Surfactant catalyzed convenient and greener synthesis of tetrahydrobenzo[a]xanthene-11-ones at ambient temperature

Article abstract of DOI:10.3762/bjoc.7.9

An efficient and greener protocol for the synthesis of 12-aryl-8,9,10,12- tetrahydrobenzo[a]xanthen-11-one using tetradecyltri-methylammonium bromide (TTAB) at room temperature in water is described.

Full text of DOI:10.3762/bjoc.7.9

Surfactant catalyzed convenient and greener
synthesis of tetrahydrobenzo[a]xanthene-11-ones at
ambient temperature
Pravin V. Shinde, Amol H. Kategaonkar, Bapurao B. Shingate
and Murlidhar S. Shingare*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 53–58.
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada
University, Aurangabad-431 004, India
doi:10.3762/bjoc.7.9
Received: 14 September 2010
Accepted: 29 November 2010
Published: 13 January 2011
Email:
Murlidhar S. Shingare* - prof_msshingare@rediffmail.com
*
Corresponding author
Associate Editor: J. Murphy
Keywords:
© 2011 Shinde et al; licensee Beilstein-Institut.
License and terms: see end of document.
green chemistry; in-water synthesis; surfactant;
tetradecyltrimethylammonium bromide (TTAB); xanthene
Abstract
An efficient and greener protocol for the synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one using tetradecyltri-
methylammonium bromide (TTAB) at room temperature in water is described.
Introduction
The development of novel synthetic methodologies to facilitate for good reactivity, alternatives for improving the solubility of
the preparation of specific molecules is an intense area of organic substrates that may ultimately help in expanding the
research. In this regard, efforts have been constantly made to scope of water-based organic syntheses have been investigated
introduce new methodologies that are efficient and more [3]. Incorporation of surface-active agents (surfactants) in
compatible with the environment. One of the most desirable aqueous media has been proved to enhance the reactivity of
approaches to address this challenge is a search for surrogates water mediated reactions via the formation of micelles or vesic-
for commonly employed organic solvents from various health ular cavities. The use of micellar and vesicle forming surfac-
and environmental reasons [1]. From the green chemistry point tants as catalysts in water is widespread and has been studied
of view, water would be the perfect solvent to carry out chem- for a number of different synthetic transformations/ multicom-
ical operations since it is safe, non-toxic, inexpensive and poses ponent reactions in water [4].
no threat to the environment [2]. However, water is rarely used
or even considered as a solvent for organic reactions. One of the Multicomponent reactions (MCRs) have emerged as an
principal reasons is undoubtedly the limited solubility of most extremely powerful tool in combinatorial chemistry and drug
organic compounds in pure water. Since solubility is important discovery, since they offer significant advantages over conven-
53

Beilstein J. Org. Chem. 2011, 7, 53–58.
tional linear stepwise syntheses, in terms of improving classical Results and Discussion
organic reactions, for promoting new reactions and for the In our initial study, reaction of 4-chlorobenzaldehyde, β-naph-
development of straightforward synthetic routes to bioactive thol and dimedone in water was considered as a standard model
heterocycles [5].
reaction (Scheme 1). During this investigation, efforts were
mainly focused on a variety of surfactants. In this regard,
Xanthenes and benzoxanthenes constitute important classes of different cationic surfactants such as cetyltrimethylammonium
biodynamic heterocycles and their synthesis has received much bromide (CTAB), methyltriphenylphosphonium bromide
attention especially in the field of medicinal/pharmaceutical (MTPPB) and cetylpyridinium chloride (CPC) as well as an
chemistry due to their wide range of biological/pharmacologi- anionic surfactant, sodium dodecyl sulfate (SDS), were utilized
cal activities, e.g., antibacterial [6], anti-inflammatory [7] and at ambient temperature.
antiviral [8]. Some xanthene based compounds have found
application as antagonists for inhibiting the action of
zoxalamine and in photodynamic therapy [9,10]. In addition,
their derivatives can be used as dyes [11,12], pH sensitive fluo-
rescent materials for the visualization of biomolecular assem-
blies [13] and in laser technologies [14,15].
Among the xanthene based compounds, tetrahydro-
benzo[a]xanthene-11-ones are of interest and have great poten-
Scheme 1: Standard model reaction.
tial for further synthetic transformations [16,17]. Some novel
methods for the synthesis of tetrahydrobenzo[a]xanthene-11-
ones via multicomponent condensation reaction have been From these preliminary studies, it was observed that the anionic
developed and catalysts such as NaHSO4.SiO2 [18], strontium surfactant SDS and cationic surfactants CPC and MTPPB gave
triflate [19], Zr(HSO4)4 [20], dodecatungstophosphoric acid the desired product albeit in low yields, i.e., 57%, 32% and 59%
(
PWA) [21], iodine [22], InCl3/P2O5 [23] and p-toluenesul- respectively (Table 1, entries 1–3). In contrast, the cationic
fonic acid/ionic liquid([bmim]BF4) [24] have been employed surfactant CTAB accelerated the model reaction to afford the
for their synthesis. However, in an era where green methods are desired product in good 81% yield (Table 1, entry 4). From this,
desirable many of these methods are unsatisfactory as they it was concluded that cationic surfactants, particularly quater-
involve the use of halogenated solvents, catalyst loadings of up nary ammonium where the counterion is bromide are far supe-
to 30 mol %, low yields, drastic reaction conditions, prolonged rior to other surfactants for efficient catalysis.
reaction times and tedious isolation procedures. All of these
disadvantages make further improvements for the synthesis of Encouraged by these results, we then investigated some more
such molecules essential. Recently, synthetic methods that cationic surfactants, particularly quaternary ammonium bro-
involve tetra(n-butyl)ammonium fluoride (TBAF) [25] and mides. For this purpose, we utilized TEAB, TBAB and TTAB
proline triflate [26] in water have been described. However, the in the model reaction. After a careful study, decreased reaction
major problems associated with these routes are the need for times and increased product yields were observed with
higher/reflux conditions and longer reaction times. Therefore, it increasing alkyl chain length of the surfactant up to a C14 chain
was thought worthwhile to develop a new greener and more length. Longer alkyl chains led to a slight decrease in product
convenient method for the preparation of tetrahydro- yield. TEAB did not afford more than 43% yield of product,
benzo[a]xanthene-11-ones.
even after 4 h (Table 1, Entry 5), whereas TBAB gave a 68%
yield (Table 1, Entry 6). By comparison, TTAB influenced both
Considering the significance of surfactants and in continuation the yield and the reaction time and gave the product in an excel-
of our program [27-33] to develop new and convenient syn- lent 85% yield (Table 1, Entry 7) within only 2.5 h and proved
thetic protocols for the construction of bioactive heterocycles, to be a better catalyst than CTAB. The success of TTAB as an
herein we wish to report a highly efficient synthesis of 12-aryl- efficient catalysis could be related to the number of carbon
8
,9,10,12-tetrahydrobenzo[a]xanthen-11-ones using tetradecyl- atoms in the hydrophobic alkyl chain of surfactant, which
trimethylammonium bromide (TTAB) in aqueous micellar reaches saturation, in this case, C14, after which the reaction
form. The pronounced catalytic effect of TTAB in organic syn- yield and reaction time are independent of the surfactant with
thesis is described for the first time.
alkyl chains larger than C14.
54

Beilstein J. Org. Chem. 2011, 7, 53–58.
Table 1: Screening of surfactants.a
Entry
Surfactant
Temperature (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
SDS
RTc
RTc
RTc
RTc
RTc
RTc
4
4
4
3
4
4
57
CPC
32
MTPPB
CTAB
TEAB
TBAB
TTAB
TTAB
TTAB
TTAB
TTAB
59
81
43
68
RTc (2.5, 5, 10, 15, 20)d 2.5
55, 72, 85, 88, 89
60
3
3
3
3
85
81
76
71
80
10
1
100
Reflux
1
areaction conditions: 1 (1 mmol), 2b (1 mmol), 3 (1 mmol), surfactant (10 mol %), in water (5 mL); bisolated yields; croom temperature (RT) was 40 °C;
dnumbers in parentheses indicate concentration of surfactant and corresponding yields are given in the "Yield" column.
It is of note to point out that the addition of TTAB converted an increase in the effective concentration of the organic reac-
the initially floating reaction mass into a homogeneous mixture, tants, which might increase the reaction rate via a concentration
which on stirring became a white turbid emulsion. This obser- effect. In micellar solution, organic substrates are pushed away
vation implies, that there was formation of micelles or micelle- from water molecules towards the hydrophobic core of micelle
like colloidal aggregates. Indeed, formation of spherical droplets thus inducing efficient collisions between organic
droplets in water was confirmed by optical microscopy [34,35] substrates which eventually enhance the reaction rate and result
(
Figure 1).
in rapid reactions in water. The hydrophobic interior of the
micelles swiftly excludes the water molecules generated during
the reaction, thus shifting the equilibrium towards the desired
product that ultimately leads to an increase in the reaction yield
[
34-36]. This explanation is schematically represented in
Figure 2.
We next investigated the effect of temperature on the rate of
reaction. For this purpose the reaction was carried at higher
temperatures, i.e., 60 °C, 80 °C, 100 °C and under reflux condi-
tions. However, increasing the temperature failed to enhance the
reaction rate substantially. In point of fact, higher temperatures
lowered the product yield slightly, accompanied by some impu-
rities (Table 1, Entries 8–11).
Figure 1: Optical micrograph of the reaction mixture. (A) normal view,
(B) magnified view. (Scale bar = 2.5 µm).
Catalyst concentration is a significant factor that exclusively
affects the reaction rate and product yield. To study this, the
It is well known that dehydration reactions are difficult to carry reaction was performed at different concentrations of TTAB,
out in water, since water generated during the reaction needs to i.e., 2.5, 5, 10, 15, and 20 mol %, and gave the product in 55%,
be removed to shift the equilibrium towards the side of the 72%, 85%, 88% and 89% yield, respectively (Table 1, entry 7).
dehydrated product. Nevertheless, by introducing a surfactant Thus, it was clear that reaction rate increased with increasing
(
TTAB), dehydration was successfully achieved in water. The catalyst concentration up to 15 mol % without any significant
catalytic effect of the micellar solution of TTAB may be attrib- difference on further increasing the catalyst concentration. It
uted to the hydrophobic nature of organic substrates. Formation means 15 mol % of surfactant was sufficient for catalyzing the
of emulsion droplets takes place in water in the presence of reaction effectively.
surfactant and substrate molecules. It is suggested that most of
the organic substrates are concentrated in these spherical In accordance with the literature [15], a plausible mechanistic
droplets, which act as a hydrophobic reaction sites and results in path for the formation of tetrahydrobenzo[a]xanthen-11-ones
55

Beilstein J. Org. Chem. 2011, 7, 53–58.
Figure 2: Schematic diagram representing the role of TTAB.
can be outlined as follows: nucleophilic addition of 2-naphthol To generalize the synthetic procedure, various electronically
to the aldehyde to give an intermediate ortho-quinone methide divergent aryl aldehydes were treated with β-naphthol and
(
o-QM), subsequent Michael addition of dimedone to the o-QM dimedone under the optimized reaction conditions and all these
followed by attack of the phenolic –OH group of the o-QM at substrates were found to be equally amenable to these condi-
the carbonyl carbon of dimedone to yield a cyclic hemiketal that tions. Interestingly, some heteroaryl aldehydes also underwent
on dehydration affords the final product.
the reaction smoothly. Representative results are summarized in
Table 2: Synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones.a
Entry
Comp.
R
Time (h)
Yieldb (%)
mp [ref.] (°C)
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
Ph
3
87
88
85
89
85
84
89
89
91
88
87
91
79
74
66
150–151 [20]
182–184 [20]
174–176 [20]
205–206 [20]
184–186 [22]
177–178 [20]
222–224 [20]
169–170 [20]
180–181 [20]
221–223 [20]
187–189 [19]
211–212 [22]
176–178c
4-Cl-C6H4
4-Me-C6H4
4-MeO-C6H4
4-F-C6H4
2-Cl-C6H4
2-NO2-C6H4
3-NO2-C6H4
4-NO2-C6H4
4-HO-C6H4
4-Br-C6H4
Piperonyl
2-Thienyl
2-Furfuryl
3-Indolyl
2.5
3
3.5
2.5
3
4g
4h
4i
2
3
2.5
3
10
11
12
13
14
15
4j
4k
4l
3
4
4mc
4nc
4oc
6
6
170–172c
8
202–204c
areaction conditions: 1 (1 mmol), 2 (1 mmol), 3 (1 mmol), TTAB (15 mol %) in water (5 mL) at RT (40 °C); bisolated yields; cformation of these com-
pounds were confirmed on the basis of spectral analyses.
56

Beilstein J. Org. Chem. 2011, 7, 53–58.
Table 2. Formation of the products was confirmed by IR, 1H 127.2, 128.7, 129.4, 130.2, 131.5, 131.9, 132.6, 143.3, 145.1,
NMR, 13C NMR and mass spectrometry.
150.8, 164.2, 196.9; IR (KBr, cm−1): v 2952, 1648, 1597, 1373,
231, 1184, 823; ES-MS: 389.14 [M+], 391.13 [M+ + 2].
1
Conclusion
In conclusion, we have developed an exceedingly simple, mild 12-(2-nitrophenyl)-9,9-dimethyl-8,9,10,12-tetrahydro-
and clean synthetic protocol for the synthesis of tetrahy- benzo[a]xanthen-11-one (4g) 1H NMR (CDCl3, 200 MHz): δ
drobenzo[a]xanthene-11-ones. In this method, the use of TTAB 0.87 (s, 3H), 1.10 (s, 3H), 2.19 (d, 2H, J = 4Hz), 2.52 (d, 2H, J
for an organic transformation has been described for the first = 4 Hz), 6.56 (s, 1H), 7.02 (d, 1H, J = 8 Hz), 7.18–7.46 (m, 5H,
time. Water is not only an inexpensive and environmentally Ar-H), 7.75–7.88 (m, 3H, Ar-H), 8.51 (d, 1 H, J = 8 Hz); 13C
benign solvent, but also plays an important role in reactivity and NMR (50 MHz, CDCl3): δ 27.2, 29.5, 32.1, 34.6, 41.1, 51.4,
selectivity. Surfactants catalyze the reaction efficiently at room 112.9, 116.6, 117.8, 124.1, 124.7, 126.9, 127.4, 128.1, 128.7,
temperature with short reaction times without using any harmful 129.5, 129.9, 131.3, 131.7, 132.2, 134.0, 141.5, 149.2, 163.9,
organic reagents and solvents.
196.1; IR (KBr, cm−1): v 2957, 1651, 1595, 1537, 1376, 1348,
226, 1174, 818; ES-MS: 400.16 [M+].
1
Experimental
General: All chemicals were purchased and used without any 12-[4-(benzo[d][1,3]dioxol-5-yl)]-9,9-dimethyl-8,9,10,12-
further purification. Melting points were determined on a Veego tetrahydrobenzo[a]xanthen-11-one (4l) 1H NMR (CDCl3,
apparatus and are uncorrected. Infrared spectra were recorded 200 MHz): δ 1.01 (s, 3H), 1.13 (s, 3H), 2.28 (s, 2H), 2.56 (s,
on a Bruker spectrophotometer as KBr discs, and the absorp- 2H), 5.62 (s, 1H), 5.80 (d, 1H, J = 1.2 Hz) 5.84 (d, 1H, J = 1.2
tion bands are expressed in cm−1. 1H NMR and 13C NMR Hz), 6.59 (d, 1H, J = 8 Hz), 6.77 (td, 2H, J = 8, 2 Hz),
spectra were recorded on an NMR spectrometer AC200 in 7.26–7.48 (m, 3H), 7.72–7.79 (m, 2H), 7.95 (d, 1H, J = 8 Hz);
CDCl3, chemical shifts (δ) are given in ppm relative to TMS. 13C NMR (50 MHz, CDCl3): δ 26.9, 29.6, 32.6, 34.7, 41.4,
Mass spectra were taken on a Macro mass spectrometer 51.7, 101.5, 111.2, 113.8, 115.1, 117.6, 122.3, 123.8, 124.8,
(
Waters) by the electrospray method (ES).
128.3, 128.8, 129.4, 130.1, 131.6, 132.5, 140.0, 145.5, 146.7,
49.1, 163.4, 197.6; IR (KBr, cm−1): v 2948, 1639, 1592, 1368,
1
Optical microscopy measurements: A drop of the turbid reac- 1234, 1178, 1039, 829; ES-MS: 399.21 [M+].
tion mixture was diluted with distilled water, and then subjected
to light microscopy measurement using an ordinary light micro- 12-(thiophen-2-yl)-9,9-dimethyl-8,9,10,12-tetrahydro-
scope under 400× magnification.
benzo[a]xanthen-11-one (4m) 1H NMR (CDCl3, 200 MHz): δ
.05 (s, 3H), 1.14 (s, 3H), 2.34 (s, 2H), 2.56 (s, 2H), 6.02 (s,
Typical experimental procedure: To a mixture of β-naphthol 1H), 6.72–6.75 (m, 2 H), 6.99 (dd, 1H, J = 4Hz), 7.40–7.51 (m,
(0.144 g, 1 mmol), 4-chlorobenzaldehyde 2b (0.140. g, 1 2H), 7.75–7.82 (m, 2H), 8.01 (d, 1H, J = 8 Hz); 13C NMR (50
1
1
mmol) and dimedone 3 (0.140 g, 1 mmol) in water (5 mL), was MHz, CDCl3): δ 27.5, 29.4, 32.9, 34.8, 41.8, 51.2, 113.4, 115.2,
added TTAB (0.050 g, 15 mol %). This reaction mixture was 117.7, 123.0, 126.5, 127.2, 127.9, 129.6, 130.3, 130.8, 132.1,
allowed to stir vigorously at room temperature. Progress of the 132.6, 135.4, 139.7, 144.6, 162.7, 197.6; IR (KBr, cm−1): v
reaction was monitored by TLC (ethyl acetate:n-hexane = 2:8). 2947, 1638, 1593, 1372, 1225, 1061, 724; ES-MS: 361.13 [M+].
After completion of the reaction (2.5 h), the solid obtained was
collected by filtration and washed successively with warm 12-(furan-2-yl)-9,9-dimethyl-8,9,10,12-tetrahydro-
water and aqueous ethanol. The crude product was recrystal- benzo[a]xanthen-11-one (4n) 1H NMR (CDCl3, 200 MHz): δ
lized from ethanol to afford the pure product which required no 1.06 (s, 3H), 1.16 (s, 3H), 2.38 (s, 2H), 2.62 (s, 2H), 6.02 (s,
further purification. (It is important to note here that some crude 1H), 6.76–6.81 (m, 2 H), 7.01 (dd, 1H, J = 4Hz), 7.46–7.58 (m,
products were obtained as sticky solids and in such cases, 2H), 7.80–7.85 (m, 2H), 8.03 (d, 1H, J = 8 Hz); 13C NMR (50
before isolation of product, they were treated with aqueous MHz, CDCl3): δ 27.6, 29.6, 33.0, 34.9, 41.6, 51.3, 112.2, 114.7,
ethanol.)
123.5, 123.9, 126.6, 127.1, 128.2, 128.8, 129.5, 129.9, 130.3,
32.2, 133.4, 136.1, 152.7, 162.9, 198.2; IR (KBr, cm−1): v
2-(4-chlorophenyl)-9,9-dimethyl-8,9,10,12-tetrahy- 2951, 1641, 1596, 1362, 1231, 1058, 735; ES-MS: 345.09 [M+].
drobenzo[a]xanthen-11-one (4b) 1H NMR (CDCl3, 200
MHz): δ 1.00 (s, 3H), 1.16 (s, 3H), 2.29 (d, 2H, J = 4 Hz), 2.59 12-(indol-3-yl)-9,9-dimethyl-8,9,10,12-tetrahydro-
d, 2H, J = 4 Hz), 5.70 (s, 1H), 7.17–7.43 (m, 7H, Ar-H), benzo[a]xanthen-11-one (4o) 1H NMR (CDCl3, 200 MHz): δ
1
1
(
7
2
.77–7.89 (m, 3H, Ar-H); 13C NMR (50 MHz, CDCl3): δ 27.1, 0.91 (s, 3H), 1.12 (s, 3H), 2.24 (d, 2H, J = 6 Hz), 2.61 (s, 2H),
9.3, 32.5, 34.3, 41.9, 50.9, 113.1, 116.8, 117.3, 123.9, 125.0, 6.00 (s, 1H), 6.98 (td, 2H, J = 8, 2 Hz), 7.16 (d, 2H, J = 4 Hz ),
57

Beilstein J. Org. Chem. 2011, 7, 53–58.
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1
1
16.9, 117.7, 119.7, 120.2, 123.9, 124.2, 125.1, 126.1, 128.5,
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Acknowledgements
2
7.Kale, B. Y.; Shinde, A. D.; Sonar, S. S.; Shingate, B. B.; Kumar, S.;
Ghosh, S.; Venugopal, S.; Shingare, M. S. Beilstein J. Org. Chem.
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The Authors thank the Head of the Department of Chemistry of
the Dr. Babasaheb Ambedkar Marathwada University,
Aurangabad, for providing laboratory facilities.
28.Sapkal, S. B.; Shelke, K. F.; Shingate, B. B.; Shingare, M. S.
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