An organocatalyzed expeditious synthetic route to tetrahydrobenzo[a] xanthen-11-ones via grinding technique

Article abstract of DOI:10.2174/157017811797249308

In the present study, successful implementation of grinding technique and camphor sulphonic acid (CSA) as a catalyst, has been demonstrated for the rapid synthesis of 12-aryl-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones at ambient temperatures

Full text of DOI:10.2174/157017811797249308

5
68
Letters in Organic Chemistry, 2011, 8, 568-572
An Organocatalyzed Expeditious Synthetic Route to Tetrahydro-
benzo[a]xanthen-11-ones via Grinding Technique
Pravin V. Shinde, Bapurao B. Shingate and Murlidhar S. Shingare*
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431004, Maharashtra, India
Received December 06, 2010: Revised February 15, 2011: Accepted February 15, 2011
Abstract: In the present study, successful implementation of grinding technique and camphor sulphonic acid (CSA) as a
catalyst, has been demonstrated for the rapid synthesis of 12-aryl-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-
ones at ambient temperatures, avoiding excessive/wasteful heating.
Keywords: Camphor sulphonic acid (CSA), clean synthesis, green chemistry, grinding technique, solvent-free synthesis, xan-
then-11-one derivatives.
INTRODUCTION
materials for the visualization of biomolecular assemblies
[
11] and in laser technologies [12].
Emerging area of green chemistry demands ecofriendly
organic chemical processes, considering the increasing
environmental pollution and its drastic impact on living
systems [1]. Hence, significance of greener pathways in
organic synthesis is ever-growing in order to attain the
sustainability. For many chemical operations, a major ad-
verse effect on the environment is consumption of energy for
heating and cooling. To overcome such problems, it is highly
enviable to develop efficient routes that utilize alternative
energy resources [2]. In this regard, a number of organic
chemists are engaged in the development of cleaner and
ecofriendly synthetic protocols. One of the thrust areas for
achieving rapid syntheses of organic compounds minimizing
the need of conventional heating and cooling, and use of
hazardous/volatile organic solvents involves the utilization
of conventional/mechanochemical grinding [3].
Among the xanthene based compounds, tetrahydro-
benzo[a]xanthene-11-ones hold an important place due to
their distinctive structure and great potential for further
transformations [13]. Some novel methods for the synthesis
of tetrahydrobenzo[a]xanthene-11-ones via multicomponent
condensation reaction have been reported. Several catalysts
like NaHSO
[16], dodecatungustophosphoric acid (PWA) [17], iodine
[18], InCl /P [19], pTSA/ionic liquid([bmim]BF ) [20],
tetra butyl ammonium fluoride (TBAF) [21], and proline
triflate [22] have been utilized for this transformation. How-
ever, in an era, where green methods are warranted many of
these methods are not satisfactory, due to one or more draw-
backs such as, use of halogenated solvents, low yields,
longer reaction times, catalyst loading upto 30 mol%, drastic
reaction conditions such as reflux/higher temperature, and
tedious work-up procedure. Therefore, it was thought
worthwhile to develop a new greener and rapid method for
this condensation.
4 2 4 4
.SiO [14], strontium triflate [15], Zr(HSO )
3
2
O
5
4
The use of solid state grinding to allow the reaction be-
tween two or more components has been established by
Toda and others [4]. Indeed, it is now clear that the grinding
of two or more substances together results in the interaction
of frictional and impact forces releasing high dynamic ener-
gies which play a key role in the rapid conversion of reac-
tants into product [5]. Such solvent-free “grinding reactions”
therefore offer a very attractive way of developing environ-
mentally benevolent methods for the synthesis of biody-
namic heterocycles as well as organic transformations [6].
Interest in the field of organocatalysis has increased spec-
tacularly in the last few years as a result of both the novelty
of the concept and, more importantly, the fact that the effi-
ciency and selectivity of many organocatalytic reactions
meet the standards of established organic reactions [23].
Camphor sulphonic acid (CSA) is achieving enormous sig-
nificance in organic synthesis as this catalyst is used in the
synthesis of chromans [24a], ligands [24b], and pseudogly-
cosides [24c] as an auxillary [25a], and in some polymeriza-
tion reactions [25b]. However, to the best of our knowledge,
there is a single report on the use of Camphor sulphonic acid
as a catalyst for one-pot multicomponent reaction [26]. Con-
sidering the importance of camphor sulphonic acid and in
continuation of our endeavor [27] to provide new and con-
venient synthetic routes for the construction of bioactive
heterocycles, herein, we wish to disclose highly efficient and
rapid synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xan-
then-11-ones by grinding technique using camphor sulphonic
acid as a catalyst.
Xanthenes and benzoxanthenes constitute an important
class of bioactive heterocycles and their syntheses have been
receiving great attention in the field of medicinal and phar-
maceutical chemistry owing to their broad spectrum of bio-
logical activities such as anti-bacterial [7], and anti-viral [8].
Some of the xanthene based compounds have found applica-
tions as antagonists for paralyzing the action of zoxalamine
and in photodynamic therapy [9]. In addition, their deriva-
tives can be used as dyes [10], pH sensitive fluorescent
*
Address correspondence to this author at the Department of Chemistry, Dr.
Babasaheb Ambedkar Marathwada University, Aurangabad 431004, Ma-
harashtra, India; Tel: +91 2402403311; Fax: +91 2402403113;
E-mail: prof_msshingare@rediffmail.com
1
570-1786/11 $58.00+.00
© 2011 Bentham Science Publishers

An Organocatalyzed Expeditious Synthetic Route
Letters in Organic Chemistry, 2011, Vol. 8, No. 8
569
O
CHO
OH
O
O
catalyst
O
Cl
Cl
1
2b
3
4b
Scheme 1. Standard model reaction.
RESULTS AND DISCUSSION
technique, these conventional methods gave lower yield tak-
ing more reaction time.
For our initial study, one-pot three-component reaction of
-naphthol, 4-chloro benzaldehyde and dimedone was con-
ꢀ
In particular, this success of CSA under grinding tech-
nique could be attributed to the following facts- (i) Grinding
of organic reactants causes multiple particle rupture, which
results in their size reduction and a simultaneous generation
of large new surfaces and surface energy within the system
[5b]. (ii) Thus newly formed large surfaces during grinding
come in close contact by their collision and result in the in-
teraction of frictional and impact forces, releasing high dy-
namic energies. At this stage, hot point is formed where the
molecules reach very high vibrational excitation leading to
bond breaking and making [5a]. (iii) Consequently, the
chemical reactivity of reactants considerably increases,
which results in significant acceleration of chemical reaction
leading to the formation of desired product within short reac-
tion times, avoiding undesirable side products.
sidered as a standard model reaction (Scheme 1). To affect
the model reaction, various attempts were made under grind-
ing technique by using mortar and pestle. In this regard, dif-
ferent homogeneous catalysts such as sulfamic acid, oxalic
acid, boric acid, camphor sulphonic acid, and heterogeneous
4 2
catalysts like silica supported perchloric acid (HClO -SiO )
and silica sulfuric acid were utilized for model reaction.
When sulfamic acid and oxalic acid were employed, product
was obtained in low yields (Table 1, entries 1-2). Further-
more, use of boric acid as a catalyst resulted moderate yield
(
Table 1, entry 3), whereas, intervention of camphor sul-
phonic acid delivered fascinating results with very high 94%
yield (Table 1, entry 4). In comparison, heterogeneous cata-
lysts i.e. silica supported perchloric acid and silica sulfuric
acid (SSA) afforded the desired product in good yields (Ta-
ble 1, entries 5-6), which may be due to their large surface
area. Finally, as a matter of fact, camphor sulphonic acid
stood out as an excellent catalyst furnishing high product
yield within much shorter reaction time (15 min).
It is noteworthy to point out that, reaction mixture on
pulverization in mortar immediately turns into sticky mass,
which on further grinding for appropriate time affords the
desired product in excellent yield and purity.
Concentration of a catalyst is a significant factor that ex-
clusively affects the reaction rate and product yield. To study
this, reaction was carried out at different concentrations of
CSA such as 1, 2.5, 5, and 10 mol% for 15 minutes and
product was formed in 58%, 77%, 93% and 94% yield, re-
spectively. Moreover, uncatalyzed reaction afforded the
product in trace amounts. From this, it was clear that reaction
rate enhanced effectively with increase in concentration of
Encouraged by this, model reaction was further tried un-
der solvent-free heating at 120 ºC (Table 1, entry 7) and also
by stirring method, in the presence of water as solvent at
room temperature (Table 1, entry 8). Here, water was exam-
ined keeping in view, its greener and unique properties [28].
But, these results did not show any improvement in product
yield and reaction time. Rather, when compared to grinding
a
Table 1. Screening of Catalysts
b
Entry
Catalyst
Technique
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
Sulfamic acid
Oxalic acid
Boric acid
Grinding at RT
Grinding at RT
Grinding at RT
Grinding at RT
Grinding at RT
Grinding at RT
Heating at 120 ºC
30
30
30
15
30
30
30
60
32
28
55
94
81
74
81
76
CSA
C
HClO
4
-SiO
2
C
SSA
CSA
CSA+H
2
O (2 mL)
Stirring at RT
a
b
c
Reaction conditions: 1 (1 mmol), 2b (1 mmol), 3 (1 mmol), catalyst concentration (10 mol%); Isolated yields; In these cases, amount of catalyst taken was 0.1 gm.

5
70 Letters in Organic Chemistry, 2011, Vol. 8, No. 8
Shinde et al.
OH
R
H
O
H+
-
H O
2
O
R
R
HO
H
OH
H+
OH
O
O
O
(A)
H
O
O
S
H+
OH
R
H
O
(
CSA)
H+
Source
R
R
O
O
HO
OH
R
O
O
H
(
C)
-
H O
2
(B)
OH
Fig. (1). Plausible mechanism involved in the synthesis of tetrahydrobenzo-[a]xanthene-11-ones.
catalyst upto 5 mol% without any significant difference on
further increase in catalyst concentration. It means 5 mol%
of CSA was sufficient for catalyzing the reaction smoothly.
ing mild and inexpensive catalyst under grinding technique.
This technique overcomes some of the problems associated
with excessive or wasteful heating. Remarkable advantages
of this synthetic strategy over others are- (i) decreased reac-
tion times, (ii) omission of solvents, (iii) simplified work-up
procedure, (iv) ambient reaction temperature, (v) consump-
tion of low amount of catalyst in view of atom economy, and
In accordance with the literature [14], plausible mecha-
nistic path leading to the formation of tetrahydro-
benzo[a]xanthen-11-ones can be outlined as follows- nu-
cleophilic addition of 2-naphthol to aldehyde (protonated by
camphor sulphonic acid) which leads to the ortho-quinone
methide (o-QM) intermediate (A). Subsequent Michael addi-
tion of dimedone thus generated o-QM followed by the at-
tack of phenolic –OH group of o-QM to the carbonyl carbon
of dimedone (protonated with CSA) which provides cyclic
hemiketal (B) that on dehydration affords the final product
(
vi) better energy balance in comparison with those of clas-
sical performance.
EXPERIMENTAL SECTION
All chemicals were purchased and used without any fur-
ther purification. Melting points were determined on a
Veego apparatus and are uncorrected. Conventional mortar
and pestle were used for grinding the reaction mixture. Infra-
red spectra were recorded on a Bruker spectrophotometer in
(
C) (Fig. 1).
To further establish the scope of optimized reaction con-
ditions and in order to generalize the synthetic procedure,
variety of electronically divergent aryl aldehydes were
treated with ꢀ-naphthol and dimedone, and all these sub-
strates were found to be equally amenable to these condi-
tions. Notably, liquid aldehydes take slightly more time to
complete the reaction in comparison with solid aldehydes.
Interestingly, some heteryl aldehydes also underwent the
reaction efficiently. Representative results are summarized in
Table 2. Formation of the products was confirmed on the
-1
a KBr disc, and the absorption bands are expressed in cm .
1
13
H NMR and C NMR spectra were recorded on NMR spec-
trometer AC200 in CDCl . Chemical shifts (ꢁ) are in ppm
3
relative to TMS. Mass spectra were taken on a Macro mass
spectrometer (Waters) by electro-spray method (ES).
Typical Experimental Procedure
1
13
basis of IR, H NMR, C NMR and mass spectroscopic data.
A mixture of ꢀ-naphthol 1 (1 mmol), 4-chloro benzalde-
hyde 2b (1 mmol) and dimedone 3 (1 mmol) was taken in a
mortar and pulverized well in the presence of CSA (5 mol%)
using pestle and further grinding was continued until reac-
tion got completed. Progress of the reaction was monitored
by TLC (ethyl acetate:n-hexane = 2:8). After completion of
CONCLUSION
In summary, we have developed a new and clean syn-
thetic protocol for tetrahydrobenzo[a]xanthene-11-ones us-

An Organocatalyzed Expeditious Synthetic Route
Table 2. Synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones
Letters in Organic Chemistry, 2011, Vol. 8, No. 8
571
a
O
OH
O
O
O
CSA
R
H
Grinding
R
O
1
2a-o
3
4a-o
b
Entry
Compound
R
Time (min)
Yield (%)
M.P. [Ref.] (°C)
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
Ph
25
15
25
20
20
25
15
15
15
20
20
15
40
40
88
93
91
88
90
89
91
89
94
90
87
92
81
79
67
150-151 [18]
182-184 [18]
174-176 [18]
205-206 [18]
184-186 [20]
177-178 [18]
222-224 [18]
169-170 [18]
180-181 [18]
221-223 [18]
187-189 [17]
211-212 [20]
4-Cl-Ph
4-Me-Ph
4-OMe-Ph
4-F-Ph
2-Cl-Ph
4g
4h
4i
2-NO
3-NO
4-NO
2
2
2
-Ph
-Ph
-Ph
1
1
1
1
1
1
0
1
2
3
4
5
4j
4-OH-Ph
4-Br-Ph
4k
4l
Piperonyl
2-Thienyl
2-Furfuryl
3-Indolyl
*
*
4m
176-178
*
*
4n
170-172
*
*
4o
50
202-204
a
b
*
Reaction conditions: 1 (1 mmol), 2 (1 mmol), 3 (1 mmol), CSA (5 mol%); Isolated yields; Formation of these compounds were confirmed on the basis of spectral analyses.
the reaction (15 min), reaction mixture was quenched with 5
mL water to get product (4b). Then it was collected by sim-
ple filtration. Thus obtained product was crystallized with
aqueous ethanol to afford pure product without need of any
further purification.
12-(4-(Benzo[d][1,3]dioxol-5-yl))-9,9-dimethyl-8,9,10,
1
12-tetrahydrobenzo-[a]xanthen-11-one (4l)
H
NMR
(CDCl , 200 MHz): ꢀ 1.01 (s, 3H), 1.13 (s, 3H), 2.28 (s, 2H),
3
2.56 (s, 2H), 5.62 (s, 1H), 5.80 (d, 1H, J=1.2 Hz) 5.84 (d,
1H, J=1.2 Hz), 6.59 (d, 1H, J=8Hz), 6.77 (td, 2H, J=8, 2Hz),
.26-7.48 (m, 3H), 7.72-7.79 (m, 2H), 7.95 (d, 1H, J=8Hz);
7
Spectral Data of Representative compounds
13
3
C NMR (50 MHz, CDCl ): ꢀ 26.9, 29.6, 32.6, 34.7, 41.4,
1
2-(4-chlorophenyl)-9,9-dimethyl-8,9,10,12-tetrahydr-
51.7, 101.5, 111.2, 113.8, 115.1, 117.6, 122.3, 123.8, 124.8,
128.3, 128.8, 129.4, 130.1, 131.6, 132.5, 140.0, 145.5, 146.7,
1
obenzo[a]xanthen-11-one (4b) H NMR (CDCl
3
, 200
-
1
MHz): ꢀ 1.00 (s, 3H), 1.16 (s, 3H), 2.29 (d, 2H, J= 4Hz),
149.1, 163.4, 197.6; IR (KBr, cm ): v 2948, 1639, 1592,
+
2
7
2
1
1
1
.59 (d, 2H, J= 4Hz), 5.70 (s, 1H), 7.17-7.43 (m, 7H, Ar-H),
.77-7.89 (m, 3H, Ar-H); C NMR (50 MHz, CDCl ): ꢀ
3
1368, 1234, 1178, 1039, 829; ES-MS: 399.21 (M ).
1
3
1
2-(thiophen-2-yl)-9,9-dimethyl-8,9,10,12-tetrahydro-
1
7.1, 29.3, 32.5, 34.3, 41.9, 50.9, 113.1, 116.8, 117.3, 123.9,
25.0, 127.2, 128.7, 129.4, 130.2, 131.5, 131.9, 132.6, 143.3,
3
benzo[a]xanthen-11-one (4m) H NMR (CDCl , 200 MHz):
-
1
ꢀ 1.05 (s, 3H), 1.14 (s, 3H), 2.34 (s, 2H), 2.56 (s, 2H), 6.02
(s, 1H), 6.72-6.75 (m, 2 H), 6.99 (dd, 1H, J=4Hz), 7.40- 7.51
45.1, 150.8, 164.2, 196.9; IR (KBr, cm ): v 2952, 1648,
+
597, 1373, 1231, 1184, 823; ES-MS: 389.14 (M ), 391.13
13
+
(m, 2H), 7.75-7.82 (m, 2H), 8.01 (d, 1H, J=8Hz); C NMR
(
M +2).
(
3
50 MHz, CDCl ): ꢀ 27.5, 29.4, 32.9, 34.8, 41.8, 51.2, 113.4,
115.2, 117.7, 123.0, 126.5, 127.2, 127.9, 129.6, 130.3, 130.8,
132.1, 132.6, 135.4, 139.7, 144.6, 162.7, 197.6; IR (KBr, cm
1
2-(2-nitrophenyl)-9,9-dimethyl-8,9,10,12-tetrahydro-
1
-
benzo[a]xanthen-11-one (4g) H NMR (CDCl
3
, 200 MHz):
1
ꢀ
2
5
0.87 (s, 3H), 1.10 (s, 3H), 2.19 (d, 2H, J=4Hz), 2.52 (d,
H, J=4 Hz), 6.56 (s, 1H), 7.02 (d, 1H, 8Hz), 7.18-7.46 (m,
H, Ar-H), 7.75-7.88 (m, 3H, Ar-H), 8.51 (d, 1 H, J=8 Hz);
): v 2947, 1638, 1593, 1372, 1225, 1061, 724; ES-MS:
+
361.13 (M ).
1
3
12-(furan-2-yl)-9,9-dimethyl-8,9,10,12-tetrahydroben-
C NMR (50 MHz, CDCl
3
): ꢀ 27.2, 29.5, 32.1, 34.6, 41.1,
1
zo[a]xanthen-11-one (4n) H NMR (CDCl
3
, 200 MHz): ꢀ
5
1
1
1
1.4, 112.9, 116.6, 117.8, 124.1, 124.7, 126.9, 127.4, 128.1,
28.7, 129.5, 129.9, 131.3, 131.7, 132.2, 134.0, 141.5, 149.2,
1
.06 (s, 3H), 1.16 (s, 3H), 2.38 (s, 2H), 2.62 (s, 2H), 6.02 (s,
-
1
1H), 6.76-6.81 (m, 2 H), 7.01 (dd, 1H, J=4Hz), 7.46- 7.58
63.9, 196.1; IR (KBr, cm ): v 2957, 1651, 1595, 1537,
13
+
(m, 2H), 7.80-7.85 (m, 2H), 8.03 (d, 1H, J=8Hz); C NMR
376, 1348, 1226, 1174, 818; ES-MS: 400.16 (M ).
(
3
50 MHz, CDCl ): ꢀ 27.6, 29.6, 33.0, 34.9, 41.6, 51.3, 112.2,

5
72 Letters in Organic Chemistry, 2011, Vol. 8, No. 8
Shinde et al.
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14.7, 123.5, 123.9, 126.6, 127.1, 128.2, 128.8, 129.5, 129.9,
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-
1
1
)
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[
+
3
45.09 (M ).
1
2-(indol-3-yl)-9,9-dimethyl-8,9,10,12-tetrahydroben-
1
zo[a]xanthen-11-one (4o) H NMR (CDCl
3
, 200 MHz): ꢀ
[14]
0
6
7
.91 (s, 3H), 1.12 (s, 3H), 2.24 (d, 2H, J=6Hz), 2.61 (s, 2H),
.00 (s, 1H), 6.98 (td, 2H, J=8, 2Hz), 7.16 (d, 2H, J=4 Hz ),
.31-7.37 (m, 3H), 7.48 (d, 1H, J=8 Hz ), 7.71 (d, 2H,
1
3
J=8Hz), 8.09 (d, 1H, J=8Hz), 8.16 (s, 1H, -NH); C NMR
50 MHz, CDCl ): ꢀ 26.5, 28.9, 31.8, 34.2, 41.2, 51.0, 111.8,
14.1, 116.9, 117.7, 119.7, 120.2, 123.9, 124.2, 125.1, 126.1,
28.5, 129.2, 129.6, 130.7, 131.9, 135.4, 141.2, 148.8, 164.2,
[15]
(
3
1
1
1
1
[16]
-
1
97.1; IR (KBr, cm ): v 3363, 1648, 1597, 1365, 1229,
+
175, 1142, 796; ES-MS: 394.19 (M ).
2
009, 1, 57-63.
[
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Authors are thankful to the Head, Department of Chemis-
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rangabad, for providing the laboratory facility.
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