Synthesis of some new 1-aryl-4-formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran- 3-yl)pyrazoles using the Vilsmeier-Haack reaction - Isolation of the key intermediate 1-aryl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl) pyrazoles

Article abstract of DOI:10.1139/V06-015

Arylhydrazones of dehydroacetic acid (DHA) underwent Vilsmeier-Haack reaction to generate the corresponding 1-aryl-4-formyl-3-(4-hydroxy-6-methyl-2- oxo-2H-pyran-3-yl)pyrazoles (1) with the pyrone moiety of the DHA remaining intact. However, when the reac

Full text of DOI:10.1139/V06-015

438
Synthesis of some new 1-aryl-4-formyl-3-(4-
hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)pyrazoles
using the Vilsmeier–Haack reaction — Isolation of
the key intermediate 1-aryl-3-(4-hydroxy-6-methyl-
2-oxo-2H-pyran-3-yl)pyrazoles
Ajay Kumar, Om Prakash, Mayank Kinger, and Shiv P. Singh
Abstract: Arylhydrazones of dehydroacetic acid (DHA) underwent Vilsmeier–Haack reaction to generate the corre-
sponding 1-aryl-4-formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)pyrazoles (1) with the pyrone moiety of the DHA
remaining intact. However, when the reaction was performed using 1 equiv. of the reagent, compounds of 1-aryl-3-(4-
hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)pyrazoles (2a–2f) were obtained, which underwent smooth conversion to 1-aryl-
4-formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)pyrazoles (1) on further treatment with another equivalent of the
Vilsmeier–Haack reagent. This unreported observation suggests the intermediacy of 1-aryl-3-(4-hydroxy-6-methyl-2-
oxo-2H-pyran-3-yl)pyrazoles (2) in Vilsmeier–Haack reactions of hydrazones with a pyrone moiety. Several analogous
compounds were also prepared using arylhydrazones of DHA.
Key words: dehydroacetic acid, pyrazoles, Vilsmeier–Haack reaction, 3-acetyl-4-hydroxy-6-methyl-2H-pyran-2-one.
Résumé : Les arylhydrazones de l’acide déhydroacétique (DHA) subissent la réaction de Vilsmeier–Haack pour
conduire à la formation des 1-aryl-4-formyl-3-(4-hydroxy-6-méthyl-2-oxo-2H-pyran-3-yl)pyrazoles (1) dans lesquelles la
portion pyrone de l’acide DHA demeure intacte. Toutefois, lorsque la réaction est effectuée en utilisant un équivalent
de réactif, on obtient plutôt les 1-aryl-3-(4-hydroxy-6-méthyl-2-oxo-2H-pyran-3-yl)pyrazoles (2a–2f) qui, sous
l’influence d’un autre équivalent de réactif de Vilsmeier–Haack, se transforment facilement en 1-aryl-4-formyl-3-(4-
hydroxy-6-méthyl-2-oxo-2H-pyran-3-yl)pyrazoles (1). Cette observation qui n’avait jamais été rapportée antérieurement
suggère que les 1-aryl-3-(4-hydroxy-6-méthyl-2-oxo-2H-pyran-3-yl)pyrazoles (2) sont des intermédiaires dans les réac-
tions de Vilsmeier-Haack comportant une portion pyrone. On a préparé plusieurs composés analogues à partir d’autres
arylhydrazones de l’acide DHA.
Mots clés : acide déhydroacétique, pyrazoles, réaction de Vilsmeier–Haack, 3-acétyl-4-hydroxy-6-méthyl-2H-pyran-2-one.
[Traduit par la Rédaction] Kumar et al. 442
Introduction
properties of new pyrazole derivatives, we became interested
in the synthesis of the pyrazoles in which the pyrone moiety
of the DHA remains intact. We now report herein the syn-
thesis of pyrazoles 4 and 5 from DHA by using the
Vilsmeier–Haack reagent.
Significant interest in the chemistry of pyrazoles is re-
flected by the design of new synthetic approaches, theoreti-
cal calculations on the reactions leading to their formation,
applications of new spectroscopic techniques and their sig-
nificance in pharmaceuticals, agrochemicals, and dye stuffs,
etc. (1–3). In the course of our studies on the synthesis of
heterocyclic compounds from the reactions of dehydroacetic
acid (DHA) and its derivatives (4–6), we have reported the
synthesis of pyrazoles (2) and bipyrazoles (3) from the rear-
rangement of hydrazones of DHA (1) (7) (Scheme 1).
In the conversion of 1 to 2, there is involvement of the
pyrone ring of DHA, leading to the formation of a β-
dicarbonyl moiety through a novel rearrangement process. In
continuation of our studies dealing with the synthesis and
Ar
Ar
N
N
O
OH
N
N
OH
O
CHO
O
O
CH3
CH3
5
4
Received 8 December 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 31 March 2006.
A. Kumar, O. Prakash,1 M. Kinger, and S.P. Singh. Department of Chemistry, Kurukshetra University, Kurukshetra-136 119,
Haryana, India.
1Corresponding author (e-mail: dromprakash50@rediffmail.com).
Can. J. Chem. 84: 438–442 (2006)
doi:10.1139/V06-015
© 2006 NRC Canada

Kumar et al.
439
Scheme 1.
OH NNHAr
CH3
OH
O
O
CH3
AcOH
ArNHNH2
EtOH
H3C
O
O
H3C
O
1
DHA
Ar
Ar
N
N
N
N
Ar'NHNH2
HO
CH3
Ar'
HO
CH3
EtOH, AcOH
O
N
N
O
CH3
H3C
2
3
There are several recent reports involving the cyclization
of some ketone hydrazones to 1-substituted 4-formylpyrazoles
by using the Vilsmeier–Haack reagent (POCl3–DMF,
Scheme 2) (8–10). This transformation involves double
formylation and its mechanistic pathways are not certain.
Based on this important variation of the Vilsmeier–Haack re-
action, we have adopted the same approach for the synthesis
of 4.
Scheme 2.
Ar
NNHAr
CH3
N
N
DMF
POCl3
R
R
CHO
pyrazole 5a. Interestingly, the reaction afforded the desired
product 1-aryl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)py-
razole (5a), in 60% yield (Scheme 4). Using identical reac-
tion conditions, syntheses of similarly constituted pyrazoles
(5b–5e) were achieved.
To the best of our knowledge, all the reported cases of
Vilsmeier–Haack reactions of hydrazones give 4-formylpy-
razoles, as shown in Scheme 2. This study represents the
first example of the preparation of pyrazoles 5 from the cor-
responding hydrazones.
Results and discussion
1-Aryl-4-formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-
3-yl)pyrazoles (1a–1e)
Initially, DHA phenylhydrazone (1a) was prepared by the
condensation of phenylhydrazine and DHA in ethanolic so-
lution containing a small amount of acetic acid, and was
subjected to the Vilsmeier–Haack reaction under standard
conditions, i.e., treating 2 equiv. of POCl3–DMF with
hydrazone 4a at 55–60 °C for 5 h. The usual work up, fol-
lowed by the purification of the resulting crude product by
recrystallization from ethanol, gave the expected 4-
formylpyrazole derivative 1a as a crystalline solid in 75%
yield. Similarly, several DHA hydrazones (1b–1e), which
were obtained by treating DHA with arylhydrazines, also
underwent the Vilsmeier–Haack reaction to afford the corre-
sponding 1-aryl-4-formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-
pyran-3-yl)pyrazoles (4b–4e) in good yields (Scheme 3).
The structures of all products were established by their spec-
Conversion of 5 to 4
It is significant that pyrazoles 5a–5e upon Vilsmeier–
Haack reaction underwent smooth conversion into the corre-
sponding 4-formylpyrazoles (4a–4e). The physical and spec-
tral data of 4a–4e were found to be identical with those
obtained directly from 1. The synthesis of 4 from 5, not only
provides additional examples of the Vilsmeier–Haack
formylation of pyrazole derivatives, but also gives a clear in-
dication of the involvement of 5 as an intermediate in the
conversion of 1 to 4.
1
tral properties, i.e., IR, H NMR, MS, and elemental analy-
ses.
The successful conversion of 3 to 4 reveals that the 4-
hydroxy-6-methyl-2H-pyran-2-one system remains intact in
the presence of the Vilsmeier–Haack reagent.
Mechanistic pathways
Although the mechanism of the Vilsmeier–Haack reaction
is not certain, two alternative pathways (Scheme 5) can be
envisaged. According to Scheme 5 (path a), formation of the
products 4 involves double formylation at the CH3 group of
the hydrazones 1, followed by cyclization. In the second
pathway (path b), single formylation at the CH3 group fol-
lowed by cyclization, initially gives 4-unsubstituted pyrazoles
(5), which subsequently undergo another formylation to pro-
duce 4.
1-Aryl 3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)pyra-
zoles (5a–5e)
After the successful conversion of 1 to 4, we focused our
attention on the synthesis of the related compounds 5. The
reaction was carried out using controlled conditions, such as
the use of 1 equiv. of Vilsmeier–Haack reagent to carry out
the monoformylation of hydrazone 1a to generate the
© 2006 NRC Canada

440
Can. J. Chem. Vol. 84, 2006
Scheme 3.
Ar
OH NNHAr
CH3
N
N
OH
2.2 equiv.
POCl3–DMF
stirring for 5 h
at 55-60 °C
CHO
O
H3C
O
O
O
CH3
4
1
Compounds Ar
Compounds Ar
1a, 4a
1b, 4b
1c, 4c
C6H5
1d, 4d
1e, 4e
4-NO2C6H4
4-CH3C6H4
4-ClC6H4
2,4-(NO2)2C6H3
Scheme 4.
Ar
OH NNHAr
CH3
N
N
O
OH
1.1 equiv.
POCl3–DMF
stirring for 1 h
at 55-60 °C
H3C
O
O
O
CH3
5
1
Compounds Ar
Compounds Ar
1a, 5a
1b, 5b
1c, 5c
C6H5
4-CH3C6H4
4-ClC6H4
1d, 5d
1e, 5e
4-NO2C6H4
2,4-(NO2)2C6H3
Scheme 5.
H
NHAr
CH2
POCl3–DMF
Me2N CHCl
NNHAr
N
N
NHAr
H
Py
CH2
Py
Py
CH2CH NMe2
a
Ar
b
N
N
N
NHAr
CH NMe2
Py
NMe2
Ar
N
H
Py
CH
CH NMe2
N
H
NMe2
Py
NMe2
H
-
Me2NH
Ar
N
N
POCl3
DMF
hydr.
5
4
Py
NMe2
Since we were able to isolate the products 5 by using
1 equiv. of the Vilsmeier–Haack reagent (in this study), path
b (Scheme 5) appears to be more reasonable. The observa-
tion in the present work that the second formylation of the
pyrazole 5, using similar reaction conditions and the reagent,
indeed, afforded 4, supports this pathway. However, in the
presence of 2 equiv. of the Vilsmeier–Haack reagent, this
study does not permit us to speculate on the operation of ei-
ther of the two mechanisms as outlined in Scheme 5.
It is to be noted that in contrary to our observations, Kira
© 2006 NRC Canada

Kumar et al.
441
Scheme 6.
common chemicals such as dehydroacetic acid, aldehydes,
phenylhydrazine, 2,4-dinitrophenylhydrazine, etc., were ob-
tained from commercial suppliers.
NH
N
Ph
Ph
CH3
N
N
General procedure for the preparation of the
hydrazone of DHA
POCl3–DMF
Ph
DHA (0.02 molar equiv.) was dissolved in ethanol
(50 mL) by warming, and arylhydrazine (0.02 molar equiv.)
was added to the mixture while it was being shaken. The
contents were stirred for 10 min and allowed to stand at
room temperature for 2 h. The yellow solid (1) obtained was
filtered and crystallized from acetonitrile.
6
CHO
8 (80%)
Ph
N
Compounds 1a–1e were similarly prepared.
N
POCl3–DMF
8 (9%)
Ph
General procedure for the preparation of 1-aryl-3-(4-
hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)pyrazoles (5)
Hydrazone 1 (0.004 molar equiv.) was added to the
Vilsmeier–Haack reagent (prepared from DMF (10 mL) and
POCl3 (0.0044 molar equiv.)), and the reaction mixture was
stirred at 55–60 °C for 1 h and then poured into ice-cold wa-
ter. The solid that separated upon neutralization with
NaHCO3 was filtered, washed with water, and crystallized
from ethanol to afford 5.
7
et al. (11) reported that in the case of Vilsmeier–Haack reac-
tion of acetophenone phenylhydrazone (6), pyrazoles 7 are
not intermediates in the reaction. They obtained 4-formyl-
1,3-diphenylpyrazole (8) in 80% yield from the Vilsmeier–
Haack reaction of 6. However, formylation of 1,3-diphenyl-
pyrazole (7) under identical conditions gave the same
product (8), only in 9% yield (Scheme 6). Repetition of this
work confirmed these observations.
Obviously, the presence of the 4-hydroxypyran-2-one
moiety plays an important role in the conversions of 1 to 4
and of 5 to 4. Further studies on related examples might be
helpful to determine the actual effect of structural changes
on the mechanistic pathways of these transformations.
Finally, noteworthy features of the present study are as
follows:
(i) New pyrazole derivatives of 4 and 5 containing the 2-
pyrone moiety are available through the Vilsmeier–
Haack reaction using a simple experimental procedure.
The mild reaction conditions do not allow attack at
other nucleophilic positions (C(6) and C(4)-OH) of the
pyrone moiety.
(ii) The synthesis of 5 represents the first example of mono-
formylation in the Vilsmeier–Haack reaction of the hy-
drazones of the R(CH3)C:NNHAr type.
Compounds 5a–5e were similarly prepared.
3-(4-Hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-1-
phenylpyrazole (5a)
Yield: 65%; mp 158 to 159 °C. IR (KBr, cm–1): 1720
1
(C=O). H NMR (CDCl3, 300 MHz) δ: 2.4 (s, 3H, CH3), 6.0
(s, 1H, C5′-H, DHA), 7.3 (d, 1H, C4-H, J = 2.1 Hz), 7.31 (m,
1H, Ph), 7.50 (m, 2H, Ph), 7.62 (m, 2H, Ph), 8.0 (d, 1H, C5-
H, J = 2.1 Hz), 13.0 (s, 1H, OH). MS m/z: 268. Elemental
anal. calcd. for C15H12N2O3: C 67.16, H 4.48, N 10.44;
found: C 67.28, H 4.49, N 10.34.
3-(4-Hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-1-(4-
tolyl)pyrazole (5b)
Yield 60%; mp 122 to 123 °C. IR (KBr, cm–1): 1723
1
(C=O). H NMR (CDCl3, 300 MHz) δ: 2.3 (s, 3H, CH3), 2.4
(s, 3H, CH3), 6.0 (s, 1H, C5′-H, DHA), 7.3 (d, 1H, C4-H, J =
2.1 Hz), 7.0 (d, 2H, Ph, J = 8.0 Hz), 7.1 (d, 2H, Ph, J =
8.0 Hz), 8.0 (d, 1H, C5-H, J = 2.1 Hz), 13.0 (s, 1H, OH). MS
m/z: 282. Elemental anal. calcd. for C16H14N2O3: C 68.08, H
4.96, N 9.92; found: C 68.24, H 4.95, N 9.94.
(iii) The isolation of 5 and its subsequent formylation pro-
vides a valuable clue regarding the mechanistic path-
ways followed in these transformations.
(iv) The new pyrazole derivatives 4 and 5, synthesized in the
present work, are needed as precursors for the synthesis
of several novel fused pyrazoloquinolone derivatives,
particularly through the use of hypervalent iodine re-
agents.
1-(4-Chlorophenyl)-3-(4-hydroxy-6-methyl-2-oxo-2H-
pyran-3-yl)pyrazole (5c)
Yield 62%; mp 167 to 168 °C. IR (KBr, cm–1): 1722
1
(C=O). H NMR (CDCl3, 300 MHz) δ: 2.4 (s, 3H, CH3), 6.0
(s, 1H, C5′-H, DHA), 7.3 (d, 1H, C4-H, J = 2.1 Hz), 7.1 (d,
2H, Ph, J = 8.1 Hz), 7.4 (d, 2H, Ph, J = 8.1 Hz), 8.0 (d, 1H,
C5-H, J = 2.1 Hz), 13.0 (s, 1H, OH). MS m/z: 302 (M+), 304
(M+ + 2). Elemental anal. calcd. for C15H11ClN2O3: C 59.50,
H 3.63, N 9.25; found: C 59.53, H 3.64, N 9.29.
Experimental
Instrumentation and general procedures
Melting points were taken in open capillaries and are un-
corrected. 1H NMR spectra were recorded on a Bruker
300 MHz instrument using TMS as an internal standard. IR
spectra were recorded on a Buck Scientific IR M500
spectrophotometer. Elemental analyses were carried out in a
PerkinElmer 2400 instrument, and mass spectra were re-
corded on a Kratos MS-50 mass spectrometer. Most of the
3-(4-Hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-1-(4-
nitrophenyl)pyrazole (5d)
Yield 68%; mp 204 to 205 °C. IR (KBr, cm–1): 1725
1
(C=O). H NMR (CDCl3, 300 MHz) δ: 2.4 (s, 3H, CH3), 6.0
(s, 1H, C5′-H, DHA), 7.3 (d, 1H, C4-H, J = 2.1 Hz), 7.6 (d,
2H, Ph, J = 8.4 Hz), 8.3 (d, 2H, Ph, J = 8.4 Hz), 8.0 (d, 1H,
© 2006 NRC Canada

442
Can. J. Chem. Vol. 84, 2006
C5-H, J = 2.1 Hz), 13.0 (s, 1H, OH). MS m/z: 313. Elemen-
tal anal. calcd. for C15H11N3O5: C 57.51, H 3.51, N 13.42;
found: C 57.55, H 3.49, N 13.44.
calcd. for C16H11N3O6: C 56.30, H 3.23, N 12.32; found: C
56.28, H 3.24, N 12.34.
1-(2,4-Dinitrophenyl)-4-formyl-3-(4-hydroxy-6-methyl-2-
1-(2,4-Dinitrophenyl)-3-(4-hydroxy-6-methyl-2-oxo-2H-
pyran-3-yl)pyrazole (5e)
oxo-2H-pyran-3-yl)pyrazole (4e)
Yield 55%; mp 200 to 202 °C. IR (KBr, cm–1): 1728,
Yield 60%; mp 221 to 222 °C. IR (KBr, cm–1): 1728
1
1
1685 (C=O). H NMR (CDCl3, 300 MHz) δ: 2.4 (s, 3H,
(C=O). H NMR (CDCl3, 300 MHz) δ: 2.4 (s, 3H, CH3), 6.0
CH3), 6.0 (s, 1H, C5′-H, DHA), 7.6 (m, 1H, Ph), 8.3 (d, 1H,
Ph), 8.8 (s, 1H, Ph), 8.5 (s, 1H, C5-H), 9.9 (s, 1H, CHO),
13.0 (s, 1H, OH). MS m/z: 386. Elemental anal. calcd. for
C15H12N2O3: C 67.16, H 4.48, N 10.44; found: C 49.28, H
4.49, N 10.34.
(s, 1H, C5′-H, DHA), 7.3 (d, 1H, C4-H, J = 2.1 Hz), 7.6 (m,
1H, Ph), 8.3 (d, 1H, Ph), 8.7 (s, 1H, Ph), 8.0 (d, 1H, C5-H,
J = 2.1 Hz), 13.0 (s, 1H, OH). MS m/z: 358. Elemental anal.
calcd. for C15H10N4O7: C 50.27, H 2.79, N 15.64; found: C
50.30, H 2.77, N 15.60.
General procedure for the preparation of 1-aryl-4-
formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-
yl)pyrazoles (4)
Hydrazone 1 (0.004 molar equiv.) was added to the
Vilsmeier–Haack reagent (prepared from DMF (10 mL) and
POCl3 (0.0088 molar equiv.)), and the reaction mixture was
stirred at 55–60 °C for 5 h and then poured into ice-cold wa-
ter. The solid that separated upon neutralization with
NaHCO3 was filtered, washed with water, and crystallized
from ethanol to afford 4.
General procedure for the preparation of 1-aryl-4-
formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-
yl)pyrazoles (4) from 1-aryl-3-(4-hydroxy-6-methyl-2-
oxo-2H-pyran-3-yl)pyrazole (5)
Compound 5 (0.004 molar equiv.) was added to the
Vilsmeier–Haack reagent (prepared from DMF (10 mL) and
POCl3 (0.0044 molar equiv.)), and the reaction mixture was
stirred at 55–60 °C for 4 h and then poured into ice-cold wa-
ter. The solid that separated upon neutralization with
NaHCO3 was filtered, washed with water, and crystallized
from ethanol to afford 4 in 50% yields.
Compounds 4a–4e were similarly prepared.
Compounds 4a–4e were similarly prepared.
4-Formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-1-
phenylpyrazole (4a)
Yield 55%; mp 118 to 119 °C. IR (KBr, cm–1): 1720,
1
1683 (C=O). H NMR (CDCl3, 300 MHz) δ: 2.4 (s, 3H,
CH3), 6.0 (s, 1H, C5′-H, DHA), 7.0–7.7 (m, 5H, Ph), 8.5 (s,
1H, C5-H), 9.9 (s, 1H, CHO), 13.0 (s, 1H, OH). MS m/z:
296. Elemental anal. calcd. for C16H12N2O4: C 64.86, H
4.05, N 9.45; found: C 64.88, H 4.06, N 9.44.
Acknowledgement
We are thankful to the Defence Research and Develop-
ment Organisation (DRDO, New Delhi) (ERIP/ER/
0303447/M/01) for financial assistance.
4-Formyl-3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-1-
(4-tolyl)pyrazole (4b)
Yield 51%; mp 133 to 134 °C. IR (KBr, cm–1): 1723,
1
1687 (C=O). H NMR (CDCl3, 300 MHz) δ: 2.3 (s, 3H,
References
CH3), 2.4 (s, 3H, CH3), 6.0 (s, 1H, C5′-H, DHA), 7.0 (d, 2H,
Ph, J = 8.0 Hz), 7.1 (d, 2H, Ph, J = 8.0 Hz), 8.5 (s, 1H, C5-
H), 9.9 (s, 1H, CHO), 13.0 (s, 1H, OH). MS m/z: 310. Ele-
mental anal. calcd. for C17H14N2O4: C 65.80, H 4.51, N
9.03; found: C 65.81, H 4.49, N 9.06.
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Lett. 109 (1969).
1
1687 (C=O). H NMR (CDCl3, 300 MHz) δ: 2.4 (s, 3H,
CH3), 6.0 (s, 1H, C5′-H, DHA), 7.6 (d, 2H, Ph, J = 8.4 Hz),
8.3 (d, 2H, Ph, J = 8.4 Hz), 8.5 (s, 1H, C5-H), 9.9 (s, 1H,
CHO), 13.0 (s, 1H, OH). MS m/z: 341. Elemental anal.
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