Thermally reversible photochromic behaviour of new naphthopyrans involving an intramolecular [2+2] cyclization reaction

Article abstract of DOI:10.1016/j.tet.2009.04.070

New 4-(2,2-diphenylethenyl)naphthopyrans were synthesized and their photochromic behaviour in solution were studied under continuous UV light irradiation conditions. Although only one coloured photoproduct was expected to be formed, due to the naphthopyra

Full text of DOI:10.1016/j.tet.2009.04.070

Tetrahedron 65 (2009) 5369–5376
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Thermally reversible photochromic behaviour of new naphthopyrans
involving an intramolecular [2þ2] cyclization reaction
,
a
b
b
Paulo J. Coelho a , Luıs M. Carvalho , Gaston Vermeersch , Stephanie Delbaere
*
´
a
´
Centro de Qu´ımica-Vila Real, Universidade de Tras-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal
b University of Lille Nord de France, CNRS UMR 8009, ‘NMR and Photochemistry’ Team, F-59006, Lille, France
a r t i c l e i n f o
a b s t r a c t
Article history:
New 4-(2,2-diphenylethenyl)naphthopyrans were synthesized and their photochromic behaviour in
solution were studied under continuous UV light irradiation conditions. Although only one coloured
photoproduct was expected to be formed, due to the naphthopyran substitution pattern, NMR analysis
on degassed UV irradiated solutions, performed at low temperature, showed the formation of different
compounds. Among them, the main product is formed through an intramolecular [2þ2] cyclization re-
action and show thermally reversible photochromic properties.
Received 27 January 2009
Received in revised form 15 April 2009
Accepted 18 April 2009
Available online 3 May 2009
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
Naphthopyran
Photochromism
NMR analysis
UV light
[2þ2] Cycloaddition
1. Introduction
process. The two photoisomers, although exhibiting similar absorp-
tion spectra, show very different thermal stabilities. While the TC
The photochromic properties of naphthopyrans have been ex-
tensively studied in the last decade due to the wide range of ap-
plications with prominence in the manufacture of ophthalmic
plastic lenses and solar protection glasses.1 Under near-UV light
irradiation these uncoloured molecules undergo a pyran-ring
opening with formation of the trans,cis isomer (TC, major product)
that, upon isomerization of the double bond, leads to the trans,-
trans isomer (TT, minor product) (Scheme 1). A photostationary
state is usually reached after several minutes of irradiation.
isomer rapidly returns tothe uncoloured closed form, the TT isomer is
thermally more stable and shows a fading rate 102 to 103 times slower
and is therefore responsible for the persistence of a residual colour for
several minutes/hours after the removal of the light source.2
In order to tune the photochromic properties of these systems
a great number of substitution patterns and/or annelations have
been investigated. Although hundreds of substituted naphthopyr-
ans have been synthesized during the last 15 years, there are very
few examples of 2H-naphtho[1,2-b]pyrans substituted in position
4.3 Such substituent is expected to cause a strong steric hindrance
that interferes with the conversion from the cis-quinoidal (TC) to
the trans-quinoidal species (TT). However, the presence of a 4-(2,2-
diphenylethenyl) substituent in the 2H-naphtho[1,2-b]pyran
structure could play an interesting role since for such molecule only
one photoisomer is expected to be obtained. After the occurrence of
the photochemical pyran-ring opening, the photoisomerization of
the double bond will not, presumably, produce different
When the light source is removed the system returns to the
original colourless state, either via a thermal or a photoinduced
Ph
Ph
Ph
O
O
Ph
O
Ph
UV
Ph
+
Δ, vis
Closed Form
Colourless
TC
TT
Ph
Coloured
Ph
Ph
Ph
Ph
2
O
UV
O
Scheme 1. Photochromic equilibrium for 2H-naphtho[1,2-b]pyrans.
Δ, vis
4
Ph
Ph
Ph
* Corresponding author. Tel.: þ351 259 350284; fax: þ351 259 350480.
E-mail address: pcoelho@utad.pt (P.J. Coelho).
Scheme 2. Expected photochromic equilibrium for 4-(2,2-diphenylethenyl)-2H-
naphtho[1,2-b]pyran.
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.04.070

5370
P.J. Coelho et al. / Tetrahedron 65 (2009) 5369–5376
Ar
Ar
OH
O
OH
O
Ar
O
a
b
Ar
O
2a,b
a Ar= Ph
4a,b
b Ar= p-F-Ph
c
Ar
O
Ar
O
Ar
Ar
O
Ar
Ar
Ar
OH
O
Ar
O
Ar
Ar
e
d
Ar
OEt
+
OH
OH
OH
Ar
4a,b
3a,b
5a,b
Scheme 3. Synthesis of 4-(2,2-diphenylethenyl)-2H-naphtho[1,2-b]pyrans 5a,b. Reagents and conditions: (a) ArCOAr 1a,b, t-BuOK, toluene, reflux; (b) H2SO4, rt; (c) Zn,
BrCH2COOEt, I2, reflux; (d) ArMgBr, THF, reflux; (e) CH3COOH, reflux.
photoisomers (Scheme 2). Moreover such photoproduct is likely to
be relatively unstable due to steric hindrance and rapidly fade to
the uncoloured form thus producing a fast bleaching photochromic
compound.
In this paper we describe the synthesis and photochromic
behaviour, under near-UV irradiation, of two new 4-(2,2-diphenyl-
ethenyl)-2H-naphtho[1,2-b]pyrans 5a and 5b. Compound 5b is the
tetrafluoro derivative of 5a in which the four fluorine atoms are in
the para position of the phenyl rings. This substitution pattern
enables the photoisomerization process to be probed by 19F NMR
spectroscopy and to increase the spectral resolution in 1H NMR by
reducing the signals overlapping in the aromatic part.
Table 1
NMR spectral data for naphthopyrans 5a,b in CDCl3
R
R
4'
5a, R=H
5b, R=F
3'
1'
2'
3
2
1
O
10
10b
9
8
4
2''
10a
6
4a
5
2'''
6a
1'''
1'''
3'''
4'''
R
3''
2''''
7
4''''
R
3''''
5a
5b
5a
5b
1H (J in Hz)
13C
2
d
d
82.9
82.3
3
4
4a
5
6
6a
7
8
5.81, d (1.6)
d
5.67, d (1.4)
d
126.9
131.6
116.9
122.4
120.4
134.6
127.6
126.4
125.6
122.3
124.6
147.9
126.3
131.9
116.4
122.0
120.8
134.8
127.7
126.7
125.9
122.1
124.6
147.6
d
d
7.50
7.39
d
7.45
7.41
7.76
7.47
7.51
8.40, d (8.0)
d
7.78, d (7.9)
7.49, dd (6.9, 7.9)
7.51, dd (6.9, 8.4)
8.33, d (8.33)
d
9
10
10a
10b
d
d
10
20
30
40
d
145.2
126.7
128.0
127.2
140.5
128.4
115.0
7.10
7.20
7.20
7.06, dd (5.3, 8.8)
6.91, dd (8.8, 8.8)
d
162.1, d (3JF–C¼249 Hz)
200
6.86, d (1.6)
d
6.80, d (1.4)
d
124.8
146.2
140.2
130.2
128.6
127.1
142.4
128.3
127.8
127.6
124.7
144.5
135.9
131.7
300
1%
2%
3%
4%
1&
2&
3&
4&
d
d
7.29
7.35
7.35
d
7.18, dd (5.4, 8.7)
7.00, dd (8.7, 8.7)
d
115.7
162.7, d (3JF–C¼250 Hz)
d
138.1
129.6
115.3
7.35
7.35
7.35
7.32, dd (5.4, 8.7)
7.05, dd (8.7, 8.7)
d
162.2, d (3JF–C¼250 Hz)

P.J. Coelho et al. / Tetrahedron 65 (2009) 5369–5376
5371
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
2. Results and discussion
2.1. Synthesis
After UV Irradiation
Before UV irradiation
Compounds 5a and 5b were prepared in three steps from 1-
hydroxy-2-acetonaphthone according to Scheme 3. The reaction of
1-hydroxy-2-acetonaphthone with benzophenone in the presence
of sodium tert-butoxide is known to give a a,b-unsaturated ketone
that cyclizes under acidic treatment to give 2,2-diphenylnaph-
thopyran-4-one 2a.4 However a very large excess of benzophenone
is used, which renders very difficult the isolation of the product. A
typical experiment uses 12 equiv of benzophenone, 9 equiv of base,
20 h of reflux and after treatment with HBr/HOAc and steam dis-
tillation of the unreacted starting materials, the naphthopyran-4-
one is isolated by column chromatography with 57% yield.5 We
have found an easier methodology using only 3 equiv of the ketone.
Thus a mixture of 1-hydroxy-2-acetonaphthone (1 equiv), diary-
lketone 1a,b (3 equiv) and t-BuOK (5 equiv) was heated under
300
350
400 450
Wavelength (nm)
500
550
600
Figure 1. UV–vis absorption spectra of naphthopyran 5a before and after UV
irradiation.
reflux for 3 h giving rise to the red coloured a,b-unsaturated ke-
tones 4a,b. Treatment of these compounds with H2SO4 for 3 h at
room temperature afforded the naphthopyran-4-ones 2a,b, which
were easily purified by column chromatography, in 56–61% yield.
The Reformatsky reaction of 2a,b with ethyl bromoacetate/Zn in
the presence of iodine gave, after hydrolysis, mainly the naph-
thopyran-4-ols 3a,b (69–70%) resulting from the addition to the
Dark
Visible light on
0.12
0.10
ketone, along with a,b-unsaturated ketones 4a,b formed by isom-
erization of the starting naphthopyran-4-ones (17–20%). The
naphthopyran-4-ols 3a,b were then treated with the Grignard re-
agent derived from bromobenzene or 1-bromo-4-fluorobenzene, in
THF, affording after hydrolysis the corresponding diols that were
dissolved in acetic acid and heated under reflux for 20 min affording
naphthopyrans 5a,b in 47–58% yield. The complete 1H and 13C as-
signments for compounds 5a,b are given in Table 1. The spectrum of
the tetrafluoro compound 5b exhibited improved resolution by re-
ducing overlapping of signals, so enabling spectral assignments and
measurements of coupling constants to be performed accurately.
0.08
0.06
0.04
0.02
UV Light On
0.00
0
5
10
15
Time (min)
20
25
30
2.2. Photochromic behaviour under continuous UV
irradiation at 20 8C
Figure 2. Colour forming and colour bleaching of naphthopyran 5b.
In toluene solution (10ꢀ4 M) naphthopyrans 5a,b are nearly col-
ourless compounds with a very strong absorption in the UV region.
Figure 3. 1H NMR spectra of 5b (A) before and (B) after 8 min of UV irradiation at 225 K (
, impurities).
*

5372
P.J. Coelho et al. / Tetrahedron 65 (2009) 5369–5376
Figure 4. 19F NMR spectra of 5b (A) before and (B) after 8 min of UV irradiation at 225 K.
Continuous UV light irradiation (150 W ozone free Xe lamp), using
a filter Schott 011FG09 (259<
T¼79%) of atoluene solution of 5a,at20 ꢁC, leadstothe developmentof
a pale yellow colouration (Fig.1) with a maximal absorption at 435 nm.
The absorbance at the maximal absorption wavelength in-
creased with time but the photostationary state was not achieved
even after 30 min of irradiation (Fig. 2). When the UV irradiation
was turned off, the absorbance decreased very slowly following
a monoexponential kinetic underlining the formation of only one
relatively stable coloured species. The return to the uncoloured
state was achieved under visible irradiation (>420 nm). The pho-
tochromic behaviour of these compounds was reproducible.
l
<388 nm with lmax¼330 nm and
2.3. NMR analysis of UV irradiated solutions
To clarify the structure of the photoisomers formed upon UV
irradiation, NMR studies of fluoro-substituted naphthopyran 5b, in
degassed toluene solutions, have been carried out before and after
UV irradiation.6 Experiments have been performed at low tem-
perature (225 K) to slow the well-known thermal evolution of open
forms.7 Irradiation with filtered UV-band (259<
l<388 nm with
lmax¼330 nm) led to the appearance of a red colouration that
rapidly changed to yellow. This colour change was visible to the
naked eye. 1H and 19F NMR spectra (Figs. 3 and 4) of the same
sample of 5b were recorded before irradiation, after 4 and 8 min of
UV irradiation, within the thermal evolution at 225 K for 30 min
and finally after two periods of heating at room temperature, to
accelerate the bleaching.
By measuring the peak-intensities (Fig. 5), five different photo-
products, labelled P1 to P5, were clearly evidenced, but only
compounds named P1 and P2 presented a reversion to the initial
compound while the three other photoproducts remained at con-
stant concentration, and, therefore were attributed to side-prod-
ucts (Scheme 4).
Compound P1 appears very unstable at 225 K and its concen-
tration is very low, 2% and 3% after 4 and 8 min of irradiation, re-
spectively. It is characterized by a doublet of doublets at 8.64 ppm
in 1H NMR and four lines at ꢀ112.06, ꢀ112.32, ꢀ112.84 and
ꢀ112.87 ppm in 19F NMR. As this structure is not thermally stable
and decreases toward 5b, in parallel with the bleaching from red to
yellow colours, it can be reasonably associated with the only pos-
sible transoid isomer of photomerocyanine, strongly hindered and
Figure 5. Time-evolution of the different photoproducts of compound 5b.

P.J. Coelho et al. / Tetrahedron 65 (2009) 5369–5376
5373
Ar
Ar
Ar
O
2
Ar= p-F-Ph
H
3
Ar
Ar
Ar
O
Ar
Ar
Ar
Ar
Ar
Ar
2''
O
O
H
UV
Δ
10
7
10b
4
9
8
+
Ar
Side-products
4a
+
Ar
+
3''
Ar
5
Ar
6
P4, P5
5b
P1
P2
P3
Scheme 4. Photochromic equilibrium for naphthopyran 5b.
Figure 6. Structure of P2 with characteristic 1H–1H correlations evidenced from 2D NMR COSY (solid arrows) and NOESY (dotted arrows), and its 1H NMR subtracted spectrum.
therefore very unstable. Moreover, the chemical shifts in 19F NMR
are in complete agreement with the area characteristic of the flu-
oro-phenyl groups attached to sp2 carbon.7,8
2 and the two fluoro-phenyl groups on carbon 300 are not equivalent
because they became diastereotopic due to the C3 stereocenter.
Therefore, the only possible structure able to justify these
observations results from an electrocyclic reaction that forms
a four-membered ring: the 1,3-diene-type structure 5b photo-
cyclises toward a cyclobutene derivative through the bond forma-
tion between C3 and C300. Because of the angle strain associated
with a four-membered ring, the cyclobutene derivative P2 can only
be produced and stabilized at low temperature and then thermally
returns back to the initial compound at room temperature.
The photoproduct P2 is stable at 225 K, but it undergoes a ther-
mal reversion toward the initial compound 5b when kept at room
temperature. In our experimental conditions, the formation of P2
appears as the major part of the reactivity of 5b, as clearly indicated
in 1H and 19F NMR spectra (Figs. 3 and 4) and in Figure 5 (29% and
43% after 4 and 8 min of irradiation, respectively). Compound P2 is
evidenced in 19F NMR by four lines at the chemical shifts ꢀ114.44,
ꢀ115.53, ꢀ115.10 and ꢀ115.75 ppm, therefore in the area charac-
teristic of fluoro-phenyl attached to sp3 carbon.7,8 Compound P2 has
been characterized from one (Fig. 6) and two-dimensional (COSY for
scalar correlations, NOESY for dipolar correlations, 1H–13C HSQC for
direct correlation and HMBC for long-range correlations) NMR
experiments (Figs. 7 and 8).
More particularly, scalar couplings have been observed between
the two doublet signals at 5.82 and 6.35 ppm, with a coupling
constant value of 4J¼3.6 Hz, and between the two doublets at 7.35
and 7.50 ppm (3J¼8.4 Hz). Besides, a dipolar correlation between
the doublet at 5.82 ppm and the doublet at 7.50 ppm enabled us to
attribute these signals to proton H200 and proton H5, respectively.
Both tertiary carbon atoms C200 and C3 have been characterized at
124.7 ppm and 98.9 ppm, respectively (Fig. 7). In the HMBC map,
long-range correlations through two, three and four bonds pro-
vided further confirmation of the assignment. Particularly relevant
were the cross-peaks between H200–C3 and H3–C200 (Fig. 8). The set
of chemical shifts in 1H and in 13C is close to the data measured with
the initial compound, indicating a relatively similar structure.
However, the main change concerns the chemical shifts of atoms in
position 3. The observed values can be associated to the change in
the hybridization of the carbon C3 from sp2 to sp3, and the
deshielding of H3 can be explained by the electronic environment
of the four phenyl groups. The two fluoro-phenyl groups on carbon
Concerning the three remaining photoproducts (P3, P4 and P5),
they showed no reversion toward the initial naphthopyran;
therefore they were attributed to side-products. The photoproduct
P3 (9% and 22% after 4 and 8 min of irradiation, respectively) was
identified to a furan derivative, resulting from a ring contraction of
the pyranic structure 5b (Scheme 4). Recently, Gabbutt et al. have
reported a similar behaviour with the ring contraction during the
6p
-electrocyclisation of naphthopyrans.9 In P3, the carbon atom in
position 2 is attributed to a signal at 45.1 ppm and correlated to H2
at 5.75 ppm, in complete agreement with NMR data described in
Ref. 9. As for the position 200, the carbon atom is highly deshielded at
142.7 ppm and is attached to H at 7.12 ppm. The set of data deduced
for P3 is gathered in Table 2. As for P4 and P5, they were not un-
ambiguously identified due to their small concentrations and
overlapping signals.
3. Conclusion
Continuous UV light irradiation of a toluene solution of naph-
thopyran 5b at low temperature (225 K) led to the formation of
different photoproducts: the highly unstable coloured photo-
merocyanine P1 that rapidly fades to the uncoloured closed form
5b, a photochromic cyclobutene derivative P2 resulting from [2þ2]
intramolecular cyclization that upon heating to room temperature

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P.J. Coelho et al. / Tetrahedron 65 (2009) 5369–5376
Figure 7. 2D NMR 1H–13C direct correlations (HSQC experiment).
Figure 8. 2D NMR 1H–13C long-range correlations (HMBC experiment).
fades to 5b and side-products, one of which, P3, was identified as
a furan derivative.
the monitoring beam using an optical fiber system (77654 Oriel In-
struments); 40 W mꢀ2 light flux was used (Goldilux Photometer with
UV-A probe). Visible irradiation experiments were performed using
a long-passfilter, Schott GG 420 (Oriel 59480). Athermostated (20 ꢁC)
10 mm quartz cell (3.5 mL sample solution, 1.0ꢂ10ꢀ4 M) equipped
with magnetic stirring was used. In a preliminary experiment, the
UV–vis absorption spectra of the closed and open forms and the lmax
of the open form were determined. In a second experiment the ab-
sorbance at photostationary equilibrium, Aeq, was measured at lmax
and then the decrease in the absorbance versus time was monitored.
4. Experimental
4.1. General methods
The reactions were monitored by thin-layer chromatography on
aluminium plates precoated with Merck silica gel 60 F254 (0.25 mm).
Column chromatography (CC) was performed on silica gel 60 (70–
230 mesh). The new compounds were determined to be >95% pure
by 1H NMR spectroscopy. The 1H and 13C NMR spectrawere recorded
at 298 K in CDCl3 using a Bruker ARX400 spectrometer (at 400.13
Table 2
NMR spectral data for photoproducts P2 and P3
and 100.62 MHz). Chemical shifts (d) are reported in parts per mil-
P2
P3
lion and coupling constants (J) in hertz. UV–vis spectra were recor-
ded on a CARY 50 Varian spectrophotometer. IR spectra were
obtained on a Perkin–Elmer FTIR 1600 spectrometer using KBr disks
(wavenumbers in cmꢀ1). Electronic impact mass spectra were
measured with an AutoSpecE spectrometer.
1H
13C
71.7
1H
13C
2
3
4
4a
5
6
7
8
9
10
10b
200
300
d
5.75
d
45.1
?
6.35
d
98.9
143.8
116.1
120.0
122.0
128.4
127.8
126.0
121.9
163.8
124.7
74.6
d
122.1
122.8
118.2
123.4
128.2
125.0
126.4
119.8
151.0
142.6
d
d
7.50
7.35
7.58
7.20
7.39
8.07
d
7.17
7.42
7.72
7.30
7.40
8.53
d
4.2. Spectrokinetic studies under continuous irradiation
UV–vis irradiation experiments were made using a CARY 50 Var-
ian spectrometer coupled to a 150 W Ozone free Xenon lamp (6255
Oriel Instruments), equipped with
(259<
UV lamp was filtered using a water filter (61945 Oriel Instruments)
and then carried to the spectrophotometerholder at the right angle to
a filter Schott 011FG09
<388 nm with lmax¼330 nm and T¼79%). The light from the
5.82
d
7.12
d
l
a
?
a
Not attributed due to low concentration of the compound.

P.J. Coelho et al. / Tetrahedron 65 (2009) 5369–5376
5375
4.3. NMR studies
J¼14.5 Hz, AB system, 1H), 2.85 (d, J¼16.2 Hz, AB system, 1H), 2.25
(d, J¼16.2 Hz, AB system, 1H), 1.19 (t, J¼7.1 Hz, 3H). MS (TOF) m/z
(%): 438 (1), 420 (3), 350 (61), 333 (13), 273 (59), 180 (21), 178 (24),
170 (100), 165 (30), 114 (43).
For NMR investigations, samples (10ꢀ3 M in toluene-d8) were
irradiated directly in the NMR tube (5 mm), thermoregulated, using
a 1000 W Xe–Hg HP filtered short-arc lamp (Oriel) equipped with
filter for UV irradiation (Schott 011FG09, 259<
l
<388 nm). After
4.5.1.2. 2-(1-Oxo-3,3-diphenylprop-2-enyl)-1-naphthol 4a. Orange
solid. Mp 126–127. Yield 20%. IR: 3047, 3430, 3021, 2960,1620,1587,
1314, 1260, 1058, 808. 1H NMR: 14.2 (OH), 8.49 (dd, J¼1.3, 8.4 Hz,
1H), 7.81 (d, J¼8.8 Hz, 1H), 7.77 (d, J¼7.9 Hz, 1H), 7.65 (ddd, J¼1.3,
6.8, 7.9 Hz, 1H), 7.55 (ddd, J¼1.2, 6.8, 8.4 Hz, 1H), 7.50–7.28 (m, 10H),
7.28 (s, 1H), 7.25 (d, J¼78.8 Hz, 1H). 13C NMR: 196.99 (C]O), 163.47,
155.13, 141.43, 139.03, 137.35, 130.13, 129.75, 129.60, 128.78, 128.64,
128.55, 128.28, 127.43, 125.87, 125.35, 125.08, 124.45, 123.12, 118.18,
114.20. MS (TOF) m/z (%): 350 (21), 273 (30), 170 (100), 114 (7).
irradiation had been stopped, the sample was transferred into the
thermoregulated probe, QNP (1H–13C–19F–31P) or TXI (1H–13C–15N),
of
(
a
Bruker Avance-500 spectrometer
(
n0 (1H)¼500 MHz, n0
13C)¼125 MHz, n0
(
19F)¼470 MHz).
4.4. Synthesis of the naphthopyran-4-ones 2a,b
A suspension of diarylketone 1a,b (8.1 mmol), 1-hydroxy-2-
acetonaphthone (2.7 mmol, 0.500 g) and t-BuOK (13.4 mmol, 1.5 g)
in 20 mL of toluene was heated under reflux for 3 h. During the
heating the suspension become progressively red. After cooling to
room temperature the solution was quenched with H2O (100 mL),
extracted with Et2O (3ꢂ40 mL) and the combined organic layers
dried (Na2SO4) and concentrated under reduced pressure leaving
about 5 mL of solvent. The red residue was treated with H2SO4
(4 mL), stirred 3 h at room temperature and then poured into ice.
The pale pink mixture was then extracted 3ꢂ with EtOAc. The
combined organic layers were washed with water, dried (Na2SO4)
and concentrated under reduced pressure. The residue was purified
by CC (3% EtOAc/petroleum ether then 20% EtOAc and 20% CH2Cl2 in
petroleum ether) to give pure 2a,b as pale yellow crystals.
4.5.2. From naphthopyran-4-one 2b two compounds
were obtained: alcohol 3b and ketone 4b
4.5.2.1. Ethyl(2,2-di(4-fluorophenyl)-4-hydroxy-3,4-dihydro-2H-naph-
tho[1,2-b]pyran-4-yl)acetate 3b. Pale yellow oil. Yield 70%. IR: 3486,
3116, 2993, 1729, 1606, 1578, 1509, 1398, 1376, 1229. 1H NMR: 8.47
(d, J¼7.8 Hz, 1H), 7.80 (dd, J¼7.0, 1.5 Hz, 1H), 7.60–7.40 (m, 8H),
7.03–6.90 (m, 4H), 4.18–4.06 (m, 2H), 3.31 (s, 2H), 2.88 (d,
J¼16.3 Hz, AB system,1H), 2.26 (d, J¼16.3 Hz, AB system, 1H),1.17 (t,
J¼7.1 Hz, 3H). (M¼474). MS (TOF) m/z (%): 474 (0.2), 456 (0.1), 409
(4), 386 (39), 369 (33), 201 (44), 170 (100), 114 (85).
4.5.2.2. 2-(1-Oxo-3,3-di(p-fluorophenyl)prop-2-enyl)-1-naphthol 4b.
Orange solid. Mp 134–135. Yield 17%. IR: 3046, 1610, 1571, 1504,
1411, 1222, 1159, 833, 806. 1H NMR: 14.10 (OH), 8.47 (dd, J¼1.2,
8.3 Hz, 1H), 7.80–7.74 (m, 2H), 7.66 (dd, J¼6.8, 7.9 Hz, 1H), 7.54 (dd,
J¼6.8, 8.3 Hz, 1H), 7.40–7.36 (m, 2H), 7.26–7.21 (m, 3H), 7.19 (s, 1H),
7.11 (m, 2H), 7.04 (m, 2H). 13C NMR: 196.56 (C]O), 162.0 and 165.3,
161.3 and 164.6, 163.66, 152.9, 137.4, 137.38, 134.7, 131.6, 130.6,
130.3, 127.43, 126.0, 125.3, 124.8, 124.5, 123.06, 118.32, 115.76, 115.4,
114.0. MS (TOF) m/z (%): 386 (18), 291 (8), 201 (9), 170 (100), 114
(15). Exact mass: calculated for C25H16O2F2: 386.1118; found:
386.1126.
4.4.1. 2,2-Diphenyl-3,4-dihydro-2H-naphtho[1,2-b]pyran-4-one 2a
White solid. Mp 176–178. Yield 56%. IR: 3058, 3037, 1680. 1H
NMR: 8.57 (dd, J¼7.2, 1 Hz,1H), 7.76 (m, 2H), 7.64–7.57 (m, 2H), 7.50
(m, 4H), 7.34–7.21 (m, 7H), 3.60 (s, 2H). MS (TOF): m/z (%): 350 (22),
273 (21), 171 (11), 170 (100), 114 (17), 85 (16), 83 (26).
4.4.2. 2,2-Di(p-fluorophenyl)-3,4-dihydro-2H-naphtho-
[1,2-b]pyran-4-one 2b
White solid. Mp 187–191. Yield 61%. IR: 3066, 2981, 1672, 1600,
1508, 1380, 1287, 1234, 1150, 1114, 835. 1H NMR: 8.54 (dd, J¼1.7,
8.5 Hz, 1H), 7.80 (m, 2H), 7.67–7.63 (m, 2H), 7.48 (m, 4H), 7.39 (d,
J¼8.6 Hz, 1H), 7.01–6.97 (m, 4H), 3.57 (s, 2H). 13C NMR: 190.8
(C]O), 163.7 and 160.4 (C–F), 157.3, 138.2 (C), 137.8, 129.7, 128.1,
128.0 (C),126.6,125.2,123.0,121.4,121.3,116.0, 86.5, 48.4. MS (TOF):
m/z (%): 386 (11), 291 (4), 170 (100), 114 (28).
4.6. Reaction of naphthopyran-4-ols 3a,b with the Grignard
reagent ArMgBr
4.5. Reaction of naphthopyran-4-ones 2a,b
with ethyl bromoacetate
A solution of arylmagnesium bromide (prepared from bromo-
benzene (0.78 g, 5 mmol) or 4-bromo-1-fluorobenzene (0.87 g,
5 mmol) and magnesium (0.240 g, 10 mmol) in 20 mL of dry THF)
A
solution of 2a,b (0.86 mmol) and ethyl bromoacetate
was slowly added to a solution of naphthopyran-4-ol 3a,b
(3.64 mmol, 0.600 g) in 20 mL of ethyl ether/benzene (1:4) was
slowly added over 2 h to a mixture of zinc (9.17 mmol, 0.600 g) and
iodine (one crystal) and heated with stirring under reflux. After the
addition the orange solution was maintained under reflux with
stirring for one additional hour and then poured into water. The
aqueous phase was extracted 3ꢂ with EtOAc and the combined
organic layers dried (Na2SO4) and concentrated under reduced
pressure. The residue was purified by CC (3–10% EtOAc/petroleum
ether).
(0.40 mmol) in dry THF (15 mL). After heating under reflux for 2 h,
the solution was quenched with water, extracted with Et2O
(3ꢂ50 mL), dried (Na2SO4) and concentrated under reduced pres-
sure. Acetic acid (10 mL) was added to the residue and the solution
heated under reflux for 20 min. The red solution thus obtained was
quenched with water, extracted with Et2O (3ꢂ50 mL), dried
(Na2SO4) and concentrated under reduced pressure. The residue
was then purified by CC (0–1% EtOAc/petroleum ether) to give pure
5a,b as pale yellow solids. Recrystallization from CH2Cl2/petroleum
ether gave pure 5a,b compounds as white solids.
4.5.1. From naphthopyran-4-one 2a two compounds
were obtained: alcohol 3a and ketone 4a
4.6.1. 2,2-Diphenyl-4-(2,2-diphenylethenyl)-2H-naphtho-
[1,2-b]pyran 5a
4.5.1.1. Ethyl(2,2-diphenyl-4-hydroxy-3,4-dihydro-2H-naphtho[1,2-
b]pyran-4-yl)acetate 3a. Pale yellow oil. Yield 69%. IR: 3489, 3006,
1729, 1576, 1494, 1397, 1204. 1H NMR: 8.55 (dd, J¼8.0, 1.6 Hz, 1H),
7.80 (dd, J¼7.0, 1.4 Hz, 1H), 7.60–7.41 (m, 8H), 7.31–7.17 (m, 6H),
4.14–4.08 (m, 2H), 3.42 (d, J¼14.5 Hz, AB system, 1H), 3.32 (d,
White solid. Yield 47%. IR: 3052, 3018, 1612, 1490, 1440, 1379,
1263, 1213, 1118, 1074, 973, 754, 696. For NMR data see Table 1. MS
(TOF) m/z (%): 512 (Mþ, 43), 435 (MþꢀPh, 100), 345 (31), 333 (27),
167 (19), 165 (16). Exact mass: calculated for C39H28O: 512.2140;
found: 512.2123.

5376
P.J. Coelho et al. / Tetrahedron 65 (2009) 5369–5376
Thermochromic Compounds In. Main Photochromic Families; Crano, J. C., Gu-
glielmetti, R., Eds.; Plenum: New York, NY, 1998; Vol. 1, pp 111.
4.6.2. 2,2-(4-Fluorophenyl)-4-[2,2-di(4-fluorophenyl)-
ethenyl]-2H-naphtho[1,2-b]pyran 5b
White solid. Yield 58%. IR: 3051, 1602, 1502, 1379, 1225, 1159,
835, 815. For NMR data see Table 1. MS (TOF) m/z (%): 584 (68), 489
(100), 381 (33), 369 (59), 203 (52), 201 (41). Exact mass: calculated
for C39H24OF4: 584.1763; found: 584.1764.
2. (a) Malic, N.; Campbell, J. A.; Evans, R. A. Macromolecules 2008, 41, 1206–1214;
(b) Silva, E. F.; Coelho, P. J. Lett. Org. Chem. 2008, 5, 502–506; (c) Oliveira, M. M.;
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Tanaka, T.; Okazaki, S. JP Patent 4,112,885A, 1992; (c) Tanaka, T.; Imura, S.;
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Acknowledgements
To FCT (Portugal’s Foundation for Science and Technology) for
financial support through the research unit Centro de Quımica-Vila
´
Real (POCTI-SFA-3-616) and project PTDC/QUI/66012/2006. The
500 MHz NMR facilities were funded by the Region Nord-Pas de
Calais (France), the Ministere de la Jeunesse de l’Education Natio-
nale et de la Recherche (MJENR) and the Fonds Europeens de
Developpement Regional (FEDER).
7. Delbaere, S.; Luccioni-Houze, B.; Bocha, C.; Teral, Y.; Campredon, M.; Ver-
meersch, G. J. Chem. Soc., Perkin Trans. 2 1998, 1153–1158.
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Hursthouse, M. B. Org. Biomol. Chem. 2008, 6, 3096–3104.
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