I2-catalyzed base-free cyclization of 3-homoallylquinoline-2- thiones: Facile synthesis of tetracyclic, furothiopyrano[2,3-b]quinolines

Article abstract of DOI:10.1016/j.tetlet.2014.06.041

I₂-catalyzed base-free reactions of 3-homoallylquinoline-2- thiones have been described for the synthesis of tetracyclic quinolines, tetrahydrofuro [2′,4′:4,6]thiopyrano[2,3-b]quinolines in excellent yields. Similarly, I₂-catalyzed reactions could proceed to tricyclic quinolines from hydroxyl group protected 3-homoallylquinoline-2-thiones. However, deprotection of group in tricyclic quinoline with HI again transformed into tetracyclic quinoline. The sulfonium salt intermediate has been proposed to explain these reactions.

Full text of DOI:10.1016/j.tetlet.2014.06.041

Tetrahedron Letters 55 (2014) 4378–4381
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
I2-catalyzed base-free cyclization of 3-homoallylquinoline-2-thiones:
facile synthesis of tetracyclic, furothiopyrano[2,3-b]quinolines
⇑
Mrityunjaya Asthana, Neha Sharma, Ritush Kumar, Jay Bahadur Singh, Radhey M. Singh
Department of Chemistry, Centre of Advanced Study, Banaras Hindu University, Varanasi 221005, U.P., India
a r t i c l e i n f o
a b s t r a c t
Article history:
I2-catalyzed base-free reactions of 3-homoallylquinoline-2-thiones have been described for the synthesis
of tetracyclic quinolines, tetrahydrofuro [20,40:4,6]thiopyrano[2,3-b]quinolines in excellent yields.
Similarly, I2-catalyzed reactions could proceed to tricyclic quinolines from hydroxyl group protected
3-homoallylquinoline-2-thiones. However, deprotection of group in tricyclic quinoline with HI again
transformed into tetracyclic quinoline. The sulfonium salt intermediate has been proposed to explain
these reactions.
Received 10 May 2014
Revised 5 June 2014
Accepted 7 June 2014
Available online 16 June 2014
Keywords:
Iodine
Ó 2014 Elsevier Ltd. All rights reserved.
Catalyzed
Tetracyclic, furothiopyrano[2,3-b]quinolines
Sulfonium
3-Homoallylquinoline-2-thiones
The quinoline scaffold is prevalent in various alkaloids.1 On the
other hand, its benzo/hetero-fused analogues have attracted much
attention to both medicinal and synthetic chemists because of
their biological, pharmaceutical, and agrochemical activities and
as synthetic building blocks.2 Thiopyran-fused quinolines have
been reported to possess important biological activities.3 For
example, 3,4-dihydro-2H-thiopyrano[2,3-b]quinoline4 and 2H-
thiopyrano[2,3-b]quinolin-2-carboxylic acid,5 have exhibited mGlu 1
receptor and antioxidant activities, respectively. Consequently,
several syntheses have been reported for this class of compounds.6
Recently, we7 and Wang and co-workers8 have reported the synthesis
of 2H-thiopyrano[2,3-b]quinolines from 2-mercaptoquinoline-
3-carbaldehydes and activated alkenes via Michael addition-
cyclization and Michael–Henry reaction routes, respectively.
The electrophilic iodocyclization of alkenes with proximal
nucleophilic groups has become one of the most popular routes
for the syntheses of various carbocycles and heterocycles.9 We
have reported iodocyclization of homoallylquinolin-2-ones to the
synthesis of tri/tetracyclic, pyrano[2,3-b]quinolines.10 In contrast,
iodocyclization of their 2-thiones analogues has not been studied
so far. Thus, in continuation of our interest in developing new
methodologies to the carbo/hetero-fused quinolines,11 we herein
describe base-free iodine-catalyzed cyclization reactions of 3-
homoallylquinolin-2-thiones 3 which gives new insights into this
reaction and, in turn, affords tetracyclic thiopyrano[2,3-b]quino-
lines 4, respectively.
The starting substrates, 3-homoallylquinoline-2-thiones 3a–l15
were prepared from 2-chloroquinoline-3-carbaldehydes 1 in two
steps via Barbier reaction10a followed by sodium sulfide10b in
DMF at room temperature (82–95%) (Scheme 1).
Initially, the substrate 3a was examined for iodocyclization
reaction with and without NaHCO3 under our previously reported
conditions for analogous substrates,10c with 2.2 equiv of iodine in
THF at rt under aerobic atmosphere. Both the reactions were com-
pleted in 5 min giving the same product 4a with 82% yields
(Table 1, entries 1–3). The structure of 4a was characterized as,
2,2a,10,11-tetrahydrofuro [20,40:4,6] thiopyrano[2,3-b]quinoline,
tetracyclic quinoline from its spectral and analytical data. The
structure of 4a was further confirmed unambiguously from X-ray
crystallography (Fig. 1).12
Iodocyclization reaction of 3a at ꢀ70 °C to isolate tricyclic, thio-
pyrano[2,3-b]quinoline
5 (Scheme 4) was also unsuccessful,
although, reaction rate was slow and afforded exclusively tetracyclic
4a with 78% yield (entry 4). The exclusive formation of 4a in a very
short period could be explained by rapid conversion of 5 into more
reactive intermediate for intramolecular nucleophilic substitution
reaction by the hydroxyl group. Further, it has been noticed that
nucleophilic substitution of 2-halomethylthiopyrans, similar to 2-
halomethylpyrrolidines,13 would proceed through sulfonium salt
intermediate. From these results, we speculated that 3a would rap-
idly provide the sulfonium salt intermediate B from intermediate 5
⇑
Corresponding author. Tel.: +91 542 670 2482; fax: +91 542 2368127.
E-mail address: rmohan@bhu.ac.in (R.M. Singh).
http://dx.doi.org/10.1016/j.tetlet.2014.06.041
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

M. Asthana et al. / Tetrahedron Letters 55 (2014) 4378–4381
4379
OH
OH
which in turn, affords the tetracylic 4a via intramolecular nucleo-
philic substitution reaction (Scheme 5).
CHO
Cl
In, DMF-H2O
rt
Na2S, DMF
rt
R2 R3
Cl
R2 R3
S
R2
Next, we carried out a series of experiments for standardization
of iodocyclization reaction conditions by varying iodine loadings,
catalysts, and different solvents. Decreasing the amount of iodine
ranging from 2.2 equiv to 0.1 equiv, the reactions proceeded
smoothly and afforded the cyclized product 4a in 80% to 85% yields
(entries 5–8, Table 1). Further, using base with catalytic iodine, the
reaction again completed in 20 min showing ineffectiveness of
base (entry 9). Further, the cyclization reaction under inert atmo-
sphere afforded only trace amount of 4a even after 24 h demon-
strating no regeneration of I2 (entry 10). Other catalysts such as
NIS, NBS, ICl, and NCS and solvents such as DCM, MeOH, benzene,
and dioxane were also examined for the reactions but none of
them led to any enhancement in the yield (Table 1, entries 11–
20). Thus, the optimal conditions for iodocyclization reaction were
found to be a combination of 1 equiv of 3a and 0.1 equiv of I2 in
THF at room temperature providing the best yield of 4a (entry 8).
Having established the optimal conditions for cyclization, we
next examined a series of substituted 3-(1-hydroxy-but-3-enyl)-
quinolin-2-thiones (3b–i) and 3-(1-hydroxy-2,2-dimethyl-but-3-
enyl)-quinolin-2-thiones (3j–l) which afforded the tetracyclic
quinolines 4b–l16 in good to excellent yields, respectively. The
results are summarized in Table 2.
R1
R1
R1
N
N
2
N
H
3
1
R3
Br
a: R1=R2=R3=H
3a 90%
3b 92%
3c 90%
3d 91%
3e 92%
3f 92%
3g 95%
3h 88%
3i 88%
R1= H, Me,OMe, Et, Cl, Br
R2= R3= H, Me
b: R1=6-Me; R2=R3=H
c: R1=7-Me; R2=R3=H
d: R1=8-Me; R2=R3=H
e: R1=6-OMe; R2=R3=H
f: R1=7-OMe; R2=R3=H
g: R1=8-Et; R2=R3=H
h: R1=7-Cl; R2=R3=H
i: R1=6-Br; R2=R3=H
j:R1=H; R2=R3=Me
3j 84%
3k 86%
3l 82%
k: R1=6-Me; R2=R3=Me
l: R1=7-Cl; R2=R3=Me
Scheme 1. Synthesis of 3-homoallyl quinoline-2-thiones 3a–l.
Table 1
Optimization of iodocyclization reaction conditions for the synthesis of 4aa
OH
OH
O
We further envisioned that the iodocyclization reaction of the
hydroxyl group protected thione 7 under the optimal reaction con-
dition could afford tricyclic quinoline 5. The starting thiones 7a–b
were prepared in 88–90% yields in two steps according to
Scheme 2. Thus, the cyclization reaction of 7a was initially exam-
ined under the optimal reaction conditions for 5 min, 40 min, and
every 4 h intervals, reactions were not completed at all and
catalyst, solvent
tempoC, time
N
H
3a
S
N
B
S
N
S
I
4a
Entry Catalyst
(equiv)
Base (equiv)
Solvent
Time (min) Yieldb (%)
1
2
3
4
5
6
7
8
I2 (2.2)
I2 (2.2)
I2 (2.2)
I2 (2.2)
I2 (1.0)
I2 (0.5)
I2 (0.25)
I2 (0.1)
I2 (0.1)
I2 (0.1)
NIS (0.1)
NBS (0.1)
ICl (0.1)
NCS (0.1)
I2 (0.1)
I2 (0.1)
I2 (0.1)
I2 (0.1)
I2 (0.1)
I2 (0.1)
NaHCO3 (2.3) THF
NaHCO3 (1.0) THF
5
5
5
8hr
5
82
82
Table 2
—
—
—
—
—
—
THF
THF
THF
THF
THF
THF
82
Iodocyclization of 3-homoallyl quinolin-2-thiones 3a for the synthesis of tetracyclic
thiopyrano[2,3-b]quinolines 4b
78c
81
5
5
80
83
85
85
OH
O
20
20
24 h
20
30
20
60
20
30
30
50
R2
R3
S
I2(0.1 equiv),
THF, rt
R2
S
R3
9
NaHCO3 (2.3) THF
6+SMd
80
N
H
R1
10
11
12
13
14
15
16
17
18
19
20
—
—
—
—
—
—
—
—
—
—
—
THF
THF
THF
THF
R1
N
3
4
78
R1= H, Me, OMe, Et, Cl, Br
R2= R3= H, Me
50
e
THF
DCM
DMF
MeOH
CH3CN
Benzene 90
Dioxane 90
80
82
80
74
65
65
O
O
O
N
4b
S
N
4c
S
N
4a
S
88%, 20 min
86%, 20 min
85%, 20 min
O
Bold values signify optimized reaction condition.
O
O
O
a
All reactions were performed using 3a (1 mmol), catalyst and 4.0 ml of solvent
at room temperature in air.
N
S
N
4e
S
N
4f
S
b
O
Isolated yields.
4d
c
reaction was performed at ꢀ70 °C.
88%, 20 min
87%, 20 min
86%, 20 min
d
reaction performed under inert atmosphere; S.M. = starting material.
e
O
O
An inseparable mixture of products was obtained.
O
Br
N
4h
S
N
4g
S
Cl
N
4i
S
85%, 20 min
90%, 20 min
82%, 20 min
O
O
O
N
S
N
4k
S
N
4j
S
Cl
O
4l
84%, 30 min
82%, 30 min
80%, 30 min
a
All reactions were performed using 3 (0.5 mmol) and I2 (0.1 equiv) in 4.0 ml of
THF at room temperature in air.
b
Isolated yields.
Figure 1. ORTEP diagram of 4a.

4380
M. Asthana et al. / Tetrahedron Letters 55 (2014) 4378–4381
OMe
S
OH
Cl
OMe
MeI, NaH
0oC, DMF
Na2S, DMF
rt
R1
N
H
R1
N
2
R1
N
6
Cl
7
a: R1=H
a: R1=H
a: R1=H, 88%
b: R1=Me
b: R1=Me
b: R1=Me, 90%
Scheme 2. Synthesis of starting material 7.
OH
S
OMe
I2 (0.1 equiv)
THF, rt
4h
+
SM
H2O
OMe
S
I
R1
N
H
N
N
S
3a
2
O
2
/
I2
1
+
I
O
H
R1
N
H
I+
H
OMe
OH
7
I
I2 (0.2 equiv)
THF, rt
40 min
N
S
I
a
4
R1
S
H
O
N
S
R1=H, 80% 8a
R1=Me, 82% 8b
H
A
I
N
S
B
I
Scheme 3. Synthesis of tricyclic thiopyrans 8.
I
HI
H
O
N
5
S
afforded the tricyclic quinoline 8a in 30%, 75%, and 80% yields
along with starting material (SM) 7a. Further, using 0.2 and
2.2 equiv of I2, the reactions were completed in 40 min and 5 min
with 80% yield (8a), respectively (Scheme 3), demonstrating I2-cat-
alyzed synthesis of tricyclic quinoline. Similarly, 8b was prepared
in 82% yield.
Scheme 5. Plausible mechanism for the formation of 4a.
quinoline 8 by protecting the hydroxyl group. The sulfonium salt
intermediate has been proposed to explain these reactions. All
reactions were completed in very short time under aerobic
conditions.
Next, the methyl group of 8a was deprotected to synthesize tri-
cyclic compound 5. However, the deprotection reaction of 8a with
HI in DMF afforded tetracyclic quinoline 4a, not tricyclic 5 which
again supports that the nucleophilic substitution reaction proceeds
through sulfonium salt intermediate to afford 4a (Scheme 4).
The plausible mechanism for the tetracyclic thiopyranoquino-
line is illustrated in Scheme 5. The electrophilic addition of I2 to
olefinic bond of substrate 3a results in the formation of iodonium
ion A. Intramolecular attack by sulfide anion, generated by Iꢀ from
thioamide, on A gave tricyclic intermediate 5, which rapidly con-
verts into sulfonium salt intermediate B by neighboring group par-
ticipation of sulfur atom on 2-iodomethyl carbon atom. Lastly,
sulfonium salt ring open by the hydroxyl group, via exo-dig, to
Acknowledgments
M.A. and J.B. thank CSIR, New Delhi for their Senior and Junior
research fellowships, respectively, and N.S. to UGC, New Delhi for
senior research fellowship. We are thankful to Dr. G. Pandey, Direc-
tor, CBMR, India for spectral help. The authors acknowledge Vik-
ram Singh and Virendra Kumar for help in solving the crystal data.
Supplementary data
14
afford tetracylic 4a. The aerobic oxidation of HI regenerates I2
to complete the reactions.
Supplementary data (spectroscopic data of selected entries
from Scheme 1 and Table 2) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.06.041.
In conclusion, we have described I2-catalyzed and base-free
facile synthesis of tetracyclic quinolines 4 in excellent yields. The
catalyzed reaction could be extended to the synthesis of tricyclic
OH
OMe
I
N
S
HI, DMF
5
100 oC
I
N
S
O
8a
N
S
4a
Scheme 4. Synthesis of tetracyclic 4a using HI.

M. Asthana et al. / Tetrahedron Letters 55 (2014) 4378–4381
4381
A.; Singh, R. M. Tetrahedron 2011, 67, 244; (g) Chandra, A.; Singh, B.; Singh, R.
M. J. Org. Chem. 2009, 74, 5664; (h) Chandra, A.; Singh, B.; Upadhyay, S.; Singh,
R. M. Tetrahedron 2008, 64, 11680; (i) Singh, B.; Chandra, A.; Upadhyay, S.;
Puerta, M. C.; Valerga, P.; Singh, R. M. Tetrahedron Lett. 2008, 49, 6423.
References and notes
1. (a) Elderfleld, R. C. In Heterocyclic Compounds; Elderfleld, R. C., Ed.; John Wiley:
New York, NY, London, 1960; Vol. 4,. Chapter 1 (b) Sekar, M.; Rajendra Prasad,
K. J. J. Nat. Prod. 1998, 61, 294; (c) Wright, C. W.; Kyereme, J. A.; Breen, G.;
Brown, J. E.; Cox, M. F.; Croft, S. L.; Gokcek, Y.; Kendrick, H.; Phillips, R. M.;
Pollet, P. L. J. Med. Chem. 2001, 44, 3187; (d) Sahu, N. S.; Pal, C.; Mandal, N. B.;
Nanerjee, S.; Raha, M.; Kundu, A. P.; Basu, A.; Ghosh, M.; Roy, K.;
Bandyopadhyay, S. Bioorg. Med. Chem. 2002, 10, 1687; (e) Bringman, G.;
Reichert, Y.; Kane, V. Tetrahedron 2004, 60, 3539; (f) Kournetsov, V. V.; Mendez,
L. Y. V.; Gomez, C. M. M. Curr. Org. Chem. 2005, 9, 141.
2. (a) Yates, F. S. In Comprehensive Heterocyclic Chemistry; Katritzky, A. R., Rees, C.
W., Eds.; Pergamon: New York, NY, 1984; Vol. 2,. Chapter 2.09 (b) Bentley, K. W.
The Isoquinoline Alkaloids; Harwood: Amsterdam, 1998; (c) Kalita, P. K.; Baruah,
B.; Bhuyan, P. J. Tetrahedron Lett. 2007, 47, 777; (d) Zhong, W.; Lin, F.; Chen, R.;
Su, W. Synthesis 2009, 2333.
3. (a) Jasinski, P.; Welsh, B.; Galvez, J.; Land, D.; Zwolak, P.; Ghandi, L.; Terai, K.;
Dudek, A. Z. Invest. New Drugs 2008, 26, 223; (b) Geies, A. A.; Bakhite, E. A.; El-
Kashef, H. S. Pharmazie 1998, 53, 686; (c) Wagner, G.; Vieweg, H.; Leistner, S.
Pharmazie 1993, 48, 576; (d) Shiraki, H.; Kozar, M. P.; Mlelndez, V.; Hudson, T.
H.; Ohrt, C.; Magill, A. J.; Lin, A. J. J. Med. Chem. 2011, 54, 131; (e) Kaur, K.; Jain,
M.; Reddy, R. P.; Jain, R. Eur. J. Org. Chem. 2010, 45, 3245; (f) Harrison, E. A., Jr.;
Rice, K. C. J. Heterocycl. Chem. 1977, 14, 909.
4. Lesage, A. S. J.; Bischoff, F. P.; Janssen, C. G. M.; Lavreysen, H. Patent WO
2003082350, 2003.
5. Naik, H. R. P.; Naik, H. S. B.; Naik, T. R. R.; Bindu, P. J.; Naika, H. R.; Aravinda, T.;
Lamani, D. S. Med. Chem. 2009, 5, 148.
6. (a) Prabhuswamy, B.; Ambekar, S. Y. Synth. Commun. 1999, 29, 3477; (b)
Mahata, P. K.; Venkatesh, C.; Syam Kumar, U. K.; Ila, H.; Junjappa, H. J. Org.
Chem. 2003, 68, 3966; (c) Wang, X.-S.; Li, Q.; Wu, J.-R.; Tu, S.-J. J. Comb. Chem.
2009, 11, 433; (d) Nandeshwarappa, B. P.; Kumar, D. B. A.; Kumaraswamy, M.
N.; Kumar, Y. S. R.; Naik, H. S. B.; Mahadevan, K. M. Phosphorus Sulfur Silicon
2006, 181, 1545; (e) Ramesh, E.; Vidhya, T. K. S.; Raghunathan, R. Tetrahedron
Lett. 2008, 49, 2810; (f) Zhong, W.; Ma, W.; Liu, Y. Tetrahedron 2011, 67, 3509.
7. Singh, B.; Chandra, A.; Asthana, M.; Singh, R. M. Tetrahedron Lett. 2012, 53,
3242.
12. Crystal data for 4a: empirical formula, C13H11NOS; formula weight 229.30;
crystal colour: colourless, block; crystal system, orthorhombic; lattice
parameters, a = 8.7137(8), b = 11.5146(13), c = 21.820(3),
a = b = c = 90, space
group P b c a; Z = 8; Dcalcd = 1.391 g/cm3; R(reflections) = 0.0469 (1951); wR2
(reflections) = 0.1256 (2518); refinement method full-matrix least-squares on
F2; Crystallographic data (excluding structure factors) for the structures in this
Letter have been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication number CCDC 951596. Copies of the data can be
obtained free of charge on an application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK fax: +44 1223336033 or e-mail: deposit@ccdc.cam.ac.uk.
13. Diaba, F.; Ricou, E.; Bonjoch, J. Org. Lett. 2007, 9, 2633.
14. Gabriele, B.; Veltri, L.; Maltese, V.; Spina, R.; Mancuso, R.; Salerno, G. Eur. J. Org.
Chem. 2011, 5626.
15. General procedure for the synthesis of 3-homoallylquinoline-2-thiones 3: Indium
metal (2.5 mmol) and allyl bromide (3 mmol) were added to a solution of
aldehydes 1 (1 mmol) in DMF (8 ml) and stirred for 8–12 h. After completion of
the reaction, the reaction mixture was quenched with a few drops of 2N HCl,
diluted with water, and extracted with ethyl acetate. The organic layer was
dried with anhydrous sodium sulfate, filtered, and concentrated in vacuum.
The products 2-chloro-3-homoallylquinolines 2 were recrystallized from ethyl
acetate-hexane. The 2-chloro-3-homoallylquinolines 2 were dissolved in DMF
and Na2S (1.5 equiv) was added to it. After completion of the reaction, acetic
acid was added into the reaction mixture and the corresponding 2-thiones 3
precipitated out which was filtered out. The solid products were pure enough
for further use.
3-(1-Hydroxy-but-3-enyl)-1H-quinoline-2-thione (3a). Yield: 207.90 mg (90%),
yellow solid; mp 149–150 °C; 1H NMR (300 MHz, CDCl3): d = 2.57–2.64 (m,
1H), 2.95–2.98 (m, 1H), 3.57 (s, 1H), 5.15–5.24 (m,3H), 5.89-5.97 (m, 1H), 7.34–
7.43 (m, 2H), 7.59 (t, J = 7.5 Hz, 1H), 7.68 (d, J = 7.8 Hz, 1H), 7.87 (s, 1H), 11.51
(s, 1H); 13C NMR (75 MHz, CDCl3): d = 39.5, 70.7, 116.0, 117.4, 122.6, 124.2,
127.6, 130.6, 132.1, 134.9, 138.2, 141.6, 179.6; HRMS calcd for C13H13NOS
[M+Na]+ 254.0616, found 254.0610.
16. General procedure for the synthesis of tetracyclic derivatives 4a–l: To a solution of
3a–l (0.5 mmol) in THF (4 ml) was added I2 (0.1 equiv). The reaction mixture
was continuously stirred at room temperature. After completion of reaction
mixture (as monitored by TLC), the reaction mixture was quenched with
saturated solution of aqueous sodium thiosulfate. The resulting solution was
extracted with EtOAc, washed with water, saturated brine, dried over Na2SO4,
and evaporated under reduced pressure. The residue was purified by column
chromatography on basic alumina using hexane/ethyl acetate (97:3) as eluent
to afford corresponding tetracyclic derivatives 4a–l.
8. Wu, L.; Wang, Y.; Song, H.; Tang, L.; Zhou, Z.; Tang, C. Adv. Synth. Catal. 2013,
355, 1053.
9. (a) French, A. N.; Bissmire, S.; Wirth, T. Chem. Soc. Rev. 2004, 33, 354; (b)
Parvatkar, P. T.; Parameswaran, P. S.; Tilve, S. G. Chem. Eur. J. 2012, 18, 5460; (c)
Cardillo, G.; Orena, M. Tetrahedron 1990, 46, 3321; (d) Frederickson, M.; Grigg,
R. Org. Prep. Proced. Int. 1997, 29, 33; (e) Mphahlele, M. J. Molecules 2009, 14,
4814; (f) Bedford, S. B.; Bell, K. E.; Bennett, F.; Hayes, C. J.; Knight, D. W.; Shaw,
D. E. J. Chem. Soc., Perkin Trans. 1 1999, 2143; (g) Bedford, S. B.; Bell, K. E.;
Fenton, G.; Hayes, C. J.; Knight, D. W.; Shaw, D. Tetrahedron Lett. 1992, 33, 6511.
10. (a) Chandra, A.; Singh, M. K.; Singh, R. M. Synth. Commun. 2007, 37, 1689; (b)
Srivastava, A.; Singh, R. M. Ind. J. Chem. 2005, 44B, 1868; (c) Singh, M. K.;
Chandra, A.; Singh, B.; Singh, R. M. Tetrahedron Lett. 2007, 48, 5987.
11. (a) Singh, R. M.; Kumar, R.; Sharma, N.; Asthana, M. Tetrahedron 2013, 69, 9443;
(b) Sharma, N.; Asthana, M.; Nandini, D.; Singh, R. P.; Singh, R. M. Tetrahedron
2013, 69, 1822; (c) Singh, R. M.; Sharma, N.; Kumar, R.; Asthana, M.; Upadhyay,
S. Tetrahedron 2012, 68, 10318; (d) Singh, R. M.; Chandra, A.; Sharma, N.; Singh,
B.; Kumar, R. Tetrahedron 2012, 68, 9206; (e) Chandra, A.; Upadhyay, S.; Singh,
B.; Sharma, N.; Singh, R. M. Tetrahedron 2011, 67, 9219; (f) Singh, B.; Chandra,
2,2a,10,11-Tetrahydrofuro[20,40:4,6]thiopyrano[2,3-b]quinoline
(4a).
Yield
97.32 mg (85%), white solid, mp 124–125 °C; 1H NMR (300 MHz, CDCl3):
d = 2.47 (d, J = 12.3 Hz, 1H), 2.62 (ddd, J = 12.3 Hz, 6.0 Hz, 1H), 3.73–3.76 (m,
1H), 4.32 (dd, J = 8.4 Hz, 4.2 Hz, 1H), 4.54 (d, J = 8.4 Hz, 1H), 5.16 (d, J = 6.0 Hz,
1H), 7.40 (t, J = 7.8 Hz, 1H), 7.62–7.68 (m, 3H), 7.86 (d, J = 8.4 Hz, 1H); 13C
(75 MHz, CDCl3) d = 33.9, 41.6, 76.2, 80.1, 125.3, 125.5, 127.4, 127.6, 129.9,
131.9, 133.5, 148.0, 157.8; HRMS calcd for C13H11NOS; [M+H]+ 230.0640;
Found: 230.0615.