2-Nitrofurans as dienophiles in Diels-Alder reactions

Article abstract of DOI:10.1016/j.tetlet.2005.10.042

α-Nitrofuran derivatives are studied in Diels-Alder reactions under thermal conditions. In contrast to α-acylfurans, they proved to be efficient dienophiles.

Full text of DOI:10.1016/j.tetlet.2005.10.042

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8711–8714
2
-Nitrofurans as dienophiles in Diels–Alder reactions
Claudia Della Rosa, Mar ´ı a N. Kneeteman and Pedro M. E. Mancini*
Area de Qu ı´ mica Org a´ nica, Departamento de Qu ı´ mica, Facultad de Ingenier ı´ a Qu ı´ mica, Universidad Nacional del Litoral,
Santiago del Estero 2829, 3000 Santa Fe, Argentina
Received 6 September 2005; revised 5 October 2005; accepted 12 October 2005
Abstract—a-Nitrofuran derivatives are studied in Diels–Alder reactions under thermal conditions. In contrast to a-acylfurans, they
proved to be efficient dienophiles.
Ó 2005 Published by Elsevier Ltd.
1. Introduction
Taking into account that the exposure of 2,5-dimethyl-
-nitrofuran to isoprene at elevated temperature yields
3
It has been known for some time that aromatic hetero-
cycles, such as furan, thiophene and pyrrole undergo
Diels–Alder (DA) reactions despite their aromaticity
and hence expected inertness. In view of their electron-
rich constitution and electron-donor properties, they
have been mostly involved as the diene component in
the isomeric cycloadducts (with thermal extrusion
of nitrous acid accompanying the DA reaction), which
7
is in agreement with the reactivity showed by pyr-
roles and furans b-substituted with electron-withdraw-
ing groups, the purpose of the present work is to
explore the behaviour of 2-nitrofuran, methyl 5-nitro-
furan-3-carboxylate and methyl 5-nitrofuran-2-carboxy-
late in their exposure to dienes (strongly, moderately
and poorly activated) under thermal conditions.
The goal of this work is to determine if a strong
electron-withdrawing group in a-position of furans
induces reactivity at cycloaddition reactions like
nitro-thiophenes.
1
the cycloaddition process. However, there exists a limi-
ted number of examples of five-membered, aromatic
heterocycles acting as dienophiles in DA reactions with
normal electron demand, for example, b-acylated furans
have shown to be excellent dienophiles in interaction
with isoprene. In contrast, a-acylfuran proved to be a
very poor dienophile toward isoprene.2 In view of
our interest in cycloaddition chemistry of substituted
aromatic heterocycles with electron withdrawing
groups, we have reported studies on the dienophilic
character of other aromatic systems such as nitro-
,3
2. Results and discussion
4
5
indoles and nitronaphthalenes in DA reactions with
normal electron demand. More recently, we have shown
that 2-nitrothiophene can act as dienophile in thermal
DA reactions with DanishefskyÕs diene and isoprene
leading to benzothiophenol or pyrrolyl-thiophene
depending on the reaction conditions. Besides, the
results demonstrate that a-nitro substituted thiophenes
To test the efficacy of 2-nitrofurans as dienophile in
DA reactions, the following compounds were used:
2-nitrofuran (1a), methyl 5-nitrofuran-3-carboxylate
(1b) and methyl 5-nitrofuran-2-carboxylate (1c). Isoprene
(2), 1-N-acetyl-N-propyl-1,3-butadiene (3), 1-diethyl-
amino-3-tert-butyldimethyl-siloxy-1,3-butadiene (RawalÕs
diene) (4) and 1-methoxy-3-trimethysilyloxy-1,3-butadi-
ene (DanishefskyÕs diene) (5) were chosen as the diene
partners (Scheme 1).
6
show higher dienophilicity than the b-nitro isomers.
The DA reactivity of thiophenes appears to be opposite
to that reported for furans where the b-substituted bond
was found to be more dienophilic.
When 2-nitrofurans were reacted with the above-men-
8
tioned dienes under different reaction conditions, they
showed their dienophilic character taking part in DA
cycloaddition reactions. Treatment of 1a with isoprene
2 gave a mixture of dihydrobenzofurans 6a and 6b (with
thermal extrusion of nitro group as nitrous acid from
Keywords: Diels–Alder; Dienophiles; Nitrofurans.
*
Corresponding author. Tel.: +54 3424571164; fax: +54 342
571162; e-mail: pmancini@fiqus.unl.edu.ar
4
0040-4039/$ - see front matter Ó 2005 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2005.10.042

8712
C. Della Rosa et al. / Tetrahedron Letters 46 (2005) 8711–8714
Pr N COMe
On the other hand, reactions of 1c with 2 gave a mixture
of aromatic cycloadducts of simple addition 7e and 7f
9
with moderate yields (Table 1, entries 5 and 6). Table
R2
R1
a R =R =H
1
shows that, in general, the yield increases with the
increase of temperature in every experience.
O
NO2
2
3
It should be noted that the isolation of the primary nitro-
adduct was never achieved under these conditions.
1
1
2
b R =H, R =CO Me
1
2
2
N(Et)2
OMe
c R =CO Me, R =H
1
2
2
The reaction of 1a with 1-N-acetyl-N-propyl-1,3-butadi-
ene 3 afforded benzofuran 10a with loss of N-acetyl-N-
propylamino and nitro groups. In the same way, in the
reactions with RawalÕs and DanishefskyÕs diene cycload-
ducts 11a and 12a were obtained with moderate to high
OTBS
OTMS
4
5
9
yield and complete regioselectivity (Table 2).
Scheme 1.
When 3, 4 and 5 reacted with 1b, they yielded 10b, 11b
and 12b, respectively. In the reactions with 1c, 10c, 11c
and 12c were obtained. These reactions proceeded by
the initial DA adduct) and benzofurans 7a and 7b as
principal products.
9
9
addition of the diene selectively to the nitro-substituted
double bond of the furan. All addition products showed
extrusion of the nitro group as nitrous acid. It is
noteworthy that only 1:1 adducts whose structure re-
vealed site selectivity and regioselectivity were obtained
(Scheme 3).
Exposure of 1b to isoprene yielded the mixture of iso-
meric cycloadducts 7c and 7d as principal products
(
(
60%) and a mixture of double addition adducts 8a–d
10%) and 9a–d (30%), in both cases of regioisomer mix-
9
tures (Scheme 2, Table 1).
G1
G2
G1
R1
R1
G2
R2
R2
O
O
6
a R =R =H, G =Me,G =H
7a R =R =H, G =Me, G =H
1 2 1 2
1
2
1
2
b R =R =H, G =H, G =Me
b R =R =H, G =H, G =Me
1 2 1 2
R1
1
2
1
2
c R =CO Me, R =H, G =Me, G =H
1
2
2
1
2
d R =CO Me, R =H, G =H, G =Me
2
1
2
2
1
2
R2
O
NO2
e R =H, R =CO Me, G =Me, G =H
1 2 2 1 2
f R =H, R =CO Me, G =H, G =Me
1
2
2
1
2
1
a R =R =H
1
2
b R =CO Me, R =H
1
2
2
c R =H, R =CO Me
1
2
2
G3
G3
G1
G1
G2
R1
O
R1
O
G4
G4
G2
8
a R =CO Me, G =G =Me, G =G =H 9a R =CO Me, G =G =Me, G =G =H
1 2 1 3 2 4
1 2 1 3 2 4
b R =CO Me, G =G =Me, G =G =H
b R =CO Me, G =G =Me, G =G =H
1
2
2
3
1
4
1 2 2 3 1 4
c R = CO Me, G =G =Me, G =G =H c R = CO Me,G =G =Me, G =G =H
1
2
1
4
2
3
1
2
1
4
2
3
d R =CO Me, G =G =Me, G =G =H
d R =CO Me, G =G =Me, G =G =H
1
2
2
4
1
3
1 2 2 4 1 3
Scheme 2. Diels–Alder reactions of 2-nitrofurans derivatives and isoprene.
Table 1.
a
b
Entry
1
Dienophile
Conditions
Product
Product ratio
Yield (%)
1a
200 °C, 72h
°C, 72h
200 °C, 72h
150 °C, 72h
200 °C, 72h
150 °C, 72h
6a,b; 7a,b
6a,b; 7a,b
7c,d; 8a,b,c,d; 9a,b,c,d
7c,d; 8a,b,c,d; 9a,b,c,d
7e,f
7e,f
1:1:3:3
1:1:3:3
1:3; 1:3:1:2; 1:3:1:2
1:3; 1:3:1:2; 1:3:1:2
1:3
1:3
62
53
70
60
55
51
2
3
4
5
6
150
1b
1c
a
1
2equiv of isoprene in benzene.
b
Based on consumed dienophile.

C. Della Rosa et al. / Tetrahedron Letters 46 (2005) 8711–8714
8713
Table 2.
Entry Diene Dienophile D/D Condition
ally, the more powerful the electronic effect of the sub-
stituents, the more regioselective the reaction.
a
Product,
yield
b
Moreover, a very strong electron-acceptor group, such
as nitro group induces in the furan ring similar reactivity
at a- and b-positions.
1
2
3
4
5
6
7
8
9
3
3
3
4
4
4
5
5
5
1a
1b
1c
1a
1b
1c
1a
1b
1c
3:1
3:1
3:1
3:1
3:1
3:1
2:1
2:1
2:1
120 °C, 72h
120 °C, 72h
120 °C, 72h
RefluxTol. 120 h 11a, 82 %
RefluxTol. 120 h 11b, 45%
RefluxTol. 120 h 11c, 32 %
120 °C, 72h
120 °C, 72h
120 °C, 72h
10a, 58%
10b, 51%
10c, 2 8%
Acknowledgements
12a, 50%
12b, 55%
12c, 43%
This work received financial support from the Science
and Technology Secretariat-UNL-CAI+D Program
and Doctoral Grant Programs at Universidad Nacional
del Litoral.
a
Diene/dienophile ratio.
Based on consumed nitrofuran.
b
3. Conclusions
References and notes
It has been demonstrated that in contrast to the 2-acyl-
ated furans, 2-nitrofurans react efficiently with the
above-mentioned dienes in normal electron demand
Diels–Alder reactions, with the nitro group inducing
side selectivity. This substituent is easily extrused under
thermal conditions, giving cycloadducts of high interest
as intermediaries in the synthesis of some alkaloid fami-
lies as morphine, kreysiginine and codeine. Selecting the
appropriate reagents (diene-dienophile), the DA adducts
for the total synthesis of these natural products could be
obtained in a simple way.
1
. Fringuelli, F.; Taticchi, A. Dienes in the Diels–Alder
reaction; Wiley Interscience: New York, 1990; pp 276–280.
2. Wenkert, E.; Piettre, S. J. Org. Chem. 1988, 53, 5850–
5853.
3. Wenkert, E.; Moeller, P.; Piettre, S. J. Am. Chem. Soc.
1
988, 110, 7188–7194.
4
5
. (a) Biolatto, B.; Kneeteman, M.; Paredes, E.; Mancini, P. J.
Org. Chem. 2001, 66, 3906; (b) Biolatto, B.; Kneeteman,
M.; Mancini, P. Tetrahedron Lett. 1999, 40, 3343–3346.
. (a) Paredes, E.; Biolatto, B.; Kneeteman, M.; Mancini, P.
Tetrahedron Lett. 2000, 41, 8079–8982; (b) Paredes, E.;
Biolatto, B.; Kneeteman, M.; Mancini, P. Tetrahedron Lett.
2
002, 43, 4601–4603; (c) Paredes, E.; Kneeteman, M.;
The reactions of 1b with isoprene underwent cycloaddi-
tion with the formation of 1:1 and 2:1 adducts. With the
same diene, 1c gave only 1:1 adducts. The behaviour of
Gonzalez Sierra, M.; Mancini, P. Tetrahedron Lett. 2003,
44, 2943–2945.
6. Della Rosa, C.; Paredes, E.; Kneeteman, M.; Mancini, P.
Lett. Org. Chem. 2004, 1, 369–371.
7. Della Rosa, C.; Kneeteman, M.; Mancini, P. M. E.,
unpublished results.
. General procedure. The temperature, the length of the
reaction and the diene/dienophile ratio are given in Tables 1
and 2. An ampoule containing 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
2
-nitrofurans with carbomethoxy group substituent at
0
b position is in agreement with the difference between
b-acylfurans and their a-acyl isomers.
8
Surprisingly, exposure of every 2-nitrofuran to electron-
rich dienes yielded 1:1 adducts with the cycloaddition
taking place on the nitrated bond. These results would
show that dienes with strong electron-donor substituents
induce the Diels–Alder cycloaddition selectively. Gener-
R1
R2
O
NO2
1
a R =R =H
1 2
b R =CO Me, R =H
1
2
2
c R =H, R =CO Me
1
2
2
3
4
5
R1
OTBS
OH
R1
R1
R2
O
R2
11a R =R =H
R2
O
O
1
0a R =R =H
12a R =R =H
1 2
1
2
1
2
b R =CO Me, R =H
b R =CO Me, R =H
b R =CO Me, R =H
1
2
2
1
2
2
1 2 2
c R =H, R =CO Me
c R =H, R =CO Me
c R =H, R =CO Me
1 2 2
1
2
2
1
2
2
Scheme 3.

8714
C. Della Rosa et al. / Tetrahedron Letters 46 (2005) 8711–8714
ꢀ
1
1
solution was evaporated and the residue purified by column
3
(C@O) 1730 cm . 9a H NMR (300 MHz, CDCl ) d: 1.68
chromatography in silica gel or alumina using hexane/ethyl
acetate mixtures as eluent. Biolatto, B.; Kneeteman, M.;
Paredes, E.; Mancini, P. J. Org. Chem. 2001, 66, 3906–
(s, 3H), 2.22 (s, 3H), 2.31–2.63 (m, 4H), 3.66 (s, 3H), 4.75
(m, 1H), 5.35 (br s, 1H), 6.58 (d, 1H, J = 8.6 Hz), 6.79 (dd,
1H, J = 8.6 Hz, J = 2.2 Hz), 6.82 (d, 1H, J = 2.2 Hz);
1
3
C
3
912.
. Spectral data: 6a d H NMR (500 MHz, CDCl
H), 3.15 (m, 2H), 3.35 (m, 2H), 5.56 (m, 1H), 6.25 (d, 1H,
NMR (300 MHz) d: 23.4, 24.5, 28.1, 37.5, 53.1, 61.6, 78.0,
1
9
3
) d: 1.86 (s,
114.5, 123.5, 124.5, 127.1, 130.5, 134.1, 158.1, 173.1; IR
ꢀ
1
1
3
(C@O) 1729 cm . 9b H NMR (300 MHz, CDCl ) d: 1.68
3
1
3
J = 3.1 Hz), 7.05 (d, 1H, J = 3.1Hz); C NMR (500 MHz)
(s, 3H), 2.24 (s, 3H), 2.33–2.65 (m, 4H), 3.65 (s, 3H), 4.77
d: 22.6, 24.5, 27.3, 110.7, 114.4, 122.5, 134.6, 140.6, 153.0.
(m, 1H), 5.40 (br s, 1H), 6.51 (d, 1H, J = 2.2 Hz), 6.54 (dd,
1
13
6
2
7
3
b H NMR (500 MHz, CDCl ) d: 1.88 (s, 3H), 3.16 (m,
1H, J = 8.7 Hz, J = 2.2 Hz), 7.01 (d, 1H, J = 8.7 Hz);
C
3
H), 3.35 (m, 2H), 5.54 (m, 1H), 6.25 (d, 1H, J = 3.1Hz),
NMR (300 MHz) d: 23.5, 24.6, 29.1, 35.4, 52.7, 59.0, 80.8,
1
3
.5 (d, 1H, J = 3.1 Hz); C NMR (500 MHz) d: 20.1, 23.0,
113.1, 120.7; 121.6, 123.5, 128.9, 134.1, 136.5, 160.9, 172.6;
1
ꢀ1
1
1.8, 110.9, 114.6, 122.4, 135.1, 140.6, 153.0. 7a H NMR
3
IR (C@O) 1729 cm . 9c H NMR (300 MHz, CDCl ) d:
(
500 MHz, CDCl ) d: 2.48 (s, 3H), 6.68 (d, 1H, J = 3.5Hz),
1.71 (s, 3H), 2.21–2.46 (m, 2H), 2.35 (s, 3H), 2.47–2.72 (m,
2H), 3.67 (s, 3H), 4.86 (m, 1H), 5.39 (br s, 1H), 6.50 (d, 1H,
J = 2.4 Hz), 6.53 (dd, 1H, J = 8.7 Hz, J = 2.4 Hz), 7.00 (d,
3
7
.01 (dd, 1H, J = 8.6; 2.4 Hz), 7.27 (d, 1H, J = 2.4 Hz), 7.31
13
(
d, 1H, J = 8.6 Hz), 7.52(d, 1H, J = 3.5 Hz); C NMR
1
3
(
500 MHz) d: 24.9, 107.1, 112.0, 121.0, 125.2, 128.8, 131.8,
1H, J = 8.7 Hz); C NMR (300 MHz) d: 24.4, 25.6, 29.2,
1
1
3
2
8
2
7
6
46.2, 153.8. 7b H NMR (500 MHz, CDCl ) d: 2.52 (s,
35.2, 52.5, 58.7, 80.7, 113.2, 120.7, 121.5, 123.4, 128.8,
3
ꢀ
1
1
H), 6.67 (d, 1H, J = 3.3 Hz), 6.89 (dd, 1H, J = 8.7;
134.0, 136.2, 160.8, 172.5; IR (C@O) 1730 cm . 9d
H
.4 Hz), 7.25 (d, 1H, J = 3.5 Hz), 7.38 (d, 1H, J =
NMR (300 MHz, CDCl ) d: 1.71 (s, 3H), 2.25 (s, 3H), 2.32–
3
1
3
.6 Hz), 7.50 (d, 1H, J = 3.5Hz); C NMR (500 MHz) d:
2.63 (m, 4H), 3.65 (s, 3H), 4.88 (m, 1H), 5.40 (br s, 1H),
4.7, 106.5, 111.8, 121.1, 124.1, 126.0, 132.5, 145.2, 156.5.
6.51 (d, 1H, J = 2.2 Hz), 6.54 (dd, 1H, J = 8.7 Hz,
1
13
c H NMR (300 MHz, CDCl
3
) d: 2.33 (s, 3H), 3.89 (s, 3H),
J = 2.2 Hz), 7.01 (d, 1H, J = 8.7 Hz);
C
NMR
.97 (dd, 1H, J = 8.6 Hz; J = 2.3 Hz), 7.28 (d, 1H,
(300 MHz) d: 24.4, 26.1, 29.4, 38.8, 53.1, 63.2, 78.5, 115.0,
J = 2.3 Hz), 7.31 (d, 1H; J = 8.6 Hz), 8.05 (s, 1H); IR
122.1, 123.9, 124.6, 131.4, 136.2, 137.8, 162.3, 173.0; IR
ꢀ
1
13
ꢀ1
1
(
1
C@O) 1729 cm
;
C NMR (300 MHz) d: 24.8, 52.5,
3
(C@O) 1729 cm . 10b H NMR (300 MHz, CDCl ) d: 3.86
09.5, 111.2, 119.8, 125.2, 125.7, 129.5, 153.2, 155.8, 160.2;
(s, 3H), 7.28–7.32 (m, 2H), 7.43–7.46 (m, 1H), 7.86–7.90 (m,
ꢀ
1
1
13
IR (C@O) 1728 cm . 7d H NMR (300 MHz, CDCl
3
) d:
.33 (s, 3H), 3.88 (s, 3H), 6.94 (dd, 1H, J = 8.6; 2.4 Hz),
.20 (d, 1H, J = 2.4 Hz), 7.41 (d, 1H, J = 8.6 Hz), 8.01 (s,
1H), 8.03 (s, 1H); C NMR (300 MHz) d: 52.8, 112.5;
2
7
1
1
1
3
2
5
1
118.2, 123.1, 124.6, 125.3, 127.1, 157.8, 158.3, 160.2; IR
ꢀ
1
1
(C@O) 1729 cm . 10c H NMR (300 MHz, CDCl
3
) d: 3.85
1
3
H); C NMR (300 MHz) d: 24.7, 52.5, 109.9, 112.3, 120.1,
22.5, 124.1, 132.1, 157.0, 157.7, 160.3; IR (C@O)
(s, 3H), 7.25–7.29 (m, 2H), 7.41–7.46 (m, 1H), 7.84–7.90 (m,
1H), 8.01 (s, 1H); C NMR (300 MHz) d: 52.8, 113.5,
1
3
ꢀ1
1
728 cm . 7e H NMR (500 MHz, CDCl ) d: 2.45 (s,
119.0, 123.1, 124.7, 126.1, 127.1, 157.9, 158.4, 160.2; IR
3
ꢀ
1
1
H), 3.89 (s, 3H), 7.03 (dd, 1H, J = 8.7, 2.3 Hz), 7.34 (m,
H), 7.62 (s, 1H); C NMR (500 MHz, CDCl
0.0, 112.4, 113.6, 121.2, 126.3, 129.4, 132.3, 146.1, 157.6,
3
(C@O) 1726 cm . 11a H NMR (300 MHz, CDCl ) d: 0.20
1
3
3
) d: 24.6,
(s, 6H), 1.05 (s, 9H), 6.90 (dd, 1H, J = 8.8 Hz, J = 2.4Hz),
6.93 (d, 1H; J = 3.8Hz), 7.07 (d, 1H, J = 2.4Hz), 7.39 (d,
ꢀ1
1
13
60.3; IR (C@O) 1739 cm . 7f H NMR (500 MHz,
) d: 2.48 (s, 3H), 3.89 (s, 3H), 6.98 (dd, 1H, J = 8.7,
.3 Hz), 7.23 (d, 1H, J = 2.3 Hz), 7.40 (d, 1H, J = 8.6 Hz),
1H, J = 8.8Hz), 7.45 (d, 1H, J = 3.8Hz);
C NMR
CDCl
2
7
3
(300 MHz) d: ꢀ4.4, 18.7, 25.7, 109.3, 111.6, 112.0, 119.1,
ꢀ
1
1
130.1, 145.5, 149.2, 150.1; IR (C@O) 1730 cm . 11b H
ꢀ
1
1
.61 (s, 1H); IR (C@O) 1739 cm . 8a H NMR (300 MHz,
) d: 1.71 (s, 6H), 2.34 –2.61 (m, 4H), 3.15 (m, 2H),
.30 (m, 2H), 3.66 (s, 3H), 4.76 (m, 1H), 5.54 (m, 1H),
NMR (300 MHz, CDCl ) d: 0.20 (s, 6H), 1.05 (s, 9H), 3.89
3
CDCl
3
(s, 3H), 6.90 (dd, 1H, J = 8.8 Hz, J = 2.4Hz), 7.12 (d, 1H,
1
3
3
5
3
1
J = 2.4Hz), 7.39 (d, 1H, J = 8.8Hz), 8.01 (s, 1H); C NMR
(300 MHz) d: ꢀ4.6, 18.8, 25.7, 53.4, 109.3, 112.2, 116.7,
119.6, 130.1, 151.3, 151.8, 157.3, 161.1; IR (C@O)
1
3
.61 (br s, 1H); C NMR (300 MHz) d: 24.5, 28.9, 30.3,
2.4, 35.7, 52.5, 60.2, 73.5, 121.6, 123.8, 131.8, 134.2, 141.3,
ꢀ1
1
ꢀ1
1
73.0; IR (C@O) 1730 cm . 8b H NMR (300 MHz,
) d: 1.69 (s, 3H), 1.72 (s, 3H), 2.37–2.65 (m, 4H), 3.16
m, 2H), 3.32 (m, 2H), 3.68 (s, 3H), 4.86 (m, 1H), 5.44 (m,
1729 cm . 11c H NMR (300 MHz, CDCl ) d: 3.90 (s,
3
CDCl
(
1
2
1
3
3H), 6.87 (dd, 1H, J = 8.7, 2.4 Hz), 7.10 (d, 1H, J =
2.4 Hz), 7.36 (d, 1H, J = 8.7 Hz), 7.61 (s, 1H); IR (C@O)
1
3
ꢀ1
1
H), 5.60 (br s, 1H); C NMR (300 MHz) d: 22.4, 22.7,
4.5, 30.8, 36.4, 52.2, 59.9, 75.3, 105.7, 120.1, 122.0, 130.7,
33.4, 144.2, 173.1; IR (C@O) 1730 cm . 8c H NMR
300 MHz, CDCl ) d: 1.69 (s, 3H), 1.71 (s, 3H), 2.37–2.65
1726 cm . 12a H NMR (300 MHz, CDCl ) d: 5.21 (br s,
3
1H), 6.82(dd, 1H, J = 8.8 Hz, J = 2.4 Hz), 6.85 (d, 1H,
ꢀ
1
1
J = 3.8 Hz), 7.07 (d, 1H, J = 2.4 Hz), 7.40 (d, 1H, J =
1
3
(
8.8 Hz), 7.62(d, 1H, J = 3.8 Hz); C NMR (300 MHz) d:
3
1
(
m, 4H), 3.16 (m, 2H), 3.32 (m, 2H), 3.68 (s, 3H), 4.86 (m,
105.5, 106.1, 115.2, 116.8, 131.4, 147.8, 150.2, 152.5. 12b H
1
3
1
2
1
1
6
H), 5.44 (m, 1H), 5.60 (br s, 1H); C NMR (300 MHz) d:
2.5, 22.7, 29.7, 32.5, 34.7, 39.8, 54.7, 63.8, 72.6, 105.7,
22.0, 124.1, 132.3, 136.3, 142.1, 172.1; IR (C@O)
NMR (300 MHz, CDCl
3
) d: 3.90 (s, 3H), 5.18 (br s, 1H),
6.75 (dd, 1H, J = 8.4 Hz, J = 2.4 Hz), 7.05 (d, 1H,
1
3
J = 2.4 Hz), 7.75 (d, 1H, J = 8.4 Hz), 8.15 (s, 1H);
C
ꢀ1
1
731 cm . 8d H NMR (300 MHz, CDCl
3
) d: 1.72(s,
NMR (300 MHz) d: 51.6, 105.9, 113.1, 114.5, 116.3, 127.4,
ꢀ1
H), 2.06–2.36 (m, 4H), 3.24 (m, 2H), 3.41 (m, 2H), 3.67
150.2, 153.8, 156.9, 160.1; IR (C@O) 1730 cm . 12c d: 3.85
(s, 3H), 5.20 (br s, 1H), 6.73 (dd, 1H, J = 8.8, 2.4 Hz), 7.01
(d, 1H, J = 2.4 Hz), 7.33 (d, 1H, J = 8.8 Hz), 7.65 (s, 1H);
1
3
(
s, 3H), 4.60 (m, 1H), 5.83 (m, 1H), 6.10 (m, 1H); C NMR
(
300 MHz) d: 22.4, 23.5, 26.0, 30.1, 38.5, 38.8, 52.8, 63.1,
ꢀ
1
71.0, 103.5, 121.2, 123.5, 131.9, 134.2, 141.5, 171.1; IR
IR (C@O) 1726 cm .