HCl
DDQ (2.0 equiv)
R
R
R
O
benzene
ClCH2CH2Cl
reflux, 8-20 h
reflux, 24 h
1
2
3
Scheme 1.
Keywords: 2H-Indazoles; Baylis–Hillman adducts; DDQ; Pyrazoles; Carbazoles.
*
Corresponding author. Tel.: +82 62 530 3381; fax: +82 62 530 3389; e-mail: kimjn@chonnam.ac.kr
0040-4039/$ - see front matter Ó 2005Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.05.149

5388
K. Y. Lee et al. / Tetrahedron Letters 46 (2005) 5387–5391
Table 1. Synthesis of 2H-indazole derivatives 3a–g
Entry
1
B–H adduct 1a
Pyrazole 2 (%)b,c
Indazole 3 (%)c
Ph
Ph
OH
O
N
N
N
N
1a6b
2a (51)3
3a (69)2h
F
F
F
F
N
N
N
N
2
1a
2b (48)3
Ph
3b (75)
Ph
OH
O
N
N
N
N
3
Me
Me
Me
1b6d
2c (50)
Ph
3c (74)
Ph
OH
O
N
N
N
N
4
5
MeO
MeO
MeO
1c6c
2d (54)
3d (74)
Ph
OMe
Ph
OMe
OMe OH
O
N
N
N
N
1d6e
2e (48)
3e (80)
H
N
N
N
N
N
N
6
7
1a
,
3f' (57)
2f (49)
3f (20)
Ph
N
Ph
N
N
N
OH
O
1e6b
2g (47)
3g (69)
a B–H adducts were synthesized from the reaction of aldehydes and 2-cyclohexen-1-one with DMAP according to Ref. 6.
b Pyrazole derivatives were prepared from 1 and appropriate hydrazine derivatives according to Ref. 3.
c Isolated yields in parenthesis.
1
might be produced by the combination of oxidation and
methyl group migration. Instead, we obtained alcohol
derivative 4 in 58% yield. The plausible mechanism for
the formation of 4 is depicted in Scheme 2. As reported,
the DDQ oxidation involved the hydride transfer from
substrate to DDQ to form the carbocationic species as
the first step.4,5 For the substrate 2h hydride abstraction
at the 4-position occurred selectively to generate the
carbocationic intermediate (I). The intermediate (I)
could be stabilized by the nearby enamine moiety and
could be formed selectively, which in turn react with
moisture in the reaction mixture to produce the hydroxy-
lated compound 4. The position of OH group of 4 could
be easily assigned from its H NMR spectrum by the
disappearance of the singlet methylene peak
(d = 2.38 ppm, –CH2– at 4-position) of 2h.7 In addition,
disappearance of OH peak (d, d = 1.74 ppm) and the
change of the doublet (H-4, d = 4.17 ppm) into singlet
with D2O treatment showed the position of OH as
correct.7
Unfortunately, we could not obtain the corresponding
pyrazole derivative in the reaction of 1g and phenyl-
hydrazine hydrochloride. Instead, carbazole com-
pound 5 was prepared in 61% yield by following
the mechanism proposed in Scheme 3.8 Consecutive

K. Y. Lee et al. / Tetrahedron Letters 46 (2005) 5387–5391
5389
OH
Ph
Ph
O
N
N
N
N
DDQ (2.0 equiv)
PhNHNH2.HCl
ClCH2CH2Cl
reflux, 3 days
benzene
reflux, 24 h
HO
1f
2h (45%)
4 (58%)
O
Cl
Cl
CN
CN
H2O
Ph
N
N
O
OH
Cl
Cl
CN
CN
H
(I)
O
Scheme 2.
H
N
H
N
OH
OH
O
HN
OH
N
PhNHNH2.HCl
ClCH2CH2Cl
reflux, 4 h
O
O
O
1g
H2N
NH
1,5 H
shift
HN
OH
HN
- NH3
- H2O
[3,3]
O
O
O
5 (61%)
Scheme 3.
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K. J. Med. Chem. 2000, 43, 41; (h) Li, X.; Chu, S.; Feher, V.
A.; Khalili, M.; Nie, Z.; Margosiak, S.; Nikulin, V.; Levin,
J.; Sprankle, K. G.; Fedder, M. E.; Almassy, R.; Appelt,
K.; Yager, K. M. J. Med. Chem. 2003, 46, 5663; (i)
Giragossian, C.; Sugg, E. E.; Szewczyk, J. R.; Mierke, D. F.
J. Med. Chem. 2003, 46, 3476; (j) Wrobleski, S. T.; Chen,
P.; Hynes, J., Jr.; Lin, S.; Norris, D. J.; Pandit, C. R.;
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M.; Kiener, P. A.; McIntyre, K. W.; Patil-koota, V.;
formation of the corresponding hydrazone, conversion
to carbazole skeleton by following the typical Fischer
indole synthesis mechanism,8 and finally 1,5-hydrogen
shift might generate the carbazole derivative 5. How-
ever, we could not explain the differences for the synthe-
sis of pyrazoles between 1g and 1a–e at this stage.
In summary, we disclosed a new procedure for the prep-
aration of 2H-indazole derivatives starting from the
Baylis–Hillman adducts of 2-cyclohexen-1-one by using
DDQ oxidation procedure.
Acknowledgements
This work was supported by grant (R05-2003-000-
10042-0) from the Basic Research Program of the
Korea Science and Engineering Foundation (Now
controlled under the authority of Korea Research
Foundation).
References and notes
1. For the biological activities of indazole derivatives, see, (a)
Lien, J.-C.; Lee, F.-Y.; Huang, L.-J.; Pan, S.-L.; Guh, J.-H.;
Teng, C.-M.; Kuo, S.-C. J. Med. Chem. 2002, 45, 4947; (b)
Baraldi, P. G.; Balboni, G.; Pavani, M. G.; Spalluto, G.;
Tabrizi, M. A.; De Clercq, E.; Balzarini, J.; Bando, T.;

5390
K. Y. Lee et al. / Tetrahedron Letters 46 (2005) 5387–5391
Shuster, D. J.; Turk, L. A.; Yang, G.; Leftheris, K. J. Med.
Chem. 2003, 46, 2110.
(70 eV) m/z (rel. intensity) 63 (15), 76 (21), 128 (11), 152
(10), 306 (M+, 100).
2. For the synthesis of indazole derivatives, see (a) Lebedev,
A. Y.; Khartulyari, A. S.; Voskoboynikov, A. Z. J. Org.
Chem. 2005, 70, 596; (b) Shirtcliff, L. D.; Weakley, T. J. R.;
Haley, M. M.; Kohler, F.; Herges, R. J. Org. Chem. 2004,
69, 6979; (c) Antilla, J. C.; Baskin, J. M.; Barder, T. E.;
Buchwald, S. L. J. Org. Chem. 2004, 69, 5578; (d) Cheung,
M.; Boloor, A.; Stafford, J. A. J. Org. Chem. 2003, 68, 4093;
(e) Isin, E. M.; de Jonge, M.; Castagnoli, N., Jr. J. Org.
Chem. 2001, 66, 4220; (f) Sun, J.-H.; Teleha, C. A.; Yan,
J.-S.; Rodgers, J. D.; Nugiel, D. A. J. Org. Chem. 1997, 62,
5627; (g) Song, J. J.; Yee, N. K. Org. Lett. 2000, 2, 519; (h)
Alberti, A.; Bedogni, N.; Benaglia, M.; Leardini, R.; Nanni,
D.; Pedulli, G. F.; Tundo, A.; Zanardi, G. J. Org. Chem.
1992, 57, 607; (i) Lee, J. H.; Matsumoto, A.; Yoshida, M.;
Simamura, O. Chem. Lett. 1974, 951.
Compound 3c: 74%; white solid, mp 108–109 °C; IR (KBr)
1
3055, 1597, 1504 cmꢀ1; H NMR (CDCl3) d 2.37 (s, 3H),
7.08–7.47 (m, 11H), 7.70 (dt, J = 8.4 and 0.9 Hz, 1H), 7.79
(dt, J = 8.7 and 0.9 Hz, 1H); 13C NMR (CDCl3+1 drop of
CF3COOD) d 20.36, 114.55, 119.87, 120.02, 122.58, 123.80,
125.24, 128.39, 128.59, 128.65, 128.67, 128.79, 136.54,
137.12, 138.80, 144.89; FAB Mass 285(M ++1).
Compound 3d: 74%; white solid, mp 125–126 °C; IR (KBr)
1
3059, 1608, 1504 cmꢀ1; H NMR (CDCl3) d 3.79 (s, 3H),
6.89 (d, J = 9.0 Hz, 2H), 7.06–7.12 (m, 1H), 7.23–7.46 (m,
8H), 7.67 (dt, J = 8.7 and 0.9 Hz, 1H), 7.78 (dt, J = 8.7 and
0.9 Hz, 1H); 13C NMR (CDCl3) d 55.39, 114.42, 117.81,
120.75, 121.74, 122.28, 122.33, 126.13, 127.07, 128.27,
129.09, 131.07, 135.49, 140.47, 149.09, 159.73; FAB Mass
301 (M++1).
3. Lee, K. Y.; Kim, J. M.; Kim, J. N. Tetrahedron Lett. 2003,
44, 6737.
Compound 3e: 80%; white solid, mp 121–122 °C; IR (KBr)
1
3059, 1597, 1500 cmꢀ1; H NMR (CDCl3) d 3.32 (s, 3H),
4. For DDQ oxidation, see (a) Muller, P.; Rocek, J. J. Am.
Chem. Soc. 1972, 94, 2716; (b) Stoos, F.; Rocek, J. J. Am.
Chem. Soc. 1972, 94, 2719; (c) Thummel, R. P.; Cravey, W.
E.; Cantu, D. B. J. Org. Chem. 1980, 45, 1633; (d) Wurche,
F.; Sicking, W.; Sustmann, R.; Klarner, F.-G.; Ruchardt,
C. Chem. Eur. J. 2004, 10, 2707, and references cited
therein.
5. For hydroxylation and related oxidations with DDQ, see
(a) Xu, Y.-C.; Lebeau, E.; Gillard, J. W.; Attardo, G.
Tetrahedron Lett. 1993, 34, 3841; (b) Ramdayal, F. D.;
Kiemle, D. J.; LaLonde, R. T. J. Org. Chem. 1999, 64,
4607; (c) Zhang, Z.; Magnusson, G. J. Org. Chem. 1996, 61,
2394; (d) Corey, E. J.; Xiang, Y. B. Tetrahedron Lett. 1987,
28, 5403; (e) Tanemura, K.; Suzuki, T.; Nishida, Y.;
Satsumabayashi, K.; Horaguchi, T. J. Chem. Soc., Perkin
Trans. 1 2001, 3230.
6. For the synthesis of Baylis–Hillman adducts of cyclic
enones, see (a) Luo, S.; Mi, X.; Xu, H.; Wang, P. G.;
Cheng, J.-P. J. Org. Chem. 2004, 69, 8413; (b) Lee, K. Y.;
Gong, J. H.; Kim, J. N. Bull. Korean Chem. Soc. 2002, 23,
659; (c) Lee, K. Y.; Gowrisankar, S.; Kim, J. N. Tetrahe-
dron Lett. 2004, 45, 5485; (d) Patra, A.; Batra, S.; Joshi, B.
S.; Roy, R.; Kundu, B.; Bhaduri, A. P. J. Org. Chem. 2002,
67, 5783; (e) Aggarwal, V. K.; Dean, D. K.; Mereu, A.;
Williams, R. J. Org. Chem. 2002, 67, 510.
6.83 (d, J = 8.1 Hz, 1H), 7.02–7.10 (m, 2H), 7.21–7.46 (m,
8H), 7.57 (dt, J = 8.4 and 0.9 Hz, 1H), 7.79 (dt, J = 8.7 and
0.9 Hz, 1H); 13C NMR (CDCl3) d 54.72, 111.41, 141.30,
148.87, 156.62, 117.60, 119.03, 120.65, 120.79, 121.90,
122.49, 124.47, 126.65, 127.62, 128.51, 130.55, 131.57,
132.43; FAB Mass 301 (M++1).
Compound 3f: 20%; white solid, mp 109–110 °C; IR (KBr)
1
3059, 2978, 1458 cmꢀ1; H NMR (CDCl3) d 1.63 (s, 9H),
6.93–6.99 (m, 1H), 7.16 (dt, J = 8.4 and 0.9 Hz, 1H), 7.23–
7.29 (m, 1H), 7.39–7.51 (m, 5H), 7.72 (dt, J = 8.7 and
0.9 Hz, 1H); 13C NMR (CDCl3) d 31.54, 63.02, 117.00,
120.12, 121.04, 124.15, 125.84, 128.16, 128.75, 131.06,
132.87, 135.29, 145.97; FAB Mass 251 (M++1).
Compound 3f0: 57%; white solid, mp 111–112 °C; IR (KBr)
1
3178, 1620, 1342 cmꢀ1; H NMR (CDCl3) d 7.19–7.25(m,
1H), 7.34–7.56 (m, 5H), 7.98–8.05 (m, 3H), 11.03 (br s, 1H);
13C NMR (CDCl3) d 110.14, 120.97, 121.12, 121.36, 126.79,
127.67, 128.16, 128.90, 133.54, 141.67, 145.76.
Compound 3g: 69%; oil; IR (KBr) 2954, 2927, 1597,
;
1504 cmꢀ1 1H NMR (CDCl3) d 0.82 (t, J = 7.2 Hz, 3H),
1.21–1.28 (m, 4H), 1.62–1.69 (m, 2H), 3.03 (t, J = 7.5Hz,
2H), 7.04–7.10 (m, 1H), 7.28–7.34 (m, 1H), 7.48–7.55 (m,
5H), 7.67 (dt, J = 8.4 and 0.9 Hz, 1H), 7.71 (dt, J = 8.7 and
0.9 Hz, 1H); 13C NMR (CDCl3) d 13.81, 22.14, 25.23,
29.05, 31.41, 117.56, 120.21, 120.85, 121.02, 126.16, 126.60,
128.85, 129.13, 136.91, 140.07, 148.57; FAB Mass 265
(M++1).
7. Typical experimental procedure and spectroscopic data of
the prepared compounds are as follows. Synthesis of
compound 3a:
A
mixture of pyrazole 2a (130 mg,
Compound 4: 58%; white solid, mp 155–156 °C; IR (KBr)
1
0.47 mmol) and DDQ (216 mg, 0.95mmol) in benzene
(5mL) was heated to reflux for 24 h. After removal of
solvent and chromatographic purification process (hexanes/
ether, 20:1) we obtained 3a as a white solid, 88 mg (69%).
Compound 3a: 69%; white solid, mp 106–107 °C; IR (KBr)
3059, 1624, 1597, 1500 cmꢀ1; 1H NMR (CDCl3) d 7.09–7.15
(m, 1H), 7.32–7.45(m, 11H), 7.70 (dt, J = 8.4 and 1.2 Hz,
1H), 7.81 (dt, J = 8.7 and 0.9 Hz, 1H); 13C NMR (CDCl3) d
117.93, 120.68, 121.92, 122.68, 126.18, 127.16, 128.41,
128.47, 128.92, 129.14, 129.84, 130.07, 135.56, 140.40,
149.17; Mass (70 eV) m/z (rel. intensity) 51 (10), 77 (24),
134 (33), 269 (92), 270 (M+, 100).
3421, 1504, 1369 cmꢀ1; H NMR (CDCl3) d 0.90 (s, 3H),
1.11 (s, 3H), 1.49–1.57 (m, 1H), 1.74 (d, J = 4.8 Hz, OH,
1H, D2O exchangeable), 1.96–2.07 (m, 1H), 2.66–2.78 (m,
1H), 2.85–2.93 (m, 1H), 4.17 (d, J = 4.8 Hz, 1H, converted
into singlet with D2O treatment), 7.22–7.40 (m, 10H); 13C
NMR (CDCl3) d 20.30, 23.93, 25.99, 30.38, 35.27, 70.50,
119.53, 125.18, 127.18, 128.44, 128.66, 128.97, 129.78,
130.15, 140.34, 141.48, 149.28; FAB Mass 319 (M++1).
Compound 5: 61%; white solid, mp 107–108 °C; IR (KBr)
1
3433, 1604, 1504 cmꢀ1; H NMR (CDCl3) d 4.23 (s, 2H),
6.04–6.05(m, 1H), 6.27–6.29 (m, 1H), 7.15–7.27 (m, 3H),
7.34–7.39 (m, 3H), 7.97 (d, J = 7.8 Hz, 1H), 8.04 (d,
J = 7.8 Hz, 1H), 8.11 (br s, 1H, D2O exchangeable); 13C
NMR (CDCl3) d 31.23, 106.55, 110.55, 110.80, 119.04,
119.49, 119.59, 120.15, 120.34, 123.52, 123.65, 125.76,
126.29, 138.54, 139.52, 141.65, 153.36.
Compound 3b: 75%; white solid, mp 160–161 °C; IR (KBr)
3059, 1608, 1520 cmꢀ1; H NMR (CDCl3) d 6.81–6.89 (m,
1
1H), 6.94–7.01 (m, 1H), 7.11–7.17 (m, 1H), 7.31–7.42 (m,
6H), 7.52–7.60 (m, 1H), 7.72 (dt, J = 8.4 and 0.9 Hz, 1H),
7.78 (dt, J = 9.0 and 0.9 Hz, 1H); 13C NMR (CDCl3) d
104.92, 105.24, 105.27, 105.59, 111.99, 112.04, 112.29,
112.34, 117.91, 120.70, 121.06, 122.96, 125.01, 125.06,
125.17, 125.22, 127.51, 128.79, 128.99, 129.10, 129.45,
129.47, 130.27, 130.41, 137.87, 149.63, 155.14, 155.31,
158.54, 158.71, 161.25, 161.39, 164.60, 164.74; Mass
Selected spectroscopic data of starting materials 2g and 2h
are as follows.
Compound 2g: 47%; oil; 1H NMR (CDCl3) d 0.82 (t,
J = 6.9 Hz, 3H), 1.17–1.26 (m, 4H), 1.41–1.48 (m, 2H),
1.75–1.87 (m, 4H), 2.51 (t, J = 6.0 Hz, 2H), 2.60 (t,
J = 7.8 Hz, 2H), 2.73 (t, J = 6.0 Hz, 2H), 7.29–7.45(m,

K. Y. Lee et al. / Tetrahedron Letters 46 (2005) 5387–5391
5391
5H); 13C NMR (CDCl3) d 13.77, 20.72, 22.08, 23.33, 23.38,
23.43, 24.67, 28.05, 31.32, 114.75, 125.21, 127.16, 128.81,
139.21, 140.37, 149.48.
30.42, 35.46, 36.24, 116.77, 124.88, 126.68, 127.84, 128.50,
128.83, 129.38, 130.96, 138.92, 140.53, 149.41.
8. For the mechanism of carbazole synthesis, see: Sundberg,
R. J. In Comprehensive Heterocyclic Chemistry II; Katri-
tzky, A. R., Rees, C. W., Scriven, E. F. V., Eds.; Pergamon:
Oxford, 1996; Vol. 2, pp 119–206.
1
Compound 2h: 45%; oil; H NMR (CDCl3) d 1.02 (s, 6H),
1.67 (t, J = 6.6 Hz, 2H), 2.38 (s, 2H), 2.82 (t, J = 6.6 Hz,
2H), 7.15–7.31 (m, 10H); 13C NMR (CDCl3) d 20.55, 28.16,