Synthesis of bicyclic N-methylpyrazoline and pyrazole derivatives from α,β-unsaturated ketones and N,N-dimethylhydrazine: An illustration of reductive cyclization

Article abstract of DOI:10.1002/jhet.1570

The reaction of N,N-dimethylhydrazine with α,β-unsaturated keto precursors such as 2-benzylidenecyclohexanone, 2,6-bis(benzylidene) cyclohexanone, and 3,5-bis(benzylidene)-1-methyl-4-piperidone hydrochloride provided bicyclic N-methylpyrazoles instead of hydrazones or any Michael addition products. The crystal structure of a representative pyrazole is reported. The proposed mechanism for the formation of the bicyclic N-methylpyrazole 1 is outlined.

Full text of DOI:10.1002/jhet.1570

January 2014 Synthesis of Bicyclic N-Methylpyrazoline and Pyrazole Derivatives from
a,b-Unsaturated Ketones and N,N-Dimethylhydrazine: An Illustration of
Reductive Cyclization
219
a
a
b
a
*
*
A. K. Panda, U. Das, J. W. Quail, and J. R. Dimmock
aDrug Design and Discovery Research Group, College of Pharmacy and Nutrition, University of Saskatchewan,
110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada
bSaskatchewan Structural Sciences Centre, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan
S7N 5C9, Canada
*
E-mail: umashankar.das@usask.ca or jr.dimmock@usask.ca
Received June 7, 2011
DOI 10.1002/jhet.1570
Published online 31 October 2013 in Wiley Online Library (wileyonlinelibrary.com).
The reaction of N,N-dimethylhydrazine with a,b-unsaturated keto precursors such as 2-benzylidenecyclo-
hexanone, 2,6-bis(benzylidene)cyclohexanone, and 3,5-bis(benzylidene)-1-methyl-4-piperidone hydrochloride
provided bicyclic N-methylpyrazoles instead of hydrazones or any Michael addition products. The crystal
structure of a representative pyrazole is reported. The proposed mechanism for the formation of the bicyclic
N-methylpyrazole 1 is outlined.
J. Heterocyclic Chem., 51, 219 (2014).
INTRODUCTION
of new discoveries in organic chemistry. A thorough
literature survey reveals that there are only a few reports
of the reaction of dimethylhydrazine with a,b-unsaturated
ketones, which led to the corresponding hydrazone deriva-
tives. Matsumiya et al. prepared a dimethylhydrazone
derivative of 3-methyl-2-cyclopenten-1-one [14]. A number
of acyclic dimethylhydrazone derivatives of 4-phenyl-3-
buten-2-ones were obtained as a mixture of E and Z isomers
[15,16], and efforts were made to separate the geometrical
isomers [16]. Saito et al. [17] reported the TiCl4 catalyzed
in situ generation of a dimethylhydrazone from 1,3-bis
(benzylidene)acetone.
The pyrazole nucleus is a pharmaceutically important
template and constitutes the key subunit in many biologically
active compounds with a broad range of pharmacological
activities including antidepressant [1], antimicrobial [2],
and anti-inflammatory [3] activities. One of the pyrazole
variants such as tetrahydroindazoles that contain a bicyclic
ring is active against various cancers [4] and inflammation
[5]. Other bicyclic pyrazoles, for example, pyrazolopyridines
are potent antihypertensive [6] and anti-inflammatory agents
[7]. One of the common methods for preparing pyrazoles
involves condensation of hydrazine or hydrazine derivatives
with a,b-unsaturated ketones [7–9] or 1,3-diketones [10,11].
Lee et al. reported a regoselective synthesis of 1,3,4,5-
tetrasubstituted pyrazoles from Baylis–Hillman adducts
[12]. A number of Baylis–Hillman adducts obtained from
alkylvinylketones, 2-cycloalken-1-ones were converted into
the corresponding pyrazole derivatives [12]. Pyrazole
analogs were obtained by DDQ catalyzed oxidation of the
corresponding dihydropyrazoles, which were derived from
the condensation of a,b-unsaturated ketones with N-methyl-
hydrazine [13]. Despite a number of methods available for
the synthesis of pyrazole derivatives, some interesting
bicyclic pyrazoles are difficult to access, and therefore,
construction of novel methods has attracted considerable
attention in recent times.
RESULTS AND DISCUSSION
Our initial study was focused on the readily available
precursor 2-benzylidene-cyclohexanone [18], which, on
reaction with N,N-dimethylhydrazine in pure ethanol or
dry DMSO at 80–90ꢀC, produced two products 1 and 2
in 44 and 32% yields, respectively, after purification with
the use of column chromatography (Scheme 1) instead
of forming any N,N-dimethylhydrazone adducts. It is
proposed that the formation of 1 occurred by nucleophilic
attack of the primary amino group of the hydrazine on
the carbonyl function of the benzylidene group, initially
forming a dimethylhydrazone adduct, which was stabilized
by a five-membered pyrazole ring formation leading to
N-demethylation via the release of methanol (Fig. 1).
Compound 2 is believed to be the oxidized product of 1
[19,20]. Products 1 and 2 were identical spectroscopically
with the authentic samples obtained alternatively by the
In the present study, we are reporting an interesting route
towards various bicyclic N-methylpyrazole derivatives
starting from the reaction of N,N-dimethylhydrazine with
cyclic arylidene keto precursors (Scheme 1). This novel
reaction is unprecedented, and therefore, it opens up a vista
© 2013 HeteroCorporation

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A. K. Panda, U. Das, J. W. Quail, and J. R. Dimmock
Vol 51
Scheme 1
Figure 1. Proposed mechanisms for the formation of the pyrazole derivative 1.
reaction of N-methylhydrazine with 2-benzylidenecyclohex-
anone [13] and 2-benzoylcyclohex-1-ene [13], respectively.
To evaluate the generality of this reaction, we consid-
ered other cyclic bis(arylidene)keto precursors. Condensa-
tion of dimethylhydrazine with 2,6-bis(benzylidene)
cyclohexanone [18] produced the N-methylpyrazole deri-
vatives 3 [21] and 4 as major products along with a minor
quantity of 5, which is obtained by isomerization of the
benzylidene functional group of 4. In the case of 3,5-bis
(benzylidene)-1-methyl-4-piperidone [18], two products,
namely, the N-methylpyrazole derivatives 6 [7] and 7,
were obtained along with minor quantities of 8, which
is a positional isomer of 7. In this case, an additional
major product was also isolated in 19% yield, which
was characterized as N-benzylidene-N′,N′-dimethylhy-
drazine 9 [22] (Scheme 1). This product 9 is assumed
to be formed by the Michael reaction of the primary
amino group of N,N-dimethylhydrazine with the benzy-
lidene keto group of the 3,5-bis(benzylidene)-1-methyl-
4-piperidone (Fig. 2).
The compounds 1, 3, and 6 were isolated as only one
diasterioisomeric form with the trans-orientation of the
hydrogen atoms H-3 and H-3a as evidenced from 1H-NMR.
The proton H-3 in 1, 3, and 6 appeared as a doublet at 3.33,
3.68, and 3.94 ppm, respectively, with spin–spin coupling
constant values (3JH-3,3a) of 12.86, 14.05, and 13.93 Hz,
respectively, which is a characteristic of trans isomers. The
corresponding cis isomers are characterized by smaller
coupling constants (3JH-3,3a = 2.0–5.0 Hz)[23].
The X-ray crystal structures of 3, which are reported for
the first time in this study, further confirmed the stereo-
chemistry and trans orientation of H-3 and H-3a protons
(Fig. 3). The cyclohexyl ring in 3 exists in the sofa
conformation as observed from the X-ray structure.
Various other hydrazine products such as 1-aminopiperidine
and N-benzoylhydrazine, which lack an N-methyl group,
were chosen to react with 3,5-bis(benzylidene)-1-methyl-
4-piperidone hydrochloride to investigate the mechanism
of pyrazoline ring formation. The reaction of 1-aminopi-
peridine and N-benzoylhydrazine leads to the isolation of
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DOI 10.1002/jhet

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Synthesis of Bicyclic N-Methylpyrazoline and Pyrazole Derivatives
221
the nucleophilic attack at the carbonyl carbon is restricted
as a result of the steric hindrance between the bulkier
amines and phenyl groups in the substrates; only the
reaction at the b-carbon is favored that leads to the
formation of Schiff bases.
CONCLUSIONS
The reaction of N,N-dimethylhydrazine with arylidene-
ketones derived from cyclohexanone and 4-piperidone led
to the formation of bicyclic pyrazoline and N-methylpyrazole
derivatives. These reactions illustrate an interesting
phenomenon of ring cyclization via reductive N-demethylation.
A further investigation of the scope of this reaction to
diverse arylidene ketone libraries is proposed.
Figure 2. Proposed mechanisms for the formation of Schiff bases 9, 10,
and 11.
EXPERIMENTAL
All solvents are commercially available. Analytical TLC were
performed on silica gel plates (60F-254 Merck, 0.25-mm
thickness), and visualizing was accomplished with UV light
(l = 254 nm) and iodine. Column chromatography was performed
using silica gel 60 (60–250 meshes). 1H-NMR spectra were
recorded on a Bruker AMX 500 FT spectrometer at 500 MHz.
Mass spectra were obtained with a VG 7OSE spectrometer.
Elemental analyses were obtained using an Elementar’s Vario
EL III microanalyzer. Melting points were undertaken using a
Gallenkamp instrument and are uncorrected. The X-ray crystallo-
graphic diffractions were determined using a Nonius Kappa
CCD diffractometer.
3
Figure 3. An ORTEP diagram of 3 with nonhydrogen displacement ellip-
soids drawn at the 50% probability level. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
General procedure for the preparation of N-methylpyrazolines
and pyrazoles. A mixture of the unsaturated ketone (0.01 mol) and
N,N-dimethylhydrazine (0.84 g, 0.014 mol) in DMSO (24 mL) was
stirred at 80–90ꢀC for 21 h. Water (100 mL) was added to the
reaction mixture, and after stirring at room temperature for 0.25 h, it
was extracted with toluene (4 Â 30 mL). The organic layer was
washed with water (4 Â 30 mL), dried over anhydrous sodium
sulfate, and evaporated to dryness under reduced pressure at
50–55ꢀC to provide a thick oil, which was chromatographed on a
silica gel column with the use of hexane/ethyl acetate (95:5 v/v) as
eluent to afford analytically pure compounds.
mainly the starting material along with minor quantities of
the Schiff bases 10 and 11, respectively, in yields of 17
and 30% (Scheme 2), which were confirmed by comparing
them with authentic samples prepared by the condensation
of benzaldehyde with 1-aminopiperidine [22] and N-ben-
zoylhydrazine [24]. The formation of the hydrazone ad-
duct was not observed in either of the cases. A proposed
mechanism by which the Schiff bases 10 and 11 were
formed is depicted in Figure 2. These results suggest that
2-Methyl-3-phenyl-3,3a,4,5,6,7-hexahydro-2H-indazole (1).
Yield: 44%; thick oil; ir (neat): 2928, 2860, 2779, 1492,
1446, 1162, 748, 698 cmÀ1
;
1H-NMR (CDCl3):
d
7.32
(m, 5H), 3.34 (d, 1H, J = 12.86 Hz), 2.70–2.49 (m, 5H), 1.88
Scheme 2
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A. K. Panda, U. Das, J. W. Quail, and J. R. Dimmock
Vol 51
(m, 4H), 1.28 (m, 3H); ms (EI): m/z 214.14 (M+). Anal. Calcd
for C14H18N2: C, 78.46; H, 8.47; N, 13.07. Found: C, 78.39;
H, 8.42; N, 12.96.
C
21H21N3: C, 79.97; H, 6.71; N, 13.32. Found: C, 79.81; H,
6.62; N, 13.22.
N-Benzylidene-N′,N′-dimethylhydrazine (9). Yield: 9%; oil;
ir (KBr): 2951, 2854, 2825, 2785, 1589, 1561, 1469, 1442,
2-Methyl-3-phenyl-4,5,6,7-tetrahydro-2H-indazole (2). Yield:
32%; thick oil; ir (neat): 3058, 2927, 2850, 1677, 1488, 1371,
1363, 1272, 1132, 1033cmÀ1; H-NMR (CDCl3): d 7.60 (d, 2H,
1
1
761, 698 cmÀ1; H-NMR (CDCl3): d 7.44–7.22 (m, 5H), 3.70
Ar–H, J = 7.4Hz), 7.35 (t, 2H, Ar–H), 7.28 (s, 1H, ═CH), 7.25
(t, 1H, Ar–H), 3.00 (s, 6H, N (CH3)2); ms (EI): m/z 148.08 (M+);
Anal. Calcd for C9H12N2: C, 72.94; H, 8.16; N, 18.90. Found: C,
72.78; H, 8.21; N, 18.73.
Benzylidene-1-aminopiperidine (10). 1-Aminopiperidine (0.3 g,
0.005 mol) was added to a stirring solution of freshly distilled
benzaldehyde (0.5 g, 0.0047 mol) in ethanol (10 mL) at ~10ꢀC. The
mixture was refluxed for 3 h, and the ethanol was removed at 60ꢀC
under reduced pressure. From the crude mass, hot hexanes
extraction yielded the pure product 10 as oil, 0.42 g (61%);
1H-NMR (CDCl3): d 7.64 (d, 2H, Ar–H, J= 7.3 Hz), 7.56 (s, 1H,
═CH), 7.36 (t, 2H, Ar–H), 7.25 (t, 1H, Ar–H), 3.21 (t, 4H), 1.82
(p, 4H), 1.62 (p, 2H).
Benzoic acid benzylidene-hydrazide (11). A mixture of freshly
distilled benzaldehyde (2.0g, 0.019mol) and benzoylhydrazine
(2.6g, 0.019mol) was heated under reflux in dichloromethane
(20 mL) for 24h. The reaction mixture was cooled to room
temperature, and the solid obtained was filtered and washed with
dichloromethane (2 Â 10 mL). The solid was dried at 40ꢀC under
vacuum for 6 h to give 16 as a colorless solid, 3.8g (85%); mp
(s, 3H), 2.65 (t, 2H, J = 6.3 Hz), 2.41 (t, 2H, J = 6.3 Hz), 1.85–
1.60 (m, 4H); ms (EI): m/z 212.12 (M+). Anal. Calcd for
C14H16N2: C, 79.21; H, 7.60; N, 13.20. Found: C, 79.28;
H, 7.40; N, 13.01.
7-Benzylidene-2-methyl-3-phenyl-3,3a,4,5,6,7-hexahydro-2H-
indazole (3). Yield: 33%; mp 104ꢀC (isopropyl alcohol); ir
(KBr): 2918, 2850, 1576, 1541, 1465, 1178, 1019 cmÀ; 1 1H-
NMR (CDCl3): d 7.47 (d, 2H, Ar–H, J = 7.2 Hz), 7.40 (t, 2H,
Ar–H), 7.36 (m, 5H, Ar–H), 7.26 (m, 1H, Ar–H), 7.22 (d, ═CH,
J= 2.6 Hz), 3.68 (d, 1H, J= 14.0 Hz), 3.04 (d, 1H, J=15.2Hz), 2.89
(m, 1H), 2.83 (s, 3H, N–CH3), 2.42 (m, 1H), 2.05 (m, 1H), 1.91
(m, 1H), 1.55 (m, 1H), 1.41 (m, 1H); ms (EI): m/z 302.14 (M+).
Anal. Calcd for C21H22N2 0.5 H2O: C, 80.92; H, 7.38; N, 8.99.
Found: C, 80.86; H, 7.38; N, 9.12.
7-Benzylidene-2-methyl-3-phenyl-4,5,6,7-tetrahydro-2H-
indazole (4). Yield: 15%; mp 124ꢀC (hexanes); ir (KBr): 2920,
2853, 1579, 1464, 1369, 1177, 1016 cmÀ1 1H-NMR (CDCl3):
;
d 7.75 (d, 2H, Ar–H, J = 7.1 Hz), 7.43 (m, 4H, Ar–H), 7.34
(m, 4H, Ar–H), 6.86 (s, 1H, ═CH), 4.17 (s, 3H, N–CH3), 2.88
(t, 2H), 2.79 (m, 2H), 1.89 (m, 2H); ms (EI): m/z 300.16
(M+, 100). Anal. Calcd for C21H20N2: C, 83.96; H, 6.71; N, 9.33.
Found: C, 83.83; H, 6.87; N, 9.33.
1
203ꢀC [lit [24] mp 203–205ꢀC]; H-NMR (DMSO-d6): d 11.85
(s, 1H, NH), 8.43 (s, 1H, ═CH), 7.95 (d, 2H, Ar–H, J =7.5 Hz),
7.71 (d, 2H, Ar–H, J = 6.9Hz), 7.63 (t, 1H, Ar–H), 7.52 (t, 2H,
Ar–H), 7.43 (m, 3H, Ar–H).
X-ray crystallography. Compound 3 was crystallized from
isopropyl alcohol. Crystallographic data (excluding structure
factors) have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication numbers CCDC
707837. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
7-Benzyl-2-methyl-3-phenyl-4,5-dihydro-2H-indazole (5). Yield:
6%; thick oil; ir (KBr) 2913, 2849, 1576, 1540, 1466, 1370,
1
1180, 1071 cmÀ1; H-NMR (CDCl3): d 7.51 (t, 2H, Ar–H), 7.41
(m, 8H, Ar–H), 7.25 (t, 1H, ═CH), 3.91 (s, 3H, N–CH3),
2.86 (t, 2H), 2.60 (t, 2H), 1.84 (m, 2H); ms (EI): m/z
301.16 (M+ + 1), 300.16 (M+), 299.15 (M+ À 1); Anal. Calcd
for C21H20N2: C, 83.96; H, 6.71; N, 9.33. Found: C, 83.87; H,
6.53; N, 9.21.
7-Benzylidene-2,5-dimethyl-3-phenyl-3,3a,4,5,6,7-hexahydro-
2H-pyrazolo[4,3-c]-pyridine (6). Yield: 23%; thick oil; ir
(KBr): 2913, 2851, 2321, 1578, 1543, 1460, 1361, 1219,
Acknowledgments. The authors thank the Canadian Institutes for
Health Research for an operating grant to J. R. Dimmock.
1
1091 cmÀ1; H-NMR (CDCl3): d 7.46 (d, 2H, Ar–H, J=7.1Hz),
7.38 (m, 5H, Ar–H), 7.29 (t, 4H, Ar–H and ═CH), 3.94
(d, 1H, J= 13.9 Hz), 3.74 (d, 1H, J= 14.3 Hz), 3.25 (m,1H), 3.11
(dd, 1H, J=2.1Hz, J= 13.9 Hz), 3.03 (m, 1H), 2.84 (s, 3H,
N–CH3), 2.38 (s, 3H, N–CH3), 2.34 (d, 1H, J= 10.5 Hz); ms (EI):
m/z 317.18 (M+). Anal. Calcd for C21H23N3: C, 79.46; H, 7.30; N,
13.24. Found: C, 79.38; H, 7.41; N, 13.06.
7-Benzylidene-2,5-dimethyl-3-phenyl-4,5,6,7-tetrahydro-2H-
pyrazolo[4,3-c]pyridine (7). Yield: 14%; thick oil; ir (KBr): 2925,
2854, 1580, 1464, 1369, 1280, 1226, 1181, 1068, 1017cmÀ1; 1H-
NMR (CDCl3): d 7.66 (d, 2H, Ar–H, J = 7.6 Hz), 7.43 (m, 4H,
Ar–H), 7.32 (m, 4H, Ar–H), 7.01 (s, 1H,═CH), 4.18 (s, 3H, N–
CH3), 3.81 (s, 2H), 3.60 (s, 2H), 2.48 (s, 3H, N–CH3); ms (EI): m/
z 316.17 (M+ + 1), 315.17 (M+), 314.16 (M+ À 1); Anal. Calcd for
C21H21N3: C, 79.97; H, 6.71; N, 13.32. Found: C, 79.80; H, 6.53;
N, 13.17.
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Synthesis of Bicyclic N-Methylpyrazoline and Pyrazole Derivatives
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