Anionic [4 + 2] cycloaddition reactions of dihydropyrazolin-5-one dienolate with dienophiles: A novel approach for substituted and fused indazolones

Article abstract of DOI:10.1039/a903710a

Dihydropyrazolin-5-one dienolate 2 generated by deprotonation of 2,3-dimethyl-4-formyl-1-phenylpyrazolin-5-one (4-formylantipyrine) is shown to be an efficient anionic pyrazolone α-oxy-o-quinodimethane analog, which undergoes facile cycloaddition with a v

Full text of DOI:10.1039/a903710a

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Anionic [4 ؉ 2] cycloaddition reactions of dihydropyrazolin-5-one
dienolate with dienophiles: a novel approach for substituted and
fused indazolones
Amrita Roy,b Kethiri R. Reddy,b Hiriyakkanavar Ila*a and Hiriyakkanavar Junjappa*a
a Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, U.P., India.
E-mail: hila@iitk.ac.in
b Department of Chemistry, North-Eastern Hill University, Bijni Complex, Shillong-793 003,
Meghalaya, India
Received (in Cambridge, UK) 10th May 1999, Accepted 6th August 1999
Dihydropyrazolin-5-one dienolate 2 generated by deprotonation of 2,3-dimethyl-4-formyl-1-phenylpyrazolin-
5-one (4-formylantipyrine) is shown to be an efficient anionic pyrazolone α-oxy-o-quinodimethane analog, which
undergoes facile cycloaddition with a variety of dienophiles to give substituted indazolones in good yields after
dehydration–dehydrogenation of cycloadducts.
equivalents which undergo facile cycloaddition with a variety
Introduction
of dienophiles, affording a wide range of substituted carbazoles
under mild conditions with efficient control of regioselectivity.14
We have now demonstrated that dihydropyrazolinone dienolate
2 derived from 2,3-dimethyl-4-formyl-1-phenylpyrazolin-5-one
is an efficient anionic pyrazolone o-quinodimethane equivalent
which undergoes facile regioselective cycloaddition with elec-
tron deficient dienophiles to afford substituted indazolones in
good yields. We report the results of these studies in this paper.
The heterocyclic analogs of o-quinodimethane (OQDM,
o-xylylene) are of considerable interest both from a theoretical
point of view and for their potential in organic synthesis1 as
useful dienes. This has been elegantly demonstrated in the
indole series2 for the synthesis of several naturally occurring
and biologically important indole3 and carbazole4 alkaloids.
However, with the exception of indolo-2,3-quinodimethane, the
synthetic potential of other heterocyclic analogs remain largely
unexplored, although several studies involving their methods
of generation and trapping have been published in recent
years.1 Most widely used routes to these heterocyclic o-quino-
dimethanes involve flash vacuum pyrolysis,1,5 1,4-elimin-
ation1,6,7 of suitable precursors and thermal extrusion of sulfur
dioxide from heteroaromatic fused 3-sulfolenes.1,8,9 Flash pyro-
lytic 1,4-elimination requires very harsh reaction conditions
resulting in formation of polymeric products, thus precluding
trapping of o-quinodimethane intermediates except in the most
stable cases. However, several heterocyclic o-quinodimethanes
have been generated by 1,4-elimination in solution and are
efficiently trapped by various dienophiles to afford the corre-
sponding adducts in good yields.1,6,7 Similarly, heterocyclic-
fused 3-sulfolenes have also been shown to be useful precursors
for generation and trapping of heterocyclic o-quinodimethanes
(HQDM).1,8,9 However, many of these precursors are not so
easily accessible and do not lend themselves readily to struc-
tural elaboration, thus limiting the synthetic scope of these
heterocyclic o-quinodimethanes, especially for regiocontrolled
synthesis of substituted heteroaromatic compounds which have
many proven applications.
During the course of our continued interest in the synthesis
of benzoheterocycles via heteroaromatic annelation,10,11 we
became interested in the generation and reactions of anionic
heteroaromatic o-quinodimethanes which are shown to exhibit
pronounced enophilic reactivity and high regiocontrol in
cycloadditions as a result of an increased HOMO energy level
(or net atomic charge) and charge controlled orientation of two
reactive partners in the transition state.12,13 It is pertinent to
note that neutral HQDM usually affords a mixture of regio-
isomers on cycloaddition with unsymmetrical dienophiles
which limits their synthetic scope for regiocontrolled construc-
tion of substituted benzoheterocycles.1,6–9 We have recently
reported that enolates derived from 1,2-dimethylindole-3-carb-
aldehyde are useful anionic α-oxyindolo-2,3-quinodimethane
Results and discussion
In a typical experiment, the dienolate 2 was generated as a red
colored solution by deprotonation of 4-formylantipyrine15 (5
mmol) in THF with LDA (5 mmol) at Ϫ78 ЊC followed by addi-
tion of dimethyl acetylene dicarboxylate (DMAD) (5 mmol).
The reaction mixture after overnight stirring at room temper-
ature followed by work-up afforded the corresponding dimethyl
oxodihydroindazole-5,6-dicarboxylate 3 in 72% yield (Scheme
1). The structure of 3 was established with the help of spectral
and analytical data. The reaction of 2 with other electron
deficient olefins was also investigated. Thus acrylonitrile and
ethyl acrylate reacted with 2 to give the crude [1:1] adducts 6
and 7 in nearly quantitative yields. Attempted dehydration of 6
with pyridinium tosylate in refluxing benzene (1 h) gave a mix-
ture of tetrahydroindazolone 8a and 5-cyanodihydroindazol-
one 8b in 52 and 40% yields respectively, whereas the acrylate
adduct 7 yielded only oxotetrahydroindazolecarboxylate 9a
under similar conditions. The compound 9a was dehydrogen-
ated to 9b by treatment with DDQ in refluxing dioxane (Scheme
1). Similarly, β-nitrostyrene (10) and chalcone (11) were reacted
with 2 to give crude [1:1] adducts 12 and 13 which were con-
verted to 5-nitro- (14) and 5-benzoyl- (15) 6-phenyldihydro-
indazolones in high yields by treatment with pyridinium
tosylate under identical conditions (Scheme 2). The reaction of
2 with nitroketene S,S-acetal 16 was found to be very facile
and yielded directly the corresponding 5-nitro-6-(methylthio)-
dihydroindazolone 18 in 65% yield by in situ aromatization of
the adduct 17 via dehydration and elimination of methanethiol
(Scheme 2). Finally, the hitherto unknown indazole-fused
naphthoquinone ring system 21 was obtained in 54% yield by
reacting dienolate 2 with naphthoquinone 19 and subsequent
dehydration of the resulting adduct 20 under the above
described conditions (Scheme 3). Compound 21, to our
J. Chem. Soc., Perkin Trans. 1, 1999, 3001–3004
This journal is © The Royal Society of Chemistry 1999
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Scheme 2
Scheme 1
knowledge, is the first example of naphtho[2,3-f ]indazole ring
system.
In summary, the dihydropyrazolin-5-one dienolate 2 has
been shown to be an efficient anionic α-oxy heteroaromatic
o-quinodimethane undergoing facile cycloaddition with various
dienophiles in a highly regiospecific fashion to afford substi-
tuted and fused indazolones in good yields. Neutral pyrazole
o-quinodimethanes have been generated previously by dehalo-
genation of 4,5-bis(bromomethyl)pyrazole7 or by thermolysis
of pyrazole-fused 3-sulfolenes8 followed their trapping with
various dienophiles. However, the synthetic scope of these
reactions has not been investigated extensively. Furthermore it
should be noted that more recently the α-oxybenzo-o-quino-
dimethanes have been generated by base catalyzed ring opening
of 2-hydroxycyclobutabenzene16 instead of deprotonation of
o-tolualdehyde so as to minimize the complication due to alde-
hyde–enolate condensation. However, the reactions described
in this paper involving deprotonation of 3-methyl-4-formyl-
pyrazolinone 1 to give dienolate 2 were found to be very clean
and the formation of polymeric and open chain side products
was not observed in the reaction mixture. This reactivity may be
attributed to the stabilization of the dienolate arising from the
chelated structure 2 formed by an intramolecular interaction
between the OH and C-5 carbonyl group.
Scheme 3
assembling the indazolone ring system from pyrazole pre-
cursors. Indazoles are shown to have interesting chemistry and
are effective pharmacophores in medicinal chemistry, thus
making them attractive targets for synthetic organic chemists.17
The present sequence thus represents an efficient route for
3002
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Most reported methods for the synthesis of indazoles involve
construction of the pyrazole moiety on preconstructed benzen-
oid derivatives18 while the methods based on more easily
accessible pyrazole precursors are scantily described in the
literature.11,19
1,2,6,7-Tetrahydro-5-cyano-1-methyl-2-phenyl-3H-indazol-3-
one (8a). Colorless crystals; mp 204–205 ЊC (chloroform–
hexane); yield 52%; IR (KBr): νmax 2200, 1669, 1562, 1495 cmϪ1
;
1H NMR (300 MHz, CDCl3): δ 2.67–2.73 (m, 2H, CH2), 2.80–
2.86 (m, 2H, CH2), 3.23 (s, 3H, NCH3), 7.17 (s, 1H, H-4), 7.29–
7.41 (m, 3H, ArH), 7.46–7.52 (m, 2H, ArH); 13C NMR (75
MHz, CDCl3): δ 20.1, 24.3, 34.0, 98.3, 103.3, 120.1, 125.7,
128.2, 129.5, 133.8, 152.5, 161.2; MS (m/z, %): 251 (Mϩ, 100).
Anal. calc. for C15H13N3O (251.29): C, 71.70; H, 5.21; N,
16.72%. Found: C, 71.94; H, 5.13; N, 16.80%.
Experimental
Melting points were obtained on a Thomas Hoover capillary
melting point apparatus and are uncorrected. IR spectra were
recorded on a Perkin Elmer 983 spectrophotometer. NMR
spectra were recorded on Bruker ACF-300 and Varian EM 390
spectrometers. Chemical shifts are reported in δ (ppm) relative
to tetramethylsilane. Mass spectra were obtained on a JEOL
D-300 mass spectrometer. Elemental analyses were carried out
on a Heraeus CHN-O-Rapid analyzer. All reactions were con-
ducted in oven-dried (120 ЊC) glassware under a dry argon or
nitrogen atmosphere. THF was distilled over sodium benzo-
phenone ketyl prior to use. n-BuLi was purchased from
Aldrich. 2,3-Dimethyl-4-formyl-1-phenyl-3-pyrazolin-5-one 1
was prepared according to the reported procedure;12 colorless
crystals (chloroform–hexane); mp 162 ЊC (lit.12 mp 162 ЊC) IR
(KBr): νmax 1640, 1509 cmϪ1; 1H NMR (90 MHz, CDCl3): δ 2.56
(s, 3H, CH3), 3.30 (s, 3H, NCH3), 7.25–7.65 (m, 5H, ArH), 9.90
(s, 1H, CHO).
1,2-Dihydro-5-cyano-1-methyl-2-phenyl-3H-indazol-3-one
(8b). Colorless crystals; mp 196–197 ЊC (chloroform–hexane);
1
yield 40%; IR (KBr): νmax 2185, 1661, 1594, 1482 cmϪ1; H
NMR (300 MHz, CDCl3): δ 3.28 (s, 3H, NCH3), 7.36 (d, 1H,
J = 8.6 Hz, ArH), 7.49–7.53 (m, 5H, ArH), 7.82 (dd, 1H,
J = 8.6, 1.6 Hz, ArH), 8.25 (d, 1H, J = 1.5 Hz, ArH); MS
(m/z, %): 249 (Mϩ, 23.8). Anal. calc. for C15H11N3O (249.27):
C, 72.28; H, 4.45; N, 16.86%. Found: C, 72.09; H, 4.52; N,
16.97%.
1,2,6,7-Tetrahydro-5-(ethoxycarbonyl)-1-methyl-2-phenyl-3H-
indazol-3-one (9a). Colorless crystals; mp 197–198 ЊC (chloro-
form–hexane); yield 60%; IR (KBr): νmax 1691, 1660, 1548, 1204
1
cmϪ1; H NMR (300 MHz, CDCl3): δ 1.32 (t, 3H, J = 7.1 Hz,
CH3), 2.78–2.83 (m, 4H, CH2–CH2), 3.22 (s, 3H, NCH3), 4.22
(q, 2H, J = 7.1 Hz, OCH2), 7.33–7.37 (m, 3H, ArH), 7.45–7.51
(m, 2H, ArH), 7.57 (s, 1H, H-4); 13C NMR (75 MHz, CDCl3):
δ 14.4, 20.8, 22.0, 34.3, 60.4, 104.7, 119.9, 125.3, 127.6, 128.4,
129.4, 134.4, 155.1, 162.4, 167.1; MS (m/z, %): 298 (Mϩ, 100).
Anal. calc. for C17H18N2O3 (298.34): C, 68.44; H, 6.08; N,
9.39%. Found: C, 68.73; H, 6.03; N, 9.48%.
General procedure for generation of lithium (1-methyl-3-
methylene-5-oxo-2-phenylpyrazolin-4-ylidene)methanolate 2 and
its cycloaddition with dienophiles: synthesis of indazolones
To a solution of diisopropylamine (2 mL, 14 mmol) in dry
tetrahydrofuran (10 mL) under a nitrogen atmosphere, was
added n-BuLi (6.25 mL, 10 mmol, 1.6 M) at 0 ЊC and the
reaction mixture was stirred for 20 min. To the resulting solu-
tion of lithium diisopropylamide (LDA) at Ϫ78 ЊC, a solution
of 1 (1.08 g, 5 mmol) in dry THF (30 mL) was added fol-
lowed by further stirring for 45 min. To the resulting red
colored solution of dienolate 2, appropriate dienophile (5
mmol) dissolved in dry THF (15 mL) was added while main-
taining the temperature at Ϫ78 ЊC. The reaction mixture was
then brought to room temperature during 45 min and left
overnight with stirring. It was then poured into saturated
ammonium chloride solution (150 mL) and extracted with
chloroform (3 × 50 ml). The combined organic extracts were
washed with water, dried (Na2SO4) and then concentrated to
give the crude methanol adduct which was dissolved in dry
benzene (50 mL) followed by addition of pyridinium tosylate
(1.5 g, 6 mmol) and further refluxing for 1 h. The reac-
tion mixture was poured into water (100 mL) and extracted
with chloroform (3 × 50 mL), the combined organic layer
was washed with water, dried over sodium sulfate and con-
centrated. The crude product thus obtained was purified by
column chromatography over silica gel using ethyl acetate–
hexane (19:1) as eluent.
1,2-Dihydro-5-(ethoxycarbonyl)-1-methyl-2-phenyl-3H-
indazol-3-one (9b). A solution of 9a (300 mg, 1 mmol) and
DDQ (295 mg, 1.3 mmol) in dry dioxane (10 mL) was refluxed
with stirring for 2 h. The reaction mixture was then cooled,
diluted with chloroform (20 mL) and filtered. The filtrate was
evaporated to dryness and the residue obtained was purified by
passing it through a silica gel column using hexane–ethyl
acetate (19:1) as eluent; colorless crystals; mp 125–126 ЊC
(chloroform–hexane); yield 90%; IR (KBr): νmax 1680 (br),
1626, 1368, 1295 cmϪ1; 1H NMR (300 MHz, CDCl3): δ 1.42 (t,
3H, J = 7.1 Hz, CH3), 3.25 (s, 3H, NCH3), 4.41 (q, 2H, J = 7.1
Hz, OCH2), 7.29–7.37 (m, 2H, ArH), 7.48–7.58 (m, 4H, ArH),
8.31 (dd, 1H, J = 8.7, 1.5 Hz, ArH), 8.45 (d, 1H, J = 1.5 Hz,
ArH); 13C NMR (75 MHz, CDCl3): δ 14.4, 38.8, 61.2, 111.8,
118.5, 124.0, 125.3, 126.9, 127.2, 129.3, 134.0, 134.6, 153.4,
161.6, 165.9. Anal. calc. for C17H16N2O3 (296.32): C, 68.91; H,
5.44; N, 9.45%. Found: C, 69.14; H, 5.32; N, 9.61%.
1,2-Dihydro-2,6-diphenyl-1-methyl-5-nitro-3H-indazol-3-one
(14). Colorless crystals; mp 196–197 ЊC (chloroform–hexane);
1
yield 58%; IR (KBr): νmax 1675, 1623, 1522, 1344 cmϪ1; H
Attempted purification of methanol adducts (6, 7, 12, 13 and
20) by silica gel column chromatography was not successful and
gave either dihydro (8a, 9a) or fully aromatized (14, 15 and 21)
indazolones along with a mixture of several products.
NMR (90 MHz, CDCl3): δ 3.32 (s, 3H, NCH3), 7.31 (s, 1H,
H-7), 7.40–7.75 (m, 10H, ArH), 8.66 (s, 1H, H-4); MS (m/z, %):
345 (Mϩ, 100). Anal. calc. for C20H15N3O3 (345.36): C, 69.56;
H, 4.38; N, 12.17%. Found: C, 69.73; H, 4.47; N, 12.09%.
1,2-Dihydro-5,6-bis(methoxycarbonyl)-1-methyl-2-phenyl-3H-
indazol-3-one (3). Colorless crystals; mp 180–181 ЊC (chloro-
form–hexane); yield 72%; IR (KBr): νmax 3043, 2948, 1728,
1,2-Dihydro-5-benzoyl-2,6-diphenyl-1-methyl-3H-indazol-3-
one (15). Light yellow crystals; mp 58–59 ЊC (ether–hexane);
yield 80%; IR (KBr): νmax 1658 (br), 1616, 1491, 1315 cmϪ1; 1H
NMR (300 MHz, CDCl3): δ 3.27 (s, 3H, NCH3), 7.25–7.37 (m,
9H, ArH), 7.45–7.63 (m, 5H, ArH), 7.69–7.74 (m, 2H, ArH),
8.09 (s, 1H, H-4); 13C NMR (75 MHz, CDCl3): δ 39.1, 113.9,
116.9, 123.9, 126.2, 126.8, 128.0, 128.3, 128.5, 128.8, 129.3,
130.0, 133.0, 134.4, 134.7, 137.5, 139.9, 147.1, 152.0, 161.5,
196.9; MS: (m/z, %): 404 (Mϩ, 82.4), 389 (19). Anal. calc. for
C27H20N2O2 (404.47): C, 80.18; H, 4.98; N, 6.93%. Found: C,
79.91; H, 5.06; N, 7.02%.
1
1705, 1678, 1621, 1309 cmϪ1; H NMR (300 MHz, CDCl3):
δ 3.26 (s, 3H, NCH3), 3.93 (s, 3H, OCH3), 3.97 (s, 3H, OCH3),
7.33–7.38 (m, 1H, ArH), 7.47 (s, 1H, ArH), 7.51–7.53 (m, 4H,
ArH), 8.47 (s, 1H, ArH); 13C NMR (75 MHz, CDCl3): δ 38.6,
52.7, 53.1, 112.2, 119.3, 124.2, 124.4, 127.3, 127.5, 129.4, 134.2,
138.2, 151.6, 160.7, 166.2, 168.5; MS (m/z, %): 340 (Mϩ, 100),
325 (37.5). Anal. calc. for C18H16N2O5 (340.33): C, 63.53; H,
4.74; N, 8.23%. Found: C, 63.81; H, 4.78; N, 8.30%.
J. Chem. Soc., Perkin Trans. 1, 1999, 3001–3004
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and H. Fillion, Tetrahedron, 1995, 51, 9595; M. A. Hariri, F. Pautet
1,2-Dihydro-1-methyl-5-nitro-2-phenyl-6-methylthio-3H-
indazol-3-one (18). Light yellow crystals; mp 200–201 ЊC
(chloroform–hexane); yield 65%; IR (KBr): νmax 1673, 1615,
and H. Fillion, Synlett, 1994, 459; G. E. Mertzanos, J. Stephanidou-
Stephanatou, C. A. Tsoleridis and N. E. Alexandrou, Tetrahedron
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Stephanatou, Tetrahedron Lett., 1991, 32, 4603.
1
1512, 1296 cmϪ1; H NMR (300 MHz, CDCl3): δ 2.57 (s, 3H,
SCH3), 3.32 (s, 3H, NCH3), 7.01 (s, 1H, H-7), 7.33–7.39 (m, 1H,
ArH), 7.50–7.53 (m, 4H, ArH), 8.84 (s, 1H, H-4); 13C NMR (75
MHz, CDCl3): δ 16.7, 38.4, 106.9, 114.4, 124.2, 124.3, 127.4,
129.5, 134.2, 141.2, 145.5, 152.0, 160.6; MS (m/z, %): 315 (Mϩ,
100), 236 (53). Anal. calc. for C15H13N3O3S (315.35): C, 57.13;
H, 4.15; N, 13.33%. Found: C, 57.43; H, 4.07; N, 13.50%.
7 S. Mitkidou and J. Stephanidou-Stephanatou, Tetrahedron Lett.,
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10 U. K. Syam Kumar, P. K. Patra, H. Ila and H. Junjappa,
Tetrahedron Lett., 1998, 39, 2029; P. K. Patra, J. R. Suresh, H. Ila
and H. Junjappa, Tetrahedron, 1998, 54, 10167; J. R. Suresh,
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1,2,5,10-Tetrahydro-1-methyl-2-phenyl-3H-naphtho[2,3-f ]-
indazole-3,5,10-trione (21). Red crystals; mp 240–241 ЊC
chloroform–hexane); yield 50%; IR (KBr): νmax 1694, 1669,
1
1589, 1325 cmϪ1; H NMR (300 MHz, CDCl3): δ 3.41 (s, 3H,
NCH3), 7.39–7.42 (m, 1H, ArH), 7.52–7.59 (m, 4H, ArH),
7.83–7.87 (m, 2H, ArH), 8.21 (s, 1H, H-11), 8.37, 8.39 (two d,
2H, J = 8 Hz, H-6 and H-9), 8.96 (s, 1H, H-4); MS (m/z, %):
354 (Mϩ, 100). Anal. calc. for C22H14N2O3 (354.36): C, 74.57;
H, 3.98; N, 7.91%. Found: C, 74.34; H, 4.11; N, 7.79%.
11 K. R. Reddy, A. Roy, H. Ila and H. Junjappa, Tetrahedron, 1995, 51,
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12 I. Fleming, Frontier Orbitals and Organic Chemical Reactions,
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Acknowledgements
Financial assistance under the CSIR scheme is acknowledged.
H. J. and A. R. thank CSIR, New Delhi, for an Emeritus
Scientistship and a Senior Research Fellowship respectively.
16 J. J. Fitzgerald, N. E. Drysdale and R. A. Olofson, J. Org. Chem.,
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J. Org. Chem., 1997, 62, 5627 and references therein.
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