Efficient one-pot access to 2,9-dihydrothiopyrano[2,3-b]indole scaffolds showing large stokes shifts

Article abstract of DOI:10.1002/cjoc.201500572

A simple, mild and efficient one-pot approach for the construction of 2-aryl-3-nitro-2,9-dihydrothiopyrano[2,3-b]indole derivatives has been realized in CH₂Cl₂ medium at ambient temperature via three-component tandem reaction of N-pr

Full text of DOI:10.1002/cjoc.201500572

FULL PAPER
DOI: 10.1002/cjoc.201500572
Efficient One-Pot Access to 2,9-Dihydrothiopyrano[2,3-b]indole
Scaffolds Showing Large Stokes Shifts
Shivendra Singh and Sampak Samanta*
Department of Chemistry, Indian Institute of Technology Indore, Indore 452017, India
A simple, mild and efficient one-pot approach for the construction of 2-aryl-3-nitro-2,9-dihydrothiopyrano-
[
2 2
2,3-b]indole derivatives has been realized in CH Cl medium at ambient temperature via three-component tandem
reaction of N-protected-2-chloro-3-formylindoles, sodium hydrosulfide and β-substituted nitroolefins/δ-substituted
nitrodienes using DABCO (10 mol%) as an organocatalyst, followed by dehydration in the presence of activated
molecular sieves (4 Å). The significant advantages of this protocol are simple operation, shorter reaction time, high
atom economy, good to high yields (73%－89%) and wider substrate scope. In addition, all the synthesized com-
−
1
pounds have shown the large positive Stokes shift values (5632－6081 cm ).
Keywords one-pot MCR, 2,9-dihydrothiopyrano[2,3-b]indole, organocatalysis, molecular sieves, large Stokes
shifts
Introduction
gard, we seemed that the dehydration step may perform
by using activated molecular sieves (MS) which have
Highly efficient synthesis of sulfur-containing poly-
cyclic indole derivatives is a subject of growing interest
in recent times because these building blocks play im-
portant roles in organic synthesis, pharmaceuticals and
material science. Owing to their importance in various
fields, a substantial amount of efforts have been devoted
[4]
been extensively used for drying the liquids and gases
[5]
as well as promoting various organic transformations.
As part of our continued interest towards the synthe-
[6]
sis of polycyclic indole derivatives, herein, we wish to
[1]
report a mild, convenient, practical and general one-pot
method for the synthesis of N-protected-2-aryl-3-nitro-
[
1,2]
for the preparations of these frameworks.
such a progress, the synthesis of dihydrothiopyrano-
Even with
2
,9-dihydrothiopyrano[2,3-b]indole scaffolds via a tan-
dem aromatic nucleophilic thiolation/thio-Michael/
Henry reaction of N-protected-2-chloro-3-formylindoles,
[
2,3-b]indole derivative has been considerably less ex-
[3]
plored.
Towards the access of these compounds
[3a]
NaSH•H O with substituted nitroolefins/nitrodienes in
2
(
Scheme 1), Zhou et al.
published organocatalytic
CH Cl at room temperature, followed by in situ dehy-
2
2
one-pot method for the synthesis of enantioenriched
version of 3-formyl-2-aryl-2,9-dihydrothiopyrano[2,3-
b]indoles from 2-mercapto-3-formylindoles (Scheme
dration using activated molecular sieves (Scheme 1c).
Experimental
1
a). In 2015, an enantioselective synthesis of 4,9-di-
hydothiopyrano[2,3-b]indole scaffolds has been realized
via an organocatalytic cascade thio-Michael-cyclization
of indoline-2-thione with 2-benzylidemalononitrile as
General
All reactions were carried out under air and moni-
tored by TLC using Merck 60 F254 pre coated silica gel
plates (0.25 mm thickness) and the products were visu-
alized by UV detection. Flash chromatography was car-
ried out with silica gel (200－300 mesh). FT-IR spectra
were recorded on a Bruker Tensor-27 spectrometer. H
NMR and C NMR spectra were recorded on a Bruker
Avance (III) 400 MHz spectrometer. Data for H NMR
are reported as a chemical shift, multiplicity (s＝singlet,
d＝doublet, q＝quartet, m＝multiplet), coupling con-
[
3b]
reported by Wang et al. (Scheme 1b).
In our effort,
we reported a first two-step synthetic method for the
synthesis of 2,9-dihydrothiopyrano[2,3-b]indole deriva-
tive through a three-component reaction between
1
N-Boc-2-chloro-3-formylindole, NaSH•H
substituted-nitrostyrene in the presence of organic base,
and the resulting product was dehydrated by MeSO Cl/
N reagent system. Therefore, it is an ample scope
2
O and aryl-
13
1
2
[
3c]
Et
3
to develop our previous methodology towards the syn-
thesis of 2,9-dihydrothiopyrano[2,3-b]indole scaffold in
a one-pot, efficient and economical manner. In this re-
13
stant J (Hz), integration, and assignment, data for
are reported as a chemical shift. High resolutions mass
C
*
E-mail: sampaks@iiti.ac.in; Tel.: 0091-731-243-8706; Fax: 0091-731-2364182
Received August 7, 2015; accepted September 9, 2015; published online October 28, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500572 or from the author.
1244
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2015, 33, 1244—1250

Efficient One-Pot Access to 2,9-Dihydrothiopyrano[2,3-b]indole Scaffolds
Scheme 1 Various techniques for the synthesis of dihydrothiopyrano[2,3-b]indoles
Zhou's work:
CHO
CHO
Ar
CHO
Organocatalyst
+
(1a)
N
R
SH
PhCO
2
H
N
R
S
Ar
Wang's work:
Ar
NC
Ar
CN
CN
Organocatalyst
mesitylene, 4 Å
+
(1b)
N
R
S
N
S
NH2
R
Our work:
CHO
Cl
NO2
Ar
NO
2
.
NaSH H
2
O
(1c)
+
N
R
DABCO (cat.)
CH Cl , MS (4 Å), r.t.
N
S
Ar
2
2
R
spectral analyses (HRMS) were carried out using ESI-
TOF-MS. Absorption spectra were recorded using Var-
ian UV-Vis spectrophotometer (model: cary 100). Emis-
sion spectra were recorded in a fluoromax-4p fluorime-
ter from Horiba Yovin (model: FM-100).
3
(100 MHz, CDCl ) δ: 149.0, 138.6, 136.8, 136.6, 134.7,
129.6, 127.6, 126.4, 126.3, 124.7, 124.3, 117.0, 115.3,
112.5, 86.9, 42.3, 28.1, 21.1; IR (KBr) ν: 3442, 2961,
2924, 1736, 1626, 1580, 1507, 1491, 1479, 1444, 1400,
−1
1370, 1355, 1315 cm ; HRMS (ESI) m/z calcd for
＋
C
23
H
9
22
N
2
O
4
S [M＋Na] : 445.1192, found 445.1230.
-(N-tert-Butoxycarbonyl)-2-(4-methoxyphenyl)-
-nitro-2,9-dihydrothiopyrano[2,3-b]indole (3ac)
Yield 81%; m.p. 150 ℃; H NMR (400 MHz, CDCl ) δ:
.51 (s, 1H), 8.00－8.02 (m, 1H), 7.63－7.66 (m, 1H),
.30－7.34 (m, 2H), 7.22－7.25 (m, 1H), 6.75－6.78 (m,
General procedure for the synthesis of 9-(N-tert-
butoxycarbonyl)-2-phenyl-3-nitro-2,9-dihydrothio-
pyrano[2,3-b]indole (3aa)
3
1
3
A mixture of N-protected-2-chloro-3-formylindole
8
7
2
(0.25 mmol), NaSH•H
2
O (0.3 mmol), β-nitrostyrenes
1
3
(
0.3 mmol), DABCO (10.0 mol%) and activated mo-
H), 5.74 (s, 1H), 3.73 (s, 3H), 1.69 (s, 9H); C NMR
) δ: 159.8, 149.0, 138.5, 136.8, 134.9,
31.7, 127.7, 127.6, 126.2, 124.7, 124.3, 116.9, 115.3,
lecular sieves (300 mg, 4 Å) was stirred in DCM (2.0
mL) at room temperature. The reaction mixture was
monitored by TLC (Thin Layer Chromatography). Upon
completion of the reaction, the product was directly pu-
rified by column chromatography over silica gel using
EtOAc/hexane as an eluent to afford the corresponding
pure product.
(
100 MHz, CDCl
3
1
114.2, 112.4, 86.9, 55.2, 42.1, 28.1; IR (KBr) ν: 3435,
2
1
981, 2836, 1732, 1625, 1609, 1581, 1510, 1479, 1443,
399, 1371, 1356, 1315 cm ; HRMS (ESI) m/z calcd
−1
＋
for C23
H
22
N
2
O
5
S [M＋Na] : 461.1142, found 461.1087.
-(N-tert-Butoxycarbonyl)-2-(2-methoxyphenyl)-
-nitro-2,9-dihydrothiopyrano[2,3-b]indole (3ad)
Yield 77%; m.p. 157 ℃; H NMR (400 MHz, CDCl ) δ:
.62 (s, 1H), 7.96 (d, J＝8.04 Hz, 1H), 7.65 (d, J＝7.04
Hz, 1H), 7.28－7.35 (m, 2H), 7.21－7.23 (m, 1H), 7.01
7.03 (m, 1H), 6.91－6.94 (m, 1H), 6.74－6.76 (m,
9
All the products were synthesized in Table 2 and
Scheme 2 by the above procedure and characterized by
3
1
3
1
their corresponding spectroscopic data (IR, H NMR
8
1
3
and C NMR, HRMS).
-(N-tert-Butoxycarbonyl)-2-phenyl-3-nitro-2,9-
dihydrothiopyrano[2,3-b]indole (3aa) Yield 82%;
9
－
1
3
1H), 6.25 (s, 1H), 3.95 (s, 3H), 1.66 (s, 9H); C NMR
(100 MHz, CDCl ) δ: 155.3, 149.0, 140.6, 136.8, 133.2,
129.6, 127.7, 127.6, 126.7, 126.6, 124.5, 124.2, 120.3,
116.9, 115.3, 111.9, 111.1, 86.7, 55.7, 36.1, 28.1; IR
(KBr) ν: 3440, 2977, 2928, 1730, 1622, 1598, 1582,
1489, 1445, 1403, 1370, 1357, 1320 cm ; HRMS (ESI)
m/z calcd for C H N O S [M＋Na] : 461.1142, found
1
m.p. 150 ℃; H NMR (400 MHz, CDCl
3
) δ: 8.54 (s,
H), 8.00－8.01 (m, 1H), 7.64－7.66 (m, 1H), 7.30－
.36 (m, 4H), 7.24－7.26 (m, 2H), 5.78 (s, 1H), 1.68 (s,
3
1
7
9
1
1
13
H); C NMR (100 MHz, CDCl
38.2, 136.5, 134.3, 128.6, 128.3, 127.3, 126.2, 126.1,
24.4, 124.0, 111.7, 115.0, 112.3, 86.7, 42.2, 27.8; IR
3
) δ: 148.7, 139.2,
−1
＋
23
22
2
5
(
1
KBr) ν: 3443, 2924, 2852, 1721, 1618, 1495, 1480,
461.1137.
−1
446, 1402, 1361, 1321 cm ; HRMS (ESI) m/z calcd
9-(N-tert-Butoxycarbonyl)-2-(4-benzyloxy-3-
＋
for C22
H
20
N
2
O
4
S [M＋Na] : 431.1036, found 431.1037.
-(N-tert-Butoxycarbonyl)-2-(4-methylphenyl)-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3ab)
Yield 80%; m.p. 160 ℃; H NMR (400 MHz, CDCl ) δ:
.52 (s, 1H), 7.98－8.00 (m, 1H), 7.63－7.65 (m, 1H),
.29－7.34 (m, 2H), 7.19－7.23 (m, 2H), 7.04－7.06 (m,
methoxyphenyl)-3-nitro-2,9-dihydrothiopyrano[2,3-
b]indole (3ae) Yield 74%; m.p. 135 ℃; H NMR
(400 MHz, CDCl ) δ: 8.51 (s, 1H), 7.99 (d, J＝7.52 Hz,
3
1
9
1
3
1H), 7.62－7.64 (m, 1H), 7.27－7.37 (m, 7H), 6.88－
6.89 (m, 1H), 6.72－6.75 (m, 1H), 6.67－6.69 (m, 1H),
8
7
2
13
5.73 (s, 1H), 5.06 (s, 2H), 3.80 (s, 3H), 1.68 (s, 9H); C
NMR (100 MHz, CDCl ) δ: 149.7, 149.0, 148.6, 138.6,
1
3
H), 5.75 (s, 1H), 2.27 (s, 3H), 1.69 (s, 9H); C NMR
3
Chin. J. Chem. 2015, 33, 1244—1250
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
1245

Singh & Samanta
FULL PAPER
1
1
8
2
1
36.8, 136.7, 134.8, 132.3, 128.5, 127.8, 127.5, 127.1,
24.7, 124.3, 118.7, 116.9, 115.3, 113.5, 112.5, 110.3,
7.0, 70.8, 56.0, 42.5, 28.1; IR (KBr) ν: 3441, 2925,
853, 1717, 1620, 1510, 1492, 1481, 1461, 1444, 1400,
dihydrothiopyrano[2,3-b]indole (3aj) Yield 89%;
1
m.p. 145 ℃; H NMR (400 MHz, CDCl
3
) δ: 8.44 (s,
1H), 8.01－8.03 (m, 1H), 7.62－7.64 (m, 1H), 7.33－
7.36 (m, 1H),7.30－7.33 (m, 2H), 6.20－6.21 (m, 1H),
−
1
13
371, 1358, 1320 cm ; HRMS (ESI) m/z calcd for
6.14－6.15 (m, 1H), 5.91 (s, 1H), 1.70 (s, 9H);
NMR (100 MHz, CDCl
C
＋
C
30
H
28
N
2
O
6
S [M＋Na] : 567.1560, found 567.1678.
-(N-tert-Butoxycarbonyl)-2-(4-chlorophenyl)-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3af)
Yield 85%; m.p. 155 ℃; H NMR (400 MHz, CDCl ) δ:
.53 (s, 1H), 8.00－8.02 (m, 1H), 7.64－7.67 (m, 1H),
.32－7.36 (m, 2H), 7.21－7.27 (m, 3H), 5.75 (s, 1H),
3
) δ: 150.4, 149.0, 143.3, 138.5,
9
136.8, 132.2, 127.6, 126.8, 124.8, 124.3, 117.1, 115.3,
112.7, 110.6, 108.2, 87.1, 36.8, 28.1; IR (KBr) ν: 3448,
2923, 2852, 1728, 1611, 1579, 1470, 1393, 1359, 1328
1
3
−1
8
7
1
1
1
18 2 5
cm ; HRMS (ESI) m/z calcd for C20H N O S [M＋
＋
Na] : 421.0829, found 421.0864.
13
.69 (s, 9H); C NMR (100 MHz, CDCl
3
) δ: 148.9,
9-(N-Ethoxycarbonyl)-2-phenyl-3-nitro-2,9-dihy-
38.2, 138.0, 136.8, 134.5, 134.3, 129.1, 127.8, 127.5,
26.6, 124.9, 124.4, 117.1, 115.4, 112.5, 87.2, 41.9, 28.1;
drothiopyrano[2,3-b]indole (3ba) Yield 80%; m.p.
1
180 ℃; H NMR (400 MHz, CDCl
3
) δ: 8.53 (s, 1H),
IR (KBr) ν: 3441, 2977, 2924, 1732, 1627, 1581, 1506,
8.00－8.02 (m, 1H), 7.64－7.66 (m, 1H), 7.34－7.38 (m,
2H), 7.30－7.32 (m, 3H ),7.23－7.24 (m, 2H), 5.80 (s,
1H), 4.53 (q, J＝7.28, 14.16 Hz, 2H), 1.50 (t, J＝7.0,
−1
1
490, 1443, 1400, 1372, 1356, 1316 cm ; HRMS (ESI)
＋
19 2 4
m/z calcd for C22H N O ClS [M＋Na] : 465.0646,
found 465.0687.
1
3
3
14.28 Hz, 3H); C NMR (100 MHz, CDCl ) δ: 150.5,
9
-(N-tert-Butoxycarbonyl)-2-(2-chlorophenyl)-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3ag)
Yield 84%; m.p. 180 ℃; H NMR (400 MHz, CDCl ) δ:
.65 (s, 1H), 7.99－8.02 (m, 1H), 7.66－7.68 (m, 1H),
146.1, 139.3, 138.2, 136.6, 134.9, 128.9, 128.6, 127.6,
126.4, 124.9, 124.5, 117.1, 115.4, 112.9, 64.9, 42.5, 14.2;
IR (KBr) ν: 3442, 2952, 2924, 1733, 1620, 1581, 1503,
1
3
−1
8
7
1
7
1480, 1445, 1399, 1377, 1346, 1318 cm ; HRMS (ESI)
＋
.44－7.46 (m, 1H), 7.33－7.37 (m, 1H), 7.28－7.32 (m,
H), 7.18－7.22 (m, 1H), 7.11－7.14 (m, 1H), 7.05－
16 2 4
m/z calcd for C20H N O S [M＋Na] : 403.0723, found
403.0733.
13
.09 (m, 1H), 6.25 (s, 1H), 1.67 (s, 9H); C NMR (100
) δ: 148.8, 139.2, 136.9, 135.7, 133.2,
31.8, 130.5, 129.6, 127.9, 127.4, 127.1, 124.8, 124.4,
16.9, 115.4, 112.0, 87.2, 39.3, 28.1; IR (KBr) ν: 3434,
981, 2930, 1726, 1622, 1581, 1572, 1510, 1496, 1467,
9-(N-tert-Butoxycarbonyl)-2-phenyl-6-chloro-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3ca)
Yield 83%; m.p. 160 ℃; H NMR (400 MHz, CDCl ) δ:
MHz, CDCl
3
1
1
1
3
8.47 (s, 1H), 8.04－8.05 (m, 1H), 7.54 (d, J＝8.28 Hz,
2
1
1H), 7.29－7.33 (m, 4H), 7.26－7.27 (m, 2H), 5.78 (s,
−1
13
444, 1397, 1365, 1354 cm ; HRMS (ESI) m/z calcd
1H), 1.68 (s, 9H); C NMR (100 MHz, CDCl
3
) δ: 148.6,
＋
for C22
H
19ClN
2
O
4
S [M ＋ K] : 481.0386, found
139.3, 138.8, 137.1, 135.0, 130.7, 129.1, 128.9, 128.7,
126.4, 126.0, 124.8, 117.6, 115.8, 112.1, 87.7, 42.4, 28.1;
IR (KBr) ν: 3441, 2925, 2854, 1737, 1628, 1507, 1491,
4
81.0370.
-(N-tert-Butoxycarbonyl)-2-(4-cyanophenyl)-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3ah)
Yield 87%; m.p. 158 ℃; H NMR (400 MHz, CDCl ) δ:
.55 (s, 1H), 7.97－8.00 (m, 1H), 7.64－7.66 (m, 1H),
.54－7.56 (m, 2H), 7.42－7.44 (m, 2H), 7.31－7.38 (m,
9
−1
1470, 1454, 1422, 1403, 1372, 1348 cm ; HRMS (ESI)
1
＋
3
2 4
m/z calcd for C22H19ClN O S [M＋Na] : 465.0646,
found 465.0712.
8
7
2
9-(N-tert-Butoxycarbonyl)-2-styryl-3-nitro-2,9-
13
H), 5.79 (s, 1H), 1.68 (s, 9H); C NMR (100 MHz,
) δ: 148.9, 144.4, 137.9, 136.8, 133.5, 132.8,
29.9, 127.2, 127.1, 125.1, 124.6, 118.2, 117.2, 115.4,
12.6, 112.5, 87.4, 42.1, 28.1; IR (KBr) ν: 3441, 2924,
853, 2228, 1711, 1620, 1581, 1504, 1479, 1444, 1397,
dihydrothiopyrano[2,3-b]indole (3ak) Yield 81%;
1
CDCl
1
3
m.p. 148 ℃; H NMR (400 MHz, CDCl
3
) δ: 8.37 (s,
1H), 8.03－8.04 (m, 1H), 7.61－7.63 (m, 1H), 7.22－
7.32 (m, 7H), 6.54 (d, J＝15.56 Hz, 1H), 6.24 (dd, J＝
1
2
1
7.28, 15.4 Hz, 1H), 5.34 (d, J＝6.8 Hz, 1H), 1.72 (s,
−1
13
370, 1358, 1321 cm ; HRMS (ESI) m/z calcd for
9H); C NMR (100 MHz, CDCl
3
) δ: 149.1, 138.4,
＋
C
23
H
9
19
N
3
O
4
S [M＋H] : 434.1169, found 434.1222.
-(N-tert-Butoxycarbonyl)-2-(4-nitrophenyl)-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3ai)
Yield 86%; m.p. 155 ℃; H NMR (400 MHz, CDCl ) δ:
.57 (s, 1H), 8.09－8.12 (m, 2H), 7.98－8.00 (m, 1H),
.65－7.67 (m, 1H), 7.48－7.50 (m, 2H), 7.31－7.39 (m,
136.8, 135.7, 133.7, 132.1, 128.5, 128.2, 127.6, 126.8,
125.9, 124.8, 124.3, 123.9, 117.1, 115.4, 112.9, 87.1,
41.2, 28.2; IR (KBr) ν: 1731, 1619, 1579, 1502, 1489,
1
−1
3
1478, 1443, 1398, 1369, 1356, 1318, 1286 cm ; HRMS
＋
8
7
2
22 2 4
(ESI) m/z calcd for C24H N O S [M＋Na] : 457.1192,
found 457.1196.
13
H), 5.84 (s, 1H), 1.68 (s, 9H); C NMR (100 MHz,
) δ: 148.9, 147.9, 146.2, 137.8, 136.8, 133.5,
27.4, 127.3, 127.2, 125.2, 124.6, 124.2, 117.2, 115.4,
12.6, 87.5, 41.9, 28.1; IR (KBr) ν: 3440, 2982, 2926,
854, 1716, 1674, 1622, 1604, 1581, 1528, 1504, 1489,
9-(N-Ethoxycarbonyl)-2-styryl-3-nitro-2,9-dihy-
CDCl
1
1
2
1
3
drothiopyrano[2,3-b]indole (3bk) Yield 83%; m.p.
1
181 ℃; H NMR (400 MHz, CDCl
3
) δ: 8.43 (s, H),
8.10－8.12 (m, 1H), 7.68－7.70 (m, 1H), 7.40－7.42 (m,
2H), 7.27－7.35 (m, 5H), 6.60 (d, J＝15.56 Hz, 1H),
6.29 (dd, J＝7.28, 15.6 Hz, 1H), 5.42 (d, J＝7.52 Hz,
1H), 4.64 (q, J＝7.0, 14.28 Hz, 2H), 1.61 (t, J＝2.76,
−1
443, 1401, 1369, 1355, 1321 cm ; HRMS (ESI) m/z
＋
calcd for C22
76.0878.
-(N-tert-Butoxycarbonyl)-2-furyl-3-nitro-2,9-
19 3 6
H N O S [M＋Na] : 476.0887, found
1
3
4
3
6.28 Hz, 3H); C NMR (100 MHz, CDCl ) δ: 150.6,
9
138.2, 136.6, 135.6, 134.0, 132.3, 128.5, 128.3, 127.7,
1246
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Chin. J. Chem. 2015, 33, 1244—1250

Efficient One-Pot Access to 2,9-Dihydrothiopyrano[2,3-b]indole Scaffolds
＋
ClS [M＋H] : 469.0983, found
1
6
1
26.8, 125.7, 124.9, 124.6, 123.7, 117.2, 115.4, 113.3,
calcd for C24
469.0995.
H
21
N
2
O
4
4.9, 41.2, 14.3; IR (KBr) ν: 1736, 1640, 1610, 1579,
−1
498, 1485, 1446, 1397, 1376, 1348, 1319, 1298 cm ;
9-(N-tert-Butoxycarbonyl)-2-(4-methoxystyryl)-6-
chloro-3-nitro-2,9-dihydrothiopyrano[2,3-b]indole
＋
HRMS (ESI) m/z calcd for C22
07.1060, found 407.1061.
-(N-tert-Butoxycarbonyl)-2-(4-methoxystyryl)-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3al)
Yield 74%; m.p. 150 ℃; H NMR (400 MHz, CDCl ) δ:
.36 (s, 1H), 8.02－8.04 (m, 1H), 7.61－7.62 (m, 1H),
18 2 4
H N O S [M＋H] :
1
4
(3cl) Yield 73%; m.p. 162 ℃; H NMR (400 MHz,
9
3
CDCl ) δ: 8.31 (s, 1H), 8.11 (s, 1H), 7.52－7.54 (m, 1H),
7.23－7.34 (m, 3H), 6.78－6.80 (m, 2H), 6.50 (d, J＝
15.32 Hz, 1H), 6.10 (dd, J＝6.76, 16.20 Hz, 1H), 5.33
1
3
13
8
7
1
(d, J＝6.28 Hz, 1H), 3.77 (s, 3H), 1.74 (s, 9H);
C
.20－7.35 (m, 4H), 6.75－6.77 (m, 2H), 6.48 (d, J＝
5.56 Hz, 1H), 6.09 (dd, J＝7.8, 15.68 Hz, 1H), 5.31 (d,
NMR (100 MHz, CDCl ) δ: 159.8, 148.6, 138.8, 137.1,
3
134.3, 131.8, 130.7, 128.3, 128.1, 126.1, 125.2, 124.8,
121.4, 117.6, 115.8, 113.9, 112.4, 87.6, 55.3, 41.4, 28.1;
IR (KBr) ν: 1738, 1621, 1605, 1510, 1490, 1467, 1422,
1
3
J＝7.52 Hz, 1H), 3.75 (s, 3H), 1.71 (s, 9H); C NMR
100 MHz, CDCl ) δ: 159.7, 149.1, 138.5, 136.8, 133.9,
31.6, 128.4, 128.1, 127.7, 125.7, 124.8, 124.3, 121.7,
17.0, 115.4, 113.9, 112.9, 87.0, 55.3, 41.4, 28.2; IR
KBr) ν: 1732, 1618, 1606, 1510, 1478, 1445, 1398,
(
1
1
(
3
−1
1401, 1371, 1344, 1296 cm ; HRMS (ESI) m/z calcd
＋
23 2 5
for C25H N O ClS [M ＋ Na] : 521.0908, found
521.0907.
−1
1
370, 1355, 1317, 1291 cm ; HRMS (ESI) m/z calcd
＋
for C25
H
24
N
2
O
5
[M＋Na] : 487.1298, found 487.1274.
-(N-Ethoxycarbonyl)-2-(4-methoxystyrene)-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3bl)
Yield 76%; m.p. 158 ℃; H NMR (400 MHz, CDCl ) δ:
.35 (s, 1H), 8.04－8.05 (m, 1H), 7.61－7.63 (m, 1H),
Results and Discussion
9
At the beginning, we chose a model reaction be-
tween compound 1a, β-nitrostyrene (2a) and NaSH•H O
2
in methanol at room temperature using DABCO as an
organocatalyst. After 24 h, a little amount (18%, Entry 1
and Table 1) of dihydrothiopyrano[2,3-b]indole was
1
3
8
7
2
1
7
.33－7.35 (m, 2H), 7.20－7.22 (m, 2H), 6.75－6.77 (m,
H), 6.48 (d, J＝15.56 Hz, 1H), 6.08 (dd, J＝7.80,
5.56 Hz, 1H), 5.32 (d, J＝7.52 Hz, 1H), 4.57 (q, J＝
.04, 14.04 Hz, 2H), 3.75 (s, 3H), 1.53 (t, J＝2.76, 6.28
1
obtained as characterized by its spectroscopic data ( H
1
3
NMR, C NMR, IR and HRMS). In order to access the
better yield of 3aa, we did the several reactions in
common organic solvents or neat conditions using easily
13
Hz, 3H); C NMR (100 MHz, CDCl
3
) δ: 159.7, 150.6,
1
1
5
1
38.3, 136.6, 134.2, 131.8, 128.3, 128.1, 127.7, 125.5,
24.9, 124.5, 121.5, 117.1, 115.4, 113.9, 113.3, 64.9,
5.2, 41.4, 14.3; IR (KBr) ν: 1734, 1638, 1606, 1579,
510, 1490, 1446, 1397, 1375, 1349, 1321, 1291 cm ;
available basic catalysts such as DABCO, DBU, Et
3
N,
Al and KF-Al at 60 ℃. The results showed that
2
O
3
2 3
O
−1
＋
a
Table 1 Optimization of reaction
HRMS (ESI) m/z calcd for C23
59.0985, found 459.0995.
-(N-tert-Butoxycarbonyl)-2-(2-methoxystyryl)-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3am)
Yield 74%; m.p. 158 ℃; H NMR (400 MHz, CDCl ) δ:
.36 (s, 1H), 8.03－8.05 (m, 1H), 7.61－7.63 (m, 1H),
20 2 5
H N O S [M＋Na] :
4
CHO
NO2
Ph
NO
2
.
2
NaSH H
O
9
+
Catalyst
Solvent, Temp.
N
Cl
N
S
Ph
2a
1
Boc
Boc
3aa
3
1
a
8
7
1
1
1
1
1
.29－7.31 (m, 3H), 7.14－7.18 (m, 1H), 6.89 (d, J＝
5.8 Hz, 1H), 6.77－6.83 (m, 2H), 6.26 (dd, J＝7.52,
5.72 Hz, 1H), 5.34 (d, J＝7.28 Hz, 1H), 3.74 (s, 3H),
.71 (s, 9H); C NMR (100 MHz, CDCl
49.0, 138.5, 136.8, 133.9, 129.3, 127.7, 127.2, 126.9,
25.8, 124.7, 124.6, 124.3, 123.9, 120.5, 117.0, 115.3,
b
Entry Catalyst
Solvent
MeOH
MeOH
EtOH
THF
T/℃
rt
t/h Yield /%
24 18
1
2
3
4
5
6
7
8
9
1
DABCO
DABCO
DABCO
DABCO
DBU
13
3
) δ: 157.0,
60
60
60
60
60
60
60
r.t.
r.t.
r.t.
24 21
24 24
24 27
113.0, 110.9, 86.9, 55.4, 41.8, 28.2; IR (KBr) ν: 1731,
1
1
619, 1598, 1579, 1488, 1464, 1444, 1398, 1371, 1355,
317, 1292 cm ; HRMS (ESI) m/z calcd for
EtOH
EtOH
Nil
24 31
−1
Et N
3
24 19
＋
C
25
H
9
24
N
2
O
5
S [M＋Na] : 487.1298, found 487.1289.
-(N-tert-Butoxycarbonyl)-2-styryl-6-chloro-3-
nitro-2,9-dihydrothiopyrano[2,3-b]indole (3ck)
Yield 79%; m.p. 160 ℃; H NMR (400 MHz, CDCl ) δ:
.30 (s, 1H), 8.08 (s, 1H), 7.50－7.52 (m, 1H), 7.19－
c
c
Al O
2
3
4
4
32
37
KF-Al O
2
3
Nil
1
3
DABCO
CHCl3
CH Cl
24 42
24 51
8
7
7
9
1
1
4
1
0
DABCO
DABCO
2
2
2
2
.32 (m, 6H), 6.52 (d, J＝15.56 Hz, 1H), 6.21 (dd, J＝
.28, 15.56 Hz, 1H), 5.33 (d, J＝7.28 Hz, 1H), 1.71 (s,
H); C NMR (100 MHz, CDCl
37.1, 135.5, 134.1, 132.3, 130.7, 128.5, 128.3, 126.8,
26.1, 125.4, 124.8, 123.6, 117.6, 115.8, 112.4, 87.7,
1.2, 28.1; IR (KBr) ν: 1732, 1618, 1490, 1470, 1423,
d
11
CH Cl
4
82
1
3
a
3
) δ: 148.6, 138.7,
Unless otherwise specified, all reactions were carried out at
room temperature with 1a (0.25 mmol), 2a (0.3 mmol) and
NaSH•H O (0.3 mmol) using 10 mol% catalysts in the specified
2
b
solvents (2.0 mL). Isolated yields after column chromatography.
−1
c
d
395, 1371, 1350, 1313, 1274 cm ; HRMS (ESI) m/z
500 mg was used. 300 mg activated molecular sieves was used.
Chin. J. Chem. 2015, 33, 1244—1250
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
1247

Singh & Samanta
FULL PAPER
in all the cases unsatisfactory yields (19%－37%, En-
tries 2－8) were obtained. However, the yields (41%－
tected-2-aryl-3-nitro-2,9-dihydrothiopyrano[2,3-b]indole
derivatives in good to high yields (74%－86% 3aa－3ai
and 3ba － 3ca) within 4 － 8 h. Interestingly, het-
eroaryl-substituted β-nitroolefin (2j) was found to be a
good thio-Michael-acceptor in our present conditions,
resulting in excellent yield (89%) of corresponding
product 3aj. Moreover, several sensitive functional
5
5%) of 3aa were improved when the reaction was car-
ried out in chlorinated solvents (CH Cl and CHCl ) at
2
2
3
room temperature for 24 h in the presence of DABCO.
Gratifyingly, high yield (82%, Entry 11) of 3aa was ob-
tained in CH Cl with short span of time (4 h) when the
2 2
reaction was performed in the presence of activated mo-
lecular sieves (4 Å) as dehydrating agent.
Having optimal reaction conditions in hand, a series
of 2-aryl-3-nitro-2,9-dihydrothiopyrano[2,3-b]indole de-
rivatives were synthesized involving the reaction of sev-
2 2
groups such as Cl, OMe, OBn, Boc, CO Et, NO , CN,
furan, etc. were unaffected under the present conditions.
In order to expand towards more substrate possibility,
we employed the chemically challenging nitrodienes as
Michael acceptors for this one-pot MCR. As is evident
from Scheme 2, the thio-Michael reaction between
2-mercapto-3-formylindoles (in situ generated from the
2
eral N-protected-2-chloro-3-formylindoles, NaSH•H O
with aryl-substituted-β-nitrostyrenes in our established
reaction conditions. The obtained results were collected
in Table 2. By this procedure, several indole derivatives
2
reaction between 1a－1c and NaSH•H O) and δ-aryl-
substituted nitrodienes (2k－2m) occurred exclusively
at the β-positions of nitrodienes in our present condi-
tions. As a result, all the reactions led to the corre-
sponding (E)-2-styryl-substituted-3-nitro-2,9-dihydro-
thiopyrano[2,3-b]indole (3ak－3cl) in good to high
(
1a－1c) annulated smoothly with aryl-substituted-β-
nitrostyrene possessing electron donating (Me, OMe,
OBn) and electron withdrawing (Cl, NO , CN) substi-
2
tuents on aryl rings to provide the corresponding N-pro-
(
73%－84%) yields as shown in Scheme 2.
Table 2 One-pot synthesis of 2-aryl-3-nitro-2,9-dihydrothio-
pyrano[2,3-b]indole derivatives (3aa−3ca)
Physical appearance of all the synthesized com-
a
pounds is red in color which prompted us to evaluate
their spectroscopic properties. In this line, we began to
examine the solvatochromic effects of compound 3ai in
various organic solvents (spectroscopic grade). The re-
sults are plotted systematically in Figure 1a. The
compound 3ai shows maximum absorption bands in the
visible region at around 460－464 nm (approximately)
in THF, ethyl acetate, ethanol and methanol. Whereas,
X
CHO
2
O N
.
NaSH H
2
O, DABCO (10 mol%)
+
o
N
R
a - 1c
Cl
R
1
MS, 4 A
CH Cl , r.t., 4 - 8 h
2 2
2
a - 2j
1
1a: R = Boc, X = H
1
b: R = CO Et, X = H
2
1c: R = Boc, X = Cl
X
NO
2
N
R
S
R1
3
aa - 3ca
1
b
Entry
R
R
X
H
H
H
H
H
H
H
H
H
H
H
Name Yield /%
1
2
3
4
5
6
7
8
9
Boc
Boc
Boc
Boc
Boc
Boc
Boc
Boc
Boc
Boc
Ph
4-MeC
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
82
80
81
77
74
85
84
87
86
89
80
83
6
H
4
4-MeOC H
6
4
2-MeOC H
6
4
3-MeO-4-BnOC H
6
3
4-ClC H
6
4
2-ClC H
6
4
4-CNC H
6
4
4-NO C H
2
6
4
1
0
2-Furyl
3aj
3ba
11
CO Et Ph
2
1
2
Boc
Ph
Cl 3ca
a
Unless otherwise specified, all the reactions were carried out at
room temperature with compound (1a－1c, 0.25 mmol), nitrosty-
renes (2a－2j, 0.3 mmol), NaSH•H O (0.3 mmol), DABCO (10
2
mol%) and molecular sieves 4 Å (300 mg) in CH Cl (2.0 mL).
Figure 1 Solvatochromic effects of compound 3ai in absorption
and emission spectra (excited at 460 nm).
2
2
b
Isolated yields after column chromatography.
248
1
www.cjc.wiley-vch.de
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2015, 33, 1244—1250

Efficient One-Pot Access to 2,9-Dihydrothiopyrano[2,3-b]indole Scaffolds
Scheme 2 One-pot synthesis of (E)-2-styryl-substituted-3-nitro-2,9-dihydrothiopyrano[2,3-b]indole scaffolds (3ak－3cl)
.
NaSH H
2
O
X
NO
2
X
CHO O N
2
DABCO (10 mol%)
+
N
R
S
DCM, 5 - 8 h, r.t.
N
R
Cl
o
R1
R1
MS (4 A)
3ak - 3cl
1
1
1
a: R = Boc, X = H
2
k - 2m
b: R = CO Et, X = H
c: R = Boc, X = Cl
2
NO2
NO2
NO2
N
R
S
N
R
S
N
S
OMe
Boc
3
ak: R = Boc; 81%
3al
OMe
:
R = Boc; 74%
3am: 74%
3bk: R = CO Et; 84%
2
2
3bl: R = CO Et; 76%
Cl
NO
2
Cl
NO
2
N
Boc
S
N
S
Boc
OMe
3
ck: 79%
3
cl: 73%
small red shifts (10－16 nm) are shown in acetonitrile
λmax 470 nm) and chloroform (λmax 476 nm) solvents.
pyrano annulated indole derivatives may find out the
potential application in material science.
[
7]
(
On the other hand, the emission maxima for 3ai are
strongly influenced by solvent polarity and exhibit ba-
thochromic shifts as well as positive Stokes shifts with
increasing polarity of solvents (Figure 1b).
Table 3 Absorption and fluorescence spectra of compounds 3aa
a
－
3cl
b
c
SS/cm−1
5950
6081
5977
5769
5866
5818
5632
5750
5877
5782
5904
5935
5844
5966
5844
5818
5869
5941
5986
Entry Product
λ
a
/nm
λ
e
/nm
SS /nm
For example, in case of non-polar aprotic solvents
such as THF, EtOAc, CHCl and CH CN, emission
1
2
3
4
5
6
7
8
9
1
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
464
456
465
469
460
461
465
463
464
458
465
458
461
460
461
461
461
460
458
641
631
644
643
630
630
630
631
638
623
641
629
631
634
631
630
632
633
631
177
175
179
174
170
169
165
168
174
165
176
171
170
174
170
169
171
173
173
3
3
bands are shown at 580 nm, 592 nm, and 604 nm re-
spectively. The emission bands of 3ai are further in-
creased to 629 nm and 638 nm when emission spectra
are recorded in high polar solvents such as EtOH and
MeOH respectively. Thus, it is clearly indicated that the
maximum Stokes shift value (5877 cm−1 or 174 nm) of
compound 3ai is observed in MeOH as compared to
other solvents (4116－5841 cm−1 or 120－169 nm) re-
corded for this experiment.
In order to evaluate the general trends of these ab-
sorption and fluorescence spectra, we recorded all the
synthesized 2,9-dihydrothiopyrano[2,3-b]indole deriva-
tives (3aa－3cl) in Table 2 and Scheme 2 in MeOH me-
dium under similar conditions. The absorption and
emission bands as well as their corresponding Stokes
shift values are summarized in Table 3. As shown in
Table 3, the UV-visible absorption maxima are shown at
0
3aj
3ak
3al
11
1
2
13
14
3am
3ba
3bk
3bl
3ca
3ck
3cl
15
16
17
18
4
56－465 nm which is almost independent nature of
substituents and their positions on aryl rings (Entries 1
19, Table 3 and Figure 2). Similarly, all the com-
pounds emit within the range at 623－641 nm, resulting
－
19
a
−1
in large positive Stokes shifts (5632－6081 cm , Table
All the compounds were recorded their UV and fluorescence
spectra in MeOH. Excited at 460 nm. SS indicates Stokes Shift.
b
c
3
). Therefore, the large Stokes shifts of 2,9-dihydrothio-
Chin. J. Chem. 2015, 33, 1244—1250
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
1249

Singh & Samanta
FULL PAPER
Acknowledgement
The authors thank the DST, Govt. of India research
grant (Project No. SB/S1/OC-19/2013). S. Singh is
thankful to her UGC fellowship.
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Conclusions
In conclusion, we have reported a one-pot direct
method for the construction of unknown functionalized
N-protected-2-aryl/styryl-substituted-3-nitro-2,9-dihy-
drothiopyrano[2,3-b]indoles in CH Cl via a tandem
aromatic nucleophilic thiolation/thio-Michael/Henry
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2
2
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NaSH•H O and substituted-nitroolefins/nitrodienes in
2
the presence of DABCO, followed by in situ dehydra-
tion using activated molecular sieves (4 Å). The current
one-pot process has several advantageous points such as
mild reaction conditions, high yielding, no toxic
by-products, broad substrate scope, construction of three
new bonds (C－S, C－C and C＝C) by single operation.
In addition, various sensitive functional groups were
tolerated in our mild conditions. Moreover, our synthe-
sized compounds have shown the high positive Stokes
2
015, 4493.
rd
[
7] (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3 edi-
tion, Springer Science, Singapore, 2006; (b) Valeur, B. Molecular
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Chen, Y. Chin. J. Chem. 2015, 33, 852.
−1
shift values (≤6081 cm ). Additional applications of
these synthesized compounds are under progress which
will be documented in due course.
(Lu, Y.)
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Chin. J. Chem. 2015, 33, 1244—1250