Nitration of fullerene derivatives under mild conditions

Article abstract of DOI:10.1055/s-2007-977425

A new family of N-(2,4-dinitrophenyl)-2-pyrazolino[60]fullerene derivatives has been synthesized by electrophilic nitration using nitronium triflate. As evidenced by CV and OSWV experiments, these species show enhanced electron-accepting properties (up to

Full text of DOI:10.1055/s-2007-977425

LETTER
1051
Nitration of Fullerene Derivatives under Mild Conditions
N
itration of Fu
r
llerene
D
erivatives
dunder Mild
éonditionsric Oswald, Pilar de la Cruz, Fernando Langa*
Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, 45071 Toledo, Spain
Fax +34(902) 204130; E-mail: Fernando.Lpuente@uclm.es
Received 20 December 2006
The absence of reactivity of these nitrile imines towards
C60 can be explained by an increased HOMOdipole
Abstract: A new family of N-(2,4-dinitrophenyl)-2-pyrazoli-
no[60]fullerene derivatives has been synthesized by electrophilic
nitration using nitronium triflate. As evidenced by CV and OSWV
experiments, these species show enhanced electron-accepting
properties (up to 90 mV) with respect to C60.
–
LUMOalkene gap, induced by the presence, in the aryl ring,
of a new electron-withdrawing group. These results made
us turn to the second approach, outlined in Scheme 1
(route b). It is based on the nitration of 4-nitrophenylpyra-
zolinofullerenes 3a,11 3b,13 3c,14 and 3d.14 Among the
wide variety of nitrating agents available,15 we chose the
nitronium triflate16 (NO2OTf) for several reasons: the
electrophilic nitration can be carried out under mild con-
ditions (low or room temperature) in organic solvents, the
reaction is selective and these conditions are compatible
with many pendant electron-donating and electron-with-
drawing substituents found in the benzene ring.
Key words: fullerene, electrochemistry, electrophilic aromatic
substitution
Since the first report on the electrochemical properties of
C60 in 1990,1 there has been a growing interest in the elec-
tron-acceptor character of fullerenes as they can be com-
bined with electron donors for light-induced electron-
transfer processes which are of interest for the preparation
of photovoltaic devices. C60 can accept up to six electrons,
and in fact, six reversible reductions have been observed.2
The electrochemistry of fullerene derivatives is very
similar to pristine C60 but, usually, with poorer electron-
acceptor properties, resulting from both, the saturation of
one double bond (raising the energy of the LUMO) and
the inductive effect of the addend.3
The structures of the new cycloadducts 1a–d were
confirmed by spectroscopic data (i.e., UV–Vis, FTIR, 1H
NMR, 13C NMR and MALDI–MS spectra).17
Whereas nitration of 1a and 1b was achieved at low tem-
perature, formation of 1c and 1d was performed at room
temperature due to the lack of reactivity of the corre-
sponding starting material at lower temperature. In these
cases the lower yield is the result of a rapid decomposition
of the newly formed nitrated product. Polynitration could
It is clear that the improvement of the electron-accepting
properties of C60 may lead to more efficient behavior in
electron-transfer processes4 and a better efficiency in pho-
tovoltaic devices.5 Within the past years, several attempts
have been carried out directed at the synthesis of stronger
acceptors than the parent C60;6 among these attempts are:
preparation of pyrrolidinium salts,7 presence of electrone-
gative groups directly linked to the C60 core,8 incorpora-
tion of electron-acceptor substituents9 and fullerene–
metal complexes.10
R-CHO
route b
route a
i
ii
R CH N NH
O2N
NO2
R CH N NH
2a–d
NO2
4a–d
a R = Me
b R = (Me)2CH2CHCH(Me)CH2
c R = 4-NO2C6H4
d R = 3,5-(CF3)2C6H4
We report here the synthesis and electrochemical proper-
ties of N-(2,4-dinitrophenyl)-2-pyrazolino-[60]fullerenes
1a–d, which show enhanced electron-accepting ability
(up to 90 mV) with respect to C60.
iii
iii
Hc
NO2
O2N
2
N
One approach to the preparation of the target compounds
1a–d implies the formation of 2,4-dinitrophenylhydra-
zones 2a–d, then halogenation followed by the elimina-
tion reaction to form the corresponding nitrile imine11 and
ulterior 1,3-dipolar cycloaddition to C60 (Scheme 1, route
a). Nevertheless, all reaction conditions used by us (NBS
or NCS for halogenation, different solvents, classical
heating or using microwave irradiation) failed in our
hands.12
R
N
NO2
1
N
R
N
Hb
Ha
iv
3a–d
1a–d
Scheme 1 Reagents and conditions: (i) p-nitrophenylhydrazine,
AcOH, EtOH, reflux; (ii) 2,4-dinitrophenylhydrazine, AcOH, EtOH,
reflux; (iii) (a) NBS, CHCl3, r.t.; (b) C60, Et3N, toluene, reflux or r.t.;
(iv) tetramethylammonium nitrate, triflic anhydride, CH2Cl2.
SYNLETT 2007, No. 7, pp 1051–1054
Advanced online publication: 13.04.2007
DOI: 10.1055/s-2007-977425; Art ID: G37806ST
© Georg Thieme Verlag Stuttgart · New York
2
4.
0
4.
2
0
0
7

1052
F. Oswald et al.
LETTER
not be achieved, even with a large excess of the nitrating
agent. In the case of 1c, only the N-side aromatic ring,
which is the more activated ring, could be nitrated.
The meta substitution (with respect to the original NO2),
explained by the presence of a meta-directing NO2 and a
ortho(para)-directing pyrazole moiety, was confirmed by
the 13C NMR. When compounds 3 showed four 13C NMR
signals corresponding to the nitrophenyl moiety, com-
pounds 1 exhibited six signals. Furthermore, the signal
attributed18 to C2 in 3 (see Scheme 1) was shifted down-
field in the case of 1, due to the introduction of the new
nitro moiety.
Figure 1 1H NMR of aromatic protons of compound 1b in deuterated
toluene
C6D5CD3) at different temperatures revealed no dynamic
exchange between the two possible conformers
(Figure 1).
As a typical example, the expansion of the aromatic re-
gion in the 1H NMR of compound 1b is shown in Figure 1.
The spectrum is well resolved at room temperature, with
sharp signals for the protons of the aromatic ring, showing
the signals corresponding to the proposed structure: the
most deshielded signal appears as a doublet (Jm = 2.5 Hz)
and is assigned to Hc (two nitro groups are in ortho posi-
tion). The signal corresponding to Hb appears as doublet
of doublets with two coupling constants, Jm = 2.5 Hz and
Jo = 9 Hz. Finally, the proton Ha appears as a doublet with
Jo = 9 Hz.
Like in other fullerene derivatives,19 the steric hindrance
resulting from the introduction of a nitro moiety at the C2
position seems to be sufficient to increase the rotational
energy barrier to the point where free rotation is almost
impossible. This fact was supported by the computational
studies performed to evaluate the relationship between
potential energy and the dihedral angle between the pyra-
zolinofullerene moiety and the phenyl ring. These studies
showed that two conformers can exist with a dihedral an-
gle of 138° and 322°, the energy barrier being higher than
30 kcal/mol, enough to prevent the rotation of the aryl
group.19 In fact, 1H NMR spectra (recorded in CDCl3 and
The electrochemical characteristics of the newly synthe-
sized compounds were examined using cyclic voltamme-
try (CV) and Osteryoung square-wave voltammetry
(OSWV), in order to evaluate their electron-accepting
abilities compared to parent C60.
The redox potentials for 1a–d, parent compounds 3a–d
and C60 are listed in Table 1. All of the pyrazoli-
no[60]fullerenes (1a–d and 3a–d) showed four quasire-
versible reduction waves for the fullerene moiety, similar
to that found for the parent C60. In addition, other nonre-
versible reduction wave(s), corresponding to the reduc-
tion of one (3a–d) or two (1a–d) nitro moieties, were
observed. Interestingly, a remarkable difference was
found between the reduction potentials corresponding to
the fullerene cage.
Unlike the standard behavior found in most of 1,2-dihy-
drofullerenes that showed reduction waves shifted to
more negative potentials than the parent C60,6,20 com-
pounds 1a–d and 3a–d exhibited reduction potentials
which were anodically shifted (30–90 mV) in comparison
with the parent C60, similar to other pyrazoli-
no[60]fullerenes.8 Furthermore, as shown in Table 1,
Table 1 Electrochemical Data for the Redox Processes of Compounds 1a–d, 3a–d and C60-Fullerenea
Compound
Ered(1)b
–0.97
–0.92
–0.94
–0.91
–0.93
–0.89
–0.91
–0.88
–0.90
Ered(2)c
–1.30
–1.29
–1.27
–1.26
Ered(3)b
–1.38
–1.40
–1.35
–1.39
–1.35
–1.48
–1.32
–1.55
–1.32
Ered(4)c
–1.62
–1.63
Ered(5)b
–1.85
–1.84
–1.77
–1.86
–1.77
–1.83
–1.72
–1.78
–1.85
Ered(6)c
Ered(7)b
–2.29
–2.30
–2.24
–2.28
–2.28
–2.30
–2.30
–2.27
C60
1a
3a
1b
3b
1c
–1.94
–1.94
–1.96
–1.94
–1.99
–1.91
–2.08
–2.24d
–1.63
–1.51
–1.64
–1.67
3c
1d
3d
a V vs. Ag/AgNO3; detected by OSWV in o-dichlorobenzene–MeCN (4:1) solution (0.1 M n-Bu4NClO4) at r.t. under identical experimental
conditions. OSWV data were obtained using a sweep width of 25 mV, a frequency of 15 Hz, a step potential of 4 mV, and a quiet time of 2 s
on a Windows-driven Autolab PGSTAT 30 electrochemical analyzer.
b C60-based reduction.
c p-Nitrobenzene-based redox process.
d The fourth C60-based reduction wave was overlapped with a p-nitrobenzene-based one.
Synlett 2007, No. 7, 1051–1054 © Thieme Stuttgart · New York

LETTER
Nitration of Fullerene Derivatives under Mild Conditions
1053
(10) (a) Nierengarten, J. F.; Solladie, N. J. Porphyrins
introduction of the second nitro moiety has a striking in-
fluence on the redox behavior. In all cases a shift to more
positive values by 20 mV with respect to the correspond-
ing mononitro compound was observed, which is a direct
consequence of an increase in the electronic effect of the
electron-accepting organic addend attached to the pyra-
zole ring.
Phtalocyanines 2005, 9, 760. (b) Nierengarten, J. F.; Hahn,
U.; Duarte, T. M. F.; Cardinali, F.; Solladie, N.; Walther, M.
E.; Van Dorsselaer, A.; Herschbach, H.; Leize, E.; Albrecht-
Gary, A. M.; Trabolsi, A.; Elhabiri, M. C. R. Chimie 2006,
9, 1022. (c) Rio, Y.; Enderlin, G.; Bourgogne, C.;
Nierengarten, J.-F.; Gisselbrecht, J.-P.; Gross, M.; Accorsi,
G.; Armaroli, N. Inorg. Chem. 2003, 42, 8783.
(11) Delgado, J. L.; de la Cruz, P.; López-Arza, V.; Langa, F.;
Kimball, D. B.; Haley, M. M.; Araki, Y.; Ito, O. J. Org.
Chem. 2004, 69, 2661.
(12) It has been previously reported that other 2,4-dinitro-
phenylhydrazones fail to give cycloaddition reactions. See:
Yap, G. P. A.; Alkorta, I.; Jarerovic, N.; Elguero, J. Aust.
J. Chem. 2004, 57, 1103; and references therein.
These results clearly indicate that these systems are stron-
ger electron acceptors than pristine C60. The remarkable
electron-acceptor character of these new molecules is of
interest for further applications in the preparation of
optoelectronic devices. Work is now in progress for the
preparation of blends with conjugated semi-conducting
polymers for photovoltaic applications.
(13) Compound 2b: A solution of p-nitrophenylhydrazine (1.0 g,
6.5 mmol), 3,5,5-trimethylhexanal (0.82 g, 6.5 mmol) and
two drops of AcOH in EtOH (50 mL) was heated under
reflux for 2 h. The solid was filtered off and recrystallized in
EtOH to give 2b as a red solid (1.47 g, 82%); mp 82.3–82.7
°C. 1H NMR (200 MHz, CDCl3): d = 8.13 (d, J = 9.2 Hz, 2
H), 7.90 (s, 1 H), 7.19 (t, 1 H), 6.99 (d, J = 9.2 Hz, 2 H),
2.10–2.40 (m, 2 H), 1.85 (m, 1 H), 1.32 (dd, J1 = 3.4 Hz,
J2 = 14.0 Hz, 1 H), 1.13 (dd, J1 = 6.2 Hz, J2 = 14.0 Hz, 1 H),
1.01 (d, J = 6.6 Hz, 3 H), 0.91 (s, 9 H). 13C NMR (50 MHz,
CDCl3): d = 150.3, 145.3, 144.5, 140.4, 139.7, 126.4, 126.3,
112.0, 111.3, 50.8, 50.6, 41.6, 35.7, 31.3, 31.3, 30.1, 28.2,
27.7, 23.0, 22.8. FT-IR (ATR): 3289, 2941, 1617, 1511,
1335, 1266 cm–1. Anal. Calcd for C15H23N3O2: C, 64.95; H,
8.36; N. 15.15. Found: C, 64.43; H, 8.13; N, 15.21.
Compound 3b: To a solution of hydrazone 2b (115 mg, 0.41
mmol) in anhyd CHCl3 (10 mL) was added, under an Ar
atmosphere, NBS (148 mg, 0.82 mmol). The mixture was
kept under agitation during 2 h at r.t. The solvent was
removed and an anhyd toluene (250 mL) solution of C60 (300
mg) and Et3N in excess were then added. The reaction was
kept under agitation at r.t. for 1.75 h. The toluene was then
evaporated under vacuum. The remaining solid was purified
by column chromatography (silica gel; toluene–hexane, 2:1)
to give 3b in 41% yield. 1H NMR (200 MHz, CDCl3): d =
8.30 (d, J = 9.6 Hz, 2 H), 8.16 (d, J = 9.6 Hz, 2 H), 3.15 (dd,
J1 = 5.9 Hz, J2 = 16.0 Hz, 1 H), 2.99 (dd, J1 = 8.0 Hz, J2 =
16.0 Hz, 1 H), 2.53 (m, 1 H), 1.65 (dd, J1 = 4.2 Hz, J2 = 14.6
Hz, 1 H), 1.37 (dd, J1 = 6.6 Hz, J2 = 14.7 Hz, 1 H), 1.33 (d,
J = 6.5 Hz, 3 H), 0.99 (s, 9 H). 13C NMR (50 MHz, CDCl3):
d = 150.47, 149.15, 147.94, 147.39, 146.75, 146.67, 146.56,
146.39, 146.34, 146.16, 145.76, 145.61, 145.53, 145.47,
145.34, 144.89, 144.64, 144.35, 143.74, 143.46, 143.20,
143.11, 142.64, 142.60, 142.50, 142.29, 141.82, 141.22,
141.14, 139.52, 139.44, 137.25, 136.24, 136.13, 129.28,
128.47, 125.76, 125.54, 118.58, 88.84, 84.28, 51.56, 39.37,
31.53, 31.21, 30.50, 29.96, 28.64, 23.34. FT-IR (KBr): 2941,
1597, 1535, 1335, 530 cm–1. MALDI–TOF: m/z calcd for
C75H21N3O2: 995.98; found: 995.9.
Acknowledgment
Financial support was provided by Ministerio de Educación y
Ciencia of Spain and FEDER funds (Project CTQ2004-00364/
BQU) and the Junta de Comunidades de Castilla-La Mancha
(Project PAI-05-068).
References and Notes
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Pan, C.; Xiao, Z.; Billups, W. E.; Ciufolini, M. A.; Hauge, R.
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(2) Xie, Q.; Pérez-Cordero, E.; Echegoyen, L. J. Am. Chem. Soc.
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E. Acc. Chem. Res. 1998, 31, 593.
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Duarte, T. M.; Nierengarten, J.-F.; Palermo, V.; Samorì, P.;
Rio, Y.; Accorsi, G.; Armaroli, N. Chem. Eur. J. 2005, 11,
4405. (b) Gouloumis, A.; Oswald, F.; Langa, F.; El-Khouly,
M. E.; Araki, Y.; Ito, O. Eur. J. Org. Chem. 2006, 2344.
(c) Delgado, J. L.; El-Khouly, M. E.; Araki, Y.; Gómez-
Escalonilla, M. J.; de la Cruz, P.; Oswald, F.; Ito, O.; Langa,
F. Phys. Chem. Chem. Phys. 2006, 35, 4104.
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Zhang, F.; Langa, F.; Andersson, M.; Inganäs, O. Appl.
Phys. Lett. 2004, 85, 5081. (b) Wang, X.; Perzon, E.;
Oswald, F.; Langa, F.; Admassie, S.; Andersson, M.;
Inganäs, O. Adv. Funct. Mater. 2005, 15, 1665. (c) Gadisa,
A.; Wang, X.; Admassie, S.; Perzon, E.; Oswald, F.; Langa,
F.; Andersson, M. R.; Inganas, O. Org. Electronics 2006, 7,
195.
(6) Illescas, B. M.; Martín, N. C. R. Chimie 2006, 9, 1038.
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F.; Roffia, S. J. Am. Chem. Soc. 1998, 120, 11645.
(b) Carano, M.; Da Ros, T.; Fanti, M.; Kordatos, K.;
Marcaccio, M.; Paolucci, F.; Prato, M.; Roffia, S.; Zerbetto,
F. J. Am. Chem. Soc. 2003, 125, 7139.
(8) (a) Langa, F.; Oswald, F. C. R. Chimie 2006, 9, 1058.
(b) Keshavarz, K. M.; Knight, B.; Haddon, R. C.; Wudl, F.
Tetrahedron 1996, 52, 5149.
(14) Delgado, J. L.; de la Cruz, P.; Lopez-Arza, V.; Langa, F.;
Gan, Z.; Araki, Y.; Ito, O. Bull. Chem. Soc. Jpn. 2005, 78,
1500.
(15) March, J. Advanced Organic Chemistry: Reactions,
Mechanisms and Structure, 5th ed.; McGraw-Hill: New
York, 2001, 696–699.
(16) Shackelford, S. A.; Anderson, M. B.; Christie, L. C.;
Goetzen, T.; Guzman, M. C.; Hananel, M. A.; Kornreich, W.
D.; Li, H.; Pathak, V. P.; Rabinovich, A. K.; Rajapakse, R.
J.; Truesdale, L. K.; Tsank, S. M.; Vazir, H. N. J. Org. Chem.
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(9) (a) de la Cruz, P.; de la Hoz, A.; Langa, F.; Martín, N.; Pérez,
M. C.; Sánchez, L. Eur. J. Org. Chem. 1999, 3433.
(b) Deviprasad, G. R.; Rahman, M. S.; Souza, F. D. Chem.
Commun. 1999, 849.
(17) Experimental Procedure: To an anhyd CH2Cl2 solution of
the corresponding fullerene derivative 2 (1 equiv) in CH2Cl2,
Synlett 2007, No. 7, 1051–1054 © Thieme Stuttgart · New York

1054
F. Oswald et al.
LETTER
tetramethylammonium nitrate (2 equiv) was added under Ar.
The solution was cooled to the desired temperature and
triflic anhydride (2 equiv) was added. NO2OTf was formed
in situ and the solution was kept at this temperature until the
complete disappearance of the starting material (the reaction
progression was followed by TLC). The reaction was then
quenched by adding a Na2CO3 solution (pH 8). The organic
layer was separated, dried and evaporated. The remaining
solid was purified by column chromatography affording the
desired compounds 1a–d.
Compound 1a: reaction temperature: –5 °C for 4 h. Column
chromatography (silica gel; toluene–hexane, 1:2); yield:
90%. 1H NMR (500 MHz, CDCl3): d = 8.80 (d, J = 2.5 Hz,
1 H), 8.45 (dd, J1 = 2.5 Hz, J2 = 9.0 Hz, 1 H), 8.40 (sd, J =
9.0 Hz, 1 H), 2.79 (s, 3 H).13C NMR (125 MHz, CDCl3): d =
149.36, 147.79, 147.21, 146.38, 146.32, 146.08, 145.96,
145.34, 145.49, 145.29, 145.28, 145.24, 144.41, 144.23,
144.09, 143.97, 143.21, 143.07, 143.04, 142.95, 142.93,
142.84, 142.40, 142.24, 142.18, 142.05, 142.02, 141.06,
139.81, 137.31, 136.55, 127.10, 126.28, 122.22, 88.90,
83.39, 15.03. FT–IR (KBr): 2917, 2357, 1601, 1527, 1331,
526 cm–1. UV–Vis: l (log e) = 255.0 (6.1), 320.0 (5.6) nm.
MALDI–TOF: m/z calcd for C68H6N4O4: 942.94; found:
942.0.
30.04, 28.07, 22.98. FT–IR (KBr): 2941, 2337, 1597, 1540,
1335, 526 cm–1. UV–Vis: l (log e) = 255.0 (6.2), 320.0 (5.7)
nm. MALDI–TOF: m/z calcd for C75H20N4O4: 1040.98;
found: 1040.1.
Compound 1c: reaction temperature: r.t. for 2 h. Column
chromatography (silica gel; toluene–hexane, 2:1); yield:
65%. 1H NMR (500 MHz, CDCl3): d = 8.87 (d, J = 2.5 Hz,
1 H), 8.52 (dd, J1 = 2.5 Hz, J2 = 9.0 Hz, 1 H), 8.49 (sd, J =
9.0 Hz, 1 H), 8.38 (d, J = 9.0 Hz, 2 H), 8.36 (d, J = 9.0 Hz,
2 H). 13C NMR (125 MHz, CDCl3): d = 148.61, 147.79,
147.32, 147.13, 146.46, 146.42, 146.14, 146.08, 145.60,
145.56, 145.38, 145.33, 145.16, 144.69, 144.33, 144.16,
144.15, 144.08, 143.95, 143.28, 143.07, 142.99, 142.94,
142.49, 142.32, 142.26, 142.08, 141.99, 141.95, 140.64,
140.12, 137.38, 137.21, 136.90, 129.73, 127.32, 127.13,
124.24, 122.20, 91.39, 81.42. FT-IR (KBr): 2357, 1597,
1535, 1335, 526. UV–Vis: l (log e) = 245.5 (6.3), 321.5
(5.9) nm. MALDI–TOF: m/z calcd for C73H7N5O6: 1049.87;
found: 1050.2.
Compound 1d: reaction temperature: r.t. for 3 h. Column
chromatography (silica gel; toluene–hexane, 1:2); yield:
47%. 1H NMR (500 MHz, CDCl3): d = 8.88 (d, J = 2.5 Hz,
1 H), 8.67 (br s, 2 H), 8.53 (dd, J1 = 2.5 Hz, J2 = 9.0 Hz, 1
H), 8.48 (sd, J = 9.0 Hz, 1 H), 8.00 (br s, 1 H). 13C NMR (125
MHz, CDCl3): d = 148.07, 147.58, 146.72, 146.68, 146.40,
146.35, 145.97, 145.79, 145.66, 145.59, 145.12, 144.59,
144.41, 144.16, 144.13, 143.54, 143.33, 143.25, 143.18,
142.73, 142.58, 142.53, 142.31, 142.25, 140.89, 140.40,
137.64, 137.29, 133.46, 132.90, 132.63, 128.93, 127.57,
124.26, 123.99, 122.43, 122.11, 91.81, 81.44. FT-IR (KBr):
2921, 2332, 1597, 1531, 1331, 1274, 1135, 526 cm–1. UV–
Vis: l (log e) = 254.5 (6.1), 319.0 (5.7) nm. MALDI–TOF:
m/z calcd for C75H6F6N4O4: 1140.86; found: 1141.2.
(18) Langa, F.; De la Cruz, P.; Delgado, J. L.; Espildora, E.;
Gómez-Escalonilla, M. J.; De la Hoz, A. J. Mater. Chem.
2002, 12, 2130.
Compound 1b: reaction temperature: –5 °C for 5 h. Column
chromatography (silica gel; toluene–hexane, 1:2); yield:
85%. 1H NMR (500 MHz, CDCl3): d = 8.78 (d, J = 2.4 Hz,
1 H), 8.44 (dd, J1 = 2.5 Hz, J2 = 9.0 Hz, 1 H), 8.40 (sd, J =
9.0 Hz, 1 H), 3.11 (dd, J1 = 4.6 Hz, J2 = 16.5 Hz, 1 H), 2.95
(dd, J1 = 8.8 Hz, J2 = 16.5 Hz, 1 H), 2.45 (m, 1 H), 1.54 (dd,
J1 = 4.5 Hz, J2 = 14.0 Hz, 1 H), 1.33 (dd, J1 = 6.2 Hz, J2 =
14.0 Hz, 1 H), 1.27 (d, J = 6.6 Hz, 3 H), 0.99 (s, 9 H). 13
NMR (125 MHz, CDCl3): d = 151.63, 147.77, 147.20,
146.37, 146.36, 146.31, 146.31, 146.07, 146.06, 146.05,
145.96, 145.95, 145.55, 145.53, 145.51, 145.46, 145.43,
145.42, 145.32, 145.29, 145.28, 145.26, 145.25, 144.40,
144.29, 144.24, 144.10, 144.09, 144.06, 143.21, 143.01,
142.97, 142.85, 142.83, 142.75, 142.42, 142.40, 142.26,
142.24, 142.19, 142.16, 142.12, 142.10, 142.01, 142.00,
140.95, 140.90, 139.84, 139.79, 137.31, 136.45, 136.26,
C
(19) Ajamaa, F.; Figueira Duarte, T. M.; Bourgogne, C.; Holler,
M.; Fowler, P. W.; Nierengarten, J. F. Eur. J. Org. Chem.
2005, 3766.
(20) Illescas, B. M.; Martin, N. J. Org. Chem. 2000, 65, 5986.
126.99, 126.00, 122.19, 88.84, 83.84, 51.39, 39.25, 31.26,
Synlett 2007, No. 7, 1051–1054 © Thieme Stuttgart · New York

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