Efficient one-pot synthesis of 14-aryl-14H-dibenzo[a,j]xanthene derivatives promoted by niobium pentachloride

Article abstract of DOI:10.2478/s11696-014-0597-8

A simple and efficient procedure for the synthesis of 14-aryl-14H-dibenzo[a,j ]xanthene derivatives through one-pot condensation of 2-naphthol with aryl aldehydes in the presence of niobium pentachloride is described. The reactions were carried out using

Full text of DOI:10.2478/s11696-014-0597-8

Chemical Papers
DOI: 10.2478/s11696-014-0597-8
ORIGINAL PAPER
Efficient one-pot synthesis of 14-aryl-14 -dibenzo[ , ]xanthene
derivatives promoted by niobium pentachloride
Aloisio de Andrade Bartolomeu*, Manoel Lima de Menezes,
Luiz Carlos da Silva Filho
Department of Chemistry, Faculty of Sciences, Laboratory of Organic Synthesis and Processes (LaOSP),
Sa˜o Paulo State University (UNESP), 17033-360, Bauru, Sa˜o Paulo, Brazil
Received 30 January 2014; Revised 26 March 2014; Accepted 4 April 2014
A simple and efficient procedure for the synthesis of 14-aryl-14H-dibenzo[a,j ]xanthene deriva-
tives through one-pot condensation of 2-naphthol with aryl aldehydes in the presence of niobium
pentachloride is described. The reactions were carried out using 25 mole % of NbCl5, at ambient
temperature, and in acceptable reaction times, affording excellent yields of the products.
c
ꢀ 2014 Institute of Chemistry, Slovak Academy of Sciences
Keywords: niobium pentachloride, Lewis acid, one-pot condensation, 14-aryl-14H-dibenzo[a,j ]xanthene
derivatives
Introduction
The synthesis of xanthenes and their derivatives
has received significant attention due to their various
pharmacological activities, such as antibacterial and
antifungal (Limsuwan et al., 2009; Omolo et al., 2011),
analgesic and anti-inflammatory (Hafez et al., 2008;
Banerjee et al., 2011), anti-viral (Jamison et al., 1990;
Naidu et al., 2012), antioxidant (Nishiyama et al.,
1998), anti-cancer (Chatterjee et al., 1996; Song et al.,
2013), cytotoxic (Bhattacharya et al., 2009) and anti-
proliferative properties (Kumar et al., 2010). These
compounds have also been employed as sensitisers in
Fig. 1. Examples of natural xanthenes.
photodynamic therapy (Neckers, 1989; Gutiérrez &
García, 1998; Chang et al., 2008), in the food industry
as additives (Tanaka, 2001; Chequer et al., 2012; Odo
et al., 2012; Qi et al., 2012), in laser technologies (Ah-
mad et al., 2002; De et al., 2005) and as fluorescent
materials for the visualisation of biomolecules (Knight
& Stephens, 1989; Liu et al., 2001; Hilderbrand &
Weissleder, 2007). In addition, xanthenes and their
derivatives can be used as sensitisers in dye-sensitised
solar cells (DSSCs) (Hara et al., 2000; Pradhan et al.,
2007; Guillén et al., 2008; Sharma et al., 2009) and as
hole-transporting materials in organic light-emitting
devices (OLEDs) (Chu et al., 2012).
Xanthenes are rare in natural plants and have been
isolated from only two plant families, Compositae and
Fabaceae (Ravindranath & Seshadri, 1973; Thangadu-
rai et al., 2001; Cao et al., 2007, 2008; Huang et
al., 2010). Compounds I, II and III are examples
of natural xanthenes (Fig. 1). Blumeaxanthene (I )
*Corresponding author, e-mail: aloisio andrade@fc.unesp.br

ii
A. de Andrade Bartolomeu et al./Chemical Papers
and blumeaxanthene II (II ) have been isolated from
Blumeariparia DC (Compositae), a Chinese medici-
nal herb traditionally used to treat gynecological dis-
orders (Huang et al., 2010) and the 3-isopropyl-9a-
methyl-1,2,4a,9a-tetrahydroxanthene (III ) has been
isolated from Indigofera longeracemosa Boiv. Ex.
Baill. (Fabaceae) for use in traditional Indian medicine
as an antidote for all snake venoms (Thangadurai et
al., 2001).
Due to the applicability of the xanthenes and ben-
zoxanthenes, several synthetic protocols have been
reported, including the reaction of alkylphenoxy-
magnesium halides with triethylorthoformate (Casir-
aghi et al., 1973), the palladium-catalysed cyclisa-
tion of polycyclic aryltriflate esters (Wang & Har-
vey, 2002), the cyclocondensation reaction between
2-tetralone and 2-hydroxyarylaldehydes under acidic
conditions (Jha & Beal, 2004), and the reaction of
the condensation of cyclic 1,3-diketones with aryl
aldehydes catalysed by molybdate sulphonic acid
(Karami et al., 2013). Furthermore, 14-aryl-14H-
dibenzo[a,j]xanthene derivatives can be prepared by
the condensation reaction of 2-naphthol with aryl
aldehydes in the presence of different Lewis acids
(Ding et al., 2007; Wang et al., 2008; Urinda et
al., 2009; Wang et al., 2010; Soleimani et al., 2011;
Tabatabaeian et al., 2011; Mirjalili et al., 2011; Zolfigol
et al., 2012; Rao et al., 2012; Cao et al., 2013) and
Brønsted acids (Gong et al., 2009; Hajipour et al.,
2010; Rahmatpour, 2011; Ghassamipour & Sardarian,
2012; Prasad et al., 2012; Zare et al., 2012).
Table 1. Optimisation of reaction between 2-naphthol (IV )
and benzaldehydea (Va)
Entry Amount of catalyst/mole % Time/h Yieldb/%
1
2
3
4
0
10
25
50
120
48
48
5
60
90
75
24
a) Reaction conditions: 2-naphthol (1.0 mmol), benzaldehyde
(0.5 mmol) and NbCl5 (0–50 mole %) in CH2Cl2 at ambient
temperature; b) yield of isolated products.
NbCl5 used was re-crystallised by sublimation follow-
ing the procedure detailed in the literature (Alves,
1986). Thin-layer chromatography was performed on
0.2 mm Merck 60 F254 silica gel aluminium sheets,
which were visualised with an ammonium cerium(IV)
sulphate/phosphomolybdic acid/sulphuric acid/water
mixture. Infrared (IR) spectra were recorded on a
Bruker Vertex 70 Fourier Transform spectrometer us-
ing a Bruker Platinum ATR unit. Bruker DPX 300,
DRX 400 and DRX 500 spectrometers were employed
for the NMR spectra (CDCl3 solutions) using TMS
as internal reference for 1H and CDCl3 as an inter-
nal reference for 13C NMR spectra. MS analyses were
performed on a PerkinElmer Clarus 680 GC equipped
with a Clarus SQ 8T mass spectrometer. The melt-
ing point of compounds were determined by DSC
analyses. DSC measurements were performed using a
PerkinElmer DSC 8500 with power compensation.
Several metallic Lewis acids (Saini et al., 2006;
Wang et al., 2008, 2010; Urinda et al., 2009; Soleimani
et al., 2011; Tabatabaeian et al., 2011; Mirjalili et al.,
2011; Rao et al., 2012; Zolfigol et al., 2012; Cao et al.,
2013) have been employed as catalysts in this type of
reaction whilst, in the current work, studies on the
General procedure for one-pot synthesis of 14-
aryl-14H-dibenzo[a,j]xanthene derivatives
To a solution of NbCl5 (25 mole %) in anhydrous
CH2Cl2 (2.0 mL), maintained at ambient tempera-
ture under N2 atmosphere, were added a solution of
2-naphthol (IV ) (1.0 mmol) and the respective alde-
hyde (Va–Vu) (0.5 mmol) in anhydrous CH2Cl2 (4.0
mL). Once the addition was completed, stirring was
continued at the same temperature for 24 or 48 h
(Table 2). The reaction mixture was quenched with
H2O addition (3.0 mL) and the product was extracted
with CH2Cl2 (10.0 mL). The organic layer was sepa-
rated and washed with saturated aqueous NaHCO3
(3 × 10.0 mL), brine (2 × 10.0 mL), dried over an-
hydrous MgSO4, filtered and the solvent from the
filtrate evaporated under vacuum. The residue was
re-crystallised from ethanol to afford pure products
(VIa–VIu). The full experimental details and spectro-
scopic characterisations (1H and 13C NMR, infrared
and mass spectrometry) for compounds VIa–VIu can
be found in the Supplementary Content section of this
article’s web page.
use of niobium pentachloride (NbCl
5) as a catalyst
in these one-pot reactions are reported. NbCl5 forms
chloro-bridged dimers in its solid state in which each
niobium atom is surrounded by a distorted octahe-
dron of chlorine atoms. NbCl5 is highly electrophilic
and can act as a Lewis acid, promoting several organic
transformations (Batista et al., 1997; Kobayashi et al.,
2000; Constantino et al., 2001, 2004, 2005, 2007; An-
drade, 2004; da Silva-Filho et al., 2005, 2008; Andrade
& Rocha, 2006; Wang et al., 2007; Polo et al., 2008;
Cai et al., 2010; Hou et al., 2010, 2011; da Silva et
al., 2012; dos Santos & da Silva, 2012; Lacerda et al.,
2012; Liu et al., 2013; Martins et al., 2013; dos Santos
et al., 2013; Bian et al., 2014; Nasseri et al., 2014).
Experimental
All reactions were performed under an atmosphere
of N2. CH2Cl2 was distilled from CaH2. All chemi-
cals were purchased from Sigma–Aldrich (St. Louis,
MO, USA) and used without further purification.
Results and discussion
The reaction between 2-naphthol (IV ) and ben-

A. de Andrade Bartolomeu et al./Chemical Papers
iii
Table 2. Synthesis of 14-aryl-14H-dibenzo[a,j ]xanthene derivatives promoted by NbCl5
M.p./◦C
Reportedb
Product
R
Time/h
Yielda/%
Found
VIa
VIb
VIc
VId
VIe
VIf
VIg
VIh
VIi
C6H5
4-FC6H4
3-FC6H4
2-FC6H4
4-ClC6H4
3-ClC6H4
2-ClC6H4
4-BrC6H4
3-BrC6H4
48
48
48
24
48
48
24
48
48
24
48
24
24
24
24
24
24
24
24
48
48
90
91
93
95
95
96
96
97
97
96
70
91
93
98
97
95
96
98
92
96
88
190
247
222
233
295
200
220
300
195
215
201
182
261
329
225
233
201
208
302
285
213
190–191 (Ding et al., 2007)
245 (Poupelin et al., 1978a)
220 (Poupelin et al., 1978a)
234 (Poupelin et al., 1978a)
295 (Rostamizadeh et al., 2008)
199–200 (Kumari et al., 2008)
221 (Poupelin et al., 1978a)
300–302 (Jin et al., 2006)
194–195 (Rostamizadeh et al., 2008)
216–217 (Nasr-Esfahani & Abdizadeh, 2013)
202–203 (Gong et al., 2009)
180–181 (Bhattacharya & Rana, 2007)
258–259 (Mirjalili et al., 2011)
326–328 (Tabatabaeian et al., 2011)
224–226 (Ding et al., 2007)
233–234 (Rostamizadeh et al., 2008)
197–198 (Mirjalili et al., 2011)
208 (Poupelin et al., 1978a)
VIj
VIk
VIl
2-BrC6H4
4-OCH3C6H4
3-OCH3C6H4
2-OCH3C6H4
4-NO2C6H4
3-NO2C6H4
4-CH3C6H4
3-CH3C6H4
2-CH3C6H4
4-[(CH3)3C]C6H4
4-C6H5C6H4
4-SCH3C6H4
Vim
VIn
VIo
VIp
VIq
VIr
VIs
VIt
VIu
303 (Prasad & Nath, 2012)
281 (Poupelin et al., 1978b)
216 (Hunnur et al., 2008)
a) Yield of isolated products; b) all products are known and their melting points were compared with values already reported.
Fig. 2. NbCl5-promoted synthesis of 14-aryl-14H-dibenzo[a,j ]xanthenes.
zaldehyde (Va) was used as a reaction model in order
to develop a protocol for the optimisation of the re-
action conditions. The results are summarised in Ta-
ble 1, where it can be seen that this reaction was
strongly influenced by the amount of catalyst. The
best results were obtained using 25 mole % of cata-
lyst in 48 hours (Table 1, entry 3) and 50 mole % in
24 hours (Table 1, entry 4). Accordingly, 25 mole %
of catalyst was selected for use in these reactions due
to its low concentration, which resulted in excellent
yields.
After optimisation of the reaction conditions, other
benzaldehydes (Vb–Vu) were examined in the pres-
ence of 25 mole % of NbCl5. These one-pot reactions
were carried out under an inert atmosphere of N2, at
ambient temperature and using anhydrous CH2Cl2 as
solvent. The products obtained were purified by re-
crystallisation in ethanol and characterised by spec-
troscopic (1H NMR, 13C NMR and FTIR) and spec-
trometric (GC-MS) methods. The results are sum-
marised in Fig. 2 and Table 2. In general, the results
given in Table 2 show that excellent yields of 14-aryl-
14H-dibenzo[a,j]xanthene derivatives were obtained
under mild conditions in 24 or 48 hours. Furthermore,
it was observed that this protocol permitted the use
of aryl aldehydes containing electron-withdrawing and
electron-donating groups.
In Table 3, the result obtained in this work (En-
try 1) for the synthesis of 14-(4-chlorophenyl)-14H-
dibenzo[a,j]xanthene (VIe) is compared with other
studies detailed in the literature (Khosropour et al.,
2005; Das et al., 2006; Nagarapu et al., 2007; Dabiri
et al., 2008a, 2008b; Urinda et al., 2009; Wang et
al., 2010; Rahmatpour & Aalaie, 2011; Zolfigol et al.,
2012). The results presented in Table 3 show that,
regardless of the catalyst used, in the presence or ab-
sence of solvent, high temperatures are necessary to
obtain good yields of xanthene VIe. When compared
with other catalysts, NbCl5 proved to be the only one
capable of promoting this one-pot reaction with good
yields at ambient temperature.
A possible mechanism for the condensation reac-
tion of 2-naphthol (IV ) with aryl aldehydes (Va–Vu)
promoted by NbCl5 is presented in Fig. 3. The mech-

iv
A. de Andrade Bartolomeu et al./Chemical Papers
Table 3. Comparison of efficiency of several catalysts for synthesis of 14-(4-chlorophenyl-14H-dibenzo[a,j ]xanthene (VIe)
Entry
Catalyst
Solvent
Temp/◦C
Time/h
Yield c/%, reference
1
2
3
4
5
6
7
8
9
NbCla5
Alumb
DCM
DCM
DCM
DCE
H2O
a.t.
a.t.
Reflux
Reflux
100
100
120
125
90
48
24
6
18
7
4
2
2.5
3
2
95 (This paper)
Traces (Dabiri et al., 2008a)
Traces (Rahmatpour & Aalaie, 2011)
93 (Khosropour et al., 2005)
80 (Urinda et al., 2009)
90 (Dabiri et al., 2008a)
89 (Dabiri et al., 2008b)
95 (Khosropour et al., 2005)
90 (Das et al., 2006)
PS/AlClb3
PTSAb
In(OTf)b3
Alumb
H2O
MK-10a
PTSAb
I2a
None
None
None
None
None
None
10
11
12
WO3/ZrOb2
FeCla3
125
100
110
92 (Nagarapu et al., 2007)
96 (Wang et al., 2010)
97 (Zolfigol et al., 2012)
10
0.1
WClb6
a) 4-Chlorobenzaldehyde/2-naphthol = 0.5 mmol : 1.0 mmol; b) 4-chlorobenzaldehyde/2-naphthol = 1.0 mmol : 2.0 mmol; c) yield
of isolated products.
Fig. 3. Proposed reaction mechanism for one-pot synthesis of 14-aryl-14H-dibenzo[a,j ]xanthenes promoted by NbCl5.
anism for the reaction is assumed to be the same as
that proposed by Zolfigol et al. (2012) using WCl6 as
a Lewis acid. Initially, the aryl aldehyde is activated
by NbCl5. Next, the carbonyl group of the activated
aldehyde is attacked by 2-naphthol, producing the in-
termediate VII. Intermediate VIII is produced by the
hydrogen transfer in VII. Subsequently, after elimi-
nation of a water molecule, o-quinone methide (IX )
is generated. Intermediate IX is activated by NbCl5
and acts as a Michael acceptor. Subsequently, this in-
termediate undergoes a Michael addition of another
molecule of 2-naphthol and produces X, which is then

A. de Andrade Bartolomeu et al./Chemical Papers
v
converted into XI by the NbCl5-promoted ring clos-
ing of X. Finally, after elimination of a water molecule
from XI, the dibenzoxanthenes (VIa–VIu) are formed.
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Solvent-free synthesis of 14-aryl-14H-dibenzo[a,j ]xanthenes
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In conclusion, a simple and efficient procedure
for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthene
derivatives under mild conditions was described. The
reactions were carried out using 25 mole % of NbCl5,
at ambient temperature, and in acceptable reaction
times, affording excellent yields of the products.
Acknowledgements. The authors would like to thank Funda-
¸ca˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP)
(Procs. 2012/23821-7 and 2013/08697-0), Conselho Nacional
de Desenvolvimento Científico e Tecnológico (CNPq), Coor-
denadoria de Aperfei¸coamento de Pessoal do Nível Superior
(CAPES), and Pró-Reitoria de Pesquisa da UNESP (PROPe-
UNESP) for their financial support. The authors would also
like to thank CBMM – Companhia Brasileira de Metalurgia e
Minerac¸a˜o for the NbCl5 samples. Special thanks are accorded
to T. M. R. Gianeti and A. W. Ballarinin from the Sa˘o Paulo
State University in Botucatu for the MS and DSC analyses.
Supplementary data
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(Efficient one-pot synthesis of 14-aryl-14H-dibenzo
[a,j]xanthene derivatives promoted by niobium pen-
tachloride) can be found in the online version of this
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