Multicomponent one-pot synthesis of 2-naphthol derivatives and evaluation of their anticancer activity

Article abstract of DOI:10.1007/s00044-011-9884-x

Treatment of 2-naphthol with aldehydes and allyl tributyl stannane or anisole in the presence of AlCl3 at 0°C to room temperature afforded its 1-alkyl derivatives in high yields (76-83%) within 4-8 h. The products were evaluated for their cytotoxic activi

Full text of DOI:10.1007/s00044-011-9884-x

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2012) 21:3321–3325
DOI 10.1007/s00044-011-9884-x
ORIGINAL RESEARCH
Multicomponent one-pot synthesis of 2-naphthol derivatives
and evaluation of their anticancer activity
•
Biswanath Das Cheruku Ravindra Reddy
•
•
•
Jajula Kashanna Suman Kumar Mamidyala
Chityal Ganesh Kumar
Received: 23 April 2011 / Accepted: 4 November 2011 / Published online: 16 November 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Treatment of 2-naphthol with aldehydes and
allyl tributyl stannane or anisole in the presence of AlCl3 at
0°C to room temperature afforded its 1-alkyl derivatives in
high yields (76–83%) within 4–8 h. The products were
evaluated for their cytotoxic activity against four human
cancer cell lines. The most potent compound (5d) showed
IC50 of 1.2 1.1, 1.6 1.0, 0.9 0.1, and 0.8 0.4 lM
against Hep G2, A549, MDA 231, and HeLa cell lines,
respectively, and its activity was found to be comparable to
that of doxorubicin.
et al., 1999, Khodaei et al., 2006, Selvam and Perumal,
2006, Das et al., 2007, 2009, 2010, Srihari et al., 2007,
Shaterian et al., 2008, Mahdavinia and Bigdeli, 2009, Zhao
et al., 2007). However, the multistep procedures are the
disadvantages in many of these methods. Herein, we report
a simple and efficient one-pot synthesis of different
2-naphthol derivatives. The anticancer activity of these
compounds has also been evaluated.
Chemistry
Keywords 2-Naphthol derivatives Á
Multicomponent synthesis Á AlCl3 Á Anticancer activity Á
Doxorubicin
In continuation of our work (Das et al., 2007, 2009) on the
development of useful synthetic methodologies, we have
observed that 2-naphthol (1) when treated with aldehydes
(2) and allyl tributyl stannane (3) or anisole (4) using AlCl3
as a catalyst afforded its 1-alkyl derivatives (5 or 6) at 0°C
to room temperature (Scheme 1).
Introduction
2-naphthol derivatives are valuable pharmacologic com-
pounds possessing various important biological properties
such as antiinflammatory, antibacterial, hypotensive, and
bradycardiac activities (Shen et al., 1999, Huang et al.,
2003, Chopade et al., 2010). Some of these compounds
have recently been prepared by different workers (Shen
Various 2-naphthol derivatives (5, 6) have been pre-
pared using different aldehydes (2) following the above
three-component one-pot synthetic method (Table 1).
These derivatives included 4,4-diarylbent-1-ene com-
pounds (5a–5g) and triaryl methanes (6a–6d). The former
were formed within 4–6 h, while the latter in 6–8 h. The
yields of the products were high (76–83%). The structures
of these derivatives were established from their spectral
1
(IR, H, 13C NMR, and MS) and analytical data. All the
This article is a Part 222 in the series ‘‘Studies on novel synthetic
methodologies’’.
prepared compounds were optically inactive.
The mechanism of this multicomponent synthesis
involves the initial reaction of 2-naphthol (1) with alde-
hydes (2) in the presence of AlCl3 to form the orthoquinone
methide intermediate A (Van De Water and Pettus, 2002)
which was subsequently attacked by the nucleophile, allyl
tributyl stannane (3) or anisole (4) to afford the products
(Scheme 2).
B. Das (&) Á C. R. Reddy Á J. Kashanna
Organic Chemistry Division-I, Indian Institute of Chemical
Technology, Tarnaka, Hyderabad 500 607, India
e-mail: biswanathdas@yahoo.com
S. K. Mamidyala Á C. G. Kumar
Chemical Biology Laboratory, Indian Institute of Chemical
Technology, Tarnaka, Hyderabad 500 607, India
123

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Med Chem Res (2012) 21:3321–3325
Scheme 1
AlCl3
MeCN
OH
SnBu3
RCHO
00C - r.t
2
OH
4-6 h
4
5
R
3
1
2
3
1
76-83 %
OMe
AlCl3
MeCN
OH
RCHO
00C - r.t
6-8 h
OH
R
OMe
1
2
6
4
79-82 %
It works under mild reaction conditions. The reaction was
conducted at 0°C to room temperature. The ether linkage
was not affected.
Table 1 Synthesis of 2-naphthol derivatives (5, 6)
Entry
R
Time (h)
Product
Yieldc(%)
1
C6H5
5.0
7.0
4.0
4.5
6.0
6.0
6.5
7.0
8.0
6.0
8.0
5aa
6ab
5ba
5ca
6bb
5da
5ea
5fa
79
81
83
81
82
78
77
80
79
76
79
2
C6H5
Bioactivity
3
4-Me–C6H4
4-i–Pr–C6H4
4-i–Pr–C6H4
4-F–C6H4
2-F–C6H4
3-F–C6H4
4-F–C6H4
4-Cl–C6H4
4-Cl–C6H4
4
The synthetic 2-naphthol derivatives (5, 6) were evaluated
for their in vitro cytotoxic activity against four human
cancer cell lines including liver (Hep G2), lung (A 549),
breast (MDA 231), and cervical (HeLa) by applying MTT
assay (Navarra et al., 2010). Doxorubicin was used as a
positive control and DMSO as a negative control. IC50
values were determined for each compound (Table 2;
Fig. 1). The data indicate that the 2-naphthol derivatives
containing a propylene chain at the C-1 alkyl group
(compounds 5) are generally more active compared with
those having an aromatic ring at the same position (com-
pounds 6). Moreover, among the products 5, the compound
which contains an unsubstituted phenyl ring (5a) exhibits
low activity (Table 2, entry 1). However, a compound with
a substituted phenyl ring showed higher activity (entries 3,
4, 6, 8). The most active compound, 5d (entry 6) contains a
4-fluoro phenyl ring and it possess IC50 of 1.2 1.1,
1.6 1.0, 0.9 0.1, and 0.8 0.4 lM against Hep G2,
5
6
7
8
9
6cb
5ga
6db
10
11
The structures of the products were established from their spectral
1
(IR, H, 13C NMR, and ESI–MS) data
a
2-naphthol (1 mmol), aldehyde (1 mmol), allyl tributyl stannane
(1.2 mmol), and AlCl3 (10 mol %) were used
b
2-naphthol (1 mmol), aldehyde (1 mmol), anisole (1.2 mmol), and
AlCl3 (10 mol %) were used
c
Isolated yields after purification
The catalyst, AlCl3 is easily available and less expen-
sive. It catalyses the present multicomponent synthesis
efficiently for the preparation of 2-naphthol derivatives.
SnBu3
OH
3
R
AlCl3
5
AlCl3
RCHO
OCH3
O
CH3CN
OH
4
AlCl3
R
OH
1
2
A
R
OCH3
6
Scheme 2
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Med Chem Res (2012) 21:3321–3325
3323
with aldehydes and allyl tributyl stannane or anisole in the
presence of AlCl3 at 0°C to room temperature. The mild
reaction conditions, high yields, operational simplicity, and
one-pot conversion are the advantages of the method. The
evaluation of cytotoxic property of these 2-naphthol
derivatives identified a compound having activity compa-
rable to that of doxorubicin.
Table 2 IC50 values (lM) of test compounds against different cancer
cell lines
Entry Sample
code
IC50 values (lM)a
HepG2 A549
MDA231
HeLa
1
5a
6a
5b
5c
6b
5d
5e
5f
53.0 2.1 59.0 3.2 33.0 1.4 76.0 3.6
6.2 1.6 6.1 2.4 5.8 2.5 6.3 1.7
5.6 1.5 6.8 1.2 6.0 1.4 6.2 1.5
15.0 2.1 13.5 3.2 16.5 4.5 12.5 3.2
44.0 3.5 50.0 2.1 37.8 2.8 33.0 4.6
1.2 1.1 1.6 1.0 \1 0.1 \1 0.4
9.7 1.5 7.8 1.9 6.6 1.2 6.8 2.1
6.5 0.9 8.6 1.3 9.1 1.9 8.8 1.1
2
3
4
Experimental
5
6
General
7
8
All commercially available reagents were used directly
without further purification unless otherwise stated. The
solvents used were all of AR grade and were distilled under
a positive pressure of dry nitrogen atmosphere wherever
necessary. The progress of the reactions was monitored by
analytical thin layer chromatography (TLC) performed on
Merck Silica Gel 60 F254 plates. Column chromatography
was carried out using silica gel 60–120 mesh (Qingdao
Marine Chemical, China). IR spectra were recorded on a
Perkin-Elmer RX1 FT-IR spectrophotometer and mass
spectra on VG-Autospec micromass. NMR spectra were
recorded on Gemini 200 MHz spectrometer with tetra-
methylsilane as internal standard using CDCl3. The
chemical shifts are expressed as d values in parts per
million (ppm) and the coupling constants (J) are given in
Hertz (Hz). Yields were of purified compounds and were
not optimized. Optical rotations were measured with
JASCO DIP 300 digital polarimeter at 25°C.
9
6c
5 g
6d
14.0 3.5 17.2 4.1 12.5 2.1
11 2.0
10
11
12
7.4 1.1 9.6 1.2 7.6 1.1 6.4 1.2
42.0 3.7 44.6 2.4 33.2 2.1 29.4 2.7
Doxorubicin 1.2 0.5 \1 0.1 \1 0.1 \1 0.2
No inhibition of growth was observed when 1% DMSO was used as
negative control
a
All tests were performed in duplicate (n = 2) and values are represented
as mean standard deviations
General experimental procedure
AlCl3 (10 mol %) was added to a solution of a 2-naphthol
(1 mmol), aldehyde (1 mmol), and allyl tributyl stannane
or anisole (1.2 mmol) in MeCN (5 ml) at 0°C under N2
atmosphere. The mixture was stirred at room temperature
and the reaction was monitored by TLC. After completion,
the solvent was removed from the reaction mixture, and
water and EtOAc (10 ml each) were added. The organic
layer was separated and concentrated. The residue was
subjected to column chromatography (silica gel, hexane–
EtOAc) to obtain a pure product (5 or 6).
Fig. 1 IC50 values of the test compounds
A 549, MDA 231, and HeLa cell lines, respectively. The
activity of the compound is comparable to that of doxo-
rubicin (Table 2). The regioisomers 5e (containing a
2-fluoro phenyl ring) and 5f (containing a 3-fluoro phenyl
ring) of 5a exhibited lower activity. The other three com-
pounds 5b, 5g, 6a also possess impressive cytotoxic
activity.
The spectral and analytical data of the unknown
products
1-[1-(4-isopropylphenyl)but-3-enyl]naphthalen-2-ol (5c)
Conclusions
1
IR: 3449, 1637, 1462, 1219 cm-1; H NMR (200 MHz,
In conclusion, we have developed an efficient method for
the conversion of 2-naphthol into its derivatives by reaction
CDCl3): d 8.02, (1H, d, J = 8.0 Hz, Ar–H), 7.71 (1H, d,
J = 8.0 Hz, Ar–H), 7.60 (1H, d, J = 8.0 Hz, Ar–H), 7.40
123

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Med Chem Res (2012) 21:3321–3325
(1H, t, J = 8.0 Hz, Ar–H), 7.31–7.20 (4H, m, Ar–H), 7.12
(2H, d, J = 8.0 Hz, Ar–H), 6.93 (1H, d, J = 8.0 Hz, Ar–
H), 5.69 (1H, m, H-2), 5.10–4.92 (2H, m, H2-1), 4.87 (1H,
brd, J = 8.0 Hz, H-4), 3.13 (1H, m, CHMe2), 3.01–2.81
(2H, m, H2-3), 1.22 (6H, t, J = 7.0 Hz, CHMe2); 13C NMR
(50 MHz, CDCl3): d 152.2 (Ar–C), 147.0 (Ar–C), 143.4
(Ar–C), 137.0 (Ar–C), 134.6 (C-2), 129.9 (Ar–C), 128.8
(Ar–C), 128.2 (Ar–C), 127.9 (Ar–C), 127.1 (Ar–C), 126.8
(Ar–C), 123.2 (Ar–C), 122.1 (Ar–C), 119.4 (Ar–C), 117.9
(Ar–C), 116.4 (C-1), 40.3 (C-4), 36.2 (C-3), 34.2 (CHMe2),
24.1 (CHMe2); ESIMS: m/z 317 [M ? H]?.; Anal. Calc for
C23H24O: C 87.34, H 7.60%; Found: C 87.45, H 7.58%.
Ar–H), 7.36–6.92 (4H, m), 6.88–6.78 (3H, m, Ar–H), 5.62
(1H, m, H-2), 5.12–4.96 (2H, m, H2-1), 4.83 (1H, brd,
J = 7.0 Hz, H-4), 3.17–3.02 (2H, m, H2-3); 13CNMR
(50 MHz, CDCl3): d 162.7 (d, J = 280.0 Hz, Ar–C–F),
151.2 (Ar–C), 143.8 (Ar–C), 134.1 (C-2), 131.3 (Ar–C),
131.1 (Ar–C), 129.5 (Ar–C), 128.7 (Ar–C), 126.4 (Ar–C),
124.2 (Ar–C), 124.1 (Ar–C), 123.7 (Ar–C), 122.8 (Ar–C),
121.6 (Ar–C), 118.8 (Ar–C), 118.3 (d, J = 8.0 Hz, Ar–C),
116.7 (C-1), 114.2 (d, J = 8.0 Hz, Ar–C), 41.0 (C-4), 36.7
(C-3); ESIMS: m/z 293 [M ? H]?.; Anal. Calc for
C20H17FO: C 82.19, H 5.82%; Found: C 82.31, H 5.86%.
1-[1-(4-chlorophenyl)but-3-enyl]naphthalen-2-ol (5g)
1-[1-(4-fluorophenyl)but-3-enyl]naphthalen-2-ol (5d)
1
IR: 3448, 1629, 1491, 1462, 1366, 1237 cm-1; H NMR
1
IR: 3448, 1630, 1509, 1461, 1270, 1224 cm-1; H NMR
(200 MHz, CDCl3): d 7.91 (1H, d, J = 8.0 Hz, Ar–H),
7.70 (1H, d, J = 8.0 Hz, Ar–H), 7.58 (1H, d, J = 8.0 Hz,
Ar–H), 7.40–7.04 (6H, m, Ar–H), 6.93 (1H, d, J = 8.0 Hz,
Ar–H), 5.68 (1H, m, H-2), 5.08–4.92 (2H, m, H2-1), 4.82
(1H, brd, J = 7.0 Hz, H-4), 3.18–2.98 (2H, m, H2-3); 13C
NMR (50 MHz, CDCl3): d 151.8 (Ar–C–Cl), 142.2
(Ar–C), 137.0 (Ar–C), 135.3 (Ar–C), 133.4 (C-2), 129.9,
(Ar–C), 129.1 (Ar–C), 128.2 (Ar–C), 126.4 (Ar–C), 124.2
(Ar–C), 124.0 (Ar–C), 122.8 (Ar–C), 121.7 (Ar–C), 119.1
(Ar–C), 118.9 (Ar–C), 116.2 (C-1), 40.7 (C-4), 36.2 (C-3);
ESIMS: m/z 309, 311 [M ? H]?.; Anal. Calc for
C20H17ClO: C 77.80, H 8.15%; Found: C 77.89, H 8.09%.
(200 MHz, CDCl3): d 7.91 (1H, d, J = 8.0 Hz, Ar–H),
7.70 (1H, d, J = 8.0 Hz, Ar–H), 7.58 (1H, d, J = 8.0 Hz,
Ar–H), 7.39–7.02 (4H, m, Ar–H), 6.98–6.82 (3H, m, Ar–
H), 5.69 (1H, m, H-2), 5.10–4.98 (2H, m, H2-1), 4.82 (1H,
brd, J = 7.0 Hz, H-4), 3.19–3.01 (2H, m, H2-3); 13C NMR
(50 MHz, CDCl3): d 160.2 (d, J = 280.0 Hz, Ar–C–F),
151.9 (Ar–C), 137.1 (Ar–C), 133.5 (C-2), 131.1 (Ar–C),
131.0 (Ar–C), 129.9 (Ar–C), 128.8 (Ar–C), 126.8 (Ar–C),
124.3 (Ar–C), 123.2 (Ar–C), 123.0 (Ar–C), 122.0 (Ar–C),
119.1 (Ar–C), 119.0 (d, J = 8.0 Hz, Ar–C), 116.2 (C-1),
40.5 (C-4), 36.9 (C-3); ESIMS: m/z 293 [M ? H]?.; Anal.
Calc for C20H17FO: C 82.19, H 5.82%; Found: C 82.28, H
5.78%.
1-[(4-fluorophenyl)(4-methoxyphenyl)methyl]naphthalen-
2-ol (6c)
1-[1-(2-fluorophenyl)but-3-enyl]naphthalen-2-ol (5e)
1
IR: 3468, 1626, 1603, 1508, 1463, 1248 cm-1; H NMR
IR: 3450, 1632, 1514, 1465, 1231 cm-1
;
1H NMR
(200 MHz, CDCl3): d 7.86 (1H, d, J = 8.0 Hz, Ar–H),
7.73–7.61 (2H, m, Ar–H), 7.55 (1H, d, J = 8.0 Hz, Ar–H),
7.38–7.12 (4H, m, Ar–H), 7.06 (2H, d, J = 8.0 Hz, Ar–H),
7.02–6.91 (2H, m, Ar–H), 6.80 (2H, d, J = 8.0 Hz, Ar–H),
6.27 (1H, s, Ar–H), 5.22 (1H, brs, CHAr3), 3.72 (3H, s,
(200 MHz, CDCl3): d 7.89 (1H, d, J = 8.0 Hz, Ar–H),
7.72 (1H, d, J = 8.0 Hz, Ar–H), 7.58 (1H, d, J = 8.0 Hz,
Ar–H), 7.55 (1H, t, J = 8.0 Hz, Ar–H), 7.36–6.92 (3H, m,
Ar–H), 6.89–6.80 (3H, m, Ar–H), 5.71 (1H, m, H-2), 5.17–
4.92 (2H, m, H2-1), 4.87 (1H, brd, J = 7.0 Hz, H-4), 3.20–
3.04 (2H, m, H2-3); 13C NMR (50 MHz, CDCl3): d 160.5
(d, J = 280.0 Hz, Ar–C–F), 151.7 (Ar–C), 132.8 (C-2),
131.2 (Ar–C), 131.4 (d, J = 8.0 Hz, Ar–C), 129.5 (Ar–C),
128.7 (Ar–C), 128.0 (Ar–C), 127.2 (Ar–C), 126.5 (Ar–C),
125.2 (Ar–C), 124.8 (Ar–C), 123.1 (Ar–C), 123.0 (Ar–C),
121.7 (Ar–C), 119.1 (Ar–C), 118.6 (d, J = 8.0 Hz, Ar–C),
116.2 (C-1), 40.3 (C-4), 36.5 (C-3); ESIMS: m/z 293
[M ? H]?.; Anal. Calc for C20H17FO: C 82.19, H 5.82%;
Found: C 82.12, H 5.84%.
OMe); 13C NMR (50 MHz, CDCl3):
d 156.1, (d,
J = 280.0 Hz Ar–C), 154.0 (Ar–C), 152.9 (Ar–C), 137.8
(Ar–C), 132.2 (Ar–C), 130.8 (Ar–C), 130.1 (Ar–C), 130.0
(Ar–C), 129.1 (Ar–C), 127.9 (Ar–C), 126.9 (Ar–C), 126.6
(Ar–C), 123.2 (Ar–C), 122.8 (Ar–C), 120.0 (Ar–C), 118.1
(Ar–C), 116.2, (d, J = 10.0 Hz, Ar–C), 114.7 (Ar–C), 54.9
(OMe), 47.1 (CHAr3),; ESIMS: m/z 381 [M ? Na]?.;
Anal. Calc for C24H19FO2: C 80.45, H 5.31%; Found: C
80.58, H 5.27%.
1-[(4-chlorophenyl)(4-methoxyphenyl)methyl]naphthalen-
2-ol (6d)
1-[1-(3-fluorophenyl)but-3-enyl]naphthalen-2-ol (5f)
1
IR: 3447, 1628, 1512, 1464, 1218 cm-1
;
1H NMR
IR: 3468, 1627, 1603, 1509, 1463, 1246 cm-1; H NMR
(200 MHz, CDCl3): d 7.87 (1H, d, J = 8.0 Hz, Ar–H),
7.68 (1H, d, J = 8.0 Hz, Ar–H), 7.54 (1H, d, J = 8.0 Hz,
(200 MHz, CDCl3): d 7.78 (1H, d, J = 8.0 Hz, Ar–H),
7.63 (1H, d, J = 8.0 Hz, Ar–H), 7.61 (1H, d, J = 8.0 Hz,
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Med Chem Res (2012) 21:3321–3325
3325
Ar–H), 7.30–7.12 (4H, m, Ar–H), 7.10 (2H, d, J = 8.0 Hz,
Ar–H), 7.01 (2H, d, J = 8.0 Hz, Ar–H), 6.92 (1H, d,
J = 8.0 Hz, Ar–H), 6.72 (2H, d, J = 8.0 Hz, Ar–H), 6.19
(1H, s, Ar–H), 5.09 (1H, brs, CHAr3), 3.69 (3H, s, OMe);
13C NMR (50 MHz, CDCl3): d 159.0 (Ar–C), 152.9
(Ar–C), 140.4 (Ar–C), 133.1 (Ar–C), 133.0 (Ar–C),
130.5 (Ar–C), 130.0(Ar–C), 129.9 (Ar–C), 128.9 (Ar–C),
128.0 (Ar–C), 127.1 (Ar–C), 124.7 (Ar–C), 124.4
(Ar–C), 123.5 (Ar–C), 122.9 (Ar–C), 119.9 (Ar–C), 119.3
(Ar–C), 114.9 (Ar–C), 55.2 (OMe), 47.1 (CHAr3; ESIMS:
m/z 375, 377 [M ? H]?.; Anal. Calc for C24H19ClO2: C
76.90, H 5.07%; Found: C 76.82, H 5.13%.
absorbance from that of the blanks. The IC50 values (50%
inhibitory concentration) were calculated from the plotted
absorbance data for the dose–response curves. IC50 values
(in lM) are expressed as the average of two independent
experiments.
Acknowledgments The authors thank CSIR and UGC, New Delhi
for financial assistance.
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Cell lines used for testing in vitro cytotoxicity included
HepG2 derived from human hepatocellular liver carcinoma
cells (ATCC No. HB-8065), A549 derived from human
lung adenocarcinoma epithelial cells (ATCC No. CCL-
185), MDA231 derived from human breast adenocarci-
noma cells (ATCC No. HTB-26) and HeLa derived from
human cervical cancer cells (ATCC No. CCL-2). These
cell lines were obtained from American Type Culture
Collection, Manassas, VA, USA.
All tumor cell lines were maintained in a Modified
Eagle’s medium (Sigma-Aldrich, USA) supplemented with
10% fetal bovine serum (Sigma), along with 1% non-
essential amino acids without L-glutamine (Sigma), 0.2%
sodium bicarbonate, 1% sodium pyruvate (Sigma), and 1%
of antibiotic mixture (10,000 units penicillin and 10 mg
streptomycin per ml, Sigma). The cells were washed and
resuspended in the above medium and 100 ll of this sus-
pension was seeded in 96-well flat bottom plates. The cells
were maintained at 37°C in a humidified 5% CO2 incubator
(Model 2406 Shellab CO2 incubator, Sheldon manufac-
turing, Cornelius, OR, USA). After 24 h of incubation, the
cells were treated for 2 days with eleven test compounds at
concentrations ranging from 0.1 to 100 lM in DMSO (1%
final concentration) and were assayed at the end of the
second day. Each assay was performed with two internal
controls: (1) an IC0 with cells only, (2) an IC100 with media
only. After 48 h of incubation, the cells were subjected to
MTT colorimetric assay (5 mg ml-1 MTT). The effect of
the different test compounds on the viability of tumor cell
lines was measured at the wavelength of 540 nm on a
multimode reader (InfiniteÒ M200, Tecan, Switzerland).
Dose–response curves were plotted for the test compounds
and controls after correction by subtracting the background
Zhao X-L, Chen Y-J, Liu L, Wang D (2007) Three-component Barbier
allylation-Friedel-Crafts alkylation promoted by BuPyCl/
SnCl2Á2H2O: application to the synthesis of 4-(substituted phenyl)
chromans. Chinese J Chem 25:1312
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