Diels-Alder reactions of nitrobenzofurans: A simple dibenzofuran Synthesis. Theoretical studies using DFT methods

Article abstract of DOI:10.1016/j.tetlet.2011.02.074

2- and 3-nitrobenzofurans are studied in polar thermal Diels-Alder reactions with normal electron demand using several structurally different dienes. A very strong electron-acceptor group, such as the nitro group, pushes the dienophilic character of these heterocyclic compounds. Since this substituent is easily extruded under thermal conditions, this reaction sequence becomes a simple method for the preparation of families of organic compounds with heteroatom rings. Part of this work is specifically concerned with theoretical studies using DFT methods. The global and local electrophilicity and nucleophilicity indices were calculated for the dienophiles and dienes used in this study. When 2-nitrobenzofuran and 3-nitrobenzofuran were reacted with isoprene, 1-trimethylsilyloxy-1,3-butadiene and Danishesfky's diene, under different thermal reaction conditions they showed their dienophilic character taking part in normal-demand polar DA cycloaddition reactions.

Full text of DOI:10.1016/j.tetlet.2011.02.074

Tetrahedron Letters 52 (2011) 2316–2319
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Diels–Alder reactions of nitrobenzofurans: a simple dibenzofuran
Synthesis. Theoretical studies using DFT methods
⇑
Claudia D. Della Rosa, Juan P. Sanchez, María N. Kneeteman, Pedro M. E. Mancini
Chemistry Department, Facultad de Ingeniería Química, Universidad Nacional del Litoral, Santiago del Estero 2829, 3000 Santa Fe, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
2- and 3-nitrobenzofurans are studied in polar thermal Diels–Alder reactions with normal electron
demand using several structurally different dienes. A very strong electron-acceptor group, such as the
nitro group, pushes the dienophilic character of these heterocyclic compounds. Since this substituent
is easily extruded under thermal conditions, this reaction sequence becomes a simple method for the
preparation of families of organic compounds with heteroatom rings. Part of this work is specifically con-
cerned with theoretical studies using DFT methods. The global and local electrophilicity and nucleophi-
licity indices were calculated for the dienophiles and dienes used in this study. When 2-nitrobenzofuran
and 3-nitrobenzofuran were reacted with isoprene, 1-trimethylsilyloxy-1,3-butadiene and Danishesfky’s
diene, under different thermal reaction conditions they showed their dienophilic character taking part in
normal-demand polar DA cycloaddition reactions.
Received 22 December 2010
Revised 16 February 2011
Accepted 17 February 2011
Available online 22 February 2011
Keywords:
Dibenzofurans
Nitrofurans
Dienophile
Cycloaddition
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
method for the preparation of families of organic compounds with
heteroatom rings.
The Diels–Alder (D–A) reaction is one of the most significant
and useful tools available in synthetic chemistry. It allows the sim-
ple construction of a six-membered ring from a diene and a dieno-
phile, bearing an almost unlimited number of variants.1 Due to our
interest in the cycloaddition chemistry of substituted aromatic
heterocycles with electron withdrawing groups, we have reported
that 2-nitrofuran, 2-nitrothiophene, 2-nitro-N-tosylpirrole and 2-
nitroselenophene react with strong and poor dienes under differ-
ent conditions.2
On the other hand, in the last years the density functional
theory (DFT) has been successful in explaining the reactivity and
regioselectivity of cycloaddition reactions. In this sense, the
regioselectivity of a series of experimentally studied D–A reactions
between furan derivatives and the Danishefsky diene has been
rationalized within the framework of local DFT-based descriptors.5
2. Results and discussion
Dibenzofurans are important heteroatomic compounds which
display a wide variety of biological activities. The dibenzofuran
containing phytoalexins show manifold biological activities, elicit-
ing a strong interest from chemists and biologists.3 Considerable
efforts have been devoted to the development of efficient methods
for the building of this heteroatomic ring system. Most general
procedures involve several steps and the overall yields are usually
not very good.4
To explore the normal electron demand D–A dienophilicity of
nitrobenzofurans 1a–b, we chose isoprene (2), 1-Trimethylsilyl-
oxy-1,3-butadiene (3), 1-Methoxy-3-trimethylsilyloxy-1,3-butadi-
ene (Danishefsky’s diene) (4) as diene (Fig. 1). The selection of
the dienes took into account the type of substitution present in
their structures and the relative nucleophilicity. Only with 2-nitro-
benzofuran was Rawal’s diene used as counterpart considering the
Herein we report a simple, economical and efficient one-step
procedure to synthesize the dibenzofuran ring system in good to
excellent yield through the D–A reaction of 2- and 3-nitrobenzofu-
ran and different dienes. A very strong electron-acceptor group,
such as the nitro group, pushes the dienophilic character of these
heterocyclic compounds. Since this substituent is easily extruded
under thermal conditions, this reaction sequence becomes a simple
TMS
OMe
R2
R1
O
OSi(Me)3
1a R1= NO2, R2=H
b R1=H, R2= NO2
2
3
4
⇑
Corresponding author. Fax: +54 3424571164.
E-mail address: pmancini@fiqus.unl.edu.ar (P.M.E. Mancini).
Figure 1.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.074

C. D. Della Rosa et al. / Tetrahedron Letters 52 (2011) 2316–2319
2317
R2
OTBS
Rawal`s diene
R1
NO2
O
O
O
5a R1= CH3, R2=H
b R1=H, R2= CH3
9
R2
Scheme 2.
2
R1
Table 2
Diels–Alder reactions of 1a with Rawal’s diene
O
6a R1= CH3, R2=H
b R1=H, R2= CH3
Entry
Conditionsa
Product
Yieldb
1
2
Reflux Tol. 48 h
Reflux Tol. 72 h
9
9
75
85
1a
b
3
4
a
3 equiv of Rawal’s diene in benzene.
Based on consumed dienophile.
b
O
trimethylsilyloxy and nitro groups. The yield is good (Scheme 1,
Table 1, entries 3 and 4). In the same way, in the thermal reaction
of 1a with Danishesfky’s diene aromatic cycloadduct 8a was ob-
tained with very good yield and complete regioselectivity (Scheme
1, Table 1, entries 5 and 6). In the same way, the reaction between
1a and Rawal’s diene was explored with results similar to those in
the cycloaddition using diene 4 to obtain product 8a (Scheme 2,
Table 2). In the last two processes we observed the expected aro-
matization of the initial nitro adduct promoted by the loss of nitro
and the metoxyl and dimethylamine groups, respectively.
7
G2
G1
O
8a G1=H, G2=OH
b G1=OH, G2=H
It is necessary to take into account that a difference between
both dienes lies in the fact that in the Rawal reaction in the aro-
matic product, the terbutyldimethylsilyloxy group remains.
Then, we analyzed the reactivity of 3-nitrobenzofuran 1b with
the dienes 2, 3 and 4. This allowed us to compare not only the rel-
ative reactivity of the 2- and 3-substitution of the aromatic ring but
also the regioselectivities in the reactions with dienes with proper
substitution. When 1b reacted with isoprene in thermal conditions
(150–200 °C, 72 h), we observed the same mixture of regioisomeric
products 5a, 5b, 6a, and 6b obtained in the reaction with substrate
1a. The yield of these reactions was reasonable (Scheme 2, Table 2,
entries 1 and 2). The reaction of 1b with diene 3 (120–140 °C, 72 h)
offers dibenzofuran 7 in a similar yield as above (Scheme 2, Table 2,
entries 3 and 4). On the other hand, the thermal reaction of 1b with
Danishefsky’s diene yielded the aromatic cycloadduct 8b with high
yield and complete regioselectivity, which is controlled by the ni-
tro and methoxy groups (Scheme 1, Table 3, entries 5 and 6). This
product resulted from the expected aromatization of the primary
nitroadducts promoted by the loss of the nitro and methoxyl
groups as nitrous acid and methanol, respectively. The isolation
of the intermediate cycloadduct was never achieved under these
conditions.
Scheme 1.
class of silyloxy substituent this compound has. Nitrobenzofurans
1a and 1b were prepared by nitration of benzo[b]furan and subse-
quent chromatographic separation. All reactions were performed
in sealed ampoule using benzene or toluene as solvent and differ-
ent temperatures and times of reaction.6
Firstly we explored 2-nitrobenzofuran 1a as substrate, develop-
ment the cycloaddition reactions with dienes 2, 3, and 4, respec-
tively. The presence of the nitro group enhanced the cycloaddition
by favoring aromatization, thus preventing retrocycloaddition. In
this sense, there is a difference with other electron withdrawing
groups, that is, acyl- in the heteroatomic ring, which do not promote
the cycloaddition reaction when they are in 2-position. The thermal
reactions of 1a with isoprene at 150 °C or 200 °C for 72 h, afforded
the mixture of aromatic regioisomeric cycloadducts 5a and 5b as
principal products and dihydrodibenzofurans 6a and 6b with rea-
sonable yield (Scheme 1, Table 1, entries 1 and 2).
On the other hand, reactions of 1a with 1-trimethylsilyloxy-1,3-
butadiene at 120–140 °C, 72 h, yielded dibenzofuran 7 with loss of
Table 1
Table 3
Diels–Alder reactions of 2-nitrobenzofuran with different dienes
Diels–Alder reactions of 3-nitrobenzofuran with diverse dienes
Entry
Diene
Conditionsa (°C)
Product
Yieldb
Entry
Diene
Conditionsa (°C)
Product
Yieldb
1
2
3
4
5
6
2, 12 equiv
200
150
140
120
140
120
5a,b; 6a,b
5a,b; 6a,b
7
7
8a
8a
75
70
68
65
80
75
1
2
3
4
5
6
2, 12 equiv
200
150
140
120
140
120
5a,b; 6a,b
5a,b; 6a,b
7
7
8b
8b
78
70
65
62
82
77
3, 3 equiv
4, 2 equiv
3, 3 equiv
4, 2 equiv
a
a
Reaction time 72 h.
Based on consumed dienophile.
Reaction time 72 h.
Based on consumed dienophile.
b
b

2318
C. D. Della Rosa et al. / Tetrahedron Letters 52 (2011) 2316–2319
medium-sized molecules. Hence, the gas-phase equilibrium geom-
3. Theoretical studies
etries of all the species here described were obtained by the full
optimization of the B3LYP/6-311+G(d,p) level using the GAUSS-
IAN03W program.16 All the stationary points found were character-
ized as true minima by frequency calculations.
There are several parameters which can be used as global or lo-
cal reactivity descriptors, for example, chemical hardness (
g) and
electronic chemical potential ( ) Both quantities can be approxi-
l
The chemical hardness, the electronic chemical potential and
the global electrophilicity indexes were calculated using Eqs 1
and 2. Regional Fukui functions for the dienophiles (fkþ) and for
the diene (fkꢀ) were obtained from single-point calculations on
the optimized structures at the ground state (Eq. 4). A program
was developed and tested which reads the FMO coefficients and
the overlap matrix from the Gaussian output files and performs
the required calculations. Once the Fukui functions were com-
puted, the local electrophilicity and nucleophilicity values were
calculated (Eqs 5 and 6).
mated in terms of the energies of the HOMO and LUMO frontier
molecular orbitals (Eqs. 1 and 2).7
g
¼ ðeLUMO
ꢀ
eHOMO
Þ
ð1Þ
ðeLUMO
ꢀ
eHOMO
Þ
l
¼
ð2Þ
2
The global electrophilicity index (
x
), introduced by Parr et al.8
is a useful descriptor of reactivity.9 Current studies based on the
DFT and applied to D–A reactions have shown this classification
of the diene/dienophile pair as a powerful tool to predict the feasi-
bility of the process and the type of mechanism involved. This in-
dex is defined as
3.2. Optimized geometries
l2
´
The most stable conformation of Danishefskys diene is the anti-
x
¼
ð3Þ
2g
periplanar. This conformation is more stable than the synclinal
arrangement and it was used in the calculations.
Useful information about polarity of the D–A processes may be
obtained from the difference in the global electrophilicity power of
the reactants. This difference has been proposed as a measure of
the polar character of the reaction. On the other hand, local reactiv-
ity indexes are associated with site selectivity in a chemical reac-
tion. These descriptors should reflect the sites in a molecule
where the reactivity pattern described by the global quantities
should take place. An important local reactivity parameter was
introduced by Parr et al. and was defined as the Fukui function.10
Eq. 4 provides a simple and direct formalism to obtain the Fukui
function.11
3.3. Global DFT properties of the reagents involved in these DA
reactions
The electrophilicity of isoprene 2 falls in the range of moderate
electrophiles within of the electrophilicity scale proposed by
Domingo et al.12 When electron-donating substituents, –OCH3
and –OSi(CH3)3, are incorporated into the structure of butadiene,
a decrease in the electrophilicity power is observed. Therefore,
´
the electrophilicity of Danishefskys diene 4 falls in the range of
marginal electrophiles, good nucleophiles, within the electrophilic-
ity scale.
As consequence of the high electrophilicity of these substituted
dienophiles and the high nucleophilicity of the dienes, it is ex-
pected that these DA reactions proceed with polar character. The
polarity of the process is assessed comparing the electrophilicity
X
X
2
fka
¼
jc aj þ
c
acmaS
lm
ð4Þ
l
l
m–
l
lꢀk
Eq. 5 has been introduced to analyze at which atomic site of a
molecule the maximum electrophilicity power will be developed.12
xk
¼
x
fkþ
ð5Þ
index of the diene/dienophile interacting pairs, that is
Dx Evi-
dently, the differences in the global electrophilicity power (
D
x
Furthermore, the first approaches toward
a
quantitative
are higher for Danishefsky’s diene 4 than 1-Trimethylsilyloxy-
1,3-butadiene 3 and isoprene 2. Therefore, a high reactivity and
high regioselectivity are expected for the pair dienophile/Danishef-
sky’s diene.
Finally, the flux of the electron-density in these polar cycloaddi-
tion reactions is also supported by means of a DFT analysis based
on the electronic chemical potentials of the reagents. The elec-
tronic chemical potentials of the substituted heterocyclic dieno-
philes are nearly ꢀ5 eV, and they are higher than those of the
dienes, nearly ꢀ3 eV, thereby suggesting that the net charge trans-
fer will take place from these electron-rich dienes toward the aro-
matic dienophiles.
description of nucleophilicity, in the form of a regional reactivity
index, have also been reported. Eq 6 has been developed by Domin-
go et al with the purpose of identifying the most nucleophilic site
of a molecule and assessing the activation/deactivation caused by
different substituents on the electrophilic aromatic substitution
reactions of aromatic compounds.13
Nk ¼ Nfꢀ
ð6Þ
k
N ¼ ðeHOMO;Nu
ꢀ
eHOMO;TCE
Þ
ð7Þ
where eHOMO,TCE is the HOMO energy of tetracyanoethylene (TCE)
(taken as a reference molecule because it exhibits the lowest HOMO
energy in a large series of molecules previously considered in the
framework of polar D–A cycloadditions).14 NK is the global nucleo-
philicity index and N is its local counterpart.
In this work, the polarity of the normal electron demand D–A
process proposed has been studied by means of global electrophi-
licity index difference between reactants and the regioselectivity of
the normal electron demand D–A reaction shown in Schemes 1 and
2 using the local electrophilicity index for dienophiles (electro-
philes in the reaction) and the local nucleophilicity index for dienes
(nucleophiles in the reaction).
According to the global electrophilicity index
x shown in Table
4 the dienes will act as nucleophiles and the dienophiles as electro-
philes. To study the regioselectivity, we used the local electrophi-
licity and nucleophilicity indexes for dienophiles and dienes,
respectively (Tables 5 and 6). The sites of study were C2 and C3
Table 4
Global electrophilicity indexes of the used dienophiles
and dienes
Compound
x (eV)
1a
1b
2
3
4
3.3275
3.0002
1.2746
1.0558
0.9611
3.1. Computational details
Studies show that the B3LYP method,15 using 6-311+G(d,p)
basis set, is adequate to model D–A reactions concerning

C. D. Della Rosa et al. / Tetrahedron Letters 52 (2011) 2316–2319
2319
Table 5
7075–7078; (c) Della Rosa, C.; Paredes, E.; Kneeteman, M.; Mancini, P. M. E.
Lett. Org. Chem. 2004, 1, 369–371.
3. Liu, Z.; Larock, R. Org. Lett. 2004, 6, 3739–3741.
4. (a) Kokubun, T.; Harborne, J. B.; Eagles, J.; Waterman, P. Phytochemistry 1995,
39, 1039–1042; (b) Miller, R.; Kleiman, R.; Powell, R. J. Natural Products 1988,
51, 328–330; (c) Hussain, M.; Hung, N. T.; Langer, P. Tetrahedron Lett. 2009, 50,
3929–3933.
5. Brasca, R.; Kneeteman, M. N.; Mancini, P. M. E.; Fabian, W. M. F. J. Mol.
Struct.(THEOCHEM) 2009, 911, 124–131.
Local electrophilicity indexes for dienophiles 1a and 1b
Dienophile
Site
xk (eV)
1a
2
3
2
3
0.1571
0.5359
0.7263
0.0482
1b
6. General Procedure for the thermal reactions of nitrobenzofurans.
The temperature, the length of the reaction, and the diene/dienophile ratio
were dependent on the starting material and are indicated in Tables 1 and 2. An
ampule containing a solution of 1.0 mmol of the dienophile and the required
amount of diene in 0.5 ml of dry benzene was cooled in liquid nitrogen, sealed,
and then heated in an oil bath. After the reaction time was completed, it was
cooled once more in liquid nitrogen and opened. The solution was evaporated
and the residue purified by column chromatography on silica gel or alumina
using hexane/ethyl acetate mixtures as eluent.
7. (a) Parr, R. G.; Yang, W. In: Density Functional Theory of Atoms and Molecules;
Oxford University Press: Oxford, UK, 1989; (b) Parr, R. G.; Pearson, R. G. J. Am.
Chem. Soc. 1983, 105, 7512; (c) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533;
(d) Janak, J. R. Phys. Rev. B 1978, 18, 7165.
Table 6
Local nucleophilicity index for dienes 2, 3, 4
Diene
Site
Nk (eV)
2
1
4
1
4
1
4
1.1183
0.7519
0.7496
0.9281
0.5441
1.3401
3
4
8. Parr, R. G.; Von Szentpaly, L.; Liu, S. J. Am. Chem. Soc. 1999, 121, 1922.
9. (a) Domingo, L. R.; Aurell, M. J.; Contreras, R. Tetrahedron 2002, 58, 4417; (b)
Pérez, P.; Domingo, L. R.; Aurell, M. J.; Contreras, R. Tetrahedron 2003, 59, 3117.
10. Parr, R. G.; Yang, W. J. Am. Chem. Soc. 1984, 106, 4049.
11. (a) Fuentealba, P.; Pérez, P.; Contreras, R. J. Chem. Phys. 2000, 113, 2544; (b)
Contreras, R.; Fuentealba, P.; Galván, M.; Pérez, P. Chem. Phys. Lett. 1999, 304,
405.
12. Domingo, L. R.; Aurell, M. J.; Pérez, P.; Contreras, R. J. Phys. Chem. A 2002, 106,
6871.
13. Pérez, P.; Domingo, L. R.; Duque-Noreña, M.; Chamorro, E. J. Mol.
Struct.(THEOCHEM) 2009, 895, 86.
in 1a and 1b and C1 and C4 in dienes 2, 3, and 4. The more favored
adducts are the ones where the most electrophilic and nucleophilic
sites interact first. In the reactions in which it is possible to discuss
regioselectivity, the experimental data agree with the computa-
tional results. However, the influence of the methyl group when
the diene is isoprene is smaller than that of the groups present in
Danishesfky’s diene.
14. (a) Domingo, L. R.; Chamorro, E.; Pérez, P. J. Org. Chem. 2008, 73, 4615; (b)
Jaramillo, P.; Domingo, L. R.; Chamorro, E.; Perez, P. J. Mol. Struct.(THEOCHEM)
2008, 865, 68.
4. Conclusions
15. (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648; (b) Lee, C.; Yang, W.; Parr, R. G.
Phys. Rev. B 1988, 37, 785.
16. Frisch, M. J. et al., Gaussian 03, Revision B.04, Gaussian Inc., Pittsburgh, PA,
2003.
When 2-nitrobenzofuran and 3-nitrobenzofuran were reacted
with the above mentioned dienes under different reaction condi-
tions, they showed their dienophilic character taking part in a nor-
mal demand D–A cycloaddition reactions. These reactions could be
considered a domino process that is initiated by a polar D–A reaction
and the subsequent concerted elimination of nitrous acid from the
[4 + 2] cycloadduct to yield the corresponding dibenzofurans.17,18
A very strong electron-acceptor group, such as a nitro group, in-
duces a similar reactivity at 2- and 3-positions in the benzofuran
ring. The ease of thermal extrusion of nitrous acid accompanying
the D–A reaction of 2- and 3-nitrobenzofurans followed by the fur-
ther aromatization makes this reaction sequence a simple, eco-
nomical and efficient method of dibenzofurans preparation,
which is a direct intermediate in the synthesis of important hetero-
atom compounds which display a wide variety of biological
activities.
DFT calculations of the electrophilicity and nucleophilicity in-
dexes agree with the experimental results and they are good reac-
tivity and regioselectivity predictors in these types of reactions.
The 2-nitrosubstituted benzofuran shows higher electrophilic-
ity power than the 3-nitrosubstituted benzofuran probably due
to the proximity of the nitro group with the heteroatom (see
Table 4).
17. General Aspects. 1H and 13C NMR spectra were recorded in CDCl3 on 300 and
75 MHz FT-spectrometers, respectively, using TMS as the internal standard;
GC–MS analyses were performed in an instrument equipped with a PE-5-type
column. IR spectra were recorded from NaCl cells. Melting points were
observed on a Winkle–Zeiss Gottingen microhot stage and were uncorrected.
The silica gel and neutral alumina used for chromatography were 70–
230 mesh. The syntheses of 2- and 3-nitrobenzofurans were performed
starting from benzofuran adapting a procedure proposed by Katritzky et al.
Synthesis 5, 699-706-2008. Other reagents were obtained from commercial
sources used as received or purified as required by standard methods.
18. Spectral data: Compound 5a 1H NMR (CDCl3) d: 1.57 (s, 3H), 6.91 (dd,
J = 7.6 Hz J = 2.1 Hz, 1H), 7.58 (d, J = 2.0 Hz, 1H), 7.78–7.80 (m, 2H); 8.20 (dd,
J = 8.4 Hz J = 1.1 Hz, 1H), 8.42 (d, J = 7.9 Hz, 1H), 8.56 (dd, J = 8.2 Hz, J = 1.5 Hz,
1H), 13C NMR (CDCl3) d: 23.6; 111.6; 111.8; 113.5; 121.2; 124.8; 125.9, 132.0;
132.8; 136.6, 146.5; 156.2. HRMS m/z 182.2226 (Calcd C13H10O 182.2218).
Compound 5b 1H NMR (CDCl3) d: 1.56 (s, 3H); 6.95(dd, J = 7.7 Hz, 1.2 Hz, 1H),
7.71 (d, J = 2.1 Hz, 1H); 7.78–7.80 (m, 2H); 8.18 (d, J = 7.8 Hz, 1H), 8.20 (dd,
J = 7.6 Hz, J = 1.2 Hz, 1H), 8.45 (dd, J = 7.8 Hz, J = 1.3 Hz, 1H) 13C NMR (CDCl3) d:
23.5; 111.7; 114.5; 120.8; 122.5; 123.6; 124.0; 126.3; 132.7; 135.6; 146.5;
155.2. HRMS m/z 182.2224 (Calcd C13H10O 182.2218). Compound 6a 1H NMR
(CDCl3) d: 1.93 (s, 3H); 3.31 (m, 2H); 3.44 (m, 2H), 5.64 (m, 1H), 7.52 (m, 2H);
7.65 (t, J = 1H); 7.91 (t, J = 1H). 13C NMR (CDCl3) d: 19.5; 22.5; 33.1; 112.4;
112.8; 120.3; 121.7; 123.8; 125.3; 132.2; 135.8; 150.7; 154.1. HRMS m/z
184.2415 (Calcd C13H12O 184.2399). Compound 6b 1H NMR (CDCl3) d: 1.92 (s,
3H); 3.31 (m, 2H); 3.44 (m, 2H); 5.72 (m, 1H); 7.52 (m, 2H); 7.65 (dd, J = 8.1 Hz,
0.9 Hz, 1H); 7.91 (dd, J = 7.31 Hz, 1.1 Hz, 1H). 13C NMR (CDCl3) d: 22.1; 24.2;
29.1; 112.4; 112.8; 119.8; 121.5, 123.8, 124.9, 131.5, 135.5, 149.8, 154.2. HRMS
m/z 184.2413 (Calcd C13H12O 184.2399). 7 Spectral data identical with those of
commercial material. Compound 8a 1H NMR (CDCl3) d : 5.21 (br s, 1H), 6.80
(dd, 1H, J = 7.8 Hz, J = 1.9 Hz), 7.02 (d, 1H, J = 2.1 Hz), 7.78–7.80 (m, 2H); 7.95
Acknowledgments
(d, J = 7.8 Hz, 1H); 8,21(dd, J = 7.6, J = 1.2, 1H), 8.43 (dd, J = 7.8, J = 1.3, 1H). 13
C
NMR (CDCl3) d: 107.6; 112.1; 114.5; 117.5; 122.8; 125.7; 137.2; 146.8; 150.1;
155.1. HRMS m/z 184.1952 (Calcd C12H8O2 184.1943). Compound 8b: 1H NMR
(CDCl3) d: 5.21 (br s, 1H), 6.85 (dd, J = 7.8 Hz, J = 1.9 Hz, 1H); 7.02 (d, J = 108 Hz,
1H), 7.78–7.80 (m, 2H); 7.91(d, J = 8.0 Hz, 1H), 8.20 (dd, J = 7.8 Hz J = 1.9 Hz,
1H), 8.56 (dd, J = 7.9 Hz, J = 2.0 Hz, 1H), 13C NMR (CDCl3) d: 105.5;112.1; 114.5;
118.5; 122.5; 126.7; 135.2; 144.8; 157.1; 159.1. HRMS m/z 184.1954 (Calcd
This research was supported by CAI+D at Universidad Nacional
del Litoral, Santa Fe, Rep. Argentina.
References and notes
C
12H8O2 184.1943). Compound 9 1H NMR (CDCl3) d: 0.20 (s, 6H), 1.05 (s, 9H),
1. For general review, see: (a) Carruthers, W. In: Cycloaddition Reactions in Organic
Snthesis; Pergamon Press: Oxford, UK, 1990; (b) Fringelli, F.; Tatichi, A. In: The
Diels–Alder Reaction; J. Wiley & Sons: Chichester, UK, 2002; (c) Corey, E. J.
Angew. Chem. Int. Ed. 2002, 41, 1650–1667.
2. (a) Della Rosa, C.; Kneeteman, M.; Mancini, P. Tetrahedron Lett. 2007, 48, 1435–
1438; (b) Della Rosa, C.; Kneeteman, M.; Mancini, P. Tetrahedron Lett. 2007, 48,
6.90 (dd, 1H, J = 8.0 Hz, J = 1.9 Hz), 6.93 (d, 1H; J = 3.8 Hz), 7.78–7.80 (m, 2H);
7.98 (d, J = 7.7 Hz, 1H); 8,21(dd, J = 7.6, J = 1.2, 1H), 8.43 (dd, J = 7.8, J = 1.3, 1H).;
13C NMR (CDCl3) d: ꢀ4.4, 18.7, 25.7, 109.3, 111.6, 112.0, 114.1, 120.5; 122.2;
125.2; 126.6; 138.1; 145.5; 152.2; 154.0. HRMS m/z 298.4582 (Calcd
C18H22O2Si 298.4574).