Regioselective synthesis of fluorine-containing indazolones from 2-acylcyclohexane-1,3-diones

Article abstract of DOI:10.1007/s10593-008-0044-2

New regioisomeric indazolones containing fluorine atoms in the aromatic ring have been synthesized in high yield by the interaction of 2-acylcyclohexane-1,3-diones, and of their enol methyl ethers, obtained by methylating the initial β,β′-triketones with

Full text of DOI:10.1007/s10593-008-0044-2

Chemistry of Heterocyclic Compounds, Vol. 44, No. 3, 2008
REGIOSELECTIVE SYNTHESIS
OF FLUORINE-CONTAINING
INDAZOLONES FROM 2-ACYL-
CYCLOHEXANE-1,3-DIONES
T. S. Khlebnicova1, V. G. Isakova1, F. A. Lakhvich1, and P. V. Kurman2
New regioisomeric indazolones containing fluorine atoms in the aromatic ring have been synthesized in
high yield by the interaction of 2-acylcyclohexane-1,3-diones, and of their enol methyl ethers, obtained
by methylating the initial β,β′-triketones with dimethyl sulfate in the presence of calcined potassium
carbonate, with 4-fluorophenylhydrazine hydrochloride and with pentafluorophenylhydrazine. The
1
structures of the synthesized compounds were confirmed by data of IR and H, 13C, and 19F NMR
spectra.
Keywords: 2-acylcyclohexane-1,3-diones, fluorine-containing indazolones, regioselective synthesis.
At the present time methods of synthesizing various fluorine-containing polyfunctional heterocyclic
structures as medicinal preparations are being developed vigorously [1, 2]. Indazoles are an important class of
organic compounds, many of which possess anti-inflammatory, analgesic, antiviral, and other physiological
activitities [3, 4].
The aim of the present work was to obtain new fluorine-containing indazolones from various
2-acylcyclohexane-1,3-diones.
Thanks to their polyfunctionality and depending on the structure of the acyl side chain and the cyclic
portion of the molecule the 2-acylcyclohexane-1,3-diones are widely applied in the synthesis of steroids,
prostaglandins, antibiotics, and other biologically active substances [5-8]. Chemical conversions of
2-acylcyclohexane-1,3-diones existing in the enolic form may also affect such reaction centers as the exo- and
endocyclic carbonyl groups, and also the methylene groups adjacent to them [9].
With the aim of synthesizing new derivatives of indazolones containing a fluorine atom in position 4 of
the benzene ring or a completely fluorinated benzene ring we studied the interaction of 2-acylcyclohexane-
1,3-diones 1a-e and their methyl ethers with 4-fluorophenylhydrazine and pentafluorophenylhydrazine.
The interaction of 2-acylcyclohexane-1,3-diones 1a-e with a small excess of an equimolar mixture of
4-fluorophenylhydrazine hydrochloride and sodium hydroxide in ethanol for 8 h at room temperature led in high
yield to the products of heterocyclization, indazolones 3a-e. The intermediate hydrazones 2a-e were not isolated.
_______
* Dedicated to Distinguished Scientist Academician A. A. Akhrem in his 95th Year.
_________________________________________________________________________________________
Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus, Minsk 220141; e-mail:
2
khlebnicova@iboch.bas-net.by. The Republican Scientific and Engineering Center for Remote Sensing of
Environment "EKOMIR", National Academy of Sciences of the Republic of Belarus, Minsk 220012. Translated
from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 393-403, March, 2008. Original article submitted
January 18, 2007.
0009-3122/08/4403-0301©2008 Springer Science+Business Media, Inc.
301

However on reacting triketones 1a,b with a small excess of pentafluorophenylhydrazine in ethanol for
8 h at room temperature both hydrazones 2f,g and indazolones 3f,g were isolated by preparative TLC in yields
of 79, 61 and 21, 39% respectively. Meanwhile, for compounds 1c-e under the same conditions, only the
heterocyclic products 3h-j were isolated. Indazolones 3f,g were obtained as the sole reaction products on boiling
compounds 1a,b in ethanol for 20 h with a small excess of pentafluorophenylhydrazine.
X
X
O
O
O
X
H
NH
R
NH2NH–
N
N
O
N
O
R
R
1a–e
O
3a-j
2a-j
DMS
X
X
O
O
X
NH2NH-
HN
H
N
O
O
N
N
O
R
R
R
OMe
4a-e
6a-j
5a-j
1–6 a R = Me, b R = Et, c R = Ph, d R = 2-furyl, e R = cyclo-C3H5;
2, 3, 5, 6 f R = Me, g R = Et, h R = Ph, i R = 2-furyl, j R = cyclo-C3H5;
2–6 a–e X = 4-F; f–j X = F5
The condensation of 2-acylcyclohexane-1,3-diones 1a-e with fluorine-containing phenylhydrazines
therefore takes place at the exocyclic carbonyl group with subsequent intramolcular cyclization of the
intermediate hydrazones 2a-j and the formation of indazolones 3a-j. The reaction proceeds regioselectively. In
every case one regioisomer is formed, as was shown previously for the reaction of 2-acetylcyclohexane-
1,3-diones with phenylhydrazines [10-13]. It should be mentioned that the formation of fluoro-substituted
indazolones 3a-j requires significantly more time than in the case of the unsubstituted derivatives.
With the aim of obtaining regioisomeric indazolones we undertook the synthesis of the corresponding
enol methyl ethers of 2-acylcyclohexane-1,3-diones 4a-e, since it is known that the reaction of enol ethers at the
cyclic keto group of 2-acylcyclohexane-1,3-diones with phenylhydrazine proceeds according to a vinylogous
substitution scheme [14].
A series of methods is proposed for the synthesis of methyl ethers of 2-acylcyclohexane-1,3-diones,
among which are alkylation of their silver salts with methyl iodide [15], or sodium and tetrabutylammonium
salts with dimethyl sulfate [16]. However the use of these methods in our case proved to be ineffective, since by
the first method the methyl ethers 4b-e were not isolated at all, apparently due to the instability of the silver
302

303

304

salts, and by the second the desired methyl ethers 4a-e were synthesized in low yield. It turned out that the enol
methyl ethers of 2-acylcyclohexane-1,3-diones 4a-e are unstable compounds and are readily hydrolyzed to the
initial triketones in the process of isolation and storage.
It was shown by us that the optimum variant is to carry out the alkylation of 2-acylcyclohexane-
1,3-diones 1a-e with dimethyl sulfate in the presence of calcined potassium carbonate. The enol ethers obtained
were used in the reaction without isolation in order to avoid loss due to their instability. Reactions were effected
by the interaction of the enol methyl ethers 4a-e with 4-fluorophenylhydrazine hydrochloride or with
pentafluorophenylhydrazine by stirring at room temperature in ethanol for 5 h. As was expected the interaction
of ethers 4a-e with phenylhydrazines proceeded by a vinylogous substitution mechanism with the formation of
hydrazones 5a-j, subsequent intramolcular cyclization of which led to indazolones 6a-j, regioisomeric with
indazolones 3a-j.
The structures of the synthesized compounds 2f,g, 3a-j, and 6a-j were confirmed by data of elemental
1
13
19
analysis, IR, mass, and H, C, and F NMR spectra (Tables 1, 2). Indazolones 3f,g and hydrazones 2f,g are
1
readily distinguished spectrally. In the H NMR spectra of the latter two additional signals are present for the
NH group protons at δ 6.33 and 6.60 (N–H, singlet) and at 14.63 and 14.67 ppm (singlet of N–H protons
bonded with a strong intramolcular bond to C=O) respectively. In the IR spectra of hydrazones 2f,g a low
TABLE 2. Characteristics of the Synthesized Compounds 3a-j, 6a-j
Mass-
Found, %
Calculated, %
——————
Com-
pound
Empirical
formula
spectrum
mp., °C
Yield, %
m/z, [M]+
С
Н
N
3a
3b
3c
3d
3e
3f
C16H17FN2O
C17H19FN2O
C21H19FN2O
C19H17FN2O2
C18H19FN2O
C16H13F5N2O
C17H15F5N2O
C21H15F5N2O
C19H13F5N2O2
C18H15F5N2O
C16H17FN2O
C17H19FN2O
C21H19FN2O
C19H17FN2O2
C18H19FN2O
C16H13F5N2O
C17H15F5N2O
C21H15F5N2O
C19H13F5N2O2
C18H15F5N2O
70.73
70.57
71.48
71.31
75.25
75.43
70.23
70.36
72.57
72.46
55.69
55.82
56.81
56.99
62.16
62.07
57.43
57.58
58.54
58.38
70.43
70.57
71.29
71.31
75.21
75.43
70.47
70.36
72.63
72.46
55.98
55.82
6.31
6.29
6.75
6.69
5.68
5.73
5.19
5.28
6.51
6.42
3.73
3.81
4.12
4.22
3.78
3.72
3.26
3.31
4.01
4.08
6.22
6.29
6.61
6.69
5.65
5.73
5.35
5.28
6.54
6.42
3.87
3.81
10.35
10.29
9.87
9.78
8.31
8.38
8.51
8.64
9.45
9.39
8.06
8.14
7.75
7.82
6.96
6.89
6.98
7.07
7.49
7.56
10.22
10.29
9.67
9.68
8.28
8.38
8.71
8.64
9.47
9.39
8.28
8.14
110-112
108-110
181-184
160-163
159-162
77-80
272
286
334
324
298
344
358
406
396
370
272
286
334
324
298
344
358
406
396
370
85
76
86
76
87
85
83
80
88
87
68
67
73
72
69
70
71
75
74
72
3g
3h
3i
59-61
175-177
168-171
172-175
83-86
3j
6a
6b
6c
6d
6e
6f
91-94
172-175
121-124
67-70
115-118
56-59
6g
6h
6i
57.14
56.99
62.20
62.07
57.41
57.58
58.25
58.38
4.31
4.22
3.81
3.72
3.20
3.31
3.95
4.08
7.89
7.82
6.95
6.89
6.96
7.07
7.47
7.56
127-130
101-104
98-101
6j
305

intensity absorption band is observed for conjugated carbonyl (1640 and 1660 cm-1 respectively), intense
absorption bands for the C=C bond (1640 and 1650 cm-1 respectively), and for C–N at 1530 cm-1 [17]. In the
mass spectra of compounds 2f,g peaks for the molecular ions were absent, but characteristic peaks were present
arising from the fission of water (m/z 344 [M-H2O]+ and 358 [M-H2O]+ for compounds 2f,g respectively).
The regioisomeric indazolones 3a-j and 6a-j differed in their physicochemical properties. The IR
spectra of compounds 3a-j were characterized by the presence of intense absorption bands in the regions
1670-1690 (conjugated carbonyl), 1520-1545 (C=C) and 1480-1500 cm-1 (C=N), but in the IR spectra of
regioisomers 6a-j there were intense absorption bands at 1680-1690 (conjugated carbonyl), in the 1545-1590
region (C=C), and in the 1540 cm-1 region (C=N). These absorption regions in the IR spectra of compounds 3a-j
and 6a-j are in good agreement with the data of IR spectra given in the literature for regioisomeric indazolones
obtained from 2-acetylcyclohexane-1,3-diones [10-13] and 2-acetylcyclohexane-1,3-dione ether [14]. In the 13C
NMR spectra of indazolones 3a-j and 6a-j, in addition to the carbon signals of methyl, methylene, and methine
groups, carbon signals were present in the region δ 191-196 (C=O), and carbon signals of C–N and C=N groups
in the region of δ 143-158 ppm. The presence of fluorine is confirmed by the signal of the fluorine atom in the
19F NMR spectra at δ from –112 to –113 ppm for compounds 3a-e and 6a-e and three signals for fluorine atoms
in the regions δ -145, -150, and -160 ppm for compounds 3f-j and 6f-j. Peaks were observed for the molcular
ions in the mass spectra of all compounds 3a-j and 6a-j.
Thanks to the regioselectivity of the reaction of 2-acylcyclohexane-1,3-diones 1a-e and their enol
methyl ethers 4a-e with phenylhydrazines containing fluorine atoms in various positions of the benzene ring,
new regioisomeric fluorine-containing indazolones have been obtained by us in preparative yield.
EXPERIMENTAL
1
13
19
The H, C, and F NMR spectra were recorded on a Bruker AVANCE 500 spectrometer (500, 125,
1
and 470 MHz respectively) in CDCl3, internal standard was TMS (for H and 13C NMR spectra) and
trichlorofluoromethane (19F NMR spectra). The IR spectra were recorded on a UR-20 instrument in KBr disks.
The mass spectra (EI, 70 eV) were obtained on a Hewlett-Packard 5890 gas chromatograph with a HP 5972
mass selective detector. Melting points were determined on a Boetius type heating block. A check on the
progress of reactions was effected by TLC on Silufol UV-254 plates (ether–hexane, 3:1). The synthesized
compounds 2f,g, 3a-j, and 6a-j were isolated by preparative TLC on Fluka HF254 silica gel (ether–hexane, 1:1).
2-Acylcyclohexane-1,3-diones 1a-e were obtained by O–C isomerization of the appropriate enol
acylates of dimedone under the action of acetone cyanohydrin by the method of [18, 19]. The enol acylates of
dimedone were obtained by the O-acylation of dimedone with carboxylic acid chlorides by the method of [20]
and were used for subsequent O–C isomerization without isolation. Acyldimedones 1a-c were synthesized in
86, 90, and 84% yield respectively, their constants were identical to those described in [20].
2-(2-Furoyl)-5,5-dimethylcyclohexane-1,3-dione (1d). Yield 88%; mp 41-44oC (ether–hexane). IR
1
spectrum, ν, cm-1: 1680, 1570, 1470. H NMR spectrum, δ, ppm: 1.13 (6H, s, 2CH3); 2.44 (2H, s, CH2); 2.61
(2H, s, CH2); 6.57 (1H, m, Hfuran); 7.67 (1H, m, Hfuran); 7.78 (1H, m, Hfuran); 17.42 (1H, br. s, OH). Mass
spectrum, m/z: 234 [M]+. Found, %: C 66.98; H 6.11. C13H14O4. Calculated, %: C 66.66; H 6.02.
2-Cyclopropanecarbonyl-5.5-dimethylcyclohexane-1,3-dione (1e). Yield 95%; mp 57-60oC (ether–
hexane). IR spectrum, ν, cm-1: 1660, 1550, 1450. 1H NMR spectrum, δ, ppm: 1.09 (6H, s, 2CH3); 1.12 (2H, m,
CH2cyclopropane); 1.30 (2H, m, CH2cyclopropane); 2.40 (2H, s, CH2); 2.53 (2H, s, CH2); 3.59 (1H, m, Hcyclopropane); 18.51
(1H, br. s, OH). Mass spectrum, m/z: 208 [M]+. Found, %: C 69.11; H 7.67. C12H16O3. Calculated, %: C 69.21;
H 7.74.
1-(4-Fluorophenyl)-3,6,6-trimethyl-6,7-dihydro-1H-indazol-4(5H)-one (3a), 3-Ethyl-1-(4-fluoro-
phenyl)-6,6-dimethyl-6,7-dihydro-1H-indazol-4(5H)-one (3b), 1-(4-Fluorophenyl)-6,6-dimethyl-3-phenyl-
306

6,7-dihydro-1H-indazol-4(5H)-one (3c), 1-(4-Fluorophenyl)-3-(2-furyl)-6,6-dimethyl-6,7-dihydro-1H-
indazol-4(5H)-one (3d), and 3-Cyclopropyl-1-(4-fluorophenyl)-6,6-dimethyl-6,7-dihydro-1H-indazol-
4(5H)one (3e) (Table 1). 4-Fluorophenylhydrazine hydrochloride (1.1 mmol) and sodium hydroxide (1.1 mmol)
were added with stirring to a solution of the appropriate triketone 1a-e (1 mmol) in ethanol (5 ml). The reaction
mixture was stirred for 8 h at room temperature. After removing the solvent on the rotary evaporator the residue
was dissolved in chloroform (50 ml). The solution was washed with dilute HCl (3×15 ml), with water
(2×15 ml), and dried over anhydrous magnesium sulfate. After removing the chloroform on the rotary
evaporator the product was isolated by preparative TLC as orange-red crystals.
3,6, 6-Trimethyl-1-pentafluorophenyl-6,7-dihydro-1H-indazol-4(5H)-one (3f), 3-Ethyl-6,6-dimethyl-
1-pentafluorophenyl-6,7-dihydro-1H-indazol-4(5H)-one (3g), 6,6-Dimethyl-1-pentafluorophenyl-3-phenyl-
6,7-dihydro-1H-indazol-4(5H)-one (3h), 3-(2-Furyl)-6,6-dimethyl-1-pentafluorophenyl-6,7-dihydro-1H-
indazol-4(5H)-one (3i), and 3-Cyclopropyl-6,6-dimethyl-1-pentafluorophenyl-6,7-dihydro-1H-indazol-
4(5H)-one (3j). A solution of the appropriate triketone 1a-e (1 mmol) and pentafluorophenylhydrazine
(1.1 mmol) in ethanol (5 ml) was stirred for 8 h at room temperature. The solvent was removed on the rotary
evaporator and the residue dissolved in chloroform (50 ml). The solution was washed with dilute HCl
(3×15 ml), with water (2×15 ml), and dried over anhydrous magnesium sulfate. After removing the chloroform
on the rotary evaporator the product was isolated by preparative TLC. In the case of compounds 1c-e only
indazolones 3g-j were isolated, but in the case of compound 1a hydrazone 2f (79%) and indazolone 3f (21%),
and in the case of compound 1b hydrazone 2g (61%) and indazolone 3g (39%) were obtained. The interaction of
triketones 1a,b with pentafluorophenylhydrazine on boiling for 20 h gave the desired indazolones 3f,g as the
sole products. Compounds 3a-j were isolated as orange-red crystals.
5,5-Dimethyl-2-[1-(2-pentafluorophenylhydrazono)ethyl]cyclohexane-1,3-dione (2f). Bright-yellow
1
crystals, yield 79%; mp 136-139oC. IR spectrum, ν, cm-1: 1640, 1590, 1530. H NMR spectrum, δ, ppm: 1.05
(6H, s, 2CH3); 2.38 (2H, s, CH2); 2.42 (2H, s, CH2); 2.76 (3H, s, CH3); 6.33 (1H, br. s, NH); 14.63 (1H, br. s,
13
NH). C NMR spectrum, δ, ppm (J, Hz): 16.41, 28.21, 30.37, 50.71, 53.28, 107.68, 120.71 (m); 137.28 (dm,
19
J = 250); 138.24 (dm, J = 250); 139.36 (dm, J = 246); 172.84 (C=N); 196.48 (C=O); 196.80 (C=O). F NMR
spectrum, δ, ppm (J, Hz): -156.04 (2F, m); -162.03 (2F, m); -162.85 (1F, t, J = 22). Mass spectrum, m/z: 344
[M-H2O]+. Found, %: C 53.16; H 4.23; N 7.85. C16H15F5N2O2. Calculated, %: C 53.04; H 4.17; N 7.73.
5,5-Dimethyl-2-[1-(2-pentafluorophenylhydrazono)propyl]cyclohexane-1,3-dione (2g). Bright-
yellow crystals, yield 61%; mp 147-149oC. IR spectrum, ν, cm-1: 1660, 1580, 1530. 1H NMR spectrum, δ, ppm
(J, Hz): 1.05 (6H, s, 2CH3); 1.24 (3H, t, J = 7.4, CH3); 2.40 (4H, s, CH2); 3.30 (2H, q, J = 7.4, CH2); 6.60 (1H,
br. s, NH); 14.67 (1H, br. s, NH). 13C NMR spectrum, δ, ppm (J, Hz): 11.91, 21.95, 28.18, 30.27, 51.19, 53.46,
106.65, 120.84 (m); 137.20 (dm, J = 251); 138.23 (dm, J = 251); 139.36 (dm, J = 247); 177.94 (C=N); 196.37
19
(C=O); 197.78 (C=O). F NMR spectrum, δ, ppm (J, Hz): -155.93 (2F, m); -162.10 (2F, m); -163.06 (1F, t,
J = 22). Mass spectrum, m/z: 358 [M-H2O]+. Found, %: C 54.38; H 4.60; N 7.51. C17H17F5N2O2. Calculated, %:
C 54.26; H 4.55; N 7.44.
2-(4-Fluorophenyl)-3,6,6-trimethyl-6,7-dihydro-2H-indazol-4(5H)-one (6a), 3-Ethyl-2-(4-fluoro-
phenyl)-6,6-dimethyl-6,7-dihydro-2H-indazol-4(5H)-one (6b), 2-(4-Fluorophenyl)-6,6-dimethyl-3-phenyl-6,7-
dihydro-2H-indazol-4(5H)-one (6c), 2-(4-Fluorophenyl)-3-(2-furyl)-6,6-dimethyl-6,7-dihydro-2H-indazol-
4(5H)-one (6d), 3-Cyclopropyl-2-(4-fluorophenyl)-6,6-dimethyl-6,7-dihydro-2H-indazol-4(5H)-one (6e),
3,6,6-Trimethyl-2-pentafluorophenyl-6,7-dihydro-2H-indazol-4(5H)-one (6f), 3-Ethyl-6,6-dimethyl-2-
pentafluorophenyl-6,7-dihydro-2H-indazol-4(5H)-one (6g), 6,6-Dimethyl-2-pentafluoro-phenyl-3-phenyl-
6,7-dihydro-2H-indazol-4(5H)-one (6h), 3-(2-Furyl)-6,6-dimethyl-2-pentafluorophenyl-6,7-dihydro-2H-
indazol-4(5H)-one (6i), and 3-Cyclopropyl-6,6-dimethyl-2-pentafluorophenyl-6,7-dihydro-2H-indazol-
4(5H)-one (6j). Calcined K2CO3 (6 mmol) and dimethyl sulfate (1 mmol) were added to a solution of the
appropriate triketone (1a-e) (1 mmol) in absolute toluene (20 ml). The reaction mixture was
307

boiled for 10 h, the solid was filtered off, and washed with toluene. After removing the toluene on the rotary
evaporator the residue was dissolved in ethanol (5 ml), and 4-fluorophenylhydrazine hydrochloride (1 mmol)
and sodium hydroxide (1 mmol) were added to the solution. The reaction mixture was stirred for 5 h at room
temperature. After removing the ethanol on the rotary evaporator, the residue was dissolved in chloroform (50
ml), the solution was washed with dilute HCl (3×15 ml), with water (2×15 ml), and dried over anhydrous
magnesium sulfate. After removing the chloroform on the rotary evaporator indazolones 6a-e were isolated by
preparative TLC. To obtain indazolones 6f-j pentafluorophenylhydrazine (1mmol) was used in place of
4-fluorophenylhydrazine (1 mmol) and sodium hydroxide (1 mmol). Compounds 6a-j were isolated as orange-
red crystals.
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308