A facile approach for the synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]xanthenes under solvent-free condition

Article abstract of DOI:10.1016/j.tetlet.2009.11.064

A facile, efficient and environment-friendly protocol for the synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]xanthenes has been developed by one-pot condensation of 2-naphthol with aliphatic and aromatic aldehydes in the presence of P₂O₅

Full text of DOI:10.1016/j.tetlet.2009.11.064

Tetrahedron Letters 51 (2010) 442–445
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A facile approach for the synthesis of 14-aryl- or
alkyl-14H-dibenzo[a,j]xanthenes under solvent-free condition
*
Ram Kumar, Ganesh Chandra Nandi, Rajiv Kumar Verma, M. S. Singh
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A
facile, efficient and environment-friendly protocol for the synthesis of 14-aryl- or alkyl-14H-
Received 14 October 2009
Revised 12 November 2009
Accepted 15 November 2009
Available online 1 December 2009
dibenzo[a,j]xanthenes has been developed by one-pot condensation of 2-naphthol with aliphatic and aro-
matic aldehydes in the presence of P2O5 or InCl3 as catalysts under solvent-free conditions. The present
approach offers the advantages of clean reaction, simple methodology, short reaction time, high yield,
easy purification, and economic availability of the catalyst.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Dibenzoxanthene, 2-Naphthol
Aldehydes
P2O5
InCl3
Solvent-free conditions
Organic syntheses involving greener process and under solvent-
free conditions have been investigated world wide due to stringent
environment and economic regulations.1 In addition, the imple-
mentation of several transformations in a single manipulation is
highly compatible with the goals of sustainable and green chemis-
try. The synthesis of xanthenes, especially benzoxanthenes (Fig. 1),
has attracted great interest in recent years due to their wide range
of biological and pharmaceutical properties such as antiviral,2 anti-
bacterial,3 and anti-inflammatory4 activities as well as sensitizers
in photodynamic therapy5 (PDT; a method of treating tumours
by combined use of a photosensitizer and light) and antagonists
for the paralyzing action of zoxazolamine.6 Furthermore, these
compounds can be used as leuco-dyes,7 pH-sensitive fluorescent
materials for visualization of biomolecules8 and utilized in laser
technologies.9
In view of great importance of benzoxanthenes, various meth-
ods have been reported for the construction of benzoxanthene
scaffolds, including the reaction of b-naphthol with formamide,10
2-naphthol-1-methanol,11 carbon monoxide,12 the cyclocondensa-
tion reaction of 2-hydroxyaromatic aldehydes with 2-tetralone,13
the reaction of benzaldehydes with acetophenones,14 palladium-
catalyzed cyclization of polycyclic aryltriflate esters15 and trapping
of benzynes by phenols.16 Recently, the synthesis of benzoxanth-
enes has been achieved by the condensation of aldehydes with b-
naphthol by cyclodehydration in the presence of various catalysts
(Scheme 1), such as AcOH–H2SO4,17 p-TSA,18 MeSO3H,19 sulfamic
acid,20a ionic liquid,20b iodine,21 heteropolyacid,22 silica sulfuric
acid,23 Amberlyst-15,24 cyanuric chloride,25 LiBr,26 CoPy2Cl2,27
29a
Yb(OTf)3,28a Sc[N(SO2C8F17)2]3,28b NaHSO4
and Al(HSO4)3.29b
However, these methods show varying degrees of success as
well as limitations such as prolonged reaction times, low yields,
use of toxic solvents, requirement of excess of reagents/catalysts,
laborious work-up procedures, the requirement of special appara-
tus, or harsh reaction conditions. Thus, the development of an
alternate milder and clean procedure is highly demanding for the
synthesis of benzoxanthenes, which surpasses those limitations.
The use of phosphorous pentoxide has given many advantages in
organic synthesis. P2O5 is mild and selective catalyst, easy to han-
dle and readily biodegradable, which has earlier been used widely
in various organic transformations such as Beckmann rearrange-
ment,30 olefin dimerisation,31 tetrahydropyranylation of alcohols32
and formation of 1,1-diacetate.33
As part of our research program directed towards the develop-
ment of highly expedient methods and the syntheses of diverse
2
12
1
11
10
3
4
13
H
R
14
5
9
O
7
8
6
* Corresponding author. Tel.: +91 542 6702502; fax: +91 542 2368127.
E-mail address: mssinghbhu@yahoo.co.in (M.S. Singh).
Figure 1. Benzoxanthenes.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.064

R. Kumar et al. / Tetrahedron Letters 51 (2010) 442–445
443
R
O
R
O
R
O
R
O
Fe(HSO4)3
Amberlyst-15
125 o
C
p-TSA
125 oC
MeSO3H
NaHSO4
R
O
OH
R
Yb(OTf)3
2
RCHO
PTC
ILs, 110 0C
H2SO4.SiO2
O
Ionic liquid
R
O
CoPy2Cl2
R
O
R
O
R
O
Scheme 1. Preparation of 14-substituted-14H-dibenzo[a,j] xanthenes by condensation of 2-naphthol and aldehydes.
tions with the above catalysts separately, and it was found that
the reactions proceeded smoothly in shorter reaction time giving
good yields. The results show that under solvent-free conditions,
the catalytic activities of P2O5 and InCl3 were increased and in all
the cases P2O5 was found to be a better catalyst than InCl3
(Table 2).
R
OH
InCl3 or P2O5
2
RCHO
+
°
O
80 C, Solvent-free
2
1
3
A test reaction using 4-bromobenzaldehyde and b-naphthol
(mole rate 1:2) at 80 °C without catalyst was performed in order
to establish the real effectiveness of the catalyst. It was found that
no conversion to product occurred even after 10 h of heating. We
then focused on optimising the reaction conditions. In order to
evaluate the most appropriate catalyst loading a model reaction
using 4-bromobenzaldehyde and b-naphthol (mole rate 1:2) was
carried out using 10 mol %, 15 mol % and 20 mol % of P2O5 at
80 °C without solvent. It was found that 20 mol% of the catalyst
showed maximum yield in minimum time (Table 1, entry 20). A
O
OH
OH
H
R
R
R
H
O
O
OH
o-QMs
OH
O
O
O
H
Δ
Table 1
OH
R
Comparison of the efficiency of P2O5 with other catalysts for the synthesis
dibenzoxanthenes
R
R
− H2O
Entry
Catalyst
Temp (°C)
Time
Yieldsa (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
p-TSA
125
125
125
125
125
125
125
120
110
130
100
100
110
90
90
85
80
80
80
80
80
15–24 h
4–8 h
6–11 h
6–12 h
23 min
3 h
0.5–2 h
2–4 h
2–3 h
81–93
78–95
74–93
90–95
72–90
64–78
80–94
75–87
87–93
80–84
78–91
80–91
78–95
82–95
74–91
65–94
60–90
72–84
78–92
87–97
76–88
H2SO4ÁSiO2
SelecfluorÒ
Sulfamic acid
Fe(HSO4)3
K5CoW12O40Á3H2O
Amberlyst-15
Montmorillonite K10
PTC
Scheme 2. Proposed mechanism for the synthesis of 14-substituted-14H-
dibenzo[a,j]xanthenes.
heterocyclic compounds34 of biological significance, we herein dis-
close a new, convenient, and one-pot synthesis of 14-aryl- or alkyl-
14H-dibenzo[a,j]xanthenes from aldehydes and b-naphthol cata-
lyzed by P2O5 or InCl3 under solvent-free conditions in excellent
yields (Scheme 2). To the best of our knowledge in the open liter-
ature, one-pot syntheses of benzoxanthenes catalyzed by P2O5 or
InCl3 have not been reported.
In our initial experiments, the condensation of 4-bromobenzal-
dehyde and b-naphthol (mole rate 1:2) was performed in the pres-
ence of catalysts InCl3 and P2O5 separately, in refluxing
dichloromethane (DCM). Both catalysts facilitated the formation
of dibenzo[a,j]xanthene in good yields (92% and 97%, respectively),
but require long reaction time (7 h and 6 h, respectively). Then, it
was decided to carry out the reactions under solvent-free condi-
LiBr
1–2 h
1–2 h
Dowex-50 W
Heteropoly acid
Yb(OTf)3/ILs
Iodine
NaHSO4
CoPy2Cl2
0.5–1.5 h
3–7 h
2.5–5 h
0.5–1 h
2–8 h
73 h
5–8 h
3–5 h
0.5–1 h
2–4 h
H2SO4
P2O5 (10 mol %)
P2O5 (15 mol %)
P2O5 (20 mol %)
P2O5 (25 mol %)
a
Yields of isolated pure products.

444
R. Kumar et al. / Tetrahedron Letters 51 (2010) 442–445
Table 2
P2O5 and InCl3 catalyzed synthesis of 14-substituted-14H-dibenzo[a,j]-xanthenes
Entry
R
Product
Method A (P2O5)
Method B (InCl3)
Mp (°C)/(lit.)
Time (min)/yield (%)
Time (min)/yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
4-NO2ÁC6H4
3-NO2ÁC6H4
2-NO2ÁC6H4
4-ClÁC6H4
3a
3b
3c
3d
3e
3f
3g
3h
3i
50/97
52/90
30/84
47/92
52/85
50/89
55/84
45/88
52/86
52/94
50/91
60/84
58/83
45/90
56/83
58/88
60/82
62/75
70/54
62/70
60/75
60/94
60/82
45/75
55/84
60/75
60/80
65/76
55/78
60/80
60/85
65/82
75/75
75/72
55/82
65/77
65/79
70/75
75/71
75/50
75/62
75/67
310–311 (311–312)21
210–211 (211–212)19
214–215 (214)21
287–288 (288–289)19
214–215 (215–216)19
202–203 (203–204)19
227–228 (228–229)19
182–183 (181–183)21
242–243
2-ClÁC6H4
4-OMeÁC6H4
4-MeÁC6H4
C6H5
3-OHÁC6H4
4-BrÁC6H4
3j
3k
3l
295–296 (296)26
239–240 (239)19
206–208
4-FÁC6H4
3-OEt, 2-OHÁC6H3
2-OH, 3-OMeÁC6H3
2,4-Cl2ÁC6H3
2-OMeÁC6H4
4-OHÁC6H4
2-OHÁC6H4
Me2CH
3m
3n
3o
3p
3q
3r
3s
3t
266–267
227–228 (227)23
258–260 (260–261)20b
139–140 (140)28b
127–128
152–153 (153–154)19
178–180
C6H5CH@CH
Me2CHCH2
CH3CH2
112–113 (113–114)19
149–150 (151–152)19
3u
further increasing of catalyst loading does not affect the yield, but
slightly slowed down the reaction (Table 1, entry 21). Thus,
20 mol % of catalyst was found to be the optimal quantity and suf-
ficient to push the reaction forward.
and economic availability of the catalyst make the ecofriendly pro-
cedure an attractive alternative to the existing methods for the
synthesis of 14-substituted-14H-dibenzo[a,j]-xanthene. Further
application of P2O5 and o-QM intermediate on the extension of this
protocol are ongoing in our group.
With the optimized conditions in hand, to explore the general-
ity of the reaction, we extended our study with different aromatic
and aliphatic aldehydes to prepare a series of dibenzo[a,j]xanth-
enes (3a–u, Table 2). In all the cases the corresponding benzox-
anthenes were obtained in good to excellent yields. However,
with aromatic aldehydes with electron-withdrawing groups as
substrates, the reaction time is shorter than those with electron-
donating groups. Though meta- and para-substituted aromatic
aldehydes gave good results, ortho-substituted aromatic aldehydes
(such as 2-nitrobenzadehyde) gave lower yields because of the ste-
ric effects. Interestingly, 4-hydroxybenzaldehyde also gave the de-
sired product in excellent yield. On the other hand, reactions with
Acknowledgements
R.K. is grateful to MDLLC, Michigan, USA for financial assistance.
G.C.N. and R.K.V. are thankful to Council of Scientific and Industrial
Research (CSIR), New Delhi for senior research fellowship.
References and notes
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(o-QMs)35 and the bulkiness of aldehydes.
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sults for the preparation of dibenzoxanthenes36 in Table 2, which
shows that P2O5 is the most efficient catalyst with respect to the
reaction time and temperature and exhibits broad applicability in
terms of yield.
A mechanistic rationale portraying the probable sequence of
events is given in Scheme 2. The reaction proceeds through the
in situ formation of ortho-quinone methide intermediate by the
nucleophilic addition of b-naphthol to aldehyde (a highly reactive
and ephemeral intermediate35 that have been extensively har-
nessed by nature), which is further attacked by second molecule
of b-naphthol followed by cyclodehydration to give the benzox-
anthenes. Phosphorous pentoxide acts as a water scavenger, which
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In conclusion we have developed a facile protocol for the
synthesis of 14-substituted-14H-dibenzo[a,j]-xanthenes36
. The
remarkable catalytic activity of P2O5 and InCl3 is superior to the re-
ported other catalytic methods with respect to reduced reaction
times. The pure products were obtained by column chromatogra-
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36. General procedure for synthesis of 14-substituted-14H-dibenzo[a,j]xanthenes: To a
nicely ground mixture of 2-naphthol (2 mmol) and aldehyde (1 mmol), P2O5
(20 mol %) or InCl3 (30 mol %) was added and the reaction mixture was heated
at 80 °C for the appropriate time (Table 2). After completion (monitored by
TLC), the reaction mixture was cooled to room temperature, water (50 mL) was
added and extracted with ethyl acetate (3 Â 25 mL). The combined extract was
washed with brine, dried over anhydrous MgSO4 and evaporated to dryness
under vacuum to give the crude product, which was purified by column
chromatography over silica gel (hexane/AcOEt = 19:1). The desired pure
products were characterized by spectral (IR, 1H and 13C NMR) and analytical
data, and by comparison of their physical and spectral data with those of
known benzoxanthenes.17–31Data for some selected compounds: 14-(3-
Hydroxyphenyl)-14H-dibenzo[a,j]-xanthene (3i): mp 242–243 °C. IR (KBr,
cmÀ1): 3412, 1588, 1507, 1413, 1258, 1237, 813; 1H NMR (300 MHz, CDCl3):
d 8.37 (d, J = 8.7 Hz, 2H), 7.83–7.76 (m, 4H), 7.59–7.54 (m, 2H), 7.47–7.37 (m,
4H), 7.18 (d, J = 7.8 Hz, 1H), 7.03 (t, J = 8.1 Hz, 1H), 6.86 (s, 1H), 6.64–6.43 (m,
2H), 4.54 (s, 1H); 13C NMR (75 MHz, CDCl3): d 154.8, 147.9, 145.9, 130.6, 130.2,
128.6, 128.1, 128.0, 126.0, 123.5, 121.9, 120.1, 117.2, 116.3, 114.6, 112.8, 37.0.
Anal. Calcd for C27H18O2: C, 86.61; H, 4.85. Found: C, 86.50; H, 4.73.
14-(4-Bromophenyl)-14H-dibenzo[a,j]-xanthene (3j): mp 295–296 °C. IR (KBr,
cmÀ1): 3056, 2922, 1622, 1590, 1514, 1456, 1392, 1243, 1231, 1207, 1143,
1062, 957, 816, 772, 741; 1H NMR (300 MHz, CDCl3): d 8.32 (d, J = 8.4 Hz, 2H),
7.85–7.78 (m, 4H), 7.60–7.55 (m, 2H), 7.48–7.37 (m, 5H), 7.25–7.23 (m, 3H),
6.46 (s, 1H); 13C NMR (75 MHz, CDCl3): d 147.9, 144.8, 131.2, 130.7, 130.6,
130.0, 129.2, 128.6, 127.0, 124.6, 123.2, 119.4, 117.6, 116.8, 36.8. Anal. Calcd
for C27H17BrO: C, 74.15; H, 3.92. Found: C, 74.23; H, 3.80.
14-(3-Ethoxy-2-hydroxyphenyl)-14H-dibenzo[a,j]-xanthene (3l): mp 206–208 °C.
IR (KBr, cmÀ1): 3412, 2961, 1583, 1509, 1408, 1258, 1247, 1032, 957; 1H NMR
(300 MHz, CDCl3): d 8.70 (d, J = 8.4 Hz, 2H), 7.80–7.73 (m, 4H), 7.58–7.36 (m,
6H), 6.85 (s, 1H), 6.76 (d, J = 7.2 Hz, 1H), 6.54–6.44 (m, 3H), 4.03–3.97 (m, 2H),
1.46 (t, J = 6.9 Hz, 3H). 13C NMR (75 MHz, CDCl3): d 148.7, 145.2, 140.2, 132.0,
131.6, 130.7, 128.4, 128.3, 126.7, 124.1, 123.4, 122.3, 120.3, 118.1, 117.8, 108.9,
64.5, 36.4, 14.8. Anal. Calcd for C29H22O3: C, 83.23; H, 5.30. Found: C, 83.15; H,
5.16.
14-Isopropyl-14H-dibenzo[a,j]xanthene (3r): mp 152–153 °C. IR (KBr, cmÀ1):
3057, 2953, 1593, 1509, 1438, 1388, 1245, 819, 746; 1H NMR (300 MHz,
CDCl3): d 8.70 (d, J = 8.4 Hz, 2H), 7.80–7.73 (m, 4H), 7.58–7.36 (m, 6H), 6.76 (d,
J = 7.2 Hz, 1H), 2.11 (m, 1H), 1.06 (d, J = 6.9 Hz, 6H); 13C NMR (75 MHz, CDCl3):
d 148.2, 145.7, 139.7, 132.4, 131.1, 129.3, 128.8, 127.8, 126.9, 124.7, 123.9,
122.7, 119.8, 118.6, 117.5, 109.9, 36.7, 30.4, 14.8. Anal. Calcd for C24H20O: C,
88.85; H, 6.21. Found: C, 88.67; H, 6.13.