Comparison of the reactivity between 2- and 3-nitropyrroles in cycloaddition reactions. A simple indole synthesis

Article abstract of DOI:10.1016/j.tetlet.2006.12.097

N-Tosyl-2-nitropyrroles react at high temperature with poorly and strongly activated dienes. They exhibit a dienophilic character similar to N-tosyl-3-nitropyrroles producing the corresponding indoles through a classical Diels-Alder process. A similar beh

Full text of DOI:10.1016/j.tetlet.2006.12.097

Tetrahedron Letters 48 (2007) 1435–1438
Comparison of the reactivity between 2- and 3-nitropyrroles
in cycloaddition reactions. A simple indole synthesis
Claudia Della Rosa, Maria Kneeteman and Pedro Mancini*
Area de Quı´mica Orga´nica, Departamento de Quı´mica, Facultad de Ingenierı´a Quı´mica, Universidad Nacional del Litoral,
Santiago del Estero 2829, 3000 Santa Fe, Argentina
Received 16 November 2006; revised 14 December 2006; accepted 15 December 2006
Available online 20 December 2006
Abstract—N-Tosyl-2-nitropyrroles react at high temperature with poorly and strongly activated dienes. They exhibit a dienophilic
character similar to N-tosyl-3-nitropyrroles producing the corresponding indoles through a classical Diels–Alder process. A similar
behaviour was observed in disubstituted N-tosyl-pyrroles.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
electron withdrawing groups (e.g., nitro or carboxylate)
in their exposure to dienes under thermal conditions.
In view of their electron-rich constitution and electron-
donor properties five-membered aromatic heterocycles
have been involved in cycloaddition reactions mostly
as the diene component.1 However, only a limited
number of them act as dienophiles in Diels–Alder
(DA) reactions with normal electron demand, for exam-
ple, aromatic heterocyclepentadienes holding an elec-
tron withdrawing group at b-position have shown to
be excellent dienophiles in the interaction with isoprene
at an elevated temperature. In contrast, a-acylhetero-
cycles have proved to be very poor dienophiles towards
isoprene.2,3 Due to our interest in the cycloaddition
chemistry of substituted aromatic heterocycles with elec-
tron-withdrawing groups, we have recently reported
that 2-nitrofurans and 2-nitrothiophenes react with
strong and poor dienes under different conditions.
2-Nitrofurans react with dienes strongly, moderately
and poorly activated in normal electron demand DA
reactions.4 Instead 2-nitrothiophene can act as dieno-
phile in thermal DA reactions with Danishefsky’s diene
and isoprene leading to benzothiophenol or pyrrolyl–
thiophene depending on the reaction conditions.5 Con-
sidering that there are fewer examples of tosyl-pyrrole
reacting with dienes in normal electron demand DA
reactions,2 the purpose of the present work is to explore
the behaviour of pyrroles substituted in 2-position with
This allowed us to compare not only the relative reactiv-
ity of the 2- and 3-substitution of the aromatic ring but
also the regioselectivities in the cases of successful cyclo-
addition. This study includes mono- and disubstituted
tosyl-pyrroles.
2. Results and discussion
The study was carried out using 1-tosyl-2-nitropyrrole
(1a), 1-tosyl-3-nitropyrrole (1b), methyl 5-nitro-1-tosyl-
pyrrole-3-carboxylate (1c) and methyl 4-nitro-1-tosyl-
pyrrole-2-carboxylate (1d). The diene components were
isoprene (2), 1-N-acetyl-N-propylamino-1,3-butadiene
(3) and 1-methoxy-3-trimethylsilyloxy-1,3-butadiene
(Danishefsky’s diene) (4) (Scheme 1).
When 2-nitropyrroles were reacted with the above men-
tioned dienes under different reaction conditions,6 they
showed their dienophilic character taking part in the
DA cycloaddition reactions.
When 1a was heated with less reactive isoprene 2, it gave
a mixture of dihydroindoles 5a and 5b (with thermal
extrusion of nitrous acid accompanying the DA reac-
tion) and indoles 6a and 6b as the principal products.9
The reactions of 1c with isoprene proceeded to produce
the mixture of isomeric cycloadducts 6c and 6d as
the principal products (55%) and a mixture of double
Keywords: Diels–Alder; Nitropyrroles; Indole.
*
Corresponding author. Tel.: +54 342 4571164; fax: +54 342
4571162; e-mail: pmancini@fiqus.unl.edu.ar
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.12.097

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C. Della Rosa et al. / Tetrahedron Letters 48 (2007) 1435–1438
Similarly, the reactions of 1b with isoprene yielded a
mixture of isomeric cycloadducts 5a, 5b, 6a and 6b
(Scheme 3, Table 1).
COMe
Pr
N
R2
On the other hand, the treatment of 1d with 2 afforded a
mixture of isomeric indoles 11a and 11b. These reactions
proceeded by the addition of the diene selectively to the
nitro-substituted double bond of the pyrrole. No bis-
adduct from the double cycloaddition of the diene was
detected.
R1
N
Ts
2
3
,
1a R1= NO2 R2= H
b R1= H, R2= NO2
c R1= NO2, R2= CO2Me
OMe
d R1= CO2Me, R2= NO2
The cycloaddition between 1-N-acetyl-N-propylamino-
1,3-butadiene 3 with 1a and 1b yielded only 1-tosyl-indol
with moderate to a high yield. The results of this type of
DA reactions involving the N-tosyl-2-nitropyrrole indi-
cate a possible sequential pathway where the ease of ther-
mal extrusion of nitrous acid accompanying the DA
reaction would lead to the 4-substituted-N-tosyl-dihydr-
oindol adduct, which would undergo thermal aromatiza-
tion by losing the N-acetyl-N-alkylamino substituent.7
OSiMe3
4
Scheme 1.
addition adducts 7a–d (15%) and 8a–d (35%), in both
cases of regioisomer mixtures (Scheme 2, Table 1).9
G1
R
G1
R
G2
R
N
G2
N
Ts
Ts
R
N
3
6a R=H, G1=Me, G2=H
b R=H, G1=H, G2=Me
c R=CO2Me, G1=Me, G2=H
d R=CO2Me, G1=H, G2=Me
Ts
5a R=H, G1=Me, G2=H
b R=H, G1=H, G2=Me
2
N
9a R=H
b R=CO2Me
NO2
Ts
1a R=H
c R=CO2Me
OH
4
R
G3
G1
G3
G1
R
R
N
G4
G2
G4
G2
Ts
N
N
Ts
Ts
10a R=H
b R=CO2Me
7a R=CO2Me, G1=G3=Me, G2=G4=H
b R=CO2Me, G2=G3=Me, G1=G4=H
c R=CO2Me, G1=G4=Me, G2=G3=H
d R=CO2Me, G2=G4=Me, G1=G3=H
8a R=CO2Me, G1=G3=Me, G2=G4=H
b R=CO2Me, G2=G3=Me, G1=G4=H
c R=CO2Me, G1=G4=Me, G2=G3=H
d R=CO2Me, G2=G4=Me, G1=G3=H
Scheme 2. Reactivity of N-tosyl-2-nitropyrroles with different dienes.
Table 1. Thermal reactions of N-tosyl-nitropyrroles with isoprene
Entry
Dienophile
Conditionsa
Products
Product ratio
Yieldb (%)
1
2
3
4
5
6
7
8
1a
200 °C, 72 h
150 °C, 72 h
200 °C, 72 h
150 °C, 72 h
200 °C, 72 h
150 °C, 72 h
200 °C, 72 h
150 °C, 72 h
5a, 5b, 6a, 6b
5a, 5b, 6a, 6b
5a, 5b, 6a, 6b
5a, 5b, 6a, 6b
6c, 6d, 7a, 7b, 7c, 7d, 8a, 8b, 8c, 8d
6c, 6d, 7a, 7b, 7c, 7d, 8a, 8b, 8c, 8d
11a, 11b
1:1:5:5
1:1:5:5
5:5:1:1
5:5:1:1
1:5; 1:3:1:2; 1:3:1:2
1:5; 1:3:1:2; 1:3:1:2
5:1
5:1
50
45
50
44
60
55
45
44
1b
1c
1d
11a, 11b
a 12 equiv of isoprene in benzene.
b Based on consumed dienophile.

C. Della Rosa et al. / Tetrahedron Letters 48 (2007) 1435–1438
1437
G1
N
G1
R
3
NO2
Ts
2
G2
G2
N
12a R=H
b R=CO2Me
R
N
N
R
R
Ts
Ts
Ts
4
6a R=H, G1=Me, G2=H
b R=H, G1=H, G2=Me
11a R=CO2Me, G1=Me, G2=H
b R=CO2Me, G1=H, G2=Me
1b R = H
d R= CO2Me
5a R=H, G1=Me, G2=H
b R=H, G1=H, G2=Me
OH
N
R
Ts
13a R=H
b R=CO2Me
Scheme 3. Reactivity of N-tosyl-3-nitropyrroles with different dienes.
Similarly, in the reactions with Danishefsky’s diene
cycloadduct 10a and 13a were obtained with a high yield
and complete regioselectivity, which is controlled by
the nitro and methoxy groups (Table 2). This products
resulted from the expected aromatization of the nitro-
adducts promoted by the loss of the nitro and methoxyl
groups as nitrous acid and methanol, respectively.8
these reactions, only 1:1 adducts whose structure re-
vealed site selectivity and regioselectivity were obtained.
The nitro group orients the cycloaddition selectively
towards the double bond to which it is directly attached
(Scheme 3, Table 2).
3. Conclusions
The isolation of the intermediate cycloadduct was never
achieved under these conditions.
It has been demonstrated that 1-tosyl-2-nitropyrrole
reacts efficiently with the above-mentioned dienes in
normal electron demand Diels–Alder reactions, with
the nitro group inducing side selectivity.
Exposure of 1c and 1d to 3 and 4 afforded indoles 9b,
12b, 10b and 13b, respectively.9 All addition products
showed extrusion of the nitro group as nitrous acid. In
A very strong electron-acceptor group, such as a nitro
group, induces a similar reactivity at 2- and 3-positions
in the pyrrole ring. The ease of thermal extrusion of
nitrous acid accompanying the DA reaction of 2- and
3-nitropyrroles followed by the further aromatization
makes this reaction sequence a simple method of indole
preparation, direct intermediate in the synthesis of some
alkaloids as serotonine, trytamine and gramine.
Table 2. Thermal reactions of N-tosyl-nitropyrroles with dienes 3 and
4
Entry Dienophile Diene Conditionc Product Yieldd
(°C)
(%)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
3a
140
120
140
120
140
120
140
120
9a
9a
45
53
44
52
40
41
44
48
12a
12a
9b
Acknowledgements
9b
12b
12b
This research was funded by the CAI+D and Doctoral
Grant Programs from Universidad Nacional del Litoral,
Santa Fe, Argentina.
9
10
11
12
13
14
15
16
1a
1b
1c
1d
4b
140
120
140
120
140
120
140
120
10a
10a
13a
13a
10b
10b
13b
13b
48
46
47
48
50
52
44
43
References and notes
1. Fringuelli, F.; Taticchi, A. Dienes in the Diels–Alder
reaction; Wiley Interscience: New York, 1990; pp 276–280.
2. Wenkert, E.; Moeller, P.; Piettre, S. J. Am. Chem. Soc.
1988, 110, 7188–7194.
3. Wenkert, E.; Piettre, S. J. Org. Chem. 1988, 53, 5850–5853.
4. Della Rosa, C.; Kneeteman, M. N.; Mancini, P. M. E.
Tetrahedron Lett. 2005, 46, 8711–8714.
a Diene/dienophile ratio 3:1.
b Diene/dienophile ratio 2:1.
c Reaction’s time 72 h.
d Based on consumed dienophile.

1438
C. Della Rosa et al. / Tetrahedron Letters 48 (2007) 1435–1438
5. Della Rosa, C.; Paredes, E.; Kneeteman, M.; Mancini,
P. M. E. Lett. Org. Chem. 2004, 1, 369–371.
(300 MHz, CDCl3) d: 1.71 (s, 3H); 2.24 (s, 3H); 2.35 (s,
3H); 2.05–2.7 (m, 4H); 3.69 (s, 3H); 3.79 (m, 1H); 5.40 (br s,
1H); 6.20 (d, 1H, J = 2.1 Hz); 6.35 (dd, 1H, J = 8.1–
2.1 Hz); 7.1 (d, 1H, J = 8.1 Hz); 7.32 (d, 2H, J = 8 Hz); 7.82
(d, 2H, J = 8 Hz). 13C NMR (75 MHz) d: 23.6; 24.7; 27.1;
37.2; 52.5; 53.6; 56.6; 114.2; 117.8; 124.3; 127.0; 128.7;
129.2; 130.2; 135.9; 137.3; 141.7; 144.1; 172.0. Compound
8c: 1H NMR (300 MHz, CDCl3) d: 1.71 (s, 3H); 2.22 (s,
3H); 2.35 (s, 3H); 2.1–2.7 (m, 4H); 3.68 (s, 3H); 3.82 (m,
1H); 5.41 (br s, 1H); 6.24 (d, 1H, J = 8.1 Hz); 6.60 (dd, 1H,
J = 8.1–1.5 Hz); 6.90 (d, 1H, J = 1.5 Hz); 7.32 (d, 2H,
J = 8 Hz); 7.82 (d, 2H, J = 8 Hz). 13C NMR (75 MHz) d:
23.2; 24.5; 25.4; 30.2; 34.5; 51.5; 53.4; 60.0; 113.6; 123.7;
127.4; 128.4; 129.8; 131.4; 131.6; 135.7; 137.3; 140.5; 142.6;
6. 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
solution was evaporated and the residue purified by column
chromatography in silica gel or alumina using hexane/ethyl
acetate mixtures as the eluent. Biolatto, B.; Kneeteman, M.;
Paredes, E.; Mancini, P. J. Org. Chem. 2001, 66, 3906–3912.
7. (a) Biolatto, B.; Kneeteman, M.; Paredes, E.; Mancini, P.
J. Org. Chem. 2001, 66, 3906–3912; (b) Biolatto, B.;
Kneeteman, M.; Mancini, P. Tetrahedron Lett. 1999, 40,
3343–3346.
1
172.0. Compound 8d: H NMR (300 MHz, CDCl3) d: 1.69
(s, 3H); 2.22 (s, 3H); 2.35 (s, 3H); 2.1–2.7 (m, 4H); 3.68 (s,
3H); 3.81 (m, 1H); 5.35 (br s, 1H); 6.20 (d, 1H, J = 8.1 Hz);
6.35 (dd, 1H, J = 8.1–2.1 Hz); 7.1 (d, 1H, J = 2.1 Hz); 7.32
(d, 2H, J = 8 Hz); 7.82 (d, 2H, J = 8 Hz). 13C NMR
(75 MHz) d: 23.7; 24.7; 30.2; 34.7; 51.2; 53.6; 59.3; 113.6;
118.1; 124.0; 127.6; 128.5; 130.1; 131.0; 134.2; 137.1; 137.8;
142.1; 144.6; 172.0. Compound 9b: 1H NMR (300 MHz,
CDCl3) d: 2.35 (s, 3H); 3.89 (s, 3H); 7.31–7.34 (m, 3H); 7.65
(m, 2H); 7.82 (d, 2H, J = 8 Hz); 8.1 (s, 1H); 8.3 (dd, 1H,
J = 8.1–2.1 Hz). 13C NMR (75 MHz) d: 24.6; 51.7; 109.3;
111.4; 119.6; 121.1; 123.6; 126.5; 128.4; 129.3; 131.0; 135.2;
137.8; 144.2; 167. HRMS m/z 329.3796 (calcd C17H15O4NS,
329.3789). Compound 10a: 1H NMR (300 MHz, CDCl3) d:
2.37 (s, 3H); 5.0 (br s, 1H); 6.45 (d, 1H, J = 3.1 Hz); 6.85
(dd, 1H, J = 8.1–1.2 Hz); 7.21 (d, 1H, J = 1.2 Hz), 7.27 (d,
1H, J = 8.1 Hz); 7.32 (d, 2H, J = 8 Hz); 7.82 (d, 2H,
J = 8 Hz). 13C NMR (75 MHz) d: 24.5; 102.5; 104.4; 110.9;
112.5; 118.6; 127.6; 128.7; 129.7; 130.3; 135.2; 143.6; 152.7.
HRMS m/z 311.3642 (calcd C15H13O3SN, 311.3636).
Compound 10b: 1H NMR (300 MHz, CDCl3) d: 2.35 (s,
3H); 3.90 (s, 3H); 5.20 (br s, 1H); 6.74 (dd, 1H, J = 8.4–
2.2 Hz); 7.05 (d, 1H, J = 8.4 Hz); 7.32 (d, 2H, J = 8 Hz);
7.75 (d, 1H, J = 2.2 Hz); 7.82 (d, 2H, J= 8 Hz); 8.20 (s, 1H).
13C NMR (75 MHz) d: 24.5; 52.0; 104.2; 109.1; 112.6; 113.8;
128.5; 128.8; 129.1; 129.7; 131.4; 135.0; 144.4; 153.1; 167.1.
HRMS m/z 345.3790 (calcd C17H15O5NS, 345.3783).
Compound 11a: 1H NMR (300 MHz, CDCl3) d: 2.45 (s,
6H); 3.89 (s, 3H); 7.05 (d, 1H, J = 8.6 Hz); 7.32 (d, 2H,
J = 8 Hz); 7.40 (m, 2H); 7.61 (d, 1H, J = 2.1 Hz); 7.82 (d,
2H, J = 8 Hz). 13C NMR (75 MHz) d: 24.5; 24.8; 52.1;
109.6; 112.3; 119.5; 121.7; 129.5; 130.5; 131.7; 132.4; 135.8;
8. Danishefsky, S.; Kitahara, T.; Yan, C. F.; Morris, J.
J. Am.Chem. Soc. 1979, 101, 6996–7000.
9. Spectral data. Compounds 5a, 5b, 6a and 6b, both of the
homologous benzenesulfonylated derivatives were reported
by Wenkert et al.2 Compound 6c: 1H NMR (300 MHz,
CDCl3) d: 2.35 (s, 3H); 2.40 (s, 3H); 3.89 (s, 3H); 7.20 (d,
1H, J = 8.1 Hz); 7.32 (d, 2H, J = 8 Hz); 7.41 (d, 1H,
J = 8.1 Hz); 7.71 (s, 1H); 7.82 (d, 2H, J = 8 Hz); 7.95 (s,
1H). 13C NMR (75 MHz) d: 24.4; 24.7; 51.6; 108.9; 118.8;
120.7; 126.7; 128.3; 129.2; 131.2; 133.2; 135.2; 144.1; 165.0.
HRMS m/z 343.4063 (calcd C18H17O4NS, 343.4059).
Compound 6d: 1H NMR (300 MHz, CDCl3) d: 2.35 (s,
3H); 2.40 (s, 3H); 3.89 (s, 3H); 7.1–7.30 (m, 2H); 7.32 (d,
2H, J = 8 Hz); 7.82 (d, 2H, J = 8 Hz); 7.95 (s, 1H); 8.15 (d,
1H, J = 8.1 Hz). 13C NMR (75 MHz) d: 24.3; 51.5; 108.9;
111.5; 119.3; 121.6; 123.8; 128.4; 126.9; 127.6; 128.8; 130.5;
135.3; 136.0; 143.8; 165.0. Compound 7a: 1H NMR
(300 MHz, CDCl3) d: 1.85 (s, 6H); 2.1–2.5 (m, 4H); 2.32
(s, 3H); 3.1–3.5 (m, 6H); 3.68 (s, 3H); 5.2 (br s, 1 H); 5.41
(m, 1H); 7.32 (d, 2H, J = 8 Hz); 7.82 (d, 2H, J = 8 Hz). 13
C
NMR (75 MHz) d: 23.5; 24.6; 28.1; 32.6; 33.7; 37.1; 46.6;
52.9; 59.2; 116.7; 121.6; 123.7; 127.6; 129.7; 132.5; 135.1;
134.6; 137.7; 170.0. HRMS m/z 412.5335 (calcd
C23H26O4NS, 412.5331). Compound 7b: 1H NMR
(300 MHz, CDCl3) d: 1.70 (s, 3H); 1.72 (s, 3H); 2.05–2.45
(m, 4H); 2.35 (s, 3H); 2.7 (m, 4H); 3.5 (m, 1H); 3.68 (s, 3H);
5.1 (br s, 1H); 5.4 (m, 1H); 7.32 (d, 2H, J = 8 Hz); 7.82 (d,
2H, J = 8 Hz). 13C NMR (75 MHz) d: 23.3; 23.6; 24.6; 27.5;
28.6; 37.2; 40.3; 47.8; 53.2; 58.6; 116.4; 121.6; 123.7; 128.4;
130.2; 131.6; 135.1; 135.8; 137.6; 143.5; 171.0. Compound
7c: 1H NMR (300 MHz, CDCl3) d: 1.70 (s, 3H); 1.72 (s,
3H); 2.05–2.45 (m, 4H); 2.35 (s, 3H); 2.8 (m, 4H); 3.4 (m,
1H); 3.68 (s, 3H); 5.0 (br s, 1H); 5.5 (m, 1H); 7.32 (d, 2H,
J = 8 Hz); 7.82 (d, 2H, J = 8 Hz). 13C NMR (75 MHz) d:
23.1; 23.6; 24.7; 32.1; 32.6; 34.1; 35.6; 44.4; 53.5; 61.4; 116.7;
121.6; 124.5; 128.2; 130.3; 132.2; 134.7; 135.0; 137.6; 142.7;
1
140.5; 144.6; 160.1. Compound 11b: H NMR (300 MHz,
CDCl3) d: 2.35 (s, 3H); 2.40 (s, 3H); 3.89 (s, 3H); 7.04 (s,
1H); 7.2–7.40 (m, 2H); 7.32 (d, 2H, J = 8 Hz); 7.7 (d, 1H,
J = 8.4 Hz); 7.82 (d, 2H, J = 8 Hz). 13C NMR (75 MHz) d:
24.5; 52.1; 109.7; 112.4; 119.5; 121.7; 129.6; 130.5; 131.7;
132.4; 135.9; 140.5; 145.2, 160.1. Compound 12b: 1H NMR
(300 MHz, CDCl3) d: 2.35 (s, 3H); 3.90 (s, 3H); 7.1–7.25 (m,
2H); 7.32 (d, 2H, J = 8 Hz); 7.5–7.7 (m, 3H); 7.82 (d, 2H,
J = 8 Hz). 13C NMR (75 MHz) d: 24.5; 52.1; 109.7; 112.1;
119.8; 121.3; 123.6; 128.5; 130.6; 130.8; 135.7; 142.4; 144.5;
160.1. Compound 13a: 1H NMR (300 MHz, CDCl3) d: 2.35
(s, 3H); 5.0 (br s 1H); 6.21 (d, 1H, J = 3.0 Hz); 6.63 (dd, 1H,
J = 8.1–1.1 Hz); 7.0 (d, 1H, J = 1.1 Hz); 7.25 (d, 1H,
J = 3.0 Hz); 7.32 (d, 2H, J = 8 Hz); 7.38 (d, 1H,
J = 8.1 Hz); 7.82 (d, 2H, J = 8 Hz). 13C NMR (75 MHz)
d: 24.5; 97.3; 102.6; 113.6; 118.4; 120.5; 120.8; 128.4; 130.7;
135.8; 137.7; 143.8; 150.6. Compound 13b: 1H NMR
(300 MHz, CDCl3) d: 2.35 (s, 3H); 3.90 (s, 3H); 5.12 (br
s, 1H); 6.95 (s, 1H); 7.1–7.28 (m, 2H); 7.32 (d, 2H,
J = 8 Hz); 7.80 (d, 1H, J = 8.4 Hz); 7.82 (d, 2H, J = 8 Hz).
13C NMR (75 MHz) d: 24.5; 52.6; 97.9; 110.4; 115.7; 121.6;
123.9; 130.5; 131.8; 133.7; 135.9; 143.8; 151.2.
1
171.0. Compound 7d: H NMR (300 MHz, CDCl3) d: 1.72
(s, 6H); 2.06–2.41 (m, 4H); 2.35 (s, 3H); 2.7 (m, 4H); 3.4 (m,
1H); 3.68 (s, 3H); 5.1 (br s, 1H); 5.4 (m, 1H); 7.32 (d, 2H,
J = 8 Hz); 7.82 (d, 2H, J = 8 Hz). 13C NMR (75 MHz)d:
23.1; 23.3; 24.6; 27.3; 32.5; 36.6; 40.5; 44.7; 52.4; 61.4; 116.9;
121.3; 124.5; 128.0; 130.2; 132.4; 134.8; 135.2; 137.8; 142.6;
1
171.0. Compound 8a: H NMR (300 MHz, CDCl3) d: 1.69
(s, 3H); 2.21 (s, 3H); 2.35 (s, 3H); 2.1–2.6 (m, 4H); 3.68 (s,
3H); 3.81 (m, 1H); 5.35 (br s, 1H); 6.25 (d, 1H, J = 8.1 Hz);
6.60 (dd, 1H, J = 8.1–1.5 Hz); 6.90 (d, 1H, J = 1.5 Hz); 7.32
(d, 2H, J = 8 Hz); 7.82 (d, 2H, J = 8 Hz). 13C NMR
(75 MHz) d: 23.7; 24.7; 25.4; 27.1; 37.2; 53.7; 54.8; 56.5;
114.6; 124.2; 127.5; 128.4; 130.4; 130.9; 131.1; 135; 137.7;
141.6; 142.3; 172.1. HRMS m/z 410.5227 (calcd
C23H24O4SN, 410.5223). Compound 8b: 1H NMR