Simple and cost effective acid catalysts for efficient synthesis of 9-aryl-1,8-dioxooctahydroxanthene

Article abstract of DOI:10.1016/j.tetlet.2012.11.058

CaCl₂ (20 mol %) dispersion in DMSO or aq HBr (10 mol %) is developed as an efficient and cost effective acid catalyst for the simple one-pot synthesis of 9-aryl-1,8-dioxooctahydroxanthene derivatives. Reaction of aromatic aldehyde (1 equiv) an

Full text of DOI:10.1016/j.tetlet.2012.11.058

Tetrahedron Letters 54 (2013) 491–494
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Simple and cost effective acid catalysts for efficient synthesis of
9-aryl-1,8-dioxooctahydroxanthene
A. Ilangovan a, , S. Muralidharan a, P. Sakthivel a, S. Malayappasamy a, S. Karuppusamy a, M. P. Kaushik b
⇑
a School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, India
b Process Technology Development Division, Defence R&D Establishment, Jhansi Road, Gwalior 474002, MP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
CaCl2 (20 mol %) dispersion in DMSO or aq HBr (10 mol %) is developed as an efficient and cost effective
acid catalyst for the simple one-pot synthesis of 9-aryl-1,8-dioxooctahydroxanthene derivatives. Reac-
tion of aromatic aldehyde (1 equiv) and dimedone (2 equiv) in the presence of CaCl2 (20 mol %) in CHCl3
at room temperature gave the open chain analogue of 1,8-dioxooctahydroxanthene. Similarly, aq HBr
(10 mol %) in water at reflux temperature gave a high yield of 9-aryl-1,8-dioxooctahydroxanthene deriv-
atives. The use of inexpensive, eco-friendly and readily available catalysts, easy work-up, and isolation of
the products makes these procedures a convenient method for the synthesis of either of the products.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 5 October 2012
Revised 16 November 2012
Accepted 18 November 2012
Available online 29 November 2012
Keywords:
Aldehydes
Calcium chloride
Dimedone
Eco-friendly method
1,8-Dioxooctahydroxanthene derivatives
aq HBr
Natural products with a xanthene heterocylic motif show a vari-
ety of important biological activities.1 9-Aryl-1,8-dioxooctahydr-
oxa-nthenes contain an inherently reactive pyran ring system
and find application in the field of medicinal chemistry as antibac-
terial, antiviral, anti-inflammatory agents, novel CCR1 receptor
antagonists, and anticancer agents.2 In the field of material science
it is used as dyes,3 luminescent sensors,4 and fluorescent markers
for the visualization of bio-molecules,5 and photostable laser dyes.6
Interestingly, this structurally complex molecule can be synthe-
sized by a simple one pot tandem reaction between an aldehyde
(1 equiv) and dimedone (2 equiv) or cyclohexane-1,3-dione in the
presence of a catalyst. However, in the absence of a catalyst7 the
reaction stops just after Knoevenagel type adduct formation,7 to
give an open chain intermediate, 2,20-aryl/alkyl methylene-bis(3-
hydroxy-5,5-dimethyl-2-cyclohexene-1-one). Thus, value of a syn-
thetic method for this important class of heterocycles depends
mainly on identifying a simple, cost effective, and eco-friendly cat-
alyst and a simple work-up procedure.
metals for noble tasks’ is the current trend in the development of
metal based catalysts.22 Calcium, the fifth most frequent element
in the earth’s crust (3.4 wt%) fits very well into this. This group 2
metal is biocompatible and it shows a significant dose of metal
Lewis-acidity typical for group 3 reagents. Calcium reagents are
less exploited in organic synthesis and are beginning to gain
momentum.22 CaCl2, one of the best known dehydrating agents
and a green Lewis acid catalyst was used in organic transforma-
tions only in a few instances, for example in the synthesis of
hexanoic acid 2-(diethylamino)ethyl ester,23 Mannich bases,24
synthesis of aminophosphonic esters,25,26 Biginelli reaction27, and
aldol reaction.28 Taking into account that xanthenediones are
important biologically active compounds, we considered CaCl2, a
FDA approved, biocompatible food additive and cost-effective
Lewis acid29 as the catalyst for its synthesis.
Recently, there is a growing interest in the use of protic acids such
as HCl, H2SO4, HClO4, and its salts as catalysts for the synthesis of 9-
aryl-1,8-dioxooctahydroxanthene. HBr, a strong mineral acid than
HCl is used for several organic transformations such as the direct
conversion of methane to higher hydrocarbons,30 asymmetric
hydrogenation,31 highly selective cross-cyclodimerization,32 oxida-
tion of alkynes to diketones,33 oxidation of benzyl alcohols to benz-
aldehydes,34 and the bromination of ketones with H2O2–HBr,35 etc.
This literature background shows that CaCl2 and HBr were used
as catalysts, only to a limited extent in organic synthesis and there
remains large scope for its further application. In continuation of
our interest8 toward the development of efficient acid catalysts
Majority of the catalysts used are acids which includes Lewis
acids,8–12 modified sulfonic acids,13–16 acidic clay17–19 silica based
catalysts,20 organocatalysts, and ionic liguids.21 However, many
of these reagents have their own disadvantages, such as high cost
and catalyst loading and low yield in many cases. There remains
scope for the identification of new cost effective catalysts. ‘Cheap
⇑
Corresponding author. Tel.: +91 9865436093; fax: +91 431 2407045/2412750.
E-mail address: ilangovanbdu@yahoo.com (A. Ilangovan).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.058

492
A. Ilangovan et al. / Tetrahedron Letters 54 (2013) 491–494
for the synthesis of 1,8-dioxooctahydroxanthene derivatives, we
found that CaCl2 (20 mol %) dispersed in DMSO and aq HBr
(10 mol %) could be used as cheap and efficient catalysts. Herein
we present our results.
in DMSO (0.5 mL for 17 mg or 0.1545 mmol of CaCl2) at 85–90 °C or
aq HBr (48%, 10 mol %) in water at reflux temperature as the condi-
tion for the formation of cyclized product 4d.
To illustrate the versatility of this method a series of aromatic
and heteroaroamtic aldehydes were studied under the standard-
ized reaction condition and the results are summarized in Table
2. Substituted aldehydes (1 equiv) such as 1k–1m, containing more
than one electron donating substituents in CHCl3 at rt, gave the
open chain compounds 3k–3m at faster reaction rate. Almost all
other substrates, containing electron donating or electron with-
drawing substituents reacted slowly at nearly the same time
(15 h). This infers that, unless there is a strong electron donating
substituent, CaCl2 will not be able to discriminate the reactivity
of the aldehydes. The reaction condition was tolerant to ether, phe-
nolic –OH, furfuryl ring and cinnamoyl groups.
A wide difference in the reaction rate could be observed, when
CaCl2 (20 mol %) dispersed in DMSO, was used as the catalyst at
85–90°C for the formation of cyclized products 4a–4m. When the
electron withdrawing group or no substituent group is present at
the para position the reaction was very fast (Table 2, entries 1
and 4) whereas inverse was the case with the electron donating
group present at the para position (Table 2, entries 8 and 9,). Pres-
ence of an electron withdrawing group at sterically hindered ortho
position (Table 2, entry 2, 1b, 11 h) and deactivating meta position
(Table 2, entry 3, 1c, 10 h) slowed down the reaction. The expected
cyclized products 4n and 4o were not formed in the case of furfur-
aldehyde (1n) and cinnamaldehyde (1o). Polymerisable nature of
the starting material may be the reason.
Reaction between 4-nitrobenzaldehyde (1d) and 5,5-dimethyl-
1,3-cyclohexanedione (2) was used to check and identify one of
the cheap and efficient catalysts among some of the unexplored
acids and standardization of reaction conditions (Table 1).
LnCl3ꢀ7H2O, a hard but mild Lewis acid, in DCM or in water at re-
flux temperature gave only the open chain compound 3d as the
major product (entries 1 and 2) which was confirmed by the pres-
ence of two singlet peaks at 11.87 ppm, and 5.57 ppm, correspond-
ing to –OH and methyne proton respectively in the 1H NMR
spectrum. Similarly a singlet peak at 4.82 ppm, characteristic of
the methyne proton was observed for the cyclized compound 4d.
BaCl2 in DCM, CuSO4 in DCM, and clay catalysts KSF or bentonite
in toluene at reflux temperature, gave only the uncyclized product
3d. Similar is the case with CaCl2 (20 mol %) in CHCl3 at room tem-
perature (rt) or at reflux temperature (entry 4). Without CaCl2 no
reaction was observed in CHCl3. Even after increasing the quantity
of CaCl2 to 100 mol % and heating at 140–150°C without a solvent,
complete conversion of uncyclized product 3d did not take place
(entry 5). However, after homogenizing the reaction mixture con-
taining CaCl2 (20 mol %) with DMSO or CH3CN-DMF (4:1) the
cyclized product 4d was obtained in very good yield in just 5 h.
Interestingly, the cyclized product 4d was obtained within the
same time (5 h) when the quantity of DMSO (0.5 mL for 17 mg or
0.1545 mmol of CaCl2) was decreased just to maintain a dispersion.
Using such a small quantity of DMSO is as good as using no solvent
and just quenching the reaction mixture with water gave the de-
sired product, 4d, in high yield (87%).
Aq HBr (48%, 10 mol %), in water at reflux temperature as a
catalyst worked efficiently with differently substituted benzalde-
hydes (Table 2). Aldehydes with strong electron donating groups,
especially 1k and 1p underwent faster reaction. Aldehyde 1q
with strong electron donating dimethylamino substituent under-
went reaction at room temperature itself. HBr is a metal-free
catalyst and known to be used as a green brominating agent
in water.35 Since HBr is used only in catalytic quantity and as
a dilute solution in water, it is likely to make lower impact on
the environment.
Next, the use of a protic acid, aq HBr (48%, 10 mol %) was exam-
ined. At rt and reflux temperature the uncyclized product 3d and cy-
clized product 4d respectively were obtained in very high yield. The
yield did not change after decreasing the quantity of the catalyst
from 100 mol % to 10 mol %. Based on these results it was concluded
that the use of CaCl2 (20 mol %) in CHCl3 at rt as the condition for the
formation of uncyclized product 3d and CaCl2 (20 mol %) dispersed
Table 1
Effect of different acid catalysts and solvents on the reaction of 4-nitrobenzaldehyde with 5,5-dimethyl-1,3-cyclohexanedionea
NO2
NO2
NO2
O
O
O
O
O
Catalyst, Solvent,
Tempearature, Time
+
+
O
H
O
O
OH OH
1d
2
3d
4d
Sl. no.
Catalyst (mol %)
Solvet
Temp (°C)
Time (h)
Yieldb (%)
Product 3
Product 4
1
2
3
4
5
6
7
8
LnCl3ꢀ7H2O (100)
LnCl3ꢀ7H2O (100)
BaCl2 (100)
CaCl2 (20)
CaCl2 (100)
DCM
Water
DCM
CHCl3
Neat
DMSO
Cat. DMSO
DCM
Toluene
Toluene
Water
Water
Reflux
Reflux
Reflux
Reflux (or) rt
140–150
85–90
85–90
Reflux
Reflux
Reflux
rt
12
13
14
15
24
05
05
14
15
16
14
14
76
70
85
87
10
Nil
Nil
77
80
80
76
Nil
Nil
25
Trace
Nil
80
CaCl2 (20)
CaCl2 (20)
87
87
Anhyd.CuSO4 (100)
KSF clay (10, w/w)
Bentonite (10, w/w)
Aq HBr (48%, 10)
Aq HBr (48%, 10)
Nil
Nil
20
Nil
99
9
10
11
12
Reflux
a
All reactions were performed using 4-nitrobenzaldehyde (1 mmol), dimedone (2 mmol).
Isolated yield.
b

A. Ilangovan et al. / Tetrahedron Letters 54 (2013) 491–494
493
Table 2
Acid catalyzed synthesis of 2,20 arylmethylene bis(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one) and 3,3,6,6-tetramethyl-9-aryl-3,4,5,6,7,9-hexahydro-2H-xanthene-1,8-dione
derivatives
O
Ar
O
O
Ar
O
O
O
CaCl2 (20 mol%)
DMSO, 85-90oC
CaCl2 (20 mol%)
CHCl3, rt
+
Ar-CHO
or HBr (10 mol%)
reflux
OH
OH
O
1
4
3
2
Entry
Ar
Product
Time (h) 3/4/4⁄
Yield (%) 3/4/4⁄
Melting Point °C (3)
Melting Point °C (4)
Found
Reported
Found
Reported
1
2
3
4
5
6
7
8
C6H5–(1a)
3a/4a
3b/4b
3c/4c
3d/4d
3e/4e
3f/4f
3g/4g
3h/4h
3i/4i
13/4/14
14/11/14
15/10/12
15/5/12
14/10/12
14/11/14
14/8/15
15/14/14
15/12/12
15/11/12
10/8/4
69/85/84
85/85/71
80/80/94
87/87/99
90/88/80
86/88/80
79/83/81
91/86/79
85/86/81
89/87/94
92/92/91
94/93/80
88/87/90
186–188
244–246
201–203
194–196
238–240
200–202
142–144
139–141
146–148
188–190
193–195
178–180
203–205
192–19436
248–25237
201–20336
195–19736
241–24338
202–20436
145–14736
141–14336
146–14836
201–20336
193–19536
187–18936
205–20812
203–205
246–248
171–173
225–226
226–229
230–232
230–233
212–214
242–245
244–246
227–228
179–181
186–188
204–2069
248–2499
170–1729
221–2239
2-NO2–C6H4–(1b)
3-NO2–C6H4–(1c)
4-NO2–C6H4–(1d)
2-Br–C6H4–(1e)
2-Cl–C6H4–(1f)
4-Cl–C6H4–(1g)
4-CH3–C6H4–(1h)
4-OCH3–C6H4–(1i)
4-OH–C6H4–(1j)
39
226-229
225–2279
229–2309
210–2119
242–24340
247–2489
224–22540
170–17240
182–1839
9
10
11
12
13
3j/4j
3k/4k
3l/4l
4-OH,3–OCH3–C6H3–(1k)
3,4–OCH3–C6H3–(1l)
3,4,5–OCH3–C6H2–(1m)
13/8/11
10/9/10
3m/4m
14
3n/4n
13/—/—
95/—/—
139–141
142–14438
—
—
(1n)
C6H5CH@CH–(1o)
3-CH3–4–OCH3–C6H4–(1p)
4-N(CH3)2–C6H5–(1q)
O
15
16
17
3o/4o
3p/4p
3q/4q
14/—/12
—/—/10
—/—/5
96/—/50
—/—/91
—/—/79
213–215
—
—
215–2167b
—
—
174–176
194–196
221–223
175–17741
—
225–22612
3/4/4⁄ the reaction was carried out using CaCl2 (20 mol %) in CHCl3 at rt /CaCl2 (20 mol %) dispersed in DMSO at 85–90 °C/aq HBr (48%, 10 mol %) in water at reflux
temperature.
Based on the observations mentioned, a possible mechanism is
proposed for the catalytic activity of CaCl2 as shown in the
Scheme 1. Knoevenagel type adduct I formed under the influence
of CaCl2 is expected to undergo Michael addition with another
molecule of dimedone to form intermediate II. It is likely that the
hetereogeneous nature of the reaction in CHCl3 does not facilitate
complex formation between highly polar intermediate II and CaCl2,
hence only product 3 was obtained, whereas homogenization with
DMSO helps better complexation and the formation of compound 4.
In conclusion, we have identified CaCl2 (20 mol %), a mild Le-
wis acid, and a conventional dehydrating agent, and aq. HBr
(48%, 10 mol %), a protic acid, as new, simple, and efficient cata-
lysts for the synthesis of 9-aryl-1,8-dioxooctahydroxanthene
derivatives and its open chain analogues. This simple procedure
is applicable to a wide variety of aromatic and heterocyclic
aldehydes with electron donating as well as electron withdraw-
ing substituents. A spectacular solvent effect was observed in
CaCl2 catalyzed reaction on changing the solvent from CHCl3 to
DMSO. This study further exemplifies that calcium, a bio-com-
patible and cheap metal in the form of CaCl2 could be used for
noble tasks inorganic synthesis. Water, a safe benign, cheap
and environmental friendly solvent was used in HBr catalyzed
reactions. The procedures said above offer several advantages
such as, a simple water work-up, no purification step, high yield,
inexpensive catalyst, and minimal environmental impact, thus
making it one of the attractive and practical protocols for the
synthesis of xanthenediones. Further studies on the use of xan-
thenediones as visible light sensitizers and as better excited
state electron donors to TiO2 for application in dye sensitized so-
lar cells are in progress.
Ar
OH
O
CaCl2
H2O
CaCl2
O
O
O
O
O
H O
O
2
H
Ar
O
O
Ar
H
O
1
Ar
O
+
O
4
Ar
H
CaCl2
O
H
CaCl2
O
I
O
Ar
O
O
Ar
O
H2O
DMSO
H
85-90 ºC
O
Ar
O
CHCl3
rt
OHOIII
O
OH
II
CaCl2
CaCl2
OH
3
OH
Scheme 1. Proposed mechanism for the CaCl2 catalyzed synthesis of 9-aryl-1,8-dioxooctahydroxanthene and its open chain form.

494
A. Ilangovan et al. / Tetrahedron Letters 54 (2013) 491–494
15. Li-Bin, L.; Tong-Shou, J.; Li-Sha, H.; Meng, L.; Na, Q.; Tong-Shuang, L. E. J. Chem.
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Organisation (DRDO), New Delhi for financial support (ERIP/ER/
0703613/M/01, dt. 23.05.2008). We thank DST-FIST for use of the
NMR facility at the School of Chemistry, Bharathidasan University.
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Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
11.058.
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