Convenient synthesis of naphthopyrans using montmorillonite K-10 as heterogeneous catalyst

Article abstract of DOI:10.1007/s12039-014-0670-4

A simple and mild procedure for the synthesis of different naphthopyrans providing high yields in a short reaction time was reported. The reaction of propargylic alcohols with α- or β-naphthol and dihydroxy naphthalenes in the presence of montmorillonite K-10 was studied. This reaction afforded high yields of the corresponding naphthopyrans. In addition, a number of new phenylene and biphenylen-linked bisnaphthopyrans were synthesized providing excellent yields via the one-pot reaction of bis-propargyl alcohols with β-naphthol.

Full text of DOI:10.1007/s12039-014-0670-4

c
J. Chem. Sci. Vol. 126, No. 4, July 2014, pp. 1081–1089. ꢀ Indian Academy of Sciences.
Convenient synthesis of naphthopyrans using montmorillonite K-10
as heterogeneous catalyst
RAHMAN HOSSEINZADEHa,∗, MARYAM MOHADJERANIb,
MOHAMMAD JAVAD ARDESTANIANa, MOHAMMAD REZA NAIMI-JAMALc and
ZAHRA LASEMId
aFaculty of Chemistry, Department of Organic Chemistry, University of Mazandaran, Babolsar, Iran
bDepartment of Molecular and Cell Biology, Faculty of Basic Sciences, University of Mazandaran,
Babolsar, Iran
cDepartment of Chemistry, Iran University of Science & Technology, Naarmak, 16846-11367 Tehran, Iran
dDepartment of Chemistry, Firoozkooh Branch, Islamic Azad University, Firoozkooh, Iran
e-mail: r.hosseinzadeh@umz.ac.ir
MS received 23 December 2013; revised 14 March 2014; accepted 9 April 2014
Abstract. A simple and mild procedure for the synthesis of different naphthopyrans providing high yields
in a short reaction time was reported. The reaction of propargylic alcohols with α- or β-naphthol and dihy-
droxy naphthalenes in the presence of montmorillonite K-10 was studied. This reaction afforded high yields
of the corresponding naphthopyrans. In addition, a number of new phenylene and biphenylen-linked bisnaph-
thopyrans were synthesized providing excellent yields via the one-pot reaction of bis-propargyl alcohols with
β-naphthol.
Keywords. Naphthopyran; propargylic alcohols; naphthol; montmorillonite K-10.
conditions.9 As in other methods, multistep strate-
gies initiating from chromanones,10 via Grignard reac-
tions on benzocoumarins followed by dehydration11
and reaction of 1-bromonaphthalene-2-ol with 3-
methylbut-2-enal over organolithium12 have been
reported. In addition, cycloaddition of naphthol to α, β-
unsaturated aldehydes in refluxing pyridine13 or cat-
alyzed by ethylenediamine diacetate14 have also been
reported. However, there is still a demand for general
methods that can efficiently provide naphthopyran rings
with various substituents.
1. Introduction
Naphthopyrans, also known as benzochromenes are
widely found in nature.1 These compounds have been
shown to have a wide range of significant biological
and pharmacological properties, such as cancer chemo-
preventive activity,2 and anti-viral activity against the
hepatitis B virus.3 These compounds are also interest-
ing substances due to their photochromic properties.4
This photochromic behaviour is the result of a pho-
toinduced reversible opening of the pyran ring that
converts the colorless form (the ‘closed form’) into
a highly conjugated coloured form (the ‘open form’)
(scheme 1). Therefore, naphthopyran derivatives have
a wide variety of applications such as ophthalmic plas-
tic lenses, solar protective glasses, electronic display
systems, optical switches, and temporary or permanent
memories.5
The present study reports a very smooth procedure
for the synthesis of different naphthopyrans containing
phenyl, ferrocenyl and fluorenyl substituents by using
montmorillonite K-10 as a suitable catalyst (scheme 2).
2. Experimental
Several synthetic approaches to naphthopyrans have
been reported. These methods include the catalyzed
condensation of naphthols with propargylic alcohol by
p-toluenesulfonic acid in solid phase,6 acidic alumina
in toluene,7 p-toluenesulfonic acid in the presence of
(MeO)3CH as a dehydrating agent,8 and by indiumtri-
chloride tetrahydrate under solvent-free ball-milling
Melting points were determined with a Stuart Scien-
tific SMP1, or an Electrothermal 9100 apparatus and are
uncorrected. Infrared (IR) spectra were recorded with a
Bruker Vector 22 FT-IR spectrometer using potassium
bromide pellets. 400 MHz 1H NMR and 100 MHz 13
C
NMR spectra were recorded on a Bruker Avance 400
spectrometer and 500 MHz 1H NMR and 125 MHz 13C
NMR spectra were recorded on a Bruker Avance DRX
∗For correspondence
1081

1082
Rahman Hosseinzadeh et al.
Ar
Ar
Ar
O
O
hν1
Ar
ν
h
,
2
Closed form
(colourless)
Open form
(coloured)
Scheme 1. Photochromic reaction of naphthopyrans.
500 spectrometer. The chemical shifts are reported in 2.1a 1,1,3-Triphenylprop-2-yn-1-ol (2c): Yield: 84%;
ppm (δ-scale) relative to internal TMS and coupling M.p. 79–81◦C;17 1H NMR (400 MHz, CDCl3):
constants are reported in Hertz (Hz). CHN analyses δ 7.69–7.72 (m, 4H), 7.52–7.55 (m, 2H), 7.28–7.40
were done with a CHN Elemental Analyzer LECO 600. (m, 9H), 2.89 (s, 1H). 13C NMR (100 MHz, CDCl3):
The progress of the reactions was checked by thin-layer δ 145.00, 131.79, 128.70, 128.34, 128.33, 127.76,
chromatography (TLC). Silica gel (70–230 mesh) was 126.06, 122.41, 91.68, 87.25, 74.85.
used for column chromatography.
2.1b 1-Ferrocenyl-1,3-diphenylprop-2-yn-1-ol (2e):
Yield: 78%; M.p. 86–88◦C;18 IR (KBr): 3520, 2205,
2.1 General procedure for the preparation
1510, 1450, 1300 cm−1; 1H NMR (500 MHz, CDCl3):δ
of propargyl alcohols (2)
7.71 (d, J = 8.4 Hz, 2H), 7.60 (br d, J = 6.0 Hz, 2H),
7.31–7.42 (m, 6H), 4.55 (br s, 1H), 4.43 (br s, 1H),
To a stirred solution of an acetylenic compound
(32 mmol) in dry ether (50 mL) was gradually added a
solution of 1.7 M n-BuLi in hexane (18 mL, 30 mmol)
4.37 (s, 5H), 4.29 (br s, 1H), 4.23 (br s, 1H), 3.21 (s,
1H). 13C NMR (125 MHz, CDCl3): δ 144.50, 132.14,
128.96, 128.85, 128.50, 128.06, 126.03, 123.24, 97.70,
during 0.5 h at -10◦C under argon. Stirring was con-
85.41, 72.27, 69.42, 68.97, 68.78,65.63.
tinued for 1 h at the same temperature. The result-
ing acetylide solution was added dropwise to a stirred
2.1c 9-(2-(Trimethylsilyl)ethynyl)-9H-fluoren-9-ol (2f):
Yield: 80%; M.p. 120–122◦C;19 1H NMR (400 MHz,
CDCl3): δ 7.69 (d, J = 7.2 Hz, 2H), 7.60 (d,
J = 7.2 Hz, 2H), 7.32–7.41 (m, 4H), 2.50 (s,
1H), 0.17 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ 147.06, 139.19, 129.63, 128.55, 124.33, 120.137,
104.82, 88.34, 75.05, -0.16.
solution of a ketone (28 mmol) in dry ether (100 mL)
at −10◦C under argon. The reaction mixture was then
allowed to warm up to room temperature. After com-
pletion of the reaction as monitored by TLC, water
(50 mL) was added and the reaction mixture was
extracted with ether (3 × 30 mL). The organic layer
was dried over anhydrous sodium sulfate. After fil-
tration, the solution was concentrated under reduced
pressure and the residue was recrystallized from n- 2.1d 9-(2-Phenylethynyl)-9H-fluoren-9-ol (2h): Yield:
hexane/dichloromethane to yield the desired propargyl 85%; Oil;1H NMR (400 MHz, CDCl3): δ 7.82 (d, J
alcohol (2).
= 8.0 Hz, 2H) 7.66 (d, J = 8.0 Hz, 2H), 7.38–7.49
Ar
R
Ar'
O
OH
OH
Ar'
Montmorillonite K-10
Toluene, rt
Ar
Ar'
R
Ar
+
or
O
R
R = H, Ph, SiMe3
Ar, Ar'= Ph, 4-Chlorophenyl, Fluorenyl, Ferrocenyl
Scheme 2. Synthesis of naphthopyrans using propargylic alcohols and naph-
thols in the presence of montmorillonite K-10 as catalyst.

Convenient and mild synthesis of naphthopyrans
1083
(m, 6H), 7.28–7.33 (m, 3H), 2.94 (s, 1H). 13C NMR
(100 MHz, CDCl3): δ 147.23, 139.11, 132.00, 129.74,
128.66, 128.58, 128.24, 124.49, 122.43, 120.29, 89.10,
83.19, 75.24.
2.2b 1-(4-Chlorophenyl)-1-phenylprop-2-yn-1-ol (2b):
Yield: 75%; Oil;20 1H NMR (400 MHz, CDCl3): δ
7.60–7.63 (m, 2H), 7.55–7.58 (m, 2H), 7.31–7.39 (m,
5H), 3.11 (s, 1H), 2.91 (s, 1H). 13C NMR (100 MHz,
CDCl3): δ 144.05, 143.05, 133.75, 128.46, 128.44,
128.12, 127.51, 125.93, 85.96, 75.92, 73.85.
2.1e 1,4-Bis(2-bromo-9-hydroxy-9H-fluorene-9-yl)-
ethynyl-benzen (4): 2 eq 2-Bromofluorenone reacted
with 1 eq corresponding acetylide. Yield: 90%; M.p.
254–256◦C; 1H NMR (400 MHz, CDCl3): δ = 7.87 (d,
J = 1.6 Hz, 2H), 7.74 (dd , J = 8.0, 1.2 Hz, 2H), 7.60
(d, J = 7.2 Hz, 2H), 7.48–7.55 (m, 4H), 7.35–7.44 (m,
6H), 7.26 (s, 2H), 2.70 (s, 2H). 13C NMR (100 MHz,
CDCl3):δ 148.82, 146.72, 138.08, 138.04, 132.84,
131.79, 129.99, 129.02, 127.89, 124.42, 122.40,
122.16, 121.61, 120.34, 90.13, 83.12, 74.96. Anal.
calcd for C36H20O2Br2: C, 67.10; H, 3.13; found: C,
66.52; H, 3.05.
2.2c 1-Ferrocenyl-1-phenylprop-2-yn-1-ol (2d): Yield:
78%; M.p. 90–92◦C;18 IR (KBr): 3520, 3250, 2350,
1518, 1350, 1100 cm−1; 1H NMR (500 MHz, CDCl3):
δ 7.62 ((pseudo br s, 2H,( 7.26–7.32 (m, 3H), 4.48 (br
s, 1H), 4.18–4.32 (m, 8H), 3.19 (s, 1H), 2.81 (s, 1H).
13C NMR (125 MHz, CDCl3): δ 143.83, 128.10,127.74,
125.55, 96.67, 86.87, 71.24, 69.11, 68.55, 68.35,
65.18.
2.2d 9-Ethynyl-9H-fluoren-9-ol (2g): Yield: 83%;
M.p. 107–109◦C;21 1H NMR (400 MHz, CDCl3): δ =
7.73 (dd, J = 8.8, 0.8 Hz, 2H), 7.64 (dd, J = 8.8,
0.8 Hz, 2H), 7.44 (dt, J = 9.2, 1.6 Hz, 2H), 7.38 (dt, J
= 9.2, 1.5 Hz, 2H), 2.60 (s, 1H), 2.50 (s, 1H). 13C NMR
(100 MHz, CDCl3):δ 146.60, 139.13, 129.89, 128.66,
124.27, 120.28, 83.85, 74.59, 71.41.
2.1f 4,4’-Bis(2-bromo-9-hydroxy-9H-fluorene-9-yl)
-ethynyl-biphenyl (5): 2 eq 2-Bromofluorenone
reacted with 1 eq coresponding acetylide. Yield: 77%;
M.p. 266–268◦C; 1H NMR (400 MHz, acetone-d6): δ =
7.93 (d, J = 1.2 Hz, 2H) 7.80 (d, J = 6.8 Hz, 2H), 7.75
(d, J = 8.0 Hz, 2H), 7.55–7.67 (m, 6H), 7.42–7.51 (m,
10H), 2.86 (s, 2H). 13C NMR (100 MHz, acetone-d6):
δ = 150.72, 148.20, 140.26, 138.60, 138.22, 132.57,
132.53, 129.81, 129.13, 127.97, 127.10, 124.86,
122.29, 122.14, 121.57, 120.67, 91.17, 82.36, 74.76.
Anal. calcd for C42H24O2Br2: C, 70.02; H, 3.36; found:
C, 69.60; H, 3.28.
2.3 General procedure for the preparation
of naphthopyrans
A mixture of a propargylic alcohol (1.0 mmol), β-
naphthol (1.0 mmol), and montmorillonite K-10 (0.4 g)
in toluene (10 mL) was reacted at room temperature.
After the disappearance of the starting materials (mon-
itored by TLC), ethyl acetate (20 mL) was added and
the reaction mixture was filtered. The filtrate was then
transferred into a separatory funnel, and washed with
water (50 mL). The organic phase was dried over anhy-
drous Na2SO4 and filtered and the solvent was evap-
orated in a rotary evaporator. The crude product was
purified by passing it over a column of silica gel, using
a mixture of n-hexane and ethyl acetate as the eluent
(4:1). All known products were characterized by com-
parison of their physical and spectroscopic data with
those in literature.
2.2 General procedure for the preparation
of propargylic alcohols containing terminal acetylenic
group (2, R=H)
Trimethylsilylethynyl alcohols (23.4 mmol) were
reacted with K2CO3(19.35 g, 140.28 mmol) in MeOH
(90 mL) and THF (90 mL). After completion of
the reaction, the solvent was evaporated and water
(100 mL) and ethyl acetate (100 mL) were added to the
residue. After separation of the organic layer, the sol-
vent was evaporated to give a solid, which was crys-
tallized from n-hexane/dichloromethane to obtain the
propargylic alcohols 2.
2.3a 3,3-Diphenyl-[3H]-naphtho[2,1-b]pyran (3a):
2.2a 1, 1-Diphenylprop-2-yn-1-ol (2a): Yield: 80%; Yield: 97%; M.p. 161–163◦C;6 1H NMR (400 MHz,
Oil;20 1H NMR (400 MHz, CDCl3): δ 7.57–7.61 CDCl3): δ = 7.98 (d, J = 8.4 Hz, 1H), 7.74 (d, J =
(m, 4H), 7.30–7.34 (m, 4H), 7.23–7.29 (m, 2H), 3.02 8.0 Hz, 1H), 7.68 (d, J = 8.8 Hz, 1H), 7.47–7.53 (m,
(s, 1H), 2.87 (s, 1H). 13C NMR (100 MHz, CDCl3): 5H), 7.25–7.36 (m, 8H), 7.23 (d, J = 9.2 Hz, 1H), 6.30
δ = 144.43, 128.27, 127.82, 126.01, 86.45, 75.92, (d, J = 10.0 Hz, 1H). 13C NMR (100 MHz, CDCl3):
75.52.
δ 150.56, 144.86, 129.83, 129.35, 128.52, 128.10,

1084
Rahman Hosseinzadeh et al.
127.72, 127.53, 127.02, 126.62, 123.61, 121.33, 2.3f 1’-Trimethylsilyl-spiro[9H-fluorene-9,3’-[3’H]-
119.54, 118.36, 114.00, 82.55.
naphtho[2,1-b]pyran] (3f): Yield: 66%; M.p. 171–
173◦C; IR (KBr): 3070, 2927, 1626, 1074, 952, 844,
1
745, 697 cm−1; H NMR (400 MHz, CDCl3): δ 8.22
(d, J = 8.4 Hz, 1H), 7.84 (d, J = 8.0 Hz, 1H), 7.68
(dd, J = 10.8, 3.6 Hz, 3H), 7.54 (tt, J = 8.4, 1.4 Hz,
1H), 7.38–7.45 (m, 5H), 7.16–7.20 (m, 3H), 6.32
(s, 1H), 0.34 (s, 9H). 13C NMR (100 MHz, CDCl3):
δ 152.26, 146.36, 139.69, 137.69, 136.10, 131.08,
130.10, 129.79, 129.20, 128.62, 128.31, 125.36,
125.28, 125.02, 123.63, 120.65, 120.01, 119.25, 83.53,
1.45. Anal. calcd for C28H24SiO: C, 82.96; H, 5.97;
found: C, 82.94; H, 5.99.
2.3b 3-(4-Chlorophenyl)-3-phenyl-[3H]-naphtho
[2,1-b]pyran (3b): Yield: 85%; M.p. 157–159◦C;22
IR (KBr): 3476, 3386, 2926, 1631, 1451, 812, 759,
1
717 cm−1; H NMR (400 MHz, CDCl3): δ = 7.97 (d,
J = 8.4 Hz, 1H), 7.74 (d, J = 8.0 Hz, 1H), 7.69 (d,
J = 8.8 Hz, 1H), 7.43–7.50 (m, 5H), 7.26–7.37 (m,
7H), 7.20 (d, J = 8.8 Hz, 1H), 6.24 (d, J = 10.0 Hz,
1H). 13C NMR (100 MHz, CDCl3): δ 150.34, 144.42,
143.36, 133.47, 130.02, 129.76, 129.40, 128.55,128.52,
128.27, 128.22, 127.71, 127.21, 126.90,126.75, 123.76,
121.31, 119.96, 118.23, 114.00, 82.09.
2.3g Spiro[9H-fluorene-9,3’-[3’H]-naphtho[2,1-b]
pyran] (3g): Yield: 70%; M.p. 200–203◦C;6 1H NMR
(400 MHz, CDCl3):δ 8.11 (d, J = 8.0 Hz, 1H), 7.82
(d, J = 8.0 Hz, 1H), 7.68–7.70 (m, 3H), 7.55–7.59
(m, 3H), 7.48 (d, J = 12.0 Hz, 1H), 7.39–7.45 (m,
3H),7.24–7.28 (m, 2H), 7.07 (d, J = 8.0 Hz, 1H), 5.74
(d, J = 10.0 Hz, 1H). 13C NMR (100 MHz, CDCl3):
δ = 152.10, 147.49, 139.03, 129.93, 129.82, 129.47,
128.67, 128.37, 126.74, 125.21, 124.87, 123.66,
121.16, 120.27, 120.11, 118.40, 112.98, 85.50.
2.3c 1,3,3-Triphenyl-[3H]-naphtho[2,1-b]pyran (3c):
Yield: 98%; M.p. 210–212◦C;231H NMR (400 MHz,
CDCl3): δ 7.77 (d, J = 8.0 Hz, 1H), 7.73 (d, J =
8.0 Hz, 1H), 7.62 (d, J= 8.0 Hz, 4H), 7.41 (t, J
= 4.5 Hz, 6H), 7.30–7.36 (m, 4H), 7.22–7.27 (m,
3H), 7.16 (d, J = 6.8 Hz, 1H), 7.05–7.08 (m, 1H),
6.27 (s, 1H). 13C NMR (100 MHz, CDCl3):δ 152.56,
144.48, 141.29, 137.18, 130.85, 130.26, 129.80,
129.40, 128.46, 128.37,128.02, 127.64, 127.49, 127.10,
126.48, 125.03, 123.14, 118.75, 116.57, 82.12.
2.3h 1’-Phenyl-spiro[9H-fluorene-9,3’-[3’H]-naphtho
[2,1-b]pyran] (3h): Yield: 75%; M.p. 243–245◦C;9,24
IR (KBr): 3053, 2925, 1623, 1447, 1240, 1000, 820,
2.3d 3-(Ferrocenyl)-3-phenyl-[3H]-naphtho[2,1-b]pyran
(3d): Yield: 95%; M.p. 145–147◦C;4d 1H NMR
(400 MHz, CDCl3): δ = 7.99 (d, J = 11.2 Hz, 1H),
7.69–7.76 (m, 2H), 7.56 (d, J = 10.0 Hz, 2H), 7.36–
7.49 (m, 1H), 7.19-7.34 (m, 6H), 6.43 (d, J = 13.1 Hz,
1H), 4.41 (s, 1H), 4.26 (s, 2H), 4.16 (s, 6H). 13C NMR
(100 MHz, CDCl3): δ 151.11, 145.35, 129.87, 129.71,
129.25, 128.56, 127.73, 127.33, 127.08, 126.61,
125.98, 123.47, 121.25, 118.23, 113.58, 95.22, 80.04,
69.26, 68.86, 68.28, 66.86.
1
740 cm−1; H NMR (400 MHz, CDCl3):δ 7.83 (d, J
= 8.0 Hz, 1H), 7.79 (d, J = 8.8 Hz, 1H), 7.70 (d, J
= 6.8 Hz, 2H), 7.49 (d, J = 8.0 Hz, 2H), 7.40–7.44
(m, 2H), 7.36-7.39 (m, 3H), 7.33–7.35 (m, 2H), 7.28–
7.32 (m, 2H), 7.24 (d, J = 8.0 Hz, 1H), 7.14–7.20
(m, 3H), 5.97 (s, 1H). 13C NMR (100 MHz, CDCl3):
δ 154.58, 146.39, 141.24, 139.61, 138.22, 130.65,
130.58, 129.96, 129.79, 128.63, 128.51, 128.32,
127.90, 127.63, 126.45, 126.14, 125.29, 125.18,
123.38, 120.08, 118.97, 116.49, 84.48.
2.3e 3-(Ferrocenyl)-1,3-diphenyl-[3H]-naphtho[2,1-
2.3i 1,4-Bis(spiro[2-bromo-9H-fluorene-9,3’-[3’H]-
b]pyran (3e): Yield: 70%; M.p. 193–195◦C; IR naphtho[2,1-b] pyran-3-yl]) benzene (6): Yield: 97%;
(KBr): 3070, 2928, 2859, 1626, 952, 844, 745 cm−1; 1H M.p. 297◦C (dec); IR (KBr): 3053, 1620, 1450, 1231,
NMR (400 MHz, CDCl3): δ 7.68–7.74 (m, 5H), 7.34– 1007, 815, 744 cm−1; 1H NMR (400 MHz, CDCl3): δ =
7.42 (m, 5H), 7.23–7.27 (m, 2H), 7.17–7.20 (m, 2H), 7.84 (d, J = 8.0 Hz, 2H), 7.79 (d, J = 8.8 Hz, 2H), 7.69
6.98–7.07 (m, 2H), 6.51 (s, 1H), 4.15–4.23 (m, 9H). 13C (br s, 2H), 7.65 (d, J = 7.6 Hz, 2H), 7.50–7.56 (m, 6H),
NMR (100 MHz, CDCl3): δ 153.06, 143.67, 141.67, 7.33-7.42 (m, 12H), 7.21 (d, J = 8.8 Hz, 2H), 7.14 (tt, J
136.31, 130.62, 130.15, 129.85, 129.08, 128.58, = 7.2, 1.3 Hz, 2H), 5.93 (s, 2H). 13C NMR (100 MHz,
128.36,128.08, 127.56, 127.46, 127.38, 126.55, 126.45, CDCl3): δ = 154.23, 148.61, 145.84, 140.49, 138.77,
124.97, 123.02, 118.72, 116.44, 95.45, 79.53, 69.62, 138.35, 138.10, 133.07, 131.01, 130.67, 130.17,
68.87, 68.80, 67.79, 67.63. Anal. calcd for C35H26FeO: 129.70, 128.76, 128.69, 128.61, 128.08, 126.48,
C, 81.08; H, 5.06; found: C, 81.06; H, 5.07.
125.33, 125.25, 125.14,123.57, 121.88, 121.41,120.17,

Convenient and mild synthesis of naphthopyrans
Table 1. Optimization of catalyst and solvent.a
1085
Amount of
catalyst (g)
Yield (%)
of the 3a
Entry
Catalyst
Solvent
1
2
3
4
5
6
7
8
p-Toluenesulfonic acid
Amberlyst 15
Montmorillonite K-10
Montmorillonite KSF
Silica gel
Alumina
(NH4)10H2(W2O7)6.xH2O
H3PW12O40
Montmorillonite K-10
Montmorillonite K-10
Montmorillonite K-10
Montmorillonite K-10
Montmorillonite K-10
Montmorillonite K-10
-
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
-
-
-
-
-
-
-
-
-
60
76
80
78
Trace
Trace
Trace
30
Trace
Trace
30
70
75
97
-
85
98
9
DMSO
THF
10
11
12
13
14
15
16
17
CH3CN
CH2Cl2
n-Hexane
Toluene
Toluene
Toluene
Toluene
Montmorillonite K-10
Montmorillonite K-10
0.2
1.2
aReaction condition: 1 mmol β-naphtol (1a) and 1 mmol 1,1-diphenyl-2-propyn-1-ol
(2a) were reacted at room temperature for 35 min.
118.76, 116.00, 84.27. Anal. calcd for C56H32Br2O2: C, 7.53–7.56 (m, 4H), 7.43–7.51 (m, 2H), 7.28–7.37 (m,
75.01; H, 3.60; found: C, 75.03; H, 3.58.
6H), 7.26 (t, J = 2.4 Hz, 1H), 7.18 (d, J = 8.0 Hz, 1H),
6.76 (d, J = 9.6 Hz, 1H), 6.22 (d, J = 9.6 Hz, 1H). 13C
NMR (100 MHz, CDCl3):δ 147.74, 145.20, 134.67,
128.15, 127.65, 127.47, 127.28, 126.86, 126.29,
125.56, 124.71, 124.53, 123.86, 122.02, 120.48,
115.44, 83.19.
2.3j 4,4’-Bis(spiro[2-bromo-9H-fluorene-9,3’-[3’H]-
naphtho[2,1-b] pyran-3-yl])biphenyl (7): Yield:
95%; M.p. 249◦C (dec); IR (KBr): 3048, 1674, 1621,
1
1235, 1007, 955, 817, 748 cm−1; H NMR (400 MHz,
CDCl3): δ = 7.79-7.84 (m, 4H), 7.65–7.68 (m, 7H),7.56
(d, J = 0.8 Hz, 4H), 7.32–7.42 (m, 12H), 7.13–7.23
(m, 7H), 5.95 (s, 2H). Anal. calcd for C62H36Br2O2: C,
76.55; H, 3.73; found: C, 76.59; H, 3.70.
2.3l
3,3,9,9-Tetraphenyl-3H,9H-naphtho[2,1-b:6,5-
b’]dipyran (12): Yield: 70%; M.p. 228–230◦C;6 1H
NMR (400 MHz, CDCl3): δ 7.53–7.55 (m, 10H),
7.32–7.36 (m, 8H), 7.25–7.29 (m, 4H), 7.17 (d, J
2.3k 2,2-Diphenyl-[2H]-naphtho[2,1-b]pyran (11): = 9.6 Hz, 2H), 7.08 (d, J = 8.4 Hz, 2H), 6.10 (d,
Yield: 60%; Oil;14 1H NMR (400 MHz, CDCl3): J = 9.6 Hz, 2H). 13C NMR (100 MHz, CDCl3):
δ 8.38 (d, J = 8.4 Hz, 1H), 7.74 (d, J = 8.4 Hz, 1H), δ 152.81, 144.65, 130.96, 129.01, 128.03, 127.53,
Scheme 3. Reaction of propargylic alcohols with β-naphthol catalyzed by
montmorillonite K-10 in toluene at room temperature.

1086
Rahman Hosseinzadeh et al.
1
1000, 957 cm−1; H NMR (400 MHz, CDCl3): δ
127.10, 125.71, 124.88, 124.16, 115.90, 114.75,
82.14.
7.65–7.68 (m, 8H), 7.25–7.33 (m, 22H), 7.03-7.06
(m, 2H), 6.73–6.76 (m, 2H), 6.24 (s, 2H). 13C NMR
(100 MHz, CDCl3):δ 144.59, 143.95, 141.27, 137.01,
129.38, 128.45, 128.13, 128.05, 127.60, 127.40,
127.03, 126.82, 125.86, 122.30, 118.80, 82.08. Anal.
calcd for C52H36O2: C, 90.14; H, 5.24; found: C, 90.15;
H, 5.26.
2.3m 1,3,3,6,6,8-Hexaphenyl-3H,6H-naphtho[2,1-b:3,
4-b’]dipyran (13): Yield: 98%; M.p. 250◦C; IR
(KBr): 3049, 1955, 1615, 1398, 1260, 1191, 1116,
Table 2. Cycloaddition reactions of propargylic alcohols with β-naphthol (1)
catalyzed by montmorillonite K-10.
Entry
Propargylic alcohols
Products
Time (min)
Yield (%)a
Ph
Ph
H
HO
O
1
45
97
2a
3a
H
HO
O
Cl
Cl
2
3
120
30
85
98
2b
3b
Ph
Ph
Ph
Ph
HO
O
2c
OH
H
3c
Ph
O
Fe
Fe
Fe
4
5
30
95
70
2d
3d
OH
Ph
Ph
O
Fe
120
2e
3e
3f
SiMe3
Me3Si
HO
O
6
7
8
120
120
120
66
70
75
2f
H
HO
O
2g
3g
Ph
Ph
HO
O
2h
3h
aYields refer to isolated products.

Convenient and mild synthesis of naphthopyrans
1087
Table 3. Synthesis of some new bis-naphthopyrans by the reaction of bis-acethylenes 4 and 5 with β-naphthol catalyzed
by montmorillonite K-10.
Entry
Propargylic alcohol
Naphthopyran
Time (min)
Yield (%)a
M.p. (◦C)
297(dec.)
249(dec.)
O
Br
Br
HO
Br
O
HO
Br
4
Br
6
1
2
45
45
97
95
O
Br
OH
Br
OH
7
O
5
Br
aYields refer to isolated products.
3. Results and discussion
also repeated in 10 mL of different solvents as shown in
table 1 (entries 9-14). A dramatic increase in the yield
of 3a was observed when toluene was taken as reaction
media (table 1, entry 14). The amount of the catalyst
was also investigated as shown in table 1 (entries 14-
17). As it is shown, 0.4 g catalyst for 1.0 mmol of 2a is
sufficient to provide an excellent yield of 3a. The reac-
tion did not take place in the absence of catalyst (table 1,
entry 15).
In a model reaction, 1 mmol (0.144 g) β-naphthol (1a),
1 mmol (0.208 g) 1,1-diphenyl-2-propyn-1-ol (2a), and
0.4 g of a Lewis acid catalyst were ground together
for 10 min at room temperature. Different Lewis acids
such as p-toluenesulfonic acid, amberlyst 15, mont-
morillonite K-10, montmorillonite KSF, silica gel, alu-
mina, (NH4)10H2(W2O7)6.xH2O and tungstophosphoric
acid were investigated. The yields of the products have
According to these results, the optimal reaction
been summarized in table 1 (entries 1-8). According to conditions were chosen as 1.0 mmol of propargylic
these results, montmorillonite K-10 was chosen for this alcohols, 1.0 mmol of β-naphthol, 0.4 g montmoril-
reaction due to its ease of use, availability and low cost lonite K-10 stirred in toluene at room temperature
and high yield of the product. The model reaction was (scheme 3).
Table 4. Cycloaddition reactions of propargylic alcohols with α-naphthol and dihydroxy naphthalens catalyzed by
montmorillonite K-10.
Entry
Naphthol
Propargyl alcohol
Naphthopyran
Time (min)
Yield (%)a
M.p. (◦C)
Ph
O
Ph
OH
8
11
1
2
2a
2a
120
120
60
70
Oil
Ph
Ph
H
O
OH
O
Ph
Ph
HO
H
12
9
228-230
Ph
Ph
Ph
Ph
O
OH
OH
O
Ph
Ph
10
13
3
2c
30
98
250
aYields refer to isolated products.

1088
Rahman Hosseinzadeh et al.
and the opening of both photochromic systems.16 In
order to synthesize a number of new bi-photochromic
molecules containing conjugated spacers, bis-propargyl
alcohols 4 and 5 were synthesized and reacted with β-
naphthol in the presence of montmorillonite K-10. The
yields of new naphtopyrans 6 and 7 were excellent as
tabulated in table 3.
The reaction of propargylic alcohols with dihydrox-
ynaphthalenes 8-10 also provided high yields of the
corresponding naphthopyrans 11-13 (table 4).
Table 5. Recycling studies of reaction between β-naphtol
(1a) and 1,1-diphenyl-2-propyn-1-ol (2a) in the presence of
montmorillonite K-10 to give product 3a.
Entry
Cycle
Time (min)
Yield (%)a
1
2
3
4
fresh
45
45
45
45
97
95
92
90
1
2
3
aYields refer to isolated products.
Another interesting feature of this method is that the
catalyst can be recovered at the end of the reaction and
can be used several times without losing its activity.
To recover the catalyst, after completion of the reac-
tion, the mixture was filtered and catalyst was washed
with EtOH. After activating the catalyst at 100◦C, it was
used for further reaction. This process was repeated for
three cycles and the yield of product 3a did not change
significantly (table 5).
In order to show the merits and drawbacks of this
catalyst, some of our results were compared with other
catalysts reported in literature (table 6). As shown in
table 6, naphthopyrans 3a, 3c and 3g can be prepared
at room temperature and in a short reaction time with
excellent yields according to our procedure.
Under optimized conditions, the substrate scope was
subsequently investigated. The results have been listed
in table 2. Both terminal and internal alkynes reacted
with β-naphthol smoothly and provided the desired
products in excellent yields (table 2, entries 1-3).
It is known that the photochromic properties of the
2H-chromenes can be greatly influenced by the nature
of the substituents located in the 2-position.15 Pre-
vious works have shown that, for example, when a
ferrocenyl group replaces a phenyl group in the 2-
position, the photochromic properties are modified.4d
In order to synthesize benzochromenes substituted by
the ferrocenyl group in the 2-position, propargylic alco-
hols having ferrocenyl group (2d and 2e) were reacted
with β-naphthol under optimal conditions to yield
the corresponding products 3d and 3e in substantial
yields (table 2, entries 4-5). Propargylic alcohols bear-
ing fluorenyl moieties (2f-h) have also afforded the
4. Conclusion
desired naphthopyran products in good yields (table 2 The present study presents a practical method for
entries 6-8). the efficient cyclizations of propargylic alcohols with
Recently, some interesting results have been obtained naphthols catalyzed by montmorillonite K-10 in good
on bi-photochromic molecules with two naphthopy- to high yields. The advantages of this method
rans sharing an aromatic ring linked through the two includes mild reaction conditions, high yields, short
pyran ring sp3-C atoms. These molecules have showed reaction times, easy work-up procedures, and ease
a very significant bathochromic shift of the maximum of recovering and reusing of the catalyst, which
wavelength of absorption of the open form and high make it a useful method for the preparation of
colourabilities due to the extension of p-conjugation naphthopyrans.
Table 6. Comparison of montmorillonite K-10 with other catalysts for the synthesis of 3a, 3c and 3 g.
Entry Naphthopyran
Catalyst
Reaction Conditions
Time (h) Yield (%) Ref
1
2
3
4
5
6
7
8
9
10
3a
3a
3a
3c
3c
3c
3c
3g
3g
3g
Montmorillonite K-10
PhMe /rt
rt
ClCH2CH2Cl
PhMe /rt
Dry PhMe/N2/rt
0.75
1
-
0.5
2
16
1
2
2
2
97
56
92
98
89
97
97
70
44
50
-
6
p-TsOH/Silica gel
PPTS/(MeO)3CH
Montmorillonite K-10
TsOH
8
–
23
24
9
Pentafluorophenyl boronic acid /4 Å MS CH2Cl2/rt
InCl3.4H2O
Montmorillonite K-10
TsOH
Ball milling at 30 Hz/rt
PhMe /rt
Dry PhMe/N2/rt
Dry CH2Cl2/rt
-
23
25
p-TsOH

Convenient and mild synthesis of naphthopyrans
8. Zhao W and Carreira E M 2003 Org. Lett. 5 4153
1089
Acknowledgements
9. Dong Y W, Wang G W and Wang L 2008 Tetrahedron
64 10148
Financial support of this work from the Research Coun-
cil of University of Mazandaran is gratefully acknowl-
edged.
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Kanjia M and Rahman M 1994 Tetrahedron 50 2507
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