Synthesis and photochromic behavior of novel annelated 2H-chromenes derived from hydroxy-9H-xanthen-9-ones

Article abstract of DOI:10.1002/1522-2675(20010131)84:1<117::AID-HLCA117>3.0.CO;2-F

The synthesis of four novel pyrano-xanthenones derived from hydroxy-9H-xanthen-9-ones is described, and their photochromic properties in solution are reported. All compounds synthesized exhibit a good colorability, making them good potential dyes. The presence of the chromone system fused in the 78-position seems to lead to more stabilized colored forms.

Full text of DOI:10.1002/1522-2675(20010131)84:1<117::AID-HLCA117>3.0.CO;2-F

Helvetica Chimica Acta ± Vol. 84 (2001)
117
Synthesis and Photochromic Behavior of Novel Annelated
2H-Chromenes Derived from Hydroxy-9H-xanthen-9-ones
by Paulo J. Coelhoa)*, Luis M. Carvalhoa), Jose C. Silvaa), Ana M.F. Oliveira-Camposb), Andre Samatc),
and Robert Guglielmettic)
a
Â
) Dept Química, Universidade de Tras-os-Montes e Alto Douro, 5000 Vila Real, Portugal
(e-mail pcoelho@utad.pt)
b) Centro de Química, IBQF, Universidade do Minho, 4710 Braga, Portugal
c
Â
Â
Â
) LCMOM, ESA 6114 CNRS, Universite de la Mediterranee, 13288 Marseille Cedex 9, France
The synthesis of four novel pyrano-xanthenones derived from hydroxy-9H-xanthen-9-ones is described,
and their photochromic properties in solution are reported. All compounds synthesized exhibit a good
colorability, making them good potential dyes. The presence of the chromone system fused in the 7,8-position
seems to lead to more stabilized colored forms.
Introduction. ± In the last years, the 2H-chromenes (2H-1-benzopyran derivatives)
have been largely studied on the basis of their industrial applicability in the field of the
photochromic ophthalmic plastic lenses [1]. Similar to spiropyranes and spirooxazines,
their photochromic activity is the result of their ability to undergo a reversible cleavage
of the C O bond, leading to a quasi-planar ꢀopen formꢁ with a distinct absorption
spectrum (Scheme 1).
Scheme 1
Spirooxazines and spiropyrans [2] are well-known compounds with excellent
photochromic activity and photostability, but the ꢀopen formsꢁ normally show relatively
narrow absorption bands with maxima above 500 nm. The absorption maxima of the
ꢀopen formꢁ of the 2H-chromenes are usually in the range of 400 ± 500 nm, which is
complementary to those of the above-mentioned compounds. This feature, along with a
good fatigue resistance, little temperature dependence, and reasonable activation and
fading rates, makes 2H-chromenes particularly suitable to form mixtures that produce
photochromic systems with a neutral activated color (significative absorption along all
the visible spectrum), as is required for the commercial acceptability of ophthalmic
lenses. This approach is limited, however, because the compounds of both series often
show differences in behavior (kinetics, temperature dependence, fatigue proneness,
colorability etc.). Ideally, only one class of photochromic pigments should be used.

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To improve the performance of photochromic systems, much attention has been
devoted to the study of new photochromic molecules obtained by modification of 2H-
1-benzopyrans, namely by heteroaromatic annelation [3]. In the present work, we
describe the synthesis and characterization of novel 2H-chromenes derived from
xanthone.
Results and Discussion. ± Synthesis. There are several methods for the synthesis of
2H-chromenes [3]. One of the most versatile is based on a one-pot reaction of a
phenol with an alkynol under acid catalysis (Scheme 2) [4 ± 6]. The reaction is
considered to proceed via a Claisen rearrangement of propargyl aryl ethers, formed by
phenol O-alkylation, followed by enolization, [1,5] H-migration, and electrocyclic ring
closure.
Scheme 2
According to this strategy, the preparation of 2H-chromenes, which contain the
xanthone system, requires the availability of hydroxy-9H-xanthen-9-ones 1a ± d. 1-
Hydroxy (1a) and 4-hydroxy-9H-xanthen-9-one (1d) were synthesized by improved
procedures of reported methods [7][8].
Thus, heating a mixture of salicylic acid, resorcinol, and ZnCl2 for 6 h at 1708
afforded 1a in 30% yield. Microwave irradiation of a mixture of sodium 2-
methoxyphenolate, sodium 2-chlorobenzoate, CuCl, and tris[2-(2-methoxyethoxy)-
ethyl]amine gave 2-(2'-methoxyphenoxy)benzoic acid, which was then treated with
polyphosphoric acid (PPA) to provide 4-methoxyxanthone in 41% yield. Demethyl-
ation with AlCl3 gave 1d. 2-Hydroxy- and 3-hydroxy derivatives 1b and 1c, respectively,
were prepared according to known methods [9].
Condensation of 1,1-diphenylprop-2-yn-1-ol with hydroxy-9H-xanthen-9-ones 1b ±
d under pyridinium p-toluenosulfonate (PPTS) catalysis gave the corresponding
pyrano-xanthone derivatives 2 ± 4 in 17 ± 50% yield.
Under the same conditions, no reaction was observed with 1-hydroxyxanthone (1a).
This lack of reactivity is probably due to the intramolecular H-bond between the OH
and the CˆO group. Thus, it was decided to reduce 1a to 9H-xanthen-1-ol (5) [10],
which was converted to the corresponding pyrano-xanthene 6 (Scheme 3). Oxidation of
6 by CrO3/Py gave the pyrano-xanthenone 7 in good yield.

Helvetica Chimica Acta ± Vol. 84 (2001)
119
Scheme 3
Photochromic Properties. The introduction of a fused chromone group by
heteroannelation in the 5,6- and 7,8-positions of the 2H-1-benzopyran moiety
(Scheme 1) led to compounds 2 ± 4 and 7 that exhibit photochromic behavior at room
temperature in toluene. The relevant parameters used to characterize this behavior
(spectrokinetic parameters [11]) are reported in the Table, in which we included two
corresponding naphthopyrans (Ref1 and Ref 2) as reference compounds [12] for
comparison. Flash-photolysis equipment coupled to a fast-scan spectrometer was used
for this purpose [13].
Maxima wavelengths of the colored form (lmax) describe the color that can be
achieved upon irradiation. The visible absorption spectra of the benzopyrans obtained
do not follow a general pattern. From a general point of view, a global bathochromic
shift can be observed when compared to the parameters concerning the corresponding
naphthopyrans. However, compounds 2 (heteroannelation in the 5,6-positions of the
naphthopyran) and 7 (heteroannelation in the 7,8-positions) display in toluene one
single band, and compounds 3 (heteroannelation in the 5,6-positions) and 4

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Helvetica Chimica Acta ± Vol. 84 (2001)
Table. Spectrokinetic Properties: Maxima Wavelengths of the Colored Form (l1, l2), Colorability (A01, A02),
Bleaching Rate (kD ), and Resistance towards Photodegradation (tA /2) of 2H-Chromenes 2 ± 4 and 7, and Two
0
Reference Compounds in Toluene Solutions (2.5  10 5 m at 258). Ref1 ˆ 3,3-diphenyl-3H-naphtho[2,1-b]pyran;
Ref2 ˆ 2,2-diphenyl-2H-naphtho[1,2-b]pyran
Compound
Annelation
l1 [nm]
A01
l2 [nm]
A02
kD (amplitude)
[s 1] [%]
tA /2 [min]
0
2
3
4
7
5,6
5,6
7,8
7,8
484
428
436
467
1.50
1.30
1.10
1.80
±
±
7.70 (96)
0.10 (4)
8.20 (49)
0.03 (51)
0.66 (82)
0.02 (18)
0.31 (17)
0.02 (83)
0.09
917
36
515
522
±
0.70
1.00
±
35
6
Ref1
Ref2
5,6
7,8
432
403
0.84
1.08
±
481
±
1.62
509
0.002
(heteroannelation in the 7,8-positions) display two bands, the first one located near
430 nm and the other one near 520 nm, covering a much larger range of wavelengths in
the visible region. This behavior was observed in 2H-chromenes to which different five-
membered rings were introduced in 5,6-positions [14], and it is markedly different from
the behavior of the parent naphthopyran that shows a single absorption band with a
maximum at 432 nm under the same experimental conditions. It is apparent that, when
the geometry of the fused chromone group is reversed (e.g., compound 2 to 3, and 4 to
7), a change of the electronic absorption spectra pattern occurs, leading to a splitting
into two bands. The presence of a heteroatom in positions 5 and 8 of the chromene
moiety seems to be important for this effect that results in a much larger coverage range
of visible wavelengths, leading to compounds potentially interesting as orange-color-
developing substances.
Thermal bleaching (ring closure) rate constants (kD) provide a measure of how fast
the system returns to its uncolored form and provides information on the stability of the
colored open form at room temperature. All the new compounds described show two
phases of kinetics of fading with variable amplitudes; this is different from the kinetics
exhibited by the reference compounds. The slow fading rate can be attributed to the
most stable s-trans-isomer. The thermal bleaching of compounds 2 and 3 has been
found to be a very fast process, indicating a photogenerated form poorly stabilized. This
is not suitable for application in variable optical-transmission materials. Compounds 4
and 7 show a relatively slower process of fading with interesting values. The presence of
the chromone system fused in the 7,8-positions seems to lead to more stabilized colored
open forms.
ꢀColorabilityꢁ (A0; absorbance measured just after the flash) is the photocoloration
efficiency, and a function of the quantum yield of the photocoloration process and the
molar absorptivity of the colored species. All the compounds synthesized exhibit a good
colorability, rendering them good potential dyes.
Fatigue resistance (tA /2; time necessary to observe a decrease of half of the
0
colorability) measures the tendency of the system to photodegradation under the

Helvetica Chimica Acta ± Vol. 84 (2001)
121
experimental conditions considered [15]. From the results in the Table, it is noteworthy
that compound 2 shows a good fatigue resistance, which may not indicate, however, an
unusual photodegradation resistance, as this process has its main origin in the open
form [16]. The open form of 2 seems to have a very short lifetime, as indicated by the
high bleaching-rate value that covers almost all the ring closure process. Compounds 3,
4 and 7 undergo photodegradation very easily.
Conclusion. ± The synthesis of 2H-chromenes with a fused chromone group
(pyrano-xanthenones) was achieved by usual methods. Particularly remarkable is the
result that only ꢀangularꢁ 2H-chromenes were obtained from 2-hydroxy- and 3-hydroxy-
9H-xanthen-9-ones. The photochromic compounds described exhibit improved colo-
rabilities, and, for compounds 3 and 4, a marked broadening of visible absorption
spectra is observed that makes these 9H-xanthen-9-one derivatives potentially
interesting for photochromic applications.
Experimental Part
General. Column chromatography (CC): silica gel 60 (70 ± 230 mesh). Photochromic measurements were
performed in 2.5 Â 10 5 m toluene soln. at 258 with a Warner & Swasey spectrometer (cell pathlength 10 cm);
5
flash energy 60 J. Resistance towards photodegradation was measured in 0.5 Â 10 toluene soln. at 258 (cell
pathlength 5 cm); lamp XBO 250 W‡ Schott filters WG295 and IR 114; flash energy 150 J. M.p.: uncorrected.
IR Spectra: Perkin-Elmer FTIR-1600 spectrometer, wave numbers in cm 1. 1H-NMR Spectra: in CDCl3 (except
if otherwise stated) on a Varian Unity Plus at 300 MHz. 13C-NMR Spectra: in CDCl3 in a Varian Unity Plus at
75.4 MHz. NMR Assignments were based on irradiation experiments. MS: AutoSpecE spectrometer. Elemental
analyses were carried out with a LECO 932 CHNS analyzer.
I-Hydroxy-9H-xanthen-9-one (1a). A mixture of salicylic acid (3.00 g, 21.7 mmol), resorcinol (1.00 g,
9.09 mmol), and anh. ZnCl2 (1.00 g, 9.8 mmol) was heated at 1708 for 6 h. After cooling, the mixture was
dissolved in AcOEt (50 ml), and silica gel (5 g) was added. Evaporation afforded a residue, which was purified
by CC (hexane/AcOEt 99.5 :0.5) to yield 1a (0.572 g, 30%). Yellow needles. M.p. 142 ± 1448 ([17]: 1488).
1H-NMR: 12.67 (s, OH); 8.30 (dd, J ˆ 7.3, 1.8, H C(8)); 7.78 (ddd, J ˆ 1.8, 6.9, 8.4, H C(6)); 7.61 (t, J ˆ 8.4,
H
H
C(3)); 7.50 (dd, J ˆ 0.7, 8.4, H C(5)); 7.42 (t, J ˆ 7.3, H C(7)); 6.96 (d, J ˆ 8.4, H C(4)); 6.82 (d, J ˆ 8.4,
C(2)).
4-Hydroxy-9H-xanthen-9-one (1d). A mixture of sodium 2-chlorobenzoate (5.00 g, 28.1 mmol), sodium 2-
methoxyphenolate (5.00 g, 34.2 mmol), CuCl (350 mg, 4 mmol), and tris[2-(2-methoxyethoxy)ethyl]amine
(0.2 ml) were suspended in dioxane (15 ml) and submitted to microwave irradiation for 40 min (1100 W). The
residue was treated with H2O (100 ml) and filtered. The soln. thus obtained was acidified with 2m HCl and
extracted with AcOEt (3 Â 50 ml). The org. phase was washed with H2O and dried (Na2SO4). The crude product
was dissolved in polyphosphoric acid (15 ml), stirred 1 day at 608, quenched with H2O, and extracted three times
with AcOEt. The combined org. extracts were dried (Na2SO4). After evaporation, the crude product was
purified by CC (hexane/AcOEt) to yield 4-methoxy-9H-xanthen-9-one (2.58 g, 41%). Yellow solid. M.p. 118 ±
1208. 1H-NMR: 8.36 (dd, J ˆ 7.8, 1.8, H C(8)); 7.93 (dd, J ˆ 7.8, 1.8, H C(1)); 7.76 (ddd, J ˆ 8.7, 7.5, 1.8,
H
C(6)); 7.63 (dd, J ˆ 8.7, 1.2, H C(5)); 7.41 (ddd, J ˆ 7.8, 7.5, 1.2, H C(7)); 7.33 (t, J ˆ 7.8, H C(2)); 7.27
(dd, J ˆ 7.8, 1.8, H C(3)); 4.06 (s, MeO). To a soln. of 4-methoxy-9H-xanthen-9-one (0.830 g, 3.67 mmol) in
anh. benzene (30 ml) was added AlCl3 (4 g, 30.0 mmol) under an Ar atmosphere. The mixture was stirred under
reflux for 1 h and then acidified with 10% HCl. The aq. phase was extracted with Et2O (3 Â 100 ml). The org.
phase was washed with H2O, dried (Na2SO4), and the solvent was evaporated under reduced pressure leaving
the crude 1d. Recrystallization from MeOH gave pure 1d (0.690 g, 89%). Colorless needles. M.p. 233 ± 2368
([18]: 230 ± 2338). 1H-NMR ((D6)Acetone): 9.15 (s, OH); 8.29 (dd, J ˆ 7.8, 1.8, H C(8)); 7.89 (ddd, J ˆ 8.0, 7.5,
1.8, H C(6)); 7.76 (dd, J ˆ 7.8, 2, H C(1)); 7.66 (dd, J ˆ 8.0, 1.2, H C(5)); 7.50 (ddd, J ˆ 7.8, 7.5, 1.2, H C(7));
7.40 (dd, J ˆ 7.8, 2, H C(3)); 7.32 (t, J ˆ 7.8, H C(2)).
Preparation of Pyrano-xanthenones. General Procedure. To a mixture of hydroxy-9H-xanthen-9-one 1
(212 mg, 1.00 mmol), 1,1-diphenylprop-2-yn-1-ol (312 mg, 1.50 mmol), and PPTS (25 mg), 30 ml of dry CHCl3

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Helvetica Chimica Acta ± Vol. 84 (2001)
were added. The suspension was refluxed for 3 days under Ar. The suspension was then treated with H2O, and
the aq. phase was extracted with CHCl3 (3 Â 30 ml). The org. phase was then washed with 5% NaOH soln. (3 Â
20 ml) and dried (Na2SO4). Solvent evaporation gave a yellow oil, which was purified by CC (silica gel; hexane/
AcOEt 90 :10). Recrystallization from pentane/CH2Cl2 gave a crystalline material.
3,3-Diphenyl-3H-pyrano[3,2-a]xanthen-12(12H)-one (2): 41%. M.p. 227 ± 2288. IR: 3050, 1648, 1610, 1579,
1467, 1454, 1307, 1243, 1216, 1145, 1066, 970, 759. 1H-NMR ((D6)Acetone): 8.47 (dd, J ˆ 10.2, 0.6, H C(1)); 8.24
(dd, J ˆ 8.1, 1.5, H C(11)); 7.84 (ddd, J ˆ 9.0, 7.2, 1.5, H C(9)); 7.56 (m, 6 H); 7.49 (d, J ˆ 9.0, H C(8)); 7.46
(ddd, J ˆ 8.1, 7.2, 1.5, H C(10)); 7.42 ± 7.26 (m, 6 H); 6.64 (d, J ˆ 10.2, H C(2)). 13C-NMR: 179.08 (CˆO);
155.31; 152.15; 148.54; 144.18; 134.41; 131.03; 128.09; 127.60; 127.08; 126.53; 124.44; 123.60; 122.50; 122.21;
‡
120.22; 118.65; 117.44; 116.35; 81.86 (C(3)). MS: 402 (100, M ), 401 (15), 385 (12), 325 (90), 239 (13), 191 (15),
165 (16), 77 (10). Anal. calc. for C28H18O3: C 83.58, H 4.48; found: C 83.38, H 4.75.
3,3-Diphenyl-3H-pyrano[2,3-c]xanthen-7(7H)-one (3): 48%. M.p. 194 ± 1958. IR: 3080, 1650, 1635, 1618,
1
1465, 1438, 1261, 1072, 754, 700. H-NMR ((D6)Acetone): 8.24 (dd, J ˆ 8.1, 1.5, H C(8)); 8.17 (d, J ˆ 9, 1 H);
7.86 (ddd, J ˆ 8.7, 7.2, 1.5, H C(10)); 7.70 (dd, J ˆ 8.7, 1.2, H C(11)); 7.55 (m, 4 H); 7.48 (ddd, J ˆ 8.1, 7.2, 1.2,
H
C(9)); 7.45 ± 7.26 (m, 7 H); 7.15 (d, J ˆ 9, H C(5)); 6.64 (d, J ˆ 10.2, H C(2)). 13C-NMR: 175.83 (CˆO);
158.45; 156.79; 145.23; 135.56; 129.95; 129.13; 128.65; 128.19; 127.53; 127.39; 126.93; 125.09; 122.46; 118.82;
117.09; 117.02; 114.80; 110.48; 84.50 (C(3)). MS: 402 (55, M ), 401 (15), 325 (100), 239 (12), 191 (13), 165 (14).
‡
Anal. calc. for C28H18O3: C 83.58, H 4.48; found: C 83.51, H 4.53.
2,2-Diphenyl-2H-pyrano[3,2-c]xanthen-7(7H)-one (4): 17%. M.p. 204 ± 2068. IR: 3080, 1656, 1637, 1606,
1461, 1446, 1326, 1224, 960, 744. 700. 1H-NMR ((D6)Acetone): 8.33 (dd, J ˆ 7.8, 1.5, H C(8)); 7.84 (d, J ˆ 8,
H
C(6)); 7.74 (ddd, J ˆ 8, 6.9, 1.5, H C(10)); 7.63 (d, J ˆ 8, H C(11)); 7.53 (m, 4 H); 7.42 ± 7.26 (m, 7 H); 7.05
(d, J ˆ 8, H C(5)); 6.75 (d, J ˆ 10, 1 H); 6.64 (d, J ˆ 10, 1 H). 13C-NMR: 176.83 (CˆO); 156.13; 145.61; 144.01;
140.83; 134.65; 131.98; 128.28; 127.89; 126.89; 126.63; 126.03; 123.93; 123.00; 122.64; 121.64; 121.38; 118.38;
118.26; 83.36 (C(2)). MS: 402 (100, M ), 401 (30), 385 (7), 325 (90), 239 (15), 191 (10), 165 (15), 77 (5). Anal.
‡
calc. for C28H18O3: C 83.58, H 4.48; found: C 83.15, H 4.55.
9H-Xanthen-1-ol (5). To a soln. of 1a (300 mg) in benzene (15 ml), a soln. of LiAlH4 (80 mg, 2.1 mmol) in
dry Et2O (15 ml) was added under Ar, and the mixture was heated under reflux for 2 h. After treatment with
hydrated Na2SO4, the mixture was poured into H2O (50 ml) and extracted with Et2O (3 Â 20 ml). The org. phase
was dried (Na2SO4). Evaporation afforded a residue, which was purified by CC (AcOEt/hexane 99 :1).
Evaporation gave 5 (0.180 g, 56%), which crystallized as colorless needles. M.p. 142 ± 1448. 1H-NMR: 7.20
(m, 2 H); 7.04 (m, 3 H); 6.65 (d, J ˆ 8.4, 1 H); 6.49 (d, J ˆ 8.4, 1 H); 3.99 (s, 2 H).
2,2-Diphenyl-2H-12H-pyrano[2,3-a]xanthene (6): 63%. IR: 3100, 2980, 1639, 1612, 1479, 1261, 1072, 802,
698. 1H-NMR: 7.48 (m, 4 H); 7.4 ± 7.25 (m, 6 H); 7.23 (m, 2 H); 7.02 (m, 2 H); 6.88 (d, J ˆ 8.1, 1 H); 6.60 (d, J ˆ
9.9, 1 H); 6.56 (d, J ˆ 8.1, 1 H); 6.03 (d, J ˆ 9.9, 1 H); 4.08 (s, 2 H). 13C-NMR: 152.44; 151.26; 150.33; 145.08;
129.33; 128.12; 127.59; 127.50; 126.82; 125.97; 125.08; 123.28; 122.93; 119.55; 116.38; 115.60; 108.70; 108.64;
‡
82.96 (C(2)); 22.57 (C(12)). MS: 388 (100, M ), 311 (30).
2,2-Diphenyl-2H-pyrano[2,3-a]xanthen-12(12H)-one (7). To a soln. of 6 (0.367 g, 0.946 mmol) in pyridine
(5 ml), CrO3 (0.400 g) was added. After stirring for 4 h at r.t., the mixture was poured into 5% aq. HCl (50 ml).
The aq. layer was extracted with Et2O (3 Â 100 ml). The org. phase was washed with H2O, dried (Na2SO4), and
the solvent was evaporated under reduced pressure, leaving the crude product, which was purified by CC (silica
gel; hexane/AcOEt 95 :5): 220 mg (58%) of 7. M.p. 211 ± 2148. 1H-NMR: 8.35 (dd, J ˆ 8.1, 1.5, H C(11)); 7.66
(m, 5 H); 7.41 ± 7.19 (m, 9 H); 6.93 (d, J ˆ 8.7, 1 H); 6.66 (d, J ˆ 9.6, 1 H); 6.39 (d, J ˆ 9.6, 1 H). 13C-NMR:
176.04 (CˆO); 157.40; 155.22; 153.69; 144.90; 134.20; 132.13; 128.26; 127.57; 127.34; 126.78; 126.49; 123.77;
‡
123.03; 122.94; 117.38; 117.06; 112.60; 109.75; 83.42 (C(2)). MS: 402 (100, M ), 385 (13), 325 (90), 297 (11), 239
‡
(12), 191 (14), 165 (14). MS: 402 (100, M ), 401 (10), 385 (12), 325 (90), 297 (12), 239 (15), 201 (13), 191 (15),
165 (15). Anal. calc. for C28H18O3: C 83.58, H 4.48; found: C 83.72, H 4.69.
We are grateful to FCT for financial support through project Praxis XXI/P/QUI/10021, and to R. Dubest
and Dr. J. Aubard (ESA CNRS 7086 University of Paris VII) for the evaluation of spectrokinetic parameters.
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Received July 22, 2000