Regioselective synthesis of sulfonylpyrazoles via base mediated reaction of diazosulfones with nitroalkenes and a facile entry into withasomnine

Article abstract of DOI:10.1021/ol201534f

A base mediated reaction of α-diazo-β-ketosulfone with nitroalkenes affords sulfonylpyrazoles as single regioisomers in excellent yield in a one-pot room temperature reaction. Aryl, heteroaryl, styrenyl, alkyl, hydroxymethyl, and hydrazinyl groups could b

Full text of DOI:10.1021/ol201534f

ORGANIC
LETTERS
2011
Vol. 13, No. 15
4016–4019
Regioselective Synthesis of
Sulfonylpyrazoles via Base Mediated
Reaction of Diazosulfones with
Nitroalkenes and a Facile Entry into
Withasomnine‡
Rahul Kumar and Irishi N. N. Namboothiri*
Department of Chemistry, Indian Institute of Technology, Bombay,
Mumbai 400 076, India
irishi@iitb.ac.in
Received June 8, 2011
ABSTRACT
A base mediated reaction of R-diazo-β-ketosulfone with nitroalkenes affords sulfonylpyrazoles as single regioisomers in excellent yield in a one-
pot room temperature reaction. Aryl, heteroaryl, styrenyl, alkyl, hydroxymethyl, and hydrazinyl groups could be introduced on the pyrazole ring by
the appropriate choice of nitroalkenes. Synthesis of sulfonylpyrazole fused to other heterocycles and application of the methodology to an
expedient synthesis of a pyrazole alkaloid, Withasomnine, are also reported.
Organosulfones occupy a unique niche in organic synth-
esis owing to their versatility as substrates and intermedi-
ates in numerous transformations.1 The diverse reactivity
of active methylene sulfones2 and vinyl sulfones3 has been
extensively exploited in the synthesis of complex molecules
including natural products.4 The ability of the sulfonyl
group to undergo facile removal has elevated its status to
one of the most enviable groups in functional group
manipulation.5 In the biological arena, organosulfones
function as antibacterial, antiparasitic, DNA cleaver,
anti-HIV, antiviral, and antiandrogen agents.6 The well-
known drugs Bicalutamide (antiprostate cancer) and
Dapsone (antileprosy) possess a sulfonyl group. A highly
bioactive heterocycle pyrazole is an integral part of the
‡ Dedicated to Prof. Vishwakarma Singh on the occasion of his 60th
birthday.
(1) Reviews: (a) Simpkins, N. S. Tetrahedron 1990, 46, 6951. (b)
Simpkins, N. S. Sulfones in Organic Synthesis; Pergamon Press: Oxford,
1993. (c) Back, T. G.; Clary, K. N.; Gao, D. Chem. Rev. 2010, 110, 4498.
(2) Active methylene sulfones, reviews: (a) R-amido sulfones as in situ
equivalents of activated imines: Yin, B.; Zhang, Y.; Xu, L. W. Synthesis
2010, 3583. (b) In Julia-Kocienski olefination: Zajc, B.; Kumar, R.
Synthesis 2010, 1822. (c) Aissa, C. Eur. J. Org. Chem. 2009, 1831. (c)
Varma, R. S. Green Chem. Lett. Rev. 2007, 1, 37. (d) Gais, H.-J.
Organosulfur Chemistry in Asymmetric Synthesis; Wiley-VCH:
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A. L.; Balenkova, E. S. Tetrahedron 2007, 63, 12481. R-Diazo-β-keto
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(3) Reviews: Vinyl sulfones: (a) Nakata, T. Chem. Soc. Rev. 2010, 39,
1955. (b) Zhu, Q.; Lu, Y. Aus. J. Chem. 2009, 62, 951. (c) Pathak, T.
Tetrahedron 2008, 64, 3605. (d) Charette, A. B.; Cote, A.; Desrosiers, J.-
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Nielsen, M.; Jacobsen, C. B.; Holub, N.; Paixao, M. W.; Joergensen,
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VCH: Weinheim, Germany, 2008; p 303. (d) Tiwari, M.; Kishore, D. Int. J.
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Renwick, A. G.; Mitchell, S., Ed. Biological Interactions of Sulfur
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r
10.1021/ol201534f
Published on Web 07/11/2011
2011 American Chemical Society

blockbuster drugs Celecoxib (antiarthritis), Viagra, and
Thiomethisosildenafil (phosphodiesterase inhibitors). Ap-
plications of other pyrazole containing compounds as
biological agents7 and as ligands in coordination
chemistry8 are well-documented in the literature. For
instance, Withasomnine, a pyrazole alkaloid, present in a
medicinal plant, Withania somnifera, found in India and
South Africa, exhibits analgesic and CNS depressant
properties.9 In spite of the above-mentioned significance
of sulfone and pyrazole moieties, compounds containing a
sulfonyl group directly attached to pyrazole received only
limited attention.10
enable us to easily introduce a sulfonyl group to the pyrazole
ring. Although R-diazo-β-ketosulfones have been employed
in intramolecular cyclopropanation15 and carbene
insertion16 reactions, surprisingly, their application as dia-
zoalkane equivalents in cycloadditions for the synthesis of
pyrazoles or pyrazolines remains unreported hitherto.
Table 1. Base Screening
Recently, we reported a facile and regioselective synthesis
of phosphonylpyrazoles via an alkoxide-mediated reaction
of R-diazo-β-ketophosphonate, BestmannÀOhira reagent
(BOR),11 with nitroalkenes and enones.12 One-pot three
component versions of our methodology13 and extension of
our methodology for the synthesis of pyrazole esters14 were
reported by others. We envisaged that R-diazo-β-ketosul-
fone would be an efficient S analog of BOR that would
entry
basea
solvent
% yieldb
1
2
3
4
5
6
À
EtOH
EtOH
EtOH
MeOH
MeOH
t-BuOH
no reaction
(7) Reviews: (a) Elguero, J. In Comprehensive Heterocyclic Chemis-
try; Katritzky, A. R., Rees, C. W., Scriven, E. F. V., Eds.; Pergamon: Oxford,
1996; Vol. 3, p 1. (b) Orth, R. E. J. Pharm. Sci. 1968, 57, 537. (c) Fustero,
S.; Sanz-Cervera, J. F.; Simon-Fuentes, A.; Roman, R.; Catalan, S.;
Murguia, M. ACS Symp. Ser. 2009, 1003, 182. (d) Dolzhenko, A. V.;
Dolzhenko, A. V.; Chui, W. K. Heterocycles 2008, 75, 1575. (e)
Elgemeie, G. H.; Zaghary, W. A.; Amin, K. M.; Nasr, T. M. Nucleosides,
Nucleotides Nucleic Acids 2005, 24, 1227. (f) Lamberth, C. Heterocycles
2007, 71, 1467. (g) McDonald, E.; Jones, K.; Brough, P. A.; Drysdale,
M. J.; Workman, P. Curr. Top. Med. Chem. 2006, 6, 1193. For a recent
article: (h) Lee, K. Y.; Kim, J. M.; Kim, J. N. Tetrahedron Lett. 2003, 44,
6737 and the references cited therein.
NaOEt
KOH
61
70
KOH
62
NaOMe
NaO-t-Bu
85
intractable
mixture
a 1.25 equiv. b Isolated yield after silica gel column chromatography.
The reaction conditions were optimized by treating p-
methoxynitrostyrene 1a with sulfone 2a in the absence of
any base and also in the presence of different bases and
alcohols at room temperature (Table 1). No reaction in the
absence of base (entry 1, Table 1) and formation of a
complex mixture in the presence of a non-nucleophilic base
such as NaO-t-Bu (entry 6, Table 1) confirmed that
deacylation of 2a by a nucleophilic base precedes cyclo-
additionin thesecasesasinthecaseofBOR12 andalsoasin
the concise mechanism shown in Table 1. Although the
reaction was complete in about 15 min in all of the other
cases, entries 2À5 indicate that NaOEt in EtOH (entry 2)
and KOH in EtOH or MeOH (entries 3À4) were less
efficient as compared to NaOMe in MeOH (entry 5,
Table 1). Therefore, further reactions were conducted
using NaOMe in MeOH at room temperature.
(8) (a) For a recent review: Halcrow, M. A. Dalton Trans. 2009, 2059.
For a recent article: (b) Singer, R. A.; Dore, M.; Sieser, J. E.; Berliner,
M. A. Tetrahedron Lett. 2006, 47, 3727.
(9) (a) Schroter, H.-B.; Neumann, D.; Katritzky, A. R.; Swinbourne,
F. J. Tetrahedron 1966, 22, 2895. (b) Adesanya, S. A.; Nia, R.; Fontaine,
C.; Pais, M. Phytochemistry 1994, 35, 1053. (c) Ravikanth, V.; Ramesh,
P.; Diwan, P. V.; Venkateswarlu, Y. Biochem. Syst. Ecol. 2001, 29, 753.
(d) Wube, A. A.; Wenzig, E.-M.; Gibbons, S.; Asres, K.; Bauer, R.;
Bucar, F. Phytochem. 2008, 69, 982.
(10) (a) Savant, M. M.; Pansuriya, M. A.; Bhuva, V. C.; Kapuriya,
N.; Patel, N. A.; Audichya, B. V.; Pipaliya, V. P.; Naliapara, T. Y. J.
Comb. Chem. 2010, 12, 176. (b) Padwa, A.; Wannamaker, W. M.
Tetrahedron 1990, 46, 1145. (c) Kanishchev, S. O.; Bandera, P. Y.;
Timoshenko, M. V.; Rusanov, B. E.; But, S. A.; Shermolovich, G. Y.
Chem. Heterocycl. Compd. 2007, 43, 887. (d) Ouyang, G.; Cai, X.-J.;
Chen, Z.; Song, B.-A.; Bhadury, P. S.; Yang, S.; Jin, L.-H.; Xue, W.; Hu,
D.-Y.; Zeng, S. J. Agric. Food Chem. 2008, 56, 10160. (e) Gao, D.; Zhai,
H.; Parvez, M.; Back, T. G. J. Org. Chem. 2008, 73, 8057. (f) Jeon, D. J.;
Yu, D. W.; Kim, H. R.; Ryu, E. K. Heterocycles 1998, 48, 155.
(11) (a) Ohira, S. Synth. Commun. 1989, 19, 561. (b) Mueller, S.;
Liepold, B; Roth, G. J.; Bestmann, H. J. Synlett 1996, 521. (c) Meffre, P.;
Hermann, S.; Durand, P.; Reginato, G.; Riu, A. Tetrahedron 2002, 58,
5159.
Undertheaboveoptimizedconditions, variousaromatic
nitroalkenes 1aÀk were treated with sulfones 2a and 2b to
afford sulfonylpyrazoles 3aÀm in good to excellent yields
(Table 2). Comparison of entries 1 and 6 shows that there is
no appreciable difference in the yield when sulfones 2a and
2b were used (Table 2). No major aromatic substituent
effect is also observed on the yield or rate of reaction. For
instance, nitrostyrenes possessing a strongly electron-do-
nating substituent such as OMe (1a and 1fÀh, entries 1, 6,
8À10) and a strongly electron-withdrawing substituent
such as NO2 (1iÀk, entries 11À13) provide the adducts
in high yield (75À97%, Table 2). While increasing the
number of strongly electron-donating substituents (OR)
(12) Nitroalkenes:(a)Muruganantham, R.;Mobin, S. M.;Namboothiri,
I. N. N. Org. Lett. 2007, 9, 1125. (b) Muruganantham, R.; Namboothiri,
I. N. N. J. Org. Chem. 2010, 75, 2197. (c) Enones: Verma, D.; Mobin, S. M.;
Namboothiri, I. N. N. J. Org. Chem. 2011, 76, 4764.
(13) (a) Mohanan, K.; Martin, A. R.; Toupet, L.; Smietana, M.;
Vasseur, J. Angew. Chem., Int. Ed. 2010, 49, 3196. (b) Martin, A. R.;
Mohanan, K.; Toupet, L.; Vasseur, J. J.; Smietana, M. Eur. J. Org.
Chem. 2011, 3184.
(14) Xie, J.-W.; Wang, Z.; Yang, W.-J.; Kong, L.-C.; Xu, D.-C. Org.
Biomol. Chem. 2009, 7, 4352.
(15) (a) Sawada, T.; Nakada, M. Adv. Synth. Catal. 2005, 347, 1527.
(b) Honma, M.; Nakada, M. Tetrahedron Lett. 2003, 44, 2007.
(16) (a) Shi, A.; Blake, A. J.; Lewis, W.; Campbell, I. B.; Judkins,
B. D.; Moody, C. J. J. Org. Chem. 2010, 75, 152. (b) Lacrampe, F.;
ꢀ
Franc-oise Leost, F.; Doutheau, A. Tetrahedron Lett. 2000, 41, 4773. (c)
Doyle, M.; Duffy, R.; Ratnikov, M.; Zhou, L. Chem. Rev. 2003, 103,
2861.
Org. Lett., Vol. 13, No. 15, 2011
4017

Table 2. Synthesis of Aryl Sulfonylpyrazoles 3 from β-Aryl
Nitroethylenes 1 and Diazosulfone 2a or 2b
Table 3. Synthesis of Heteroaryl and Alkyl Pyrazoles 5 from β-
Heteroaryl and Alkyl Nitroethylenes 4 and Diazosulfone 2b
entry
R, 4
2-Furyl, 4a
pyrazole 5
% yielda
entry
Ar, 1
2
pyrazole 3
% yielda
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
78
69
70
80
74
83
1
4-OMePh, 1a
Ph, 1b
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
2b
3a
3b
3c
3d
3e
3f
85
90
84
97
81
89
92
82
84
75
97
82
94
2-Thienyl, 4b
2
Cyclohexyl, 4c
Morpholino, 4d
PhCHdCH, 4e
o-OMePhCHdCH, 4f
3
4-ClPh, 1c
4
3-BrPh, 1d
5
4-CH3Ph, 1e
4-OMePh, 1a
Ph, 1b
6
a Isolated yield after purification by silica gel column
chromatography.
7
3g
3h
3i
8
3,4-(OMe)2Ph, 1f
3,4,5-(OMe)3Ph, 1g
benzo[d][1,3]dioxole, 1h
2-NO2Ph, 1i
3-NO2Ph, 1j
4-NO2Ph, 1k
9
10
11
12
13
3j
3k
3l
Table 4. Synthesis of 3,4,5-Trisubstituted Pyrazoles 7 from R,β-
Disubstituted Nitroethylenes 6 and Diazosulfone 2b
3m
a Isolated yield after purification by silica gel column chromatography.
marginally lowers the yield (entries 6, 8À10), a strongly
electron-withdrawing group (NO2) at the ortho or para
position as compared to meta has a positive effect (entries
11À13, Table 2). Parent nitrostyrene 1b (entries 2 and 7)
and nitrostyrenes possessing weakly electron-donating
substituents 1cÀ1e (entries 3À5) also deliver the corre-
sponding sulfonylpyrazoles 3b, 3g, and 3cÀe, respectively,
in excellent yield (Table 2).
Subsequently, nitroethylenes with diverse β-substituents
4aÀf were treated with sulfone 2b, under the above condi-
tions, to afford pyrazoles 5aÀf in good to high yields
(Table 3). These include β-heteroaryl nitroethylenes 4a
and 4b (entries 1À2), β-alkyl nitroethylene 4c (entry 3),
and a pushÀpull system such as β-aminonitroethylene 4d
(entry 4, Table 3). It may be noted that nitrodienes 4e
and 4f also react with 2b to afford the corresponding
sulfonylpyrazoles 5eÀf in good to high yields (entries
5À6, Table 3).
entry
R1
R2
7
time (h)
% yielda
1
2
3
4
5
Ph
Me
Me
7a
7b
7c
7d
7e
24
62
69
77
83
61
2-Furyl
4-MeOPh
2-Furyl
24
CH2OH
NENHEb
0.5
0.5
0.5
(CH2)4
a Isolated yield after purification by silica gel column chromatogra-
phy. b E = CO2Et.
as 8 with sulfone 2b aimed at synthesizing a sulfonylpyr-
azole moiety fused to other heterocycles 9 (Scheme 1).
Thus, nitrochromenes 8a and 8b reacted with sulfone 2b in
the presence NaOMe/MeOH to afford sulfonylpyrazoles
9aÀb in nearly quantitative yields within 1 h at room
temperature.
Having successfully synthesized a variety sulfonylpyra-
zoles 3 and 5 by treating various β-substituted nitroethy-
lenes 1 and 4 with sulfones2aÀb under our mild and simple
experimental conditions, we proceeded to synthesize 3,4,5-
trisubstituted pyrazoles 7 by employing R,β-disubstituted
nitroethylenes 6 (Table 4). Although less impressive yields
and longer reaction times were encountered in these cases,
we were pleased to isolate the 3,4,5-trisubstituted pyrazoles
7aÀe (entries 1À5, Table 4).
Scheme 1. Synthesis of Fused Sulfonylpyrazoles 9 from Nitro-
chromenes 8
Furthering the scope of our methodology was explored
by treating nitroethylene that is part of a heterocycle such
Due to the potent biological properties of With-
asomnine,9 the possibility of developing a general and
efficient route to its total synthesis via application of our
methodology appeared attractive. The approaches reported
in the literature involve intramolecular alkylation,17 oxidative
coupling,18 conversion of cyclopropanols to pyrazoles,19
(17) Marimoto, A.; Noda, K.; Watanabe, T.; Takasugi, H. Tetra-
hedron. Lett. 1968, 9, 5707.
(18) Onaka, T. Tetrahedron Lett. 1968, 9, 5711.
(19) Kulinkovich, O.; Masalov, N.; Tyvorskii, V.; De Kimpe, N.;
Keppens, M. Tetrahedron Lett. 1996, 37, 1095.
4018
Org. Lett., Vol. 13, No. 15, 2011

Scheme 2. Synthetic Approaches to Withasomnine 11
Scheme 3. Total synthesis of Withasomnine 11
radical cyclization,20,21 sydnone cycloaddition,22 multi-
component coupling,23 and hydrazine-1,3-dicarbonyl cyc-
lization.24 Indeed, smooth desulfonation of 3g to 10g in our
hands and the procedure available in the literature for the
transformation of 10g to Withasomnine 11 via N-alkylation
and intramolecular radical cyclization20 confirmed that
3g per se is a good intermediate for the synthesis of 11.
However, we wished to develop novel and efficient routes
to 11 and felt that 11 would be accessible via carbonyl
reduction and desulfonation of lactam 12 which in turn
would arise via intramolecular cyclization of ester 13
(Scheme 2). Synthesis of 13 could be achieved by our
methodology by reacting diazosulfone 2b with nitroester
14. An alternative strategy to synthesize 11 would be via
15, the intramolecular cyclization product of 16 whichinturn
would arise from reaction of 2b with 17 under our experi-
mental conditions.
Initially, we prepared nitroester 14 via a RauhutÀ
Curriertype reaction of β-nitrostyrene with ethyl acrylate,25
or via a high yielding condensation of ethyl 4-nitro-
butyrate with benzaldehyde.26 1,3-Dipolar cycloaddition
of nitroester 14 with sulfone 2b under our conditions
(NaOEt, EtOH) afforded pyrazole ester 13 in 77% yield
(Scheme 3). Reduction of the ester group in 1327 with LAH
afforded corresponding alcohol 16 in 91% yield which was
transformed through a one-pot reaction involving bromi-
nation and intramolecular cyclization to the Withasom-
nine precursor 15 in 83% yield. Finally, desulfonation of
15 to Withasomnine 11 was achieved in 78% yield using
NaÀHg in MeOH.
An alternative route developed for the synthesis of 15
involved condensation of nitrobutanol 1828 with benzal-
dehyde to afford nitroalcohol 17a in 71% yield. Conver-
sion of alcohol 17a to bromide 17b in 91% yield followed
by cycloaddition of 17b with sulfone 2b furnished sulfonyl
Withasomnine 15.
Besides the simplicity and efficiency of our strategy,29
synthesis of other natural and non-natural analogs of
Withasomnine 11 is an attractive prospect which will be
pursued and reported in due course.
In conclusion, a one-pot regioselective method has
been developed for the synthesis of sulfonylpyrazoles
from diazosulfones and nitroalkenes under very
mild and simple experimental conditions.30 The scope
of the method has been demonstrated by synthesizing
a variety of functionalized and fused pyrazoles.
Our methodology has been successfully employed
for the total synthesis of a pyrazole alkaloid With-
asomnine.
Acknowledgment. I.N.N.N. thanks DAE India for fi-
nancial assistance. R.K. thanks CSIR India for a senior
research fellowship. The authors thank SAIF, IIT Bombay
for selected NMR data.
(20) Allin, S. M.; Barton, W. R. S.; Bowman, W. R.; McInally, T.
Tetrahedron Lett. 2002, 43, 4191.
Supporting Information Available. Complete charac-
terization data and copies of NMR spectra for all of the
new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
(21) Allin, S. M.; Barton, W. R. S.; Bowman, B. E.; Elsegood,
M. R. J.; McInally, T.; McKee, V. Tetrahedron 2008, 64, 7745.
(22) (a) Ranganathan, D.; Bamezai, S. Synth. Commun. 1985, 15, 259.
(b) Foster, R. S.; Huang, J.; Vivat, J. F.; Browne, D. L.; Harrity, J. P. A.
Org. Biomol. Chem. 2009, 7, 4052.
(23) Majumder, S.; Gipson, K. R.; Staples, R. J.; Odom, A. L. Adv.
Synth. Catal. 2009, 351, 2013.
(24) Guzman-Perez, A.; Maldonado, L. A. Synth. Commun. 1991, 21,
1667.
(25) Dadwal, M.; Mohan, R.; Panda, D.; Mobin, S. M.; Namboothiri,
I. N. N. Chem. Commun. 2006, 338.
(26) Bhagwatheeswaran, H.; Gaur, S. P.; Jain, P. C. Indian J. Chem.
(28) Newkome, G. R.; Kim, H. J.; Moorefield, C. N.; Maddi, H.;
Yoo, K.-S. Macromolecules 2003, 36, 4345.
(29) The overall yields in our two approaches (Scheme 3) are 23%
and 25%, respectively. These compare well with those of the reported
approaches, viz. 27% in the radical cyclization method in which
commercially available bromopyrazole was used directly (ref 20) and
24% in the multicomponent coupling method (ref 23).
1976, 14B, 699.
(30) The regiochemistry of the sulfonylpyrazoles was confirmed
1
from 1HÀ H NOE observed in a representative system 5c between
(27) Attempted transformation of 13 to cyclized product 12
(Scheme 2) in the presence of excess base and/or under reflux conditions
led to a complex mixture presumably due to transesterification, hydro-
lysis, etc.
cyclohexyl protons and the aromatic protons (see the SI) and also
from synthesis of Withasomnine 11 from nitroalkene derivatives 14
and 17a.
Org. Lett., Vol. 13, No. 15, 2011
4019