Efficient approach to androstene-fused arylpyrazolines as potent antiproliferative agents. Experimental and theoretical studies of substituent effects on BF3-catalyzed intramolecular [3 + 2] cycloadditions of olefinic phenylhydrazones

Article abstract of DOI:10.1021/ja808636e

Highly diastereoselective Lewis acid induced intramolecular 1,3-dipolar cycloadditions of alkenyl phenylhydrazones (containing various substituents on the aromatic ring) obtained from a D-secopregnene aldehyde were carried out under fairly mild conditions

Full text of DOI:10.1021/ja808636e

Published on Web 02/26/2009
Efficient Approach to Androstene-Fused Arylpyrazolines as
Potent Antiproliferative Agents. Experimental and Theoretical
Studies of Substituent Effects on BF3-Catalyzed
Intramolecular [3 + 2] Cycloadditions of Olefinic
Phenylhydrazones
,†
Eva Frank,* Zolta´n Mucsi,‡ Istva´n Zupko´, Borba´la Re´thy, George Falkay,
´
Gyula Schneider,† and Ja´nos Wo¨lfling*,†
Department of Organic Chemistry, UniVersity of Szeged, Do´m te´r 8, H-6720 Szeged, Hungary,
Department of Chemistry and Chemical Informatics, Faculty of Education, UniVersity of Szeged,
Boldogasszony sgt. 6, H-6725 Szeged, Hungary, and Department of Pharmacodynamics and
Biopharmacy, UniVersity of Szeged, Eo¨tVo¨s u. 6, H-6720 Szeged, Hungary
Received November 4, 2008; E-mail: frank@chem.u-szeged.hu; wolfling@chem.u-szeged.hu
Abstract: Highly diastereoselective Lewis acid induced intramolecular 1,3-dipolar cycloadditions of alkenyl
phenylhydrazones (containing various substituents on the aromatic ring) obtained from a D-secopregnene
aldehyde were carried out under fairly mild conditions to furnish androst-5-ene-fused arylpyrazolines in
good to excellent yields. The ability of phenylhydrazones to undergo cyclization was found to be affected
significantly by the electronic features of the substituents on the aromatic moiety. The rates of the ring-
closure reactions were observed to be increased by electron-donating and decreased by electron-
withdrawing groups. The experimental findings on the BF3-catalyzed transformations were supported by
calculations of the proposed mechanism at the BLYP/6-31G(d) level of theory, indicating a noteworthy
dependence, mainly of the initial complexation step, and hence of the whole process, on the character of
the substituent. The cycloaddition was estimated to occur via a zwitterionic intermediate rather than involving
a pure concerted mechanism. The antiproliferative activities of the structurally related pyrazoline derivatives
were tested in vitro on three malignant human cell lines (HeLa, MCF7, and A431): the microculture
tetrazolium assay revealed that several compounds exerted marked cell growth-inhibitory effects. The highest
cytotoxic activities, displayed by the p-methoxyphenylpyrazoline derivative 7d (IC50 values: 2.01, 2.16, and
1.41 µM on HeLa, MCF7, and A341 cells, respectively), were better than those of cisplatin (IC50 values:
12.43, 9.63, and 2.84 µM, respectively).
1. Introduction
stituted hydrazines is a method frequently applied for the in
situ generation of azomethine imines,2 though monosubstituted
hydrazones as potential azomethine imine precursors are also
known to undergo inter- or intramolecular cycloadditions under
thermal3 or strongly acidic conditions.4 An azomethine imine
tautomer is suggested to be formed from the corresponding
hydrazone during these conversions by an N,N′-prototropic shift.
The development of 1,3-dipolar cycloaddition chemistry has
entered a new stage in recent years, with control of stereose-
The construction of five-membered heterocycles containing
two or more ring N atoms via the reactions of azomethine imines
with multiple-bond systems is one of the synthetic applications
of the well-known and exhaustively studied 1,3-dipolar cy-
cloadditions.1 However, these transformations have attracted less
attention as compared with those of other 1,3-dipoles (e.g.,
nitrones, nitrile oxides, and azomethine ylides) with unsaturated
dipolarophiles. The condensation of aldehydes with 1,2-disub-
(2) (a) Jones, R. C. F.; Hollis, S. J.; Iley, J. N. ARKIVOC 2007, 152. (b)
Grashey, R. in ref 1b, Vol. 1, p 736 and further references therein.
(3) For examples under thermal conditions, see:(a) Gergely, C.; Morgan,
J. B.; Overman, L. E. J. Org. Chem. 2006, 71, 9144. (b) Grigg, R.;
Kemp, J.; Thompson, N. Tetrahedron Lett. 1978, 31, 2827. (c) Khau,
V. V.; Martinelli, M. J. Tetrahedron Lett. 1996, 37, 4323. (d) Grigg,
R.; Dowling, M.; Jordan, M. W.; Sridharan, V.; Thianpatanagul, S.
Tetrahedron 1987, 43, 5873. (e) Sun, B.; Adachi, K.; Noguchi, M.
Tetrahedron 1996, 52, 901. (f) Snider, B. B.; Conn, R. S. E.; Sealfon,
S. J. Org. Chem. 1979, 44, 218. (g) Noguchi, M; Matsumoto, S.; Shirai,
M.; Yamamoto, H Tetrahedron 2003, 59, 4123. (h) Noguchi, M.;
Yamada, K. Synthesis 1993, 145.
† Department of Organic Chemistry.
‡ Department of Chemistry and Chemical Informatics, Faculty of
Education.
Department of Pharmacodynamics and Biopharmacy.
(1) For reviews of 1,3-dipolar cycloadditions, see:(a) Synthetic Applications
of 1,3-Dipolar Cycloaddition Chemistry Toward Heterocycles and
Natural Products; Padwa, A., Pearson, W. H., Eds.;Chemistry of
Heterocyclic Compounds; Wiley: Hoboken, NJ, 2003; Vol. 59. (b)
1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New
York, 1984. (c) Cycloaddition Reactions in Organic Synthesis;
Kobayashi, S., Jørgensen, K. A., Eds.; Wiley: Weinheim, 2002. (d)
Gothelf, K. V.; Jørgensen, K. A. Chem. ReV. 1998, 98, 863. (e) Kris,
H.; Jari, Y-K. Molecular DiVersity 2005, 9, 187. (f) Koumbis, A. E.;
Gallos, J. K. Curr. Org. Chem. 2003, 7, 771. (g) Nair, V.; Suja, T. D.
Tetrahedron 2007, 63, 12247.
(4) For examples under acidic conditions, see:(a) Le Fevre, G.; Sinbandhit,
S.; Hamelin, J. Tetrahedron 1979, 35, 1821. (b) Shimizu, T.; Hayashi,
Y.; Miki, M.; Teramura, K. J. Org. Chem. 1987, 52, 2277. (c) Fouchet,
B.; Joucla, M.; Hamelin, J. Tetrahedron Lett. 1981, 22, 1333.
9
3894 J. AM. CHEM. SOC. 2009, 131, 3894–3904
10.1021/ja808636e CCC: $40.75
2009 American Chemical Society

Substituent Effects on Phenylhydrazone Cycloadditions
A R T I C L E S
lectivity by choosing appropriate chiral substrates or applying
a chiral Lewis acid catalyst becoming the most important
challenge. Lewis acid catalysis allows the cycloaddition pathway
to proceed under milder conditions with improved diastereo-
selectivity and rate acceleration relative to those in conventional
protocols. Furthermore, in contrast with intermolecular reactions,
the intramolecular version has the advantage of higher degrees
of regio- and diastereoselectivity due to entropy factors and
limited conformational mobility in the transition state. Several
Lewis acid induced stereoselective 1,3-dipolar cycloadditions
of nitrones,5 azomethine ylides,6 and nitrile oxides,7 leading to
optically active five-membered ring systems, have previously
been reported. Similar catalytic reactions of azomethine imines,
however, have been less intensively studied.8 [3 + 2] cycload-
ditions of acylhydrazones as azomethine imine equivalents with
different Lewis acids were recently described by Kobayashi et
al.9 4-Nitrobenzoylhydrazone systems underwent intra- and
intermolecular ring closures with olefins with high stereoselec-
tivity in the presence of Zr(OTf)4, Sc(OTf)3, or chiral Zr
catalysts. The experimental findings suggested that the inter-
molecular reaction mechanism involved a [3 + 2]-concerted
pathway.9c Lower diastereoselectivity was attained in the
cycloaddition of an acylhydrazone to cyclopentadiene when a
stoichiometric amount of BF3 ·OEt2 was used.9a Nevertheless,
even a catalytic amount of BF3 ·OEt2 can be suitable for
promoting stereoselective intramolecular 1,3-dipolar cycload-
ditions, as we reported earlier in connection with a D-secoestrone
phenylhydrazone containing a propenyl side chain.10 In the
presence of this Lewis acid, the chiral sterane skeleton bearing
both a 1,3-dipole and a dipolarophile moiety afforded an
excellent possibility for control of the stereochemistry of the
process. Since unsubstituted phenylhydrazone was found to be
an appropriate precursor for the in situ generation of the quasi-
azomethine imine 1,3-dipole in Lewis acid media, the synthesis
of analogous derivatives was planned with a view to investigate
their behavior under similar catalytic conditions. Therefore, a
series of olefinic D-secopregnene phenylhydrazones containing
different substituents on the aromatic moiety were synthetized,
and their BF3 ·OEt2-induced intramolecular 1,3-dipolar cycload-
ditions were carried out. The proposed reaction mechanism was
subjected to theoretical analysis by DFT (B3LYP) calculations
with the intention of supporting the experimental findings.
Moreover, the synthetized androst-5-ene-fused pyrazoline de-
rivatives were applied in in Vitro pharmacological studies to
investigate their antiproliferative effects on three human ma-
lignant cell lines. Due to the structural diversity of the tested
compounds, structure-activity relationships were also examined.
2. Results and Discussion
2.1. Synthetic Studies. During preliminary experiments, D-
secopregnene aldehyde 2 was synthetized in a multistep pathway
from commercially available pregnadienolone acetate 1 (Scheme
1).11 The presence of the formyl group and the unsaturated side
chain makes 2 an excellent starting material for condensation
and subsequent cyclization to give fused heteroatom-containing
frameworks via intramolecular sequences.12 Thus, the phenyl-
hydrazones of 2 as olefinic azomethine imine precursors were
expected to undergo Lewis acid mediated 1,3-dipolar cycload-
ditions. Aldehyde 2 was therefore initially reacted with phe-
nylhydrazines 3a-l to furnish the corresponding phenylhydra-
zones 4a-l, respectively, which were isolated from the reaction
(5) For recent examples of Lewis acid induced nitrone [3 + 2] cycload-
ditions to unsaturated dipolarophiles, see:(a) Merino, P.; Tejero, T.;
Laguna, M.; Cerrada, E.; Moreno, A.; Lopez, J. A. Org. Biomol. Chem.
2003, 1, 2336. (b) Pe´rez, P.; Domingo, L. R.; Aurell, M. J.; Contreras,
R. Tetrahedron 2003, 59, 3117. (c) Kanemasa, S.; Ueno, N.; Shirahase,
M. Tetrahedron Lett. 2002, 43, 657. (d) Tanaka, J.; Kanemasa, S.
Tetrahedron 2001, 57, 899. (e) Domingo, L. R.; Benchouk, W.;
Mekelleche, S. M. Tetrahedron 2007, 63, 4464. (f) Desimoni, G.; Faita,
G.; Mella, M.; Boiocchi, M. Eur. J. Org. Chem. 2005, 1020. (g) Viton,
F.; Bernardinelli, G.; Ku¨ndig, E. P. J. Am. Chem. Soc. 2002, 124,
4968. (h) Zhao, Q.; Han, F.; Romero, D. L. J. Org. Chem. 2002, 67,
3317. (i) Pellissier, H. Tetrahedron 2007, 63, 3235.
mixtures in crystalline form in high yields.13 The H NMR
1
spectra recorded at different times for each compound, however,
revealed that E/Z isomerization occurred in solution,14a which
can be catalyzed by traces of acid14b or free phenylhydrazine.14c
The same equilibration was observed on acidic silica-based TLC
plates. To determine the configuration of the products arising
from the condensations, the chemical shifts of the 16-H signals
of the related isomers were compared. In all cases, 16-H
resonated at lower fields (∼7.00 ppm) in the originally formed
isomer, and at higher fields (∼6.5 ppm) for the stereomer that
appeared after a period in solution. This is in good agreement
with the Karabatsos rule,14a,d which states that the proton
attached to the CdN group in aldehyde phenylhydrazones is
more deshielded when it is cis than when it is trans to the anilino
moiety. Geometry optimization carried out on some selected
derivatives (4a,b,d,f,k) to compare the relative stabilities of the
related isomers indicated that the E isomers are theoretically
(6) For recent examples of Lewis acid induced azomethine ylide [3 + 2]
cycloadditions to unsaturated dipolarophiles, see:(a) Gothelf, A. S.;
Gothelf, K. V.; Hazell, R. G.; Jørgensen, K. A. Angew. Chem., Int.
Ed. 2002, 41, 4236. (b) Na´jera, C.; Sansano, J. M. Angew. Chem.,
Int. Ed. 2005, 44, 6272. (c) Yan, X-X.; Peng, Q.; Zhang, Y.; Zhang,
K.; Hong, W.; Hou, X-L.; Wu, Y-D. Angew. Chem., Int. Ed. 2006,
45, 1979. (d) Kanemasa, S. Synlett 2002, 1371. (e) Cabrera, S.;
Arraya´s, R. G.; Carretero, J. C. J. Am. Chem. Soc. 2005, 127, 16394.
(f) Pandey, J.; Dwivedi, N.; Singh, N.; Srivastava, A. K.; Tamarkar,
A.; Tripathi, R. P. Bioorg. Med. Chem. Lett. 2007, 17, 1321.
(7) For recent examples of Lewis acid-induced nitrile oxide [3 + 2]
cycloadditions to unsaturated dipolarophiles, see:(a) Yamamoto, H.;
Hayshi, S.; Kubo, M.; Harada, M.; Hasegawa, M.; Noguchi, M.;
Sumimoto, M.; Hori, K. Eur. J. Org. Chem. 2007, 2859. (b) Wagner,
G.; Danks, T. N.; Vullo, V. Tetrahedron 2007, 63, 5251. (c) Sibi,
M. P.; Itoh, K.; Jasperse, C. P. J. Am. Chem. Soc. 2004, 126, 5366.
(d) Fedou, N. M.; Parsons, P. J.; Viseux, E. M. E.; Whittle, A. J. Org.
Lett. 2005, 7, 3179.
(10) For Lewis acid induced [3 + 2] cycloadditions of phenylhydrazones
´
with olefins, see: (a) Frank, E.; Wo¨lfling, J.; Aukszi, B.; Ko¨nig, V.;
Schneider, T. R.; Schneider, G. Tetrahedron 2002, 58, 6843. (b) Ko¨nig,
´
V.; Schenider, T. R.; Frank, E; Aukszi, B.; Schneider, G.; Wo¨lfling,
J. Acra Crystallogr. 2002, E58, o810.
(11) Wo¨lfling, J.; Magyar, A.; Schneider, G. Monatsh. Chem. 2003, 134,
1387.
(8) For recent reports on the Lewis acid mediated 1,3-dipolar cycload-
ditions of azomethine imines, see:(a) Suga, H; Funyu, A.; Kakehi, A.
Org. Lett. 2007, 9, 97. (b) Shintani, R.; Fu, G. C. J. Am. Chem. Soc.
2003, 125, 10778.
(12) For the synthesis of modified steroid derivatives from 2, see:(a)
Magyar, A.; Wo¨lfling, J.; Kubas, M.; Seijo, J. A. C.; Sevvana, M.;
Herbst-Irmer, R.; Forgo´, P.; Schneider, G. Steroids 2004, 69, 301.
´
(9) For Lewis acid induced [3 + 2] cycloadditions of acylhydrazones with
olefins, see:(a) Kobayashi, S.; Hirabayashi, R.; Shimizu, H.; Ishitani,
H.; Yamashita, Y. Tetrahedron Lett. 2003, 44, 3351. (b) Kobayashi,
S.; Shimizu, H.; Yamashita, Y.; Ishitani, H.; Kobayashi, J. J. Am.
Chem. Soc. 2002, 124, 13678. (c) Yamashita, Y.; Kobayashi, S. J. Am.
Chem. Soc. 2004, 126, 11279. (d) Sugiura, M.; Kobayashi, S. Angew.
Chem., Int. Ed. 2005, 44, 5176. (e) Kobayashi, S. Eur. J. Org. Chem.
1999, 15.
(13) For a short communication in this topic, see:(a) Frank, E.; Kardos,
Zs.; Wo¨lfling, J.; Schneider, G. Synlett 2007, 1311.
(14) For the E/Z isomerization of phenylhydrazones, see:(a) Karabatsos,
G. J.; Taller, R. A. J. Am. Chem. Soc. 1963, 85, 3624. (b) Karabatsos,
G. J.; Shapiro, B. L.; Vane, F. M.; Fleming, J. S.; Ratka, J. S. J. Am.
Chem. Soc. 1963, 85, 2784. (c) Bellamy, A. J.; Hunter, J. J. Chem.
Soc., Perkin Trans. 1 1976, 456. (d) Trabelsi, M.; Salem, M.;
Champagne, B. Org. Biomol. Chem. 2003, 1, 3839.
9
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A R T I C L E S
Frank et al.
Scheme 1. Synthesis of Novel D Ring-Fused Steroidal Azacycles by 1,3-Dipolar Cycloaddition of Olefin-Substituted Hydrazones
5-6 kJ mol-1 more stable than the corresponding Z isomers.
With regard to the E/Z isomerism of 4a-l and the instability
even in the solid state of those containing an electron-donating
group (EDG) on the aromatic ring, the crude (E)-phenylhydra-
zones (4a-l) were subjected to intramolecular cyclization in
the presence of a catalytic amount of BF3 ·OEt2 directly, without
purification. The ring closures of 4a-j proceeded in a stereo-
selective manner to furnish androstene-fused pyrazoline deriva-
tives 6a-j as single diastereomers, with 17-H in the R and 3′-H
in the ꢀ position. No traces of the primarily formed pyrazolidines
5a-j were observed, which is not surprising as such hererocyclic
compounds are known to be often unstable and to undergo facile
dehydrogenation under the reaction conditions or during the
workup.3e,10a,15 Interestingly, 5k could be entrapped from the
reaction mixture as a product of the catalytic ring closure of
4k, but it transformed slowly to pyrazoline 6k on standing in
the air. In contrast, the 2′,4′-dinitrophenylpyrazolidine 5l
obtained from 4l by Lewis acid treatment displayed high
stability, with no subsequent dehydrogenation, probably as a
consequence of internal H-bond formation between the hetero
ring NH and the ortho NO2 group on the Ph ring. Nevertheless,
these transformations furnished evidence relating to the stereo-
structures of the primary products (5) via which pyrazoline
formation conceivably occurs. The reaction rates depended
strongly on the electronic character of the substituents on the
Ph, in correlation with the observed stability of the starting
phenylhydrazones 4a-l. Compounds 4b-e containing an EDG
readily underwent heterocycle formation within 20 min at 0 °C
to give 6b-e in excellent yields. The ortho CH3 group in 4c
seemed to have no steric influence at all on the intramolecular
ring closure. In contrast, the unsubstituted phenylhydrazone 4a
and those containing an electron-withdrawing group (EWG) on
the aromatic moiety (4f-k) proved to be more stable, and a
longer reaction time (3 h) was needed at the same temperature
for appreciable conversion. The residual phenylhydrazones
reduced the yields of the desired products 6a and 6f-k in these
latter cases. The strong dependence of the process on the nature
of the substitution is supported by the fact that increased reaction
temperature (40 °C) was required for the catalytic cyclization
of 2′,4′-dinitrophenylhydrazone 4l to afford 5l, but only a low
yield was achieved. Besides the presence of the two unfavorable
EWGs on the Ph ring in 4l, steric hindrance of the ortho NO2
group (larger than ortho CH3) can also hamper the cycloaddition.
Compounds 4m and 4n, with two EDGs on the Ph ring, were
so reactive that they were only detected by TLC and underwent
in situ cyclization in the absence of the Lewis acid under the
conditions of their formation from 2 with 3m or 3n. However,
in the absence of BF3 ·OEt2, the stereoselectivity of both
reactions proved to be the same as that observed for the catalytic
reactions, i.e. probably only one (5m or 5n) of the four possible
pyrazolidine diastereomers was produced, which was further
converted to 6m or 6n by oxidation. Similarly to phenylhydra-
(16) (a) Wilson, R. M.; Rekers, J. W. J. Am. Chem. Soc. 1979, 101, 4005.
(b) Padwa, A.; Ku, H. J. Org. Chem. 1980, 45, 3756. (c) Padwa, A.
in ref 1b, Vol. 2, p 304 and further references therein. (d) Kirmse,
W.; Ho¨mberger, G. J. Am. Chem. Soc. 1991, 113, 3925.
(15) Bishop, J. E.; Flaxman, K. A.; Orlek, B. S.; Sammes, P. G.; Weller,
D. J. J. Chem. Soc., Perkin Trans. 1 1995, 2551.
(17) Gothelf, K. V.; Jørgensen, K. A. in ref 1a, p 819.
9
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Substituent Effects on Phenylhydrazone Cycloadditions
A R T I C L E S
Scheme 2. A Simplified Model (11) of the Starting Sterane Skeleton (4), and the Four Possible Products (12, 13, 14, and 15), with the Atom
Numbering
zones 4a-l, tosylhydrazone 4o was synthetized by the conden-
sation of 2 with tosylhydrazide 3o, the cyclization of which
furnished 9 regio- and diastereoselectively through the presumed
ionic intermediate 8. Although the base-induced 1,3-dipolar
cycloaddition of olefinic tosylhydrazones has been reported to
occur under thermal conditions via a diazoalkane 1,3-dipole
generated in situ, which further reacts with the internal dipo-
larophile in a concerted mechanism, a stepwise carbocation
pathway has been presumed for the Lewis acid-catalyzed ring
closure of such systems.16 The stereostructure of compound 9,
involving an NdN double bond in its pyrazoline ring, is quite
similar to those of the primary products 5a-n derived from
phenylhydrazones 4a-n, which suggests a similar reaction
mechanism for their formation. For pharmacological studies,
the synthetized cycloadducts 6a-k,m,n and 9, but not 5l,
because of its low isolated yield from 4l, were deacetylated in
alkaline MeOH to furnish the corresponding 3-OH derivatives
7a-k,m,n and 10, respectively.
2.2. Theoretical Studies. To understand the reasons for the
observed substituent effect during the BF3 ·OEt2-promoted
cycloadditions of phenylhydrazones 4a-l and to explain the
high diastereofacial selectivity of the transformations, including
those from 4m and 4n, which were carried out without the Lewis
acid catalyst, computational analysis of the possible thermally
induced (PROCEDURE I) and BF3-catalyzed reaction (PRO-
CEDURE II) mechanisms was performed to compare the energy
profiles of the different processes. For the calculations, a
simplified alkenyl phenylhydrazone model structure 11 (derived
from the original sterane framework 4) was used, assuming that
the part ignored does not have a significant effect on the reaction
(Scheme 2). The atoms directly involved in the cycloaddition
or situated near the reaction center in 11 are arbitrarily
numbered. Theoretically, the ring closure of 11 can lead to four
pyrazolidine diastereomers (12, 13, 14, and 15) in both
PROCEDURES I and II: the trans configuration of the olefinic
dipolarophile moiety is conserved during the reaction, and thus
the opposite spatial orientation of the protons on C-7 and C-8
is seen in the products.17
2.2.1. Thermally vs Lewis Acid Induced Ring Closures. The
two kinds of procedure were investigated first for the unsub-
stituted phenylhydrazone 11a, which permitted a comparison
of the different pathways. During PROCEDURE I, an initial
isomerization is presumed to occur around the C-1-C-2 σ-bond,
with equilibration between the anti (11a) and gauche isomers
(16a), the latter existing in four conformers (I, II, III, and IV)
by twisting around the C-2-C-3 and/or C-6-C-7 axis (Scheme
3). From this point, two possible routes (Routes A and B) must
be considered via which the thermally induced ring closure can
take place. In Route A, the equilibrium step is immediately
followed by cyclization to yield four zwitterionic intermediates
(17a, 18a, 19a, and 20a) with different stereostrucures with
respect to the configurations at C-3, C-7, and C-8. The
subsequent rapid deprotonation-protonation equilibrium step
can lead finally to the corresponding pyrazolidine diastereomers
(12a, 13a, 14a, and 15a). A rather different mechanism
predominates for Route B, however, where isomer 16a first
rearranges by tautomerization to 21a, which also has four
conformational states (21a-I, 21a-II, 21a-III, and 21a-IV). The
direct ring closure of 21a eventually results in the four possible
diastereomeric products (12a, 13a, 14a, and 15a). The energy
profiles of Routes A and B are significantly different as each
transition state in Route B possesses lower energy than those
in Route A (Figure 1A). Route B, involving the in situ formation
of an azomethine imine 1,3-dipolar intermediate 21a from the
starting phenylhydrazone 11a, can therefore be suggested to be
the active path, in accordance with the Curtin-Hammet
principle.18 This is also in good agreement with the recom-
mendations of other authors3 for reactions of this type under
thermal conditions. Nevertheless, even in Route B all the
activation energies computed for the reactions leading to any
of the possible products are relatively high (∆G‡ ) 155-220
kJ mol-1), and the transformations are therefore predicted to
require an elevated reaction temperature to proceed. Similarly
as in PROCEDURE I, there are two alternative mechanisms
(18) Vincze, Z.; Mucsi, Z.; Scheiber, P.; Nemes, P. Eur. J. Org. Chem.
2008, 1092.
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A R T I C L E S
Frank et al.
Scheme 3. Two Possible Mechanisms (Route A and Route B) Suggested for the Formation of Pyrazolidines (12a-15a) under Thermal
Conditions (PROCEDURE I); TS ) Transition State
(Routes C and D) for the formation of the four possible products
(12a, 13a, 14a, and 15a) via the BF3 ·OEt2-promoted intramo-
lecular cyclization (PROCEDURE II) of model compound 11a
(Scheme 4). The initiating steps in both Routes C and D include
two fast equilibria, i.e., the complexation of N-4 in 11a with
BF3 to furnish 22a, and a subsequent conformational change
around the C-1-C-2 bond to give 23a. Complex formation at
N-5 is less preferable by 18.56 kJ mol-1. Intermediates 22a and
23a can exist in four conformers (I, II, III, and IV) by rotation
around the C-2-C-3 and/or C-6-C-7 bond axis. The direct ring
closure of 23a (Route C) can lead to cyclic intermediates 24a,
25a, 26a, and 27a, differing in their configurations at C-3, C-7,
and C-8. The subsequent deprotonation-protonation and de-
complexation result in the four possible pyrazolidines (12a, 13a,
14a, and 15a). In contrast, Route D in PROCEDURE II supposes
an additional isomerization step similarly as in thermally induced
Route B, during which a proton/BF3 interchange between N-4
and N-5 can occur to afford a quasi-azomethine imine 32a with
four conformational states (32a-I, 32a-II, 32a-III, and 32a-IV).
The following cyclization leads to intermediates 28a, 29a, 30a,
and 31a, which can further transform to the isomeric products
(12a, 13a, 14a, and 15a) by loss of BF3 during the workup.
The computed energy profiles of the hypothetical Routes C and
D in PROCEDURE II (Figure 1B) suggest that the latter would
be more favorable, due to the lower energies of all the transition
states via which the final products (12a, 13a, 14a, and 15a)
can be produced. Comparison of the preferred Route B in
PROCEDURE I and Route D in PROCEDURE II reveals that
Figure 1. Energy diagrams of the possible routes for PROCEDURE I (A) and PROCEDURE II (B).
9
3898 J. AM. CHEM. SOC. VOL. 131, NO. 11, 2009

Substituent Effects on Phenylhydrazone Cycloadditions
A R T I C L E S
Scheme 4. Two Possible Mechanisms (Route C and Route D) Suggested for the BF3 ·OEt2-Induced Formation (PROCEDURE II) of
Pyrazolidines (12a-15a); TS ) Transition State
the lowest threshold energy is required for the formation of 12a,
which means that the reaction may be expected to take place in
a stereoselective manner even under controlled thermal condi-
tions. This is in agreement with the experimental finding that
the ring closures of 4l and 4m, derived from the condensations
of 2 with 3l and 3m (Scheme 1), were diastereoselective. These
cycloadditions, which proceeded spontaneously at room tem-
perature to give 6l and 6m, can be regarded as thermally induced
transformations, as compared with most of those carried out at
0 °C with the Lewis acid catalyst. Additionally, the energy
diagram of Route D indicates that the transition states which
can be assigned to the other three diastereomers (13a, 14a, and
or 15a) have relatively high energies and the reactions leading
to these products are endothermic; hence, their formation is
negligible at room temperature or below. The overall activation
energy difference between Route B and Route D for 12a is ∼30
kJ mol-1, which makes the latter more favorable and allows it
to occur under milder conditions.
2.2.2. Computational Studies of Transition States Leading
to 12a in Routes B and D. Since the mechanisms of several
electrocyclic reactions during which a heteroatom (N or O) is
involved in ring formation have been reported to include
nucleophilic features,18 a further detailed characterization of the
transition states relating to 12a was carried out for both Route
B and Route D. The cyclization of 11a via Route B is expected
to follow a concerted mechanism, which means that the
corresponding transition state should possess an aromatic
character.19 The calculated nucleus-independent chemical shift
(19) (a) Cossio, F. P.; Morao, I.; Jiao, H.; Schleyer, P. v. R. J. Am. Chem.
Soc. 1999, 121, 6737. (b) Morao, I.; Lecea, B.; Cossio, F. P. J. Org.
Chem. 1997, 62, 7033.
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J. AM. CHEM. SOC. VOL. 131, NO. 11, 2009 3899

A R T I C L E S
Frank et al.
Figure 2. 2D cross sections of the potential energy surface and the structures of the two transition states (TS) for Route B (A) and Route D (B)
Table 1. Computed Gibbs Free Energies (kJ mol-1) of the
Individual Intermediates Relative to 11 and ∆G‡[overall] of Route
D for the Intramolecular [3 + 2] Cycloaddition of 11f12
11
22
23
32
∆G‡[overall]
28
12
a
b
c
d
e
f
g
h
i
0.00
0.00 -15.86
0.00 -12.69
-8.78
-6.64 44.54
-9.05 40.44
8.96 49.80
127.46
122.87
120.67
120.43
129.76
128.64
132.66
132.61
127.12
135.70
134.11
188.01
-3.71
-8.7
33.95
-11.56
-0.34
-3.43
3.22
-0.73
-0.31
6.25
0.26
2.23
0.00 -16.53 -13.03 40.39
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
-3.20
-4.11
2.46
0.72
-2.30
6.14
-0.55 46.98
-2.99 46.06
0.74 47.59
1.09 47.87
0.01 45.12
4.27 48.61
2.91 47.98
15.97 65.47
0.00
-3.50
-6.50
-6.17
1.19
4.42
8.76 -10.25
4.98
j
k
l
4.32
17.88
-4.50
31.89
Figure 3. Energy diagram of Route D for the intramolecular [3 + 2]
cycloadditions of some selected alkenyl phenylhydrazones (11b,d,h,j)
compared with that of 11a.
59.03
(NICS),20 which can be used as a descriptor of the aromatic or
antiaromatic character of a transition state, is -17.7 ppm for
that intermediate between 21a and 12a. The distances between
the C-3 and C-7 (d1 ) 2.20 Å) and N-5 and C-8 (d1 ) 2.26 Å)
are very similar in this transition state, and the minimum energy
pathway, which develops as an almost diagonal line on the ab
initio potential energy surface, supports the concerted reaction
theory for the thermally induced formation of 12a in Route B
(Figure 2A). In contrast, the ring closure of 11a via Route D
seems to proceed in a significantly different pathway as the
distances C-3-C-7 and N-5-C-8 are appreciably different and
the minimum energy pathway is notably bent in this case; hence,
the transition state between 32a and 28a can be regarded rather
as a zwitterionic transition state (Figure 2B). This assumption
is supported by the calculated NICS, which is considerably lower
in magnitude (-13.8 ppm) for the transition state between 32a
and 28a in Route D than for that between 21a and 12a in Route
B. Accordingly, the Lewis acid catalyzed Route D is suggested
to take place in a stepwise manner with zwitterionic character
rather than in a synchronous mechanism; i.e., electrophilic attack
of the electron-deprived C-3 to C-7 precedes the N-5-C-8 bond
formation.
2.2.3. Quantitative Analysis of Substituent Effects on
Route D. With regard to the experimental findings observed
during the BF3 ·OEt2-induced cyclization of 4a-l, the reactivity
of the structurally related olefinic phenylhydrazone 11a was
predicted to change on substitution on the Ph ring and this was
presumed to be manifested in the energy profile of the
theoretically more preferable Route D of PROCEDURE II. The
overall Gibbs free energy of activation of the reaction from
(20) (a) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes,
N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317. (b) Schleyer, P. v.
R.; Jiao, H. Pure Appl. Chem. 1996, 68, 209. (c) Jiao, H.; Schleyer,
P. v. R. J. Phys. Org. Chem. 1998, 11, 655.
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3900 J. AM. CHEM. SOC. VOL. 131, NO. 11, 2009

Substituent Effects on Phenylhydrazone Cycloadditions
A R T I C L E S
Figure 4. Correlation of the ∆G values of the individual subprocesses with the Hammett parameters σF of the different substituents on the Ph in the
reactions of alkenyl para-substituted phenylhydrazones (11a,b,d,f-k): (A) complexation equilibrium, (B) configurational equilibrium, (C) isomerization
equilibrium, (D) ring-closure step.
cordance with the experimental observations during the ring
closure of 4l. The Hammet constants σp,21 which are widely
used to describe the electronic effects of different substituents
and hence to measure the reactivities of substituted benzenes,
were found to display a relatively good linear correlation22
with the activation energies of the individual subprocesses
and also with ∆G‡[overall] for the reactions of the para-
substituted derivatives (11a,b,d,f-k) (Figures 4 and 5). The
fitted lines exhibit positive slopes for the first complexation
equilibrium (Figure 4A) and for the final ring-closure steps
(Figure 4D), which means that EWGs make these subpro-
cesses less favorable as compared with EDGs. The negative
slopes of the fitted lines for the conformation (Figure 4B)
Figure 5. Overall Gibbs free energies [∆G‡(overall)] for the ring closures
and isomerization equilibrium steps (Figure 4C), however,
show that these subprocesses are somewhat more preferable
when there are EWGs on the Ph ring.
of 11a,b,d,f-k correlated with the Hammett parameters σF of the different
Ph substituents.
11ato the transition state between 32a and 28a consists of
the energies of four subprocesses (Scheme 4, Figure 1B, eq
1), which were computed for the reactions of 11 other
phenylhydrazone derivatives (11b-l) containing substituents
at similar positions as in the experimental compounds 4b-l.
The detailed analysis of the four subprocesses indicated that
the changes in Gibbs free energy during the individual steps
differed considerably, depending on the electronic character
of the substituents on the Ph (Figure 3). The overall Gibbs
free energy, which is the resultant of the energies of the single
steps, is higher for EWGs than for EDGs on the aromatic
ring (Table 1). Accordingly, in agreement with the experi-
mental observations, an EDG seems to favor cycloaddition,
whereas a higher activation energy is needed when the Ph
bears an EWG. The computed activation energies of the four
subprocesses for the cycloaddition of 2′,4′-dinitrophenylhy-
drazone 11l are extremely high, as an indication of the
unfavorable nature of the transformation in complete ac-
Of the various subprocesses, the initial complex-formation
step (A) exhibits the largest substituent dependence (the
highest absolute value of the slope), which means that the
electron density and therefore the Lewis basicity of N-4 are
mainly determined by the electronic features of the substituent
on the Ph. Accordingly, EDGs facilitate complexation,
whereas EWGs make the BF3 complex less stable. A smaller
but opposite effect prevails for the isomerization subprocess
(C): EWGs promote H/BF3 interchange between N-4 and N-5.
This can be interpreted by means of the resonance structures
(21) (a) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165. (b)
Hammett, L. P. J. Am. Chem. Soc. 1937, 59, 96. (c) Hammett, L. P.;
Physical Organic Chemistry, 2nd ed.; McGraw-Hill: New York, 1970;
pp 347-390. (d) Wells, P. R. Linear Free Energy Relationships;
Academic Press: New York, 1968. (e) Wells, P. R. Chem. ReV. 1963,
63, 171. (f) Johnson, C. D. The Hammett Equation; Cambridge
University Press: Cambridge, 1973.
(22) Mucsi, Z.; Viskolcz, B.; Hermecz, I.; Csizmadia, I. G.; Keglevich, G.
Tetrahedron 2008, 64, 1868.
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J. AM. CHEM. SOC. VOL. 131, NO. 11, 2009 3901

A R T I C L E S
Frank et al.
Scheme 5. Some Relevant Resonance Structures of 23 and 32
of 23 and 32 (Scheme 5), which show that, while N-5 in 23
can be regarded as roughly neutral, it possesses a significant
negative character in structure 32, which is more preferable
when there are EWGs on the Ph ring. As expected, the
conformational equilibrium (B) is not highly affected by the
different substituents, which is manifested as a small slope
of the correlation line. However, it is somewhat surprising
that the terminal ring-closure step (D) is likewise almost
insensitive to the nature of the Ph group. This small
substituent dependence is also understandable in view of the
resonance structure of 32 (Scheme 5), where an EWG
increases the positive charge (electrophilicity) on C-3, but
at the same time and to roughly the same extent it decreases
the negative charge (nucleophilicity) on N-5. The opposite
effect is observed for EDGs. The atomic distances d1 and d2
in the transition state and the natural bond orbital charges
on C-3 and N-5 further support this concept. Finally, the
resultant of the two electronic effects is manifested in a small
slope (Figure 4D).
broad spectrum of their biological activities. However,
D-ring-fused pyrazolines23 like most of the products (6, 7,
9, and 10) have received less attention, at least from a
pharmacological aspect. Several different compounds con-
taining a pyrazoline structural moiety have been reported to
exert antiproliferative activity24 even on drug-resistant cancer
cells, and the synthetized derivatives are structurally related
to natural alkaloid analogues (Figure 6) which also have
cytotoxic properties25a,c or act as chemosensitizers in mul-
tidrug resistance;25d consequently, investigation of the
,e
cycloadducts for cytotoxic efficacy appeared obvious. The
activities were determined by the microplate-based MTT
[3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bro-
(23) (a) Amr, A. E-G. E.; Abdel-Latif, N. A.; Abdalla, M. M. Bioorg. Med.
Chem. 2006, 14, 373. (b) Amr, A. E-G. E.; Abdel-Latif, N. A.; Abdalla,
M. M. Acta Pharm. 2006, 56, 203.
(24) (a) Johnson, M.; Younglove, B.; Lee, L.; LeBlanc, R.; Holt, H.; Hills,
P.; Mackay, H.; Brown, T.; Mooberry, S. L.; Lee, M. Bioorg. Med.
Chem. Lett. 2007, 17, 5897. (b) Rostom, S. A. F. Bioorg. Med. Chem.
2006, 14, 6475. (c) Brzozowski, Z.; Sa¸czewski, F.; Gdaniec, M. Eur.
J. Med. Chem. 2000, 35, 1053. (d) Manna, F.; Chimenti, F.; Fioravanti,
R.; Bolasco, A.; Secci, D.; Chimenti, P.; Ferlini, C.; Scambia, G.
Bioorg. Med. Chem. Lett. 2005, 15, 4632. (e) Distefano, M.; Scambia,
G.; Ferlini, C.; Gallo, D.; De Vincenzo, R.; Filippini, P.; Riva, A.;
Bombardelli, E.; Mancuso, S. Anti-Cancer Drug Des. 1998, 13, 489.
(25) (a) Lee, K. R.; Kozukue, N.; Han, J. S.; Park, J. H.; Chang, E. Y.;
Baek, E. J.; Chang, J. S.; Friedman, M. J. Agric. Food Chem. 2004,
52, 2832. (b) Koduru, S.; Grierson, D. S.; van de Venter, M.; Afolayan,
A. J. Pharm. Biol. (Lisse, Neth) 2007, 45, 613. (c) US Patent 2003
/0114393 A1. (d) Lavie, Y.; Harel-Orbital, T.; Gaffield, W; Liscovitch,
M. Anticancer Res. 2001, 21, 1189.
∆G‡[overall] ) ∆G[11a f 22a] + ∆G[22a f 23a] +
∆G[23a f 32a] + ∆G‡[32a f TS(32a f 28a)] (1)
2.3. Pharmacological Studies. In recent years, considerable
interest has been focused on steroidal azacycles due to the
(26) (a) Mosmann, T. J. Immunol. Methods 1983, 65, 55. (b) Csomo´s, P.;
Zupko´, I.; Re´thy, B.; Fodor, L.; Falkay, G.; Berna´th, G. Bioorg. Med.
Chem. Lett. 2006, 16, 6273. (c) Re´thy, B.; Zupko´, I.; Minorics, R.;
Hohmann, J.; Ocsovszki, I.; Falkay, G. Planta Med. 2007, 73, 41. (d)
Kova´cs, A.; Forgo´, P.; Zupko´, I.; Re´thy, B.; Falkay, G.; Szabo´, P.;
Hohmann, J. Phytochemistry 2007, 68, 687.
Figure 6. Structure of the natural steroid alkaloid solanidine; the part
structurally identical with the synthetized compounds is shown in bold.
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3902 J. AM. CHEM. SOC. VOL. 131, NO. 11, 2009

Substituent Effects on Phenylhydrazone Cycloadditions
A R T I C L E S
Table 2. Cytotoxic Activities of the Tested Compounds
IC50 (µM)
compound
R1
R2
R3
HeLa
MCF-7
A431
6a
7a
7b
7c
6d
7d
7e
7f
7g
7h
6i
7i
7j
5k
7k
5l
H
H
H
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
H
OMe
OMe
H
Cl
COOK
CF3
OCF3
OCF3
NO2
CN
>30
>30
2.14
22.11
4.47
2.01
2.63
2.23
>30
>30
>30
>30
>30
H
H
H
H
H
OMe
H
H
H
H
H
H
H
H
H
H
H
H
H
27.90
7.34
26.12
3.34
1.42
8.92
13.96
>30
18.00
>30
>30
6.89
>30
7.87
>30
15.60
2.16
2.89
5.11
>30
8.34
>30
24.68
7.30
>30
3.51
>30
>30
11.88
>30
3.48
>30
>30
6.28
3.98
3.16
3.17
>30
2.45
9.71
26.09
7.39
>30
10.48
>30
CN
NO2
CH3
CH3
CH3
CH3
NO2
CH3
CH3
OMe
OMe
6m
7m
6n
7n
>30
7.93
>30
22.30
>30
9
10
14.00
0.15
12.43
29.37
0.28
9.63
7.57
0.15
2.84
doxorubicin
cisplatin
mide] colorimetric assay,26 using the HeLa, MCF7, and A431
cell lines, in comparison with doxorubicin and cisplatin as
positive controls. The cell growth-inhibitory potencies of the
investigated compounds, expressed as IC50 values in Table
2, demonstrate that several of the derivatives exert pro-
nounced antiproliferative effects on all three cell lines, these
effects being similar to or higher than those of the reference
cisplatin.
3. Summary
Androst-5-ene-fused arylpyrazoline derivatives were prepared
via the BF3 ·OEt2-catalyzed intramolecular [3 + 2] cycloaddition
of steroidal alkenyl tosyl- and phenylhydrazones under fairly
mild conditions. The rates of the ring closures depended
significantly on both the electronic features and the positions
of the substituents on the Ph moiety, although all transforma-
tions, including those in the absence of the Lewis acid, proved
highly diastereoselective. The experimental findings were sup-
ported by computational calculations performed on simplified
model compounds structurally related to the experimentally used
steroidal alkenyl phenylhydrazones. These revealed that mainly
the initial complexation with the Lewis acid depends on the
substituent character and can therefore determine the overall
Gibbs free energy of the process. The diastereofacial selectivity
of the cyclizations was confirmed theoretically, since only one
of the four possible diastereomers may be expected to form
under catalytic conditions; however, this single isomer was also
predicted to predominate even in the absence of the Lewis acid,
in accord with the experimental findings. For the BF3 ·OEt2-
promoted reactions, a stepwise rather than a pure concerted
mechanism may be suggested. The synthetized androst-5-ene
arylpyrazoline derivatives are of interest from a pharmacological
In some cases, the pyrazoline 3-acetates (6d,i,m,n and 9) were
also tested for comparison with the corresponding 3-OH-
containing analogues (7d,i,m,n and 10); as expected, the latter
proved to be more potent. The esters containing a pyrazolidine
substructure (5k and 5l) did not display any substantial activity
up to the final concentration of 30 µM, with the only exception
of 5l on the HeLa cells. The introduction of a Ph group into
the pyrazoline moiety (7) and substitution on the Ph ring in at
least the meta or para positions (7b, 7d-f, and 7h-k) enhanced
the activity, while the absence of Ph (10) or the presence of
unsubstituted Ph (7a) or ortho-substituted Ph (7c, 7m, and 7n)
resulted in a decrease in potency. The p-methoxyphenylpyra-
zoline 7d proved to be an outstandingly potent cytotoxic
compound on all three cell lines, with IC50 values lower than
those of cisplatin and close to those of doxorubicin. The
3-acetate analogue 6d may also be of interest because of its
marked antiproliferative effect, especially on the HeLa and A431
cells. The currently known structure-activity relationships offer
a promising possibility for the design and screening of further
derivatives.
(27) Frisch, M. J. et al. Gaussian03 6.0; Gaussian, Inc.: Pittsburgh, PA,
2003; seeSupporting Information.
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J. AM. CHEM. SOC. VOL. 131, NO. 11, 2009 3903

A R T I C L E S
Frank et al.
at545nmwithamicroplatereader.Sigmoidalconcentration-response
curves were fitted to the measured points, and the IC50 values were
calculated with GraphPad Prism 4 (GraphPad Software, San Diego,
CA, USA). Reported values are the averages of the results of two
independent experiments with four parallels. The highest DMSO
concentration of the medium, 0.1%, did not have any significant
effect on cell proliferation. Doxorubicin and cisplatin were used
as reference compounds.
aspect, since several analogues exerted marked in Vitro cytotoxic
activity on three malignant human cell lines.
Experimental Section
Theoretical Studies. All computations were carried out with the
Gaussian03 program package.27 Geometry optimizations and
subsequent frequency calculations were performed by applying the
B3LYP/6-31G(d) basis set.28,29 Thermodynamic parameters (U,
H, G, and S, listed in the Supporting Information, Tables S2-S4)
were computed at 298.15 K, applying a quantum chemical rather
than a conventional thermodynamic scale.
Acknowledgment. The financial support by the Hungarian
Scientific Research Fund (OTKA PD-72403, T-49366, and K-72309)
´
is gratefully acknowledged. E. Frank’s work was supported by the
Bioassays. Cytotoxic effects were measured in Vitro on three
human cell lines (ECACC; Salisbury, UK): HeLa (cervix adeno-
carcinoma), MCF7 (breast adenocarcinoma), and A431 (skin
epidermoid carcinoma). The cells were cultivated in a minimal
essential medium (Gibco BRL; Paisley, UK) supplemented with
10% fetal bovine serum, 1% nonessential amino acids, and an
antibiotic-antimycotic mixture. The cells were grown in a humidi-
fied atmosphere of 5% CO2 at 37 °C. Near-confluent cells were
seeded into a 96-well plate (5000 cells/well), and after overnight
standing, the medium containing the tested compound was added.
Following a 72-h incubation the cytotoxicity was measured with
the MTT assay.26 The precipitated formazan crystals were solubi-
lized in dimethyl sulfoxide (DMSO), and the absorbance was read
award of a Bolyai Ja´nos Research Fellowship.
Supporting Information Available: Experimental procedures
for the preparation and NMR spectra of all synthetized
compounds; the full author list for ref 27; calculated stereo-
structures of selected (E)- and (Z)-phenylhydrazones; Table S1
contains the natural bond orbital charges on C-3 and N-5 and
the d1 and d2 bond distances for compounds TS(32a-lf28a-l);
Tables S2-S4 contain the computed energies (E), zero-point
energies (EZPE), internal energies (U), enthalpies (H), and Gibbs
free energies (G) in hartree at the B3LYP/6-31G(d) level of
theory for compounds 11-32. This material is available free
of charge via the Internet at http://pubs.acs.org.
(28) (a) Beke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Head-Gordon,
M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503.
´
(29) Mucsi, Z.; Szabo´, A.; Hermecz, I.; Kucsman, A.; Csizmadia, I. J. Am.
Chem. Soc. 2005, 127, 7615.
JA808636E
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3904 J. AM. CHEM. SOC. VOL. 131, NO. 11, 2009