Synthesis and in vitro study of 14-aryl-14H-dibenzo[a.j]xanthenes as cytotoxic agents

Article abstract of DOI:10.1016/j.bmcl.2009.08.033

A simple and expedient method for the synthesis of a series of 14-aryl-14H-dibenzo[a.j]xanthenes is described through a one-pot condensation of β-naphthol with aryl aldehydes catalysed by TaCl₅ under solvent-free conventional heating. The major

Full text of DOI:10.1016/j.bmcl.2009.08.033

Bioorganic & Medicinal Chemistry Letters 19 (2009) 5590–5593
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Bioorganic & Medicinal Chemistry Letters
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Synthesis and in vitro study of 14-aryl-14H-dibenzo[a.j]xanthenes
as cytotoxic agents
a
a
b
b
Asish K. Bhattacharya a, , Kalpeshkumar C. Rana , Mohammad Mujahid , Irum Sehar , Ajit K. Saxena
*
a Division of Organic Chemistry and Combi Chem Bio-Resource Centre, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
b Pharmacological Division, Indian Institute of Integrative Medicine, Canal Road, Jammu 180 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2009
Revised 7 August 2009
Accepted 10 August 2009
Available online 13 August 2009
A simple and expedient method for the synthesis of a series of 14-aryl-14H-dibenzo[a.j]xanthenes is
described through a one-pot condensation of b-naphthol with aryl aldehydes catalysed by TaCl5 under
solvent-free conventional heating. The major advantages of the present method are: high yields, less
reaction time, solvent-free condition and easy purification of the products. The synthesized 14-aryl-
14H-dibenzo[a.j]xanthenes were evaluated against a panel of six human cancer lines of different tissues.
Synthesized compound 3o showed IC50 of 37.9 and 41.3
whereas 3g showed IC50 of 41.9 M against Colo-205.
l
M against Colo-205 and 502713, respectively,
Keywords:
Xanthenes
One-pot reaction
Condensation
Aldehyde
l
Ó 2009 Elsevier Ltd. All rights reserved.
b-Naphthol
Tantalum(V) chloride
Solvent-free
Cytotoxicity
In recent times xanthenes and, specifically benzoxanthenes
have attracted attention of medicinal chemists as well as organic
chemists due to their wide range of biological and pharmacological
activities such as antiviral,1 antibacterial,2 and anti-inflammatory
properties,3 as well as in photodynamic therapy4 and for antago-
nism of the paralyzing action of zoxazolamine.5 Further, these
compounds have found wide usage such as dyes,6 in laser technol-
ogies,7 and as pH-sensitive fluorescent materials for visualization
of biomolecules.8 Several methods have been reported for the syn-
thesis of xanthenes and benzoxanthenes, which include trapping of
benzynes by phenols,9a,b cyclodehydrations,9c cyclocondensation
between 2-hydroxyaromatic aldehydes and 2-tetralone9d and
intramolecular phenyl carbonyl coupling reactions of benzalde-
hydes and acetophenones.9e Also, 14-aryl-14H-dibenzo[a.j]xanth-
enes and related products have been prepared by the reaction of
b-naphthol with formamide,9f 2-naphthol-1-methanol9g and car-
bon monoxide.9h The reaction of b-naphthol with aldehydes or ace-
tals under acidic conditions have also been reported.10 However,
many of these existing methodologies suffer from one or more dis-
advantages such as prolonged reaction times,10b–e low yields, use
of harmful organic solvents,10d requirement of excess of catalyst/
reagents, and harsh reaction conditions. Keeping in view the disad-
vantages associated with earlier reported protocols as well as
increasing importance of benzoxanthenes in pharmaceutical and
industrial chemistry, there still remains a need for the develop-
ment of an efficient, low cost and ecofriendly protocol for the syn-
thesis of benzoxanthenes. Also, the dibenzoxanthene moieties has
not been explored for their cytotoxic activities, therefore, we
opined that investigation related to their cytotoxicity shall be of
importance.
Our continued endeavour in the development of ecofriendly
and solvent-free protocols11 prompted us to develop an efficient,
convenient and facile method for the synthesis of benzoxanthenes
by the condensation of aldehydes with b-naphthol catalyzed by
tantalum(V) chloride (Scheme 1). In recent years, tantalum(V)
chloride has been found to be an efficient Lewis acid12 in various
organic transformations due to it’s readily availability at a low cost,
high oxophilicity and relatively low toxicity.
In an initial study, 0.5 mmol of benzaldehyde (1a) was reacted
with 1 mmol of b-naphthol (2) in 1,2-dichloroethane (2 mL) for
R
OH
TaCl5 (10 mol%)
RCHO
+
2
neat
O
1a-n
2
3a-n
* Corresponding author. Tel.: +91 20 2590 2309; fax: +91 20 2590 2629.
E-mail address: ak.bhattacharya@ncl.res.in (A.K. Bhattacharya).
Scheme 1.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.08.033

A. K. Bhattacharya et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5590–5593
5591
Table 1
Solvent effect on the reaction of benzaldehyde and b-naphthol catalysed by TaCl5
Table 2 (continued)
a
Entry R–CHO
Product Time (h) Yieldb (%) Mp (°C)
Entry
Solvent
Reaction time (h)
Yieldb (%)
CHO
1
2
3
4
5
ClCH2CH2Cl
MeCN
MeOH
DMF
2
2
2
2
2
20
10
5
20
40
11
3k
1
1
2
1
95
70
88
96
241–243
F
CHO
CHCl3
a
10 mol %.
Isolated yield.
12
3l
184–185
192–193
239–240
b
Cl
Br
CHO
2 h in reflux conditions in the presence of TaCl5 (10 mol %). How-
ever, 14-phenyl-14H-dibenzo[a.j]xanthene was obtained in only
20% yield. The studies on effect of common organic solvents on
the course of the reaction as delineated in Table 1 clearly revealed
13
3m
3n
CHO
O
14
O
Table 2
TaCl5 catalyzed synthesis of 14-aryl-14H-dibenzo[a.j]xanthenesa
CHO
Entry R–CHO
1
Product Time (h) Yieldb (%) Mp (°C)
15
3o
1
59
204–205
CHO
CHO
OH
3a
1
95
186–187
O
OH
a
All the products were characterized by spectral data and also by comparison
with reported data.
b
Yield refers to pure recrystallized product.
2
3b
3c
3d
1
1
1
94
94
93
179–180
238–239
205–206
OCH3
that the use of solvent is not beneficial to improve the yield of the
desired product.
Hence, we turned our attention to the reaction under solvent-free
conditions. We carried out the reaction of benzaldehyde (1a) with
b-naphthol (2) and TaCl5 in neat condition and the reaction mixture
CHO
3
4
H3C
CHO
Table 3
H3CO
Comparison of TaCl5 with other catalysts for the synthesis of 14- aryl-
14H-dibenzo[a.j]xanthenes
Substrate
Catalyst
Catalyst (mol %) Reaction time Yielda (%)
CHO
5
6
3e
3f
1
1
80
96
210–211
194–195
Iodine
LiBr
10
15
2
2.5 h
65 min
4 h
8 h
8 h
9010c
8210g
8910d
9310b
9310e
95
CHO
OCH3
pTSA
SelectfluorÒ
10
H3CO
H3CO
CHO
Sulfamic acid 10
TaCl5
Iodine
LiBr
10
10
15
2
1 h
4 h
8910c
8010g
8010d
9210b
9210e
93
CHO
70 min
6 h
10 h
10 h
1 h
3 h
65 min
3 h
11 h
11 h
1 h
2.5 h
3 h
6 h
6 h
1 h
3 h
70 min
2.5 h
12 h
12 h
1 h
pTSA
CHO
SelectfluorÒ
10
OCH3
CHO
Sulfamic acid 10
TaCl5
Iodine
LiBr
7
3g
1
85
205–206
10
10
15
2
HO
8810c
8210g
9210d
9210b
9210e
94
OCH3
pTSA
SelectfluorÒ
10
CHO
NO2
Sulfamic acid 10
TaCl5
Iodine
OCH3
CHO
8
9
3h
3i
1
1
72
90
292–294
309–310
10
10
2
9310c
9010d
9310b
9310e
95
pTSA
SelectfluorÒ
10
CHO
Sulfamic acid 10
TaCl5
Iodine
LiBr
10
10
15
2
O2N
F
9210c
8110g
9010d
9110b
9110e
93
CHO
CHO
pTSA
SelectfluorÒ
10
10
3j
1
93
213–214
Sulfamic acid 10
TaCl5 10
NO2
NO2
a
References for earlier methods.
(continued on next page)

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A. K. Bhattacharya et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5590–5593
Table 4
IC50 values of synthesized compounds against human cancer cell lines of different tissues
S. no.
Compound code
IC50
(lM)
Colon
CNS
Lung
Prostate
PC-3
SW-620
502713
Colo-205
SK-N-SH
A-549
1
2
3
3g
3h
3o
54.4
63.2
49.1
45.4
50
41.3
41.9
—
37.9
44.3
55
44
74.3
—
127.9
61.1
82
63.9
was heated at 100 °C, and the corresponding benzoxanthene 3a was
obtained in 95% yield (Table 2, entry 1). This success then encour-
aged us to generalize the scope of this method.13 Several structurally
diverse aldehydes (Table 2) were subjected to condensation with
b-naphthol under the catalytic influence of TaCl5 (10 mol %) and sol-
vent-free conventional heating, and 14-aryl-14H-dibenzo[a.j]-
xanthenes were obtained in high yields. Also, tolerance of the pres-
ent method towards various functionalitiespresent in the substrates
viz. ethers, methylenedioxy, halides, hydroxy and nitro groups gen-
eralizes the scope of the present method. Further, the versatility and
the utility of the present method could easily be gauged by compar-
ison with the earlier reported protocols (Table 3). We have chosen
some model substrates and the comparison has been made with re-
spect to the catalytic amount used, reaction time and yields. It is
noteworthy to mention here that 3-methoxybenzaldehyde, 4-
methoxybenzaldehyde, 4-fluorobenzaldehyde and 3-nitrobenzal-
dehyde provided the desired product within 1 h with higher yield
than the earlier reported methods.
Supplementary data
Selected NMR spectra (1H NMR, 13C NMR and DEPT spectra of
compounds 3b, 3f, 3l and 3n). Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.bmcl.2009.08.033.
References and notes
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3. Poupelin, J. P.; Saint-Ruf, G.; Foussard-Blanpin, O.; Narcisse, G.; Uehida-Ernouf,
G.; Lacroix, R. Eur. J. Med. Chem. 1978, 13, 67.
4. (a) Ion, R. M. Prog. Catal. 1997, 2, 55; (b) Ion, R. M.; Frackowiak, D.; Planner, A.;
Wiktorowicz, K. Acta Biochim. Pol. 1998, 45, 833.
5. (a) Saint-Ruf, G.; De, A.; Hieu, H. T. Bull. Chim. Ther. 1972, 7, 83; (b) Saint-Ruf, G.;
Hieu, H. T.; Poupelin, J. P. Naturwissenschaften 1975, 62, 584.
6. (a) Menchen, S. M.; Benson, S. C.; Lam, J. Y. L.; Zhen, W.; Sun, D.; Rosenblum, B.
B.; Khan, S. H.; Taing, M. U.S. Patent, US 6583168, 2003; Chem. Abstr. 2003, 139,
p54287f.; (b) Banerjee, A.; Mukherjee, A. K. Stain Technol. 1981, 56, 83; (c)
Reynolds, G. A.; Tuccio, S. A.; Peterson, O. G.; Specht, D. P. Ger. Offen. DE
2109040, 1971; Chem. Abstr. 1971, 71, p81334c.
7. (a) Sirkecioglu, O.; Talinli, N.; Akar, A. J. Chem. Res. Synop. 1995, 502; Ahmad,
M.; King, T. A.; Ko, D.-K.; Cha, B. H.; Lee, J. J. Phys. D: Appl. Phys. 2002, 35, 1473.
8. Knight, C. G.; Stephens, T. Biochem. J. 1989, 258, 683.
9. (a) Knight, D. W.; Little, P. B. Synlett 1998, 1141; (b) Knight, D. W.; Little, P. B. J.
Chem. Soc., Perkin Trans. 1 2001, 1771; (c) Bekaert, A.; Andrieux, J.; Plat, M.
Tetrahedron Lett. 1992, 33, 2805; (d) Jha, A.; Beal, J. Tetrahedron Lett. 2004, 45,
8999; (e) Kuo, C.-W.; Fang, J.-M. Synth. Commun. 2001, 31, 877; (f) Papini, P.;
Cimmarusti, R. Gazz. Chim. Ital. 1947, 77, 142; (g) Sen, R. N.; Sarkar, N. J. Am.
Chem. Soc. 1925, 47, 1079; (h) Ota, K.; Kito, T. Bull. Chem. Soc. Jpn. 1976, 49,
1167.
10. (a) Patil, S. B.; Bhat, R. P.; Samant, S. D. Synth. Commun. 2006, 36, 2163; (b)
Kumar, P. S.; Kumar, B. S.; Rajitha, B.; Reddy, P. N.; Sreenivasalu, N.; Reddy, Y. T.
Arkivoc 2006, 46; (c) Das, B.; Ravikanth, B.; Ramu, R.; Laxminarayana, K.; Rao, B.
V. J. Mol. Catal. A: Chem. 2006, 255, 74; (d) Khosropour, A. R.; Khodaei, M. M.;
Moghannian, H. Synlett 2005, 955; (e) Rajitha, B.; Kumar, B. S.; Reddy, Y. T.;
Reddy, P. N.; Sreenivasulu, N. Tetrahedron Lett. 2005, 46, 8691; (f) Bhattacharya,
A. K.; Rana, K. C. Mendeleev Commun. 2007, 17, 247; (g) Saini, A.; Kumar, S.;
Sandhu, J. S. Synlett 2006, 1928; (h) Sarma, R. J.; Baruah, J. B. Dyes. Pigm. 2005,
64, 91.
All the compounds were assayed for in vitro cytotoxicity14
against a panel of six human cancer cell lines including SW-620,
502713 and Colo-205 (Colon), SKNSH (CNS), A-549 (Lung) and
PC-3 (Prostate) using sulforhodamine B. The synthesized 14-aryl-
14H-dibenzo[a.j]xanthenes were evaluated against six cancer lines
for their cytotoxic profiles and the results are summarized in Table
4. Compound 3g having 4-hydr-oxy-3-methoxy aryl showed sig-
nificant cytotoxic activity against Colo-205 with an IC50 value of
41.9 lM whereas other similarly substituted compounds in this
series (3b, 3d, 3e and 3f) were devoid of any cytotoxic activity.
O-Nitro substituted aryl (3h) showed some degree of cytotoxic
activity against different colon, CNS and prostrate cell lines
whereas p- or m-nitro substituted aryls (3i and 3j, respectively)
did not show cytotoxic activity against any human cancer cell
lines. The more polar compound having two aliphatic hydroxyl
groups (3o) showed more potent cytotoxic activity against Colo-
205 and Colon-502713 with an IC50 value of 37.9 and 41.3
respectively.
lM,
11. (a) Bhattacharya, A. K.; Kaur, T. Synlett 2007, 745; (b) Bhattacharya, A. K.;
Mujahid, M.; Natu, A. A. Synth. Commun. 2008, 38, 128; (c) Bhattacharya, A. K.;
Diallo, M. A.; Ganesh, K. N. Synth. Commun. 2008, 38, 1518; (d) Bhattacharya, A.
K.; Rana, K. C. Tetrahedron Lett. 2008, 49, 2598.
In conclusion, a novel and highly efficient procedure has been
developed for the synthesis of 14-aryl-14H-dibenzo[a.j]xanthenes
by condensation reaction of diverse aldehydes with b-naphthol in
one-pot using TaCl5 as the catalyst under solvent-free condition.
The main advantages of the present method are: solvent-free reac-
tion conditions, good to excellent yields, simple work-up and puri-
fication of the products. Also, some of the synthesized compounds
(3g, 3h and 3o) have shown cytotoxic activities in micro molar
range thereby suggesting that these moieties could be further
developed as possible anticancer agents by the structural
modifications.
12. (a) Kirihara, M.; Harano, A.; Tsukiji, H.; Takizawa, R.; Uchiyama, T.; Hatano, A.
Tetrahedron Lett. 2005, 46, 6377; (b) Chandrasekhar, S.; Takhi, M.; Reddy, Y. R.;
Mohapatra, S.; Rama Rao, C.; Reddy, K. V. Tetrahedron 1997, 53, 14997; (c)
Chandrasekhar, S.; Takhi, M.; Uma, G. Tetrahedron Lett. 1997, 38, 8089; (d)
Chandrasekhar, S.; Ramachandar, T.; Takhi, M. Tetrahedron Lett. 1998, 39, 3263;
(e) Chandrasekhar, S.; Ramachandar, T.; Jaya Prakash, S. Synthesis 2000, 1817.
13. General Synthetic procedure: to a mixture of aldehyde (1 mmol), and b-naphthol
(2 mmol), TaCl5 (10 mol %) was added and heated at 100 °C for the appropriate
time (Table 1) till the completion (TLC) of reaction. The reaction mass was
cooled to room temperature, DCM (25 mL) was added to it and washed with
water. The DCM extract was dried (anhydrous Na2SO4), filtered and evaporated
to furnish a solid residue. The pure 14-aryl-14H-dibenzo[a.j]xanthene was
obtained by recrystallization of the solid from EtOH. All the products were
characterized by spectral data.15
Acknowledgments
14. In vitro cytotoxicity of synthesized compounds against human cancer cell lines: the
human cancer cell lines were procured from National Cancer Institute,
Frederick, USA. Cells were grown in tissue culture flasks in complete growth
medium (RPMI-1640 medium with 2 mM glutamine, pH 7.4, supplemented
A.K.B. is grateful to Dr. S. Sivaram, Director, NCL, Pune and Dr.
Ganesh Pandey, Head, Division of Organic Chemistry for their con-
stant encouragement and support. K.C.R. is grateful to the Council
of Scientific and Industrial Research (CSIR), New Delhi for a Senior
Research Fellowship.
with 10% fetal calf serum, 100 lg/ml streptomycin and 100 units/ml penicillin)
in a carbon dioxide incubator (37 °C, 5% CO2, 90% RH). The cells at subconfluent
stage were harvested from the flask by treatment with trypsin [0.05% in PBS
(pH 7.4) containing 0.02% EDTA]. Cells with viability of more than 98% as
determined by trypan blue exclusion were used for determination of

A. K. Bhattacharya et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5590–5593
5593
cytotoxicity. The cell suspension of 1 Â 105 cells/ml was prepared in complete
growth medium.
7.13 (m, 3H), 7.34–7.60 (m, 6H), 7.74–7.82 (m, 4H), 8.35–8.40 (d, J = 8.4 Hz,
2H). 13C NMR (50 MHz, CDCl3): d = 38.0, 55.0, 111.0, 115.0, 117.2, 118.1, 120.8,
122.8, 124.8, 126.8, 128.8, 128.9, 129.3, 131.1, 131.5, 146.6, 148.8, 159.6. Anal.
Calcd for C28H20O2 (388.46): C, 86.57; H, 5.19. Found: C, 86.45; H, 5.06.
Stock solutions (2 Â 10À2 M) of compounds were prepared in DMSO. The stock
solutions were serially diluted with complete growth medium containing
50
l
g/ml of gentamycin to obtain working test solutions of required
14-(3,4-Dimethoxyphenyl)-14H-dibenzo[a,j]xanthene (3f): IR (CHCl3):
mmax 3018,
concentrations.
2927, 1593, 1514, 1458, 1259, 1215, 1141 cmÀ1 1H NMR (200 MHz, CDCl3):
.
In vitro cytotoxicity against six human cancer cell lines was determined16,17
d = 3.68 (s, 3H), 3.69 (s, 3H), 6.45 (s, 1H), 6.63–6.67 (m, 1H), 6.91–6.92 (m, 1H),
7.10–7.11 (m, 1H), 7.38–7.58 (m, 6H), 7.77–7.85 (m, 4H), 8.40 (d, J = 8.3 Hz,
2H). 13C NMR (50 MHz, CDCl3): d = 37.5, 55.7, 55.8, 110.8, 111.7, 117.5, 118.0,
120.4, 122.8, 124.3, 126.8, 128.9, 131.2, 131.5, 137.7, 147.6, 148.8, 149.1. Anal.
Calcd for C29H22O3 (418.48): C, 83.23; H, 5.30. Found: C, 83.03; H, 5.12.
using 96-well tissue culture plates. The 100 ll of cell suspension was added to
each well of the 96-well tissue culture plate. The cells were allowed to grow in
carbon dioxide incubator (37 °C, 5% CO2, 90% RH) for 24 h. Test materials
(100 ll) were added after 24 h of incubation to the wells containing cell
suspension. The plates were further incubated for 48 h in a carbon dioxide
incubator. The cell growth was stopped by gently layering trichloroacetic acid
14-(3-Chlorophenyl)-14H-dibenzo[a,j]xanthene (3l): IR (CHCl3):
mmax 2918, 2852,
2362, 2335, 1596, 1458, 1377, 1251 cmÀ1 1H NMR (200 MHz, CDCl3): d = 6.46
.
(50%, 50
l
l) on top of the medium in all the wells. The plates were incubated at
(s, H), 6.94–7.12 (m, 2H), 7.39–7.64 (m, 8H), 7.79–7.86 (m, 4H), 8.32 (d,
J = 8.4 Hz, 2H). 13C NMR (50 MHz, CDCl3): d = 37.8, 116.6, 118.1, 122.5, 124.5,
126.5, 126.8, 127.0, 128.4, 129.0, 129.2, 129.7, 131.1, 131.3, 134.5, 147.7, 148.8.
Anal. Calcd for C27H17ClO (392.88): C, 82.54; H, 4.36. Found: C, 82.26; H, 4.18.
14-(Benzo[d][1,3]dioxol-5-yl)-14H-dibenzo[a,j]xanthene (3n): IR (CHCl3): mmax
4 °C for 1 h to fix the cells attached to the bottom of the wells. The liquid of all
the wells was gently pipetted out and discarded. The plates were washed five
times with distilled water to remove trichloroacetic acid, growth medium, low
molecular weight metabolites, serum proteins etc and air-dried. The plates
were stained with sulforhodamine B dye (0.4% in 1% acetic acid, 100
30 min. The plates were washed five times with 1% acetic acid and then air-
dried. The adsorbed dye was dissolved in Tris–HCl Buffer (100 l, 0.01 M, pH
l
l) for
3018, 2925, 2358, 2329, 2252, 1593, 1515, 1485, 1400, 1249, 1215 cm–1 1H
.
NMR (200 MHz, CDCl3): d = 5.71 (s, 2H), 6.39 (s, 1H), 6.59 (d, J = 7.9 Hz, 1H),
6.86 (d, J = 1.8 Hz, 1H), 7.06–7.10 (m, 1H), 7.36–7.57 (m, 6 H), 7.74–7.83 (m, 4
H), 8.34 (d, J = 8.3 Hz, 2H). 13C NMR (50 MHz, CDCl3): d = 37.6, 100.8, 107.8,
108.9, 117.3, 118.1, 121.3, 122.7, 124.3, 126.8, 128.9, 131.1, 131.4, 139.1, 146.0,
147.9, 148.7. Anal. Calcd for C28H18O3 (402.44): C, 83.57; H, 4.51. Found: C,
83.39; H, 4.42.
l
10.4) and plates were gently stirred for 10 min on a mechanical stirrer. The
optical density was recorded on ELISA reader at 540 nm.
The cell growth was determined by subtracting mean OD value of respective
blank from the mean OD value of experimental set. Percent growth in presence
of test material was calculated considering the growth in absence of any test
material as 100% and in turn percent growth inhibition in presence of test
material was calculated. Adriamycin, 5-FU and paclitaxel were used as positive
control.
16. Monks, A.; Scudiero, D.; Skehan, P.; Shoemaker, R.; Paull, K.; Vistica, D.; Hose,
C.; Langley, J.; Cronise, P.; Vaigro-Wolff, A.; Gray-Goodrich, M.; Campbell, H.;
Mayo, J.; Boyd, M. J. Natl. Cancer Inst. 1991, 83, 757.
17. Skehan, P.; Storeng, R.; Scudiero, D.; Monks, A.; Mc Mohan, J.; Vistica, D.;
Warren, J. T.; Bokesch, H.; Kenney, S.; Boyd, M. R. J. Natl. Cancer Inst. 1990, 82,
1107.
15. Selected physical data: 14-(3-methoxyphenyl)-14H-dibenzo[a,j]xanthene (3b): IR
(CHCl3): mmax 3018, 2958, 2933, 1593, 1487, 1458, 1431, 1400, 1251 cm–1 1H
.
NMR (200 MHz, CDCl3): d = 3.61 (s, 3H), 6.41 (s, 1H), 6.48–6.53 (m, 1H), 7.01–