Alumina-sulfuric acid catalyzed eco-friendly synthesis of xanthenediones

Article abstract of DOI:10.1016/j.catcom.2011.12.036

Alumina-sulfuric acid has been developed as a cost-effective, recyclable, heterogeneous solid acid catalyst for the eco-friendly synthesis of 9-aryl-1,8-dioxododecahydroxanthenes and 9-aryl-3,3,6,6-tetramethyl-1,8- dioxooctahydroxanthenes through the cond

Full text of DOI:10.1016/j.catcom.2011.12.036

Catalysis Communications 20 (2012) 17–24
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
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Short Communication
Alumina-sulfuric acid catalyzed eco-friendly synthesis of xanthenediones
Amit Pramanik a, Sanjay Bhar b,
⁎
a
Department of Chemistry, Taki Government College, North 24 Pgs–743 429, India
Department of Chemistry, Organic Chemistry Section, Jadavpur University, Kolkata–700032, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Alumina-sulfuric acid has been developed as a cost-effective, recyclable, heterogeneous solid acid catalyst
for the eco-friendly synthesis of 9-aryl-1,8-dioxododecahydroxanthenes and 9-aryl-3,3,6,6-tetramethyl-
1,8-dioxooctahydroxanthenes through the condensation of cyclic 1,3-diketones with different aryl and
heteroaryl aldehydes.
Received 30 September 2011
Received in revised form 9 December 2011
Accepted 20 December 2011
Available online 5 January 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Solid acid catalyst
Xanthenedione
Eco-compatibility
Cyclic 1,3-diketone
Aldehyde
Selectivity
1. Introduction
aldehydes. Therefore, a cost-effective, catalytically efficient, recyclable,
eco-compatible protocol for the synthesis of the xanthenediones is
Organic reactions using heterogeneous acid catalysts [1,2] are
extremely important for the development of sustainable chemical
processes because of cost-effectiveness, better selectivity and opera-
tional simplicity. Various protic acids have been used as homoge-
neous catalysts [3,4] to construct organic frameworks having
synthetic and biological importance. Bronsted acids supported on
inorganic supports under heterogeneous condition have come out
as better alternatives to the conventional homogeneous protocols
[5–10].
still in demand from the environmental perspective.
Alumina-sulfuric acid [where OSO3H moiety is supposed to be
immobilized on alumina support] has not been investigated much ex-
cept in few reactions, e.g. esterification [23] and oximation followed
by Beckmann rearrangement [24]. As a continuation of our endeavor
[25] to dig out novel applications of this easily accessible, cheap and
stable solid acid catalyst, we report herein its remarkable catalytic be-
havior for the cost-effective eco-friendly synthesis of xanthenediones
(Scheme 1).
Xanthene derivatives possess agricultural, antibacterial, antiviral,
anticoagulant, anticancer, diuretic, spasmolytic and anti-inflammatory
properties which are also widely applied in photodynamic therapy
[11,12]. They are also used in cosmetics, pigments, biodegradable agro-
chemicals, components of dyes for laser technology and fluorescent
materials for visualization of biomolecules [13]. 9-Aryl-xanthene-1, 8-
diones also serve as valuable synthons due to inherent reactivity of
the in-built pyran ring. They are prepared by different methods includ-
ing TBAHSO4 in aq dioxane [12], wet cyanuric chloride [13], TiO2–SO4−2
[14], polyaniline p-toluenesulfonate [15], PPA–SiO2 [16], NaHSO4–SiO2
[17], Fe+3–montmorillonite [18], 1-methylimidazolium trifluoroace-
tate [19], ω-4°–ammoniumalkyl sulfonate [20], polytungstozincate
acid [21] and cellulose-sulfuric acid [22]. But often they require harsh
reaction conditions, use exotic, expensive and sometimes toxic reagents
having no scope of reuse and have limited applicability to simple
2. Experimental
2.1. Preparation of alumina-sulfuric acid
Freshly distilled chlorosulfonic acid (0.2 ml, 3 mmol) was slowly
added to neutral alumina (10 g, dried at 130 °C for 2 h) with stirring.
The liberated HCl was removed through a CaCl2 drying tube under re-
duced pressure to a water trap. Then it was kept in this condition for
1 h at room temperature, washed successively with water (10 times),
ethanol (3 times) and dried at 130 °C for 3 h. No precipitate was
obtained on treatment of silver nitrate solution with the aqueous ex-
tract of the solid after washing it ten times with distilled water. More-
over, when the washed solid was mixed with powdered potassium
dichromate and concentrated sulfuric acid, heated and the vapor
was treated with the ethanolic solution of 1,5-diphenylcarbazide, no
violet color was developed. The aforesaid observation conclusively
proved the absence of Cl− on the solid catalyst after thorough wash-
ing with distilled water.
⁎
Corresponding author. Fax: +91 33 24146223.
E-mail addresses: sanjaybharin@yahoo.com, sanjay_bhar@chemistry.jdvu.ac.in
(S. Bhar).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.12.036

18
A. Pramanik, S. Bhar / Catalysis Communications 20 (2012) 17–24
O
Ar
O
O
Ar
Alumina-sulfuric acid
+
H
EtOH ,reflux
R
O
R
R
O
O
R
R
R
2
1 eq
1
2 eq
3
Scheme 1. Synthesis of xanthenediones using alumina-sulfuric acid.
Chlorosulfonic acid was added in different proportions on neutral
dialkyl substituents of 7 which prevented the access of the second
molecule of 1b at close proximity. Therefore, novel mono- and bis-
xanthenedione skeletons can be constructed through proper posi-
tioning of the formyl groups on the aromatic ring of 2 as well as
alkyl groups in 1.
Aryl alkyl ketone 2m remained unaffected in the present reaction
condition (entry 16 in Table 2). Therefore, 1a underwent chemose-
lective condensation at the formyl group of 8 keeping the oxo moie-
ty intact and produced 9 with an additional oxo-functionality in
the aryl ring of the 9-aryl-xanthenedione framework. This chemos-
electivity was due to greater electrophilicity of the formyl carbon
compared to the oxo carbon as well as more steric accessibility of
the formyl carbon towards the nucleophilic cyclic 1,3-diketone. So
the acyclic precursor of the xanthenedione (vide infra) was formed
involving the formyl carbon of 8 and the oxo carbon was left behind.
This chemoselective transformation has been delineated in
Scheme 3.
alumina and the optimum concentration of H+ was determined by
titration of the aqueous suspension of the weighed amount of thor-
oughly washed support with standard NaOH solution. The optimum
concentration of H+ was 3×10−4 mol per gram of the support.
These results are shown in Table 1.
2.2. General procedure for the synthesis of xanthenediones
Alumina-sulfuric acid (200 mg) was added to a solution of
cyclohexane-1,3-dione
1 (2 mmol) and appropriate aldehyde 2
(1 mmol) in anhydrous ethanol (5 ml). The reaction mixture was
refluxed with stirring till completion (monitored with TLC). Then it
was cooled to room temperature and the catalyst was removed by fil-
tration. The residual catalyst was repeatedly washed with hot ethanol
(3×2 ml). The combined filtrate and washings, on evaporation, furn-
ished the product 3. This was further purified by crystallization from
ethanol. Detailed results have been presented in Table 2.
The reactions were carried out using different amounts of the cat-
alyst and the optimum amount has been determined, as presented in
Table 3. Determination of this optimum amount to achieve maxi-
mum yield was very essential to establish the efficacy and broaden
the applicability of the proposed process.
3. Results and discussions
As evident from Table 2, cyclic 1,3-diketones 1a and 1b under-
went efficient condensation with different aryl and heteroaryl alde-
hydes 2 to furnish 9-aryl-dodecahydroxanthene-1,8-diones 3a–3j
(entries 1 to 10) and 9-aryl-octahydroxanthene-1,8-diones 3k–3o
(entries 11 to 15) in good yield. The reactions are remarkably efficient
with less electrophilic aldehydes 2b–2f bearing electron-donating
substituents. Similarly, more electrophilic aldehydes 2g and 2h bear-
ing electron-withdrawing substituents reacted smoothly under the
present condition. Acid-sensitive heteroaryl aldehydes 2i and 2l
(entries 9 and 15), vinylogous aryl aldehyde 2j (entry 10) and methy-
lenedioxy substituted aryl aldehyde 2k (entry 13) also reacted very
efficiently with no side reaction. Therefore, the present alumina-
sulfuric acid catalyzed protocol has a general applicability accommo-
dating a variety of substitution pattern. The tetraketone 6, with two
4H-pyran moieties connected together through aromatic ring, having
immense applications in biological activity evaluation studies and
synthesis of various important heterocyclic frameworks [26], was
synthesized very efficiently from 4 using alumina-sulfuric acid.
This is presented in Scheme 2. Unprecedented regioselective discrim-
ination between two symmetrical formyl moieties was observed in
the case of 5 when it reacted with 1b to produce 7, as shown in
Scheme 2. This was probably due to steric hindrance of the gem-
The concentrations of the residual H+ on the recovered catalyst
after successive experiments were measured (vide supra) and furn-
ished in Table 4. Extremely marginal loss of H+ was observed. It im-
plied that SO3H moiety was tightly anchored with the alumina
surface, probably through a covalent linkage with the oxygen atoms
of aluminium oxide. In the presence of alumina-sulfuric acid (pre-
heated at 220 °C for 4 h) the reaction between 1a and 2b occurred
with same efficiency with>98% conversion and 93% yield of 3b with-
out any formation of 10 or any other side-product. The reaction be-
tween 1a and 2b in the presence of alumina-sulfuric acid after
having it exposed to ambient atmosphere for 3 days produced similar
observation. Obviously, there was no deteriorating effect of aerial
oxygen or moisture towards activity of the catalyst. Therefore, the
catalyst had considerable stability towards heat and moisture which
also provided evidence for covalent anchoring. So the catalyst had
the potential of efficient recycling. According to Fig. 1, alumina-
sulfuric acid was successfully recycled (after thorough washing and
activation) with little variation of yield. The morphology of the cata-
lyst was studied. SEM and AFM images have been presented in
Figs. 2 and 3. The SEM picture (Fig. 2) showed that the particles
were of irregular shape with a wide distribution of size (quite usual
because of the varying particle size of commercial neutral alumina).
From the AFM picture (Fig. 3), the widely distributed roughness of
the catalyst surface was evident which might be ascribed to its high
catalytic activity. The pH of the reaction medium after isolation of
the product was found to be 5.75.
Table 1
Extent of binding of ClSO3H with neutral alumina.
The said reaction between 1a and 2b was studied using various
catalytic systems and the results are presented in Table 5. 2,2-
Arylmethylene-bis–(3-hydroxy-2-cyclohexene-1-one) 10 was the
exclusive product in the absence of any catalyst (entry 1). Alumina-
sulfuric acid (entry 2) was found totally different from sulfuric acid
supported on alumina (entries 3 and 4) in terms of the % of con-
version of aldehyde, product spread and isolated yield of desired
Entry
Amount of neutral
alumina (gm)
ClSO3H added
(mmol)
Conc. of H+ in mol per gm of
the support
1
2
3
4
0.96
1.08
0.99
1.10
0.25
0.50
0.75
1.00
2.508×10−4
3.011×10−4
3.016×10−4
3.013×10−4

A. Pramanik, S. Bhar / Catalysis Communications 20 (2012) 17–24
19
Table 2
Synthesis of xanthenediones using alumina-sulfuric acid.
Entry
1
R
Aldehyde
Product
Time (h)
4
Yield (%)a
77
Obs. mp (oC)
Lit. mp (oC)
H(1a)
202–204
204–205 [15]
O
O
O
3a
2
H(1a)
3
92
215–216
216–218 [15]
Me
Me
O
O
O
O
CHO
2b
O
3b
3
H(1a)
3
89
240–242
241–243 [15]
OMe
OMe
O
CHO
2c
O
3c
4
H(1a)
5
84
219–221
220–222 [3]
NMe2
NMe2
O
CHO
2d
O
3d
5
H(1a)
4
76
232–233
233 [27]
6
H(1a)
5
80
224-226
225-227 [15]
(continued on next page)

20
A. Pramanik, S. Bhar / Catalysis Communications 20 (2012) 17–24
Table 2 (continued)
Entry
7
R
Aldehyde
Product
Time (h)
3
Yield (%)a
81
Obs. mp (oC)
Lit. mp (oC)
H(1a)
233–235
231–233 [15]
8
H(1a)
3
82
219–222
222 [15]
9
H(1a)
4
77
265–267
-
10
H(1a)
5
81
188–191
-
11
Me (1b)
3
88
216–217
217–218 [12,14]
12
Me (1b)
5
73
202–203
203–205 [28]
13
Me (1b)
4
88
222–224
224–225 [13,18]

A. Pramanik, S. Bhar / Catalysis Communications 20 (2012) 17–24
21
Table 2 (continued)
Entry
14
R
Aldehyde
Product
Time (h)
4
Yield (%)a
83
Obs. mp (oC)
Lit. mp (oC)
222 [12,14]
Me (1b)
220–221
15
Me (1b)
4
89
163–165
164–165 [13]
S
S
O
O
CHO
2l
Me
O
Me
Me
Me
3o
16
H (1a)
8
No
Reaction
COMe
2m
a
Yield refers to that of isolated pure product fully characterized spectroscopically.
xanthenedione product. The latter produced by-products with minor
amount of desired product. Compound 10 was the sole product using
neutral alumina as the catalyst (entry 5). Alumina-sulfuric acid sys-
tem was heterogeneous because no residue of the catalyst was left
on evaporation of the ethanolic extract of the catalyst. Moreover, no
precipitate was obtained when the aqueous extract of alumina-
sulfuric acid was treated with aqueous solution of barium chloride.
So the heterogeneity of the system was conclusively proved. When
concentrated sulfuric acid was used in homogeneous system (entry
6), 10 was the major product with minor amount of 3b which was
isolated in low yield contaminated with unidentified side-products.
Using Al-rich zeolite (entry 7), the side reactions were minimized
but the 3b was formed in much less amount with the preponderance
of 10. Other solid acids like cellulose-sulfuric acid (entry 8), SiO2
(entry 9), PPA- SiO2 (entry 10), HClO4- SiO2 (entry 11) and NaHSO4-
SiO2 (entry 12) were also used. 2b remained mostly unaffected with
Me
Me
Me
Me
O
O
H
O
O
O
O
O
Alumina-sulfuric acid
EtOH , 5h , reflux
79%
+
O
H
O
Me
Me
Me
Me
O
4
1 eq
Me
6
O
Me
1b
O
4 eq
+
Me
H
Alumina-sulfuric acid
EtOH , 8h , reflux
Me
O
O
H
62%
Me
H
Me
O
O
Me
Me
5
7
1 eq
Scheme 2. Synthesis of mono- and bis-xanthenediones from aryl dialdehydes.

22
A. Pramanik, S. Bhar / Catalysis Communications 20 (2012) 17–24
O
Me
Me
O
O
Alumina-sulfuric acid
O
83%
O
+
EtOH , 4h , reflux
O
H
O
O
1a
8
9
2 eq
1 eq
Scheme 3. Intramolecular chemoselectivity in xanthenedione formation.
Table 3
on refluxing in anhydrous ethanol in the presence of alumina-
sulfuric acid, underwent cyclization to produce the xanthenedione
compound 3b (Scheme 4). Thus the intermediacy of 10 in this
transformation is clearly established. It was also proved that the
catalyst played its crucial role in the acid-promoted dehydrative
cyclization of 10 to produce 3b. The mechanistic formulation
was depicted in Scheme 4. The cyclization of the intermediate 10
was initiated through protonation by alumina-sulfuric acid at one of
its carbonyl oxygen to produce 11a. The electrophilicity of the corre-
sponding carbonyl carbon in 11a was thereby increased. Concomitant
intramolecular nucleophilic attack by the nearby OH group led to
cyclization to give 11b which underwent prototropic shift to give
11c. Subsequent dehydration was assisted by the conjugate base of
alumina-sulfuric acid through proton abstraction from 11c where
the product 3b was formed. As a result, alumina-sulfuric acid was
regenerated and the catalytic cycle re-operated.
Effect of the amount of the catalyst towards the reaction.
Entry
Amount of solid catalyst (g)
Isolated yield (%)a
1.
2.
3.
4.
5.
0.05
0.1
0.2
0.3
0.4
70
79
89
86
84
a
1a (2 mmol), 2c (1 mmol), temperature: 80 °C, solvent: anhydrous EtOH, time: 3 h
Table 4
Concentrations of residual H+ on the used support after successive experiments.
Entry Reaction time
(h)a
Isolated yield Conc. of H+ in mol gm−1 of the residual
(%)
support
1
2
3
4
3
3
3
3
92
91
89
87
3.02×10−4
3.01×10−4
2.99×10−4
2.98×10−4
The catalytic attributes of the alumina-sulfuric acid have thus
been substantiated in terms of its selectivity, recyclability and
wider applicability through the aforesaid investigations.
a
1a (2 mmol), 2b (1 mmol), alumina-sulfuric acid (0.2 g), solvent: anhydrous EtOH,
The present alumina-sulfuric acid-catalyzed condensation reac-
tion was also studied in different solvents. Detailed results were pre-
sented in Table 6. It is interesting to note that 2b was totally
consumed in most of the organic solvents but relative proportion of
3b and 10 varied from solvent to solvent. In ethanol, 3b was exclu-
sively formed with negligible amount of 10. So ethanol (which is
an eco-compatible solvent) came out as the best choice of solvent
in terms of reaction time and yield. This reaction also occurred in
water, but took much longer time for substantial conversion of 10
to 3b. Complete transformation of 10 to 3b could not occur due to
slowing down of dehydration process in aqueous medium.
temperature: 80 °C.
SiO2 alone. In other cases, 3b was the major product but this was
formed less compared to alumina-sulfuric acid (entry 2). Compound
10 was not formed with PPA–SiO2. But with HClO4–SiO2 and
NaHSO4–SiO2, 10 was obtained in considerable amount depleting the
yield of 3b. It is important to note that compound 10 was the exclusive
product with NaCl-alumina where 3b was practically not formed.
Therefore, Cl− had no effect towards cyclization of 10 to 3b. Therefore,
the efficacy and superiority of alumina-sulfuric acid with respect to
other solid acid catalysts have thus been firmly established.
The attenuated acidity of the alumina-sulfuric acid was found to
be crucial for cyclization but insufficient to affect the acid-sensitive
moieties. Compound 10 was obtained through the reaction between
1a and 2b in the absence of alumina-sulfuric acid. This compound,
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
Number of Reactions
Fig. 1. Recycling of alumina-sulfuric acid.
Fig. 2. SEM image of alumina-sulfuric acid.

A. Pramanik, S. Bhar / Catalysis Communications 20 (2012) 17–24
23
Fig. 3. AFM image of alumina-sulfuric acid.
Table 5
Effect of different catalysts on the reaction.
a
Entry
Catalyst
% of unreacted 2bb
% of 3bb
% of 10b
Isolated yield of 3b (%)
1
2
3
4
5
6
7
8
None
b2
b2
15
b2
b2
8.0
14.1
58
93.6
b5
5
4.9
b2
b2
>98
b2
-
Alumina-sulfuric acid
Conc. H2SO4 (0.3 mmol) on alumina (1 g)
Conc. H2SO4 (1 mmol) on alumina (1 g)
Neutral alumina
Conc. sulfuric acid
Al-rich zeolite
Cellulose-sulfuric acid
SiO2
PPA–SiO2
>98
60.1
92
23.7
47c
d
d
d
-
-
-
-
b2
>98
57.3
69.3
14
4.7
-
20.1
46.1
>98
34.6
16.5
28
32c
8
12
-
9
b2
10
11
12
13
>90
74.3
48.7
b2
83c
67
32
-
HClO4–SiO2
NaHSO4–SiO2
NaCl–alumina
a
1a (2 mmol), 2b (1 mmol), catalyst (0.2 g having 0.3 mmol of catalytic reagent per gm of support in case of supported catalyst), temperature: 80 °C, solvent: anhydrous ethanol,
time: 3 h.
b
c
Determined by 1H NMR (300 MHz).
Contaminated with unidentified by-products.
Inseparable mixture of unidentified by-products without any trace of 3b and 10.
d
Therefore, the present method utilized a chemically robust, re-
this light, this highly efficient catalytic process can also be consid-
cyclable, supported inorganic Bronsted acid as catalyst and ethanol
as environmentally benign reusable organic solvent. Moreover, the
entire process was highly atom-efficient leaving the most innocu-
ous material, namely water, as the sole by-product. So the present
protocol minimizes the dispersal of the harmful chemicals in the
environment and maximizes the use of renewable resources. In
ered as a green technology.
4. Conclusion
Alumina-sulfuric acid has been efficiently utilized as a recyclable,
cost-effective and mildly acidic catalyst for eco-friendly synthesis of
Me
Me
Me
O
O
O
O
OH
Alumina-sulfuric acid
EtOH , reflux , 1h
+
EtOH , reflux , 2h
O
O
O
H
O
OH
1a
3b
2b
10
2 eq
1 eq
O
OSO3H
OSO3H
H
H
Alumina-sulfuric acid
OSO3H
Me
-
-
OSO3
OSO3
Me
Me
H
O
OH
O
OH
O
O
+
O
H
+
O
H
O
O
HO
OH
H
H
+
11a
11b
11c
Scheme 4. Mechanistic description of the catalytic role of alumina-sulfuric acid.

24
A. Pramanik, S. Bhar / Catalysis Communications 20 (2012) 17–24
Table 6
5.4. Compound 9
Effect of different solvents towards the reaction.
1
FT-IR (KBr), cm−1: 1677.8, 1655.5; H NMR (300 MHz, CDCl3): δ
1.94–2.04 (4H, m), 2.31–2.36 (4H, m), 2.52 (3H, s), 2.60–2.66 (4H,
m), 4.84 (1H, s), 7.39 (2H, d, J=8.3 Hz), 7.82 (2H, d, J=8.3 Hz); 13C
NMR (75 MHz, CDCl3): 20.3, 26.4, 27.0, 32.0, 36.8, 116.1, 128.2,
128.6, 135.3, 149.6, 164.1, 196.3, 197.7; HRMS observed for [M+
K]+ at 375.0997, calculated for C21H20O4 [M+K]+ 375.0999.
Solvent
Time (h)a
% of unreacted 2bb
% of 3bb
% of 10b
Hexane
Et2O
THF
Acetone
Ethyl acetate
DCM
MeOH
1-Propanol
EtOH
3
3
3
3
3
3
3
3
3
1
4
15
52.4
-
-
-
-
-
-
47.6
57.4
44.0
56.8
46.4
45.7
60.0
42.5
>98.0
42.3
75.0
90.0
-
42.6
56.0
43.2
53.6
54.3
40.0
-
b2.0
57.7
25.0
10.0
Acknowledgements
57.5
-
-
-
-
H2O
H2O
H2O
The authors express sincere thanks to Prof. B. C. Ranu, Mr. S.
Ahmed and Mr. N. Dutta of Indian Association for the Cultivation of
Science and Dr. K. K. Chakravarty and Mr. J. Podder of Jadavpur
University for their invaluable suggestions, assistance and support
in the course of the aforesaid investigation. Financial and infrastruc-
tural support from CSIR (Grant No. 01(2383)/10/EMR-II), DST-
PURSE Program, UGC-CAS Program and DST-FIST Program are also
gratefully acknowledged.
a
1a (2 mmol), 2b (1 mmol), alumina-sulfuric acid (0.2 g), solvent: 5 ml, tempera-
ture: boiling point of the solvent.
b
Percentage was calculated by 1H NMR (300 MHz).
xanthenediones with novel structural features through chemoselec-
tive condensation of aryl and heteroaryl aldehydes with cyclic 1,3-
diketones.
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5.1. Compound 3i (entry 9 in Table 2)
1H NMR (300 MHz, CDCl3): δ 1.96–2.05 (4H, m), 2.30–2.36 (4H,
m), 2.51–2.98 (4H, m), 4.90 (1H, s), 7.06–7.11 (1H, m), 7.63 (1H, dt,
J1 =7.6 Hz, J2 =1.6 Hz), 7.72 (1H, d, J=7.7 Hz), 8.45 (1H, d, J=
4.7 Hz); 13C NMR (75 MHz, CDCl3): 20.3, 27.2, 34.4, 36.7, 112.8,
118.5, 121.5, 136.9, 147.7, 161.2, 165.4, 197.1; HRMS observed for
[M+H]+ at 296.1282, calculated for C18H17O3N [M+H]+296.1284.
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5.2. Compound 3j (entry 10 in Table 2)
1
FT-IR (KBr), cm−1: 1669.7, 1622.1; H NMR (300 MHz, CDCl3): δ
2.02–2.08 (4H, m), 2.34–2.59 (8H, m), 4.42 (1H, s), 6.22–6.23 (2H,
m), 7.10–7.29 (5H, m); 13C NMR (75 MHz, CDCl3): 20.4, 27.2, 28.0,
36.9, 115.3, 126.3, 127.1, 128.3, 129.9, 131.1, 137.1, 164.7, 196.7;
HRMS observed for [M+Na]+ at 343.1311, calculated for C21H20O3
[M+Na]+ 343.1313.
5.3. Compound 8
FT-IR (KBr), cm−1: 1695.5, 1658.8, 1620.2; 1H NMR (300 MHz,
CDCl3): δ 0.988 (6H, s), 1.08 (6H, s), 2.19 (4H, m), 2.38 (3H, s), 2.48
(4H, s), 4.76 (1H, s), 7.43–7.48 (3H, m), 9.89 (1H, s); 13C NMR
(75 MHz, CDCl3): 20.2, 21.5, 27.1, 29.6, 30.9, 36.9, 56.5, 116.8, 123.7,
125.8, 127.1, 136.9, 143.7, 163.9, 190.8, 196.6; HRMS observed for
[M+H]+ at 393.2060, calculated for C25H28O4 [M+H]+ 393.2067.