Exploring the unexpected formation of spirobibenzopyrans and benzopyrylium salts and effect of Lewis acids on the Claisen-Schmidt reaction

Article abstract of DOI:10.1016/j.molstruc.2021.130598

Unexpected spiro cyclic products were obtained from the popular alkali catalyzed Claisen-Schmidt reaction of ketones with salicylaldehyde apart from the bis-chalcones. However, in the acid catalyzed reaction, we observe their transformation to a benzopyry

Full text of DOI:10.1016/j.molstruc.2021.130598

Journal of Molecular Structure 1240 (2021) 130598
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Exploring the unexpected formation of spirobibenzopyrans and
benzopyrylium salts and effect of Lewis acids on the Claisen-Schmidt
reaction
Swayamsiddha Kara, Prashant Raia, Sai Manohar Chellia, Abdul Akhirb,
Naveen Shivalingegowdac, Sidharth Choprab, Lokanath Neratur Krishnappagowdad,
Siva Kumar Belliraja,∗, Nageswara Rao Golakotia,∗
a Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam, Andhra Pradesh 515134, India
b Division of Microbiology, CSIR-Central Drug Research Institute, Lucknow, Uttar Pradesh 226031, India
c Department of Physics, Faculty of Engineering & Technology, Jain (Deemed-to-be University), Bangalore 562112, India
d Department of Studies in Physics, Manasagangotri, University of Mysore, Mysuru 570006, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Unexpected spiro cyclic products were obtained from the popular alkali catalyzed Claisen-Schmidt re-
action of ketones with salicylaldehyde apart from the bis-chalcones. However, in the acid catalyzed re-
action, we observe their transformation to a benzopyrylium salt. In the present study, we demonstrate
this phenomenon for three ketones namely cyclopentanone, cylcohexanone and cylcoheptanone. Each of
the corresponding products obained were characterized using UV-Vis, FT-IR, 1H NMR, 13 C NMR, and mass
spectrometry. In addition, the structure of the spiro molecule synthesized from cylcoheptanone reactant
has been determined by X-ray crystallography and further investigated for its thermal properties using
TGA/DTA technique. To rationalize this phenomenon, we first studied the effect of solvents on the reac-
tion, and further explored the effect of Lewis acid catalyst and various solvents for the same reaction
conditions. Further, we have evaluated in silico, the impact of thermodynamic parameter like entropy.
In this regard, we explored the relationship between the bis-chalcone, spiro molecule and the benzopy-
rylium salt. Consequently, the effect of the ring expansion from cyclopentanone to cycloheptanone was
investigated. In conclusion, we have presented the anti-bacterial potential of the synthsized spirobiben-
zopyrans and the benzopyrylium salts.
Received 5 December 2020
Revised 27 April 2021
Accepted 28 April 2021
Available online 4 May 2021
Keywords:
Claisen-Schmidt condensation
Diarylidenecycloalkanones
Spirobibenzopyrans
Pyrylium salts
Catalyst effect
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
stead of the corresponding bis-chalcone. This study is a repot of
the unusual findings obtained during the course of our experi-
Claisen-Schmidt condensation is a popular organic reaction be-
tween an aldehyde lacking an alpha-hydrogen and a ketone hav-
ing an alpha-hydrogen [1]. This reaction has been predominantly
used to synthesize chalcones and bis-chalcone derivatives and has
been extensively studied [2–7]. In an effort to explore the reac-
tion between a cycloalkanone and 2-hydroxybenzaldehyde via the
Claisen-Schmidt reaction, we discovered formation of unexpected
spirobibenzopyrans along with the bis-chalcone in basic medium.
Consequently, in order to avoid the spiro by-product, we attempted
the Claisen-Schmidt reaction under acidic conditions where we ob-
tained deeply coloured compounds called benzopyrylium salts in-
ments. Interestingly, we observed that the reaction products were
dependent on the ring size of the cycloalkanone reactant and form
only with 2-hydroxybenzaldehyde reactant. Effect of solvents and
catalyst also have been studied in order to understand the for-
mation of these compounds. Additionally, the effect of Lewis acids
also has been studied. To the best of our understanding, we herein
report for the first time, the formation of spirobibenzopyrans from
a TiCl4 catalysed reaction. We have further explored the effect on
structural stability with respect to the ring size of the ketone reac-
tant by employing in silico techniques.
This phenomenon of obtaining spirobibenzopyrans, benzopy-
rylium salts and diarylidenecycloalkanone derivatives was stud-
ied using cyclopentanone, cyclohexanone and cycloheptanone as
the ketone reactants in the Claisen-Schmidt reaction. The vari-
ous schemes describing the synthetic procedures along with the
∗
Corresponding authors.
E-mail
addresses:
bsivakumar@sssihl.edu.in
(S.K.
Belliraj),
gnageswararao@sssihl.edu.in (N.R. Golakoti).
https://doi.org/10.1016/j.molstruc.2021.130598
0022-2860/© 2021 Elsevier B.V. All rights reserved.

S. Kar, P. Rai, S.M. Chelli et al.
Journal of Molecular Structure 1240 (2021) 130598
2.3. Instrumentation
For DA-CHX and DA-CP, the 1HNMR spectra were obtained on
VARIAN 400 MHz, and 13C NMR on VARIAN 100 MHz with DMSO-
d6 as the solvent. For CHX-SK, DA-CHP and CHP-SK the 1HNMR
spectra were obtained on Bruker Ascend 400 MHz, and 13C NMR
on Bruker Ascend 100 MHz with CDCl3 as the solvent. TMS was
used as the internal standard. For CP-PS and CHX-PS, the 1HNMR
spectra were obtained on Bruker Avance III 800 MHz, and 13C NMR
on Bruker Avance III 200 MHz at 10 °C using CD3COOD as the sol-
vent. AGILENT 6430 Triple Quad LC/MS was employed to obtain
mass spectra. Only for CP-PS and CHX-PS, Bruker MALDI-TOF/TOF
MS was employed to obtain their mass spectra. The FT-IR spec-
tra were recorded between 400 and 4000 cm−1 using KBr pel-
lets employing Thermo-Nicolet Avatar 370 spectrophotometer. UV-
Vis spectra in methanol were recorded in the wavelength range
200–600 nm using Shimadzu 2450 spectrophotometer. The ther-
mal analysis was carried out in TA SDT Q600 TGA/DTA thermal an-
alyzer.
Scheme 1. The Claisen-Schmidt condensation reaction.
characterization data, the in silico techniques employed and the
TGA/DTA technique used are provided in the supplementary sec-
tion.
2. Materials and methods
2.1. General scheme for synthesis of the diarylidenecycloalkanone
derivatives
Single crystals of suitable dimensions were chosen carefully for
X-ray diffraction studies. The X-ray intensity data were collected at
a temperature of 293(2) K on a Bruker Proteum2 CCD diffractome-
ter [9] equipped with an X-ray generator operating at 45 kV and
The diarylidenecycloalkanone derivatives were synthesized as
shown in Scheme 1 by the Claisen–Schmidt condensation proce-
dure [8]. A solution of 15 ml of distilled ethanol and 15 ml of wa-
ter was prepared in a 100 ml round bottom flask and nitrogen was
flushed through the solution for 3–4 min. To the ethanol-water
mixture, 7.5 g of sodium hydroxide was added and stirred continu-
ously till it dissolved. Another mixture of 0.0625 moles of the alde-
hyde and 0.03125 moles of the ketone was prepared. Half of the
aldehyde-ketone mixture was then added to the vigorously stirred
ethanol-water mixture under nitrogen atmosphere. After the addi-
tion, a reddish coloration was obtained. 25 min later the rest of the
aldehyde-ketone mixture was added to the reaction mixture and
the nitrogen atmosphere was maintained. The reaction was mon-
itored by TLC for completion. While the reaction was complete in
less than a day for the cyclopentanone reactant, the cyclohexanone
reactant took around 2 days. However, for the cycloheptanone re-
actant, the completion time was over 5 days.
˚
10 mA, using CuK radiation of wavelength 1.54178 A. Data were
α
collected for 24 frames per set with different settings of ϕ (0 and
90°), keeping the scan width of 0.5°, exposure time of 2 s, the sam-
ple to detector distance of 45.10 mm and 2θ value at 46.6°. The
complete data sets were processed using SAINT PLUS. The struc-
tures were solved by direct methods and refined by full-matrix
least squares method on F2 using SHELXS and SHELXL programs
[10]. The geometrical calculations were carried out using the pro-
gram PLATON [11].
2.4. In silico studies
The spirobibenzopyrans, the benzopyrylium salts, and the di-
arylidenecyclocalkanone derivatives mentioned in the study were
modelled using GaussView (5.0.8) [12]. The energy minimization
of these molecules has been carried out using DFT (B3LYP/6–31G∗
basis set) theory in Gaussian 09 along with their frequency simu-
lations [13].
At the end of the reaction, the whole reaction mixture was
transferred into a beaker with ice cubes. Subsequently, acidification
of the mixture was carried out using 1:1 HCl solution drop wise
with vigorous stirring. After every drop, the pH was monitored us-
ing pH paper. When the pH became less than 5, a yellow precip-
itate was obtained. The precipitate was then filtered and washed
with cold water until the washings were free of acid and dried.
Since the cyclopentanone reactant gave a single product, i.e.,
the bis-chalcone, the solid obtained was recrystallized from ace-
tone.
2.5. In vitro antibiotic susceptibility testing
Antibiotic susceptibility testing was conducted according to
the CLSI guidelines using the broth micro dilution-assay [14].
10 mg/mL stock solutions of test compounds were prepared in
DMSO. Bacterial cultures were inoculated in MHBII and opti-
cal density (OD) was measured at 600 nm, followed by dilution
to achieve ~106 CFU/mL. The compounds were tested from 64–
0.5 mg/L in two-fold serial diluted fashion with 2.5 μL of each
concentration added to well of a 96-well round bottom microtiter
plate. Later, 97.5 μL of bacterial suspension was added to each
well containing either test compound or appropriate controls. The
plates were incubated at 37 °C for 18–24 h following which the
MIC was determined. The MIC is defined as the lowest concentra-
tion of the compound at which there is absence of visible growth.
For each test compound, MIC determinations were carried out in-
dependently three times using duplicate samples.
In case of the cyclohexanone reactant,
a mixture of both
spiroketal and bis-chalcone was obtained. The two compounds
were separated using column chromatography. The pure spiroke-
tal was obtained directly from the fractions after the column chro-
matography. On the other hand, the bis-chalcone was purified by
recrystallizing from methanol-water mixture.
The cyclopentanone product also gave
a mixture of both
spiroketal and bis-chalcone. In this case, the bis-chalcone was pu-
rified directly by recrystallizing from methanol.
2.2. General scheme for synthesis of acid catalyzed Claisen-Schmidt
reaction
As shown in Scheme 2, a mixture of the ketone (0.01 mol) and
salicylaldehyde (0.02 mol) was stirred in HCl saturated acetic acid
(80 ml). The above reaction mixture was left standing overnight
when crystals were observed settling down. The crystals were fil-
tered, washed thoroughly with diethyl ether and dried.
3. Results and discussion
Due to the alkaline conditions involved in the reaction, the phe-
nol group exists as phenoxide ion as shown in Fig. 1 that is exten-
sively stabilized by the resonance.
2

S. Kar, P. Rai, S.M. Chelli et al.
Journal of Molecular Structure 1240 (2021) 130598
Scheme 2. AcOH/HCl catalyzed benzopyrylium salt formation.
Table 1
Observations from the various Claisen-Schmidt reactions carried out with salicylaldehyde.
Alkaline medium products Acidic medium products
Ketone
Bis-chalcone
Spirobibenzopyran
Pyrylium Salt
Bis-chalcone
Spirobibenzopyran
Pyrylium Salt
Cyclopentanone
Cyclohexanone
Cycloheptanone
Formed
Formed
Formed
Did not form
Formed
Did not form
Did not form
Did not form
Did not form
Did not form
Did not form
Did not form
Did not form
Formed
Formed
Formed
Formed
Did not form
findings upon carrying out the Claisen-Schmidt reaction with the
cyclopentanone, cyclohexanone, and cycloheptanone with salicy-
laldehyde in both basic and acidic conditions. The tabulated ob-
servations (Table 1) highlight the different synthetic and structural
aspects of organic chemistry. Primarily, from these unusual prod-
ucts observed, it is evident that these deviations occur due to the
2-hydroxybenzaldehyde/ salicylaldehyde reactant in the Claisen-
Schmidt reaction.
Fig. 1. Phenoxide ion in alkaline medium.
It can be rationalized that the formation of the spiro molecule
in basic medium depends on the structural aspect of the ketone
reactant. The reduction in entropy due to ketal formation cannot
be countered by conformational changes in the cyclopentane ring
fragment due to the high ring strain in the cyclopentane ring itself.
Hence, no spiro molecule was observed with the cyclopentanone
reactant. On the other hand, both cyclohexane and cycloheptane
ring fragments can alter their conformation in order to counter the
entropy reduction suffering minimal conformational penalty, thus
yielding the respective spiro molecules.
In order to obtain the neutral product at the end of the Claisen-
Schmidt reaction, the pH of the reaction mixture was made acidic.
We observed the precipitation of the desired bis-chalcone deriva-
tive. It was at the end of this step that we observed the presence
of two different products by TLC. This particular phenomenon was
crucially first observed in the reaction between cyclohexanone and
salicylaldehyde. Upon purification and characterization, we could
characterize the products as (a) and (b) shown below in Fig. 2.
Spirobibenzopyrans and their synthesis have been reported in
a few publications in the literature [15,16]. However, this is the
first recorded formation of these molecules via the Claisen-Schmidt
condensation route. We propose that due to the acidic condi-
tions in the final step of the reaction to obtain the bis-chalcone
derivative, an intramolecular ketal formation occurs by the fol-
lowing mechanism as shown in Fig. 3. At the end of the reac-
tion, the obtained bis-chalcone was of only 60% yield, whereas the
rest 40% yield was the spiro compound. Given this intriguing phe-
nomenon, in the quest of higher yields of the bis-chalcone deriva-
tive while avoiding the formation of the spiro molecule, we pur-
sued the Claisen-Schmidt condensation reaction using different re-
action conditions.
Unlike the structural aspect of the ketone reactant which drives
the spiro molecule formation in the basic medium, the benzopy-
rylium salt formation in the acidic medium is due to the stabil-
ity offered by aromaticity of the product. The formation of the
benzopyrylium salt results in a highly conjugated aromatic system
which offers stability even in case of the cyclopentanone reactant.
Interestingly, cycloheptanone reactant did not yield either a ben-
zopyrylium salt or a bis-chalcone in the acidic medium. Rather, the
spiro molecule was obtained from the reaction. We propose that
since the cycloheptane ring fragment can change its conformation
to accommodate the entropy loss, it prefers the spiro structure.
We further explored the effect of the solvent on the reaction
in both the acidic and basic media. In this regard, we chose cyclo-
hexanone as the representative ketone reactant with salicylalde-
hyde, and studied the reaction using dichloromethane as moder-
ately polar and toluene as the non-polar solvents. Triethylamine
was used as the base catalyst whereas PTSA (p-toluenesulfonic
acid) was used as the acid catalyst. We observed the formation
of benzopyrylium salt in the acidic medium with both the sol-
vents. The identification of the product was confirmed by co-TLC,
UV and mass spectrometry by comparison with the authentic sam-
ple obtained previously. This confirms the influence of salicylalde-
hyde and acidic medium to generate the alternative product as
benzopyrylium salt from the Claisen-Schmidt reaction irrespective
of the solvent system. Intriguingly, we also observed the presence
of the corresponding spiro molecule on the TLC. This could be at-
tributed to the presence of the non-polar medium where the spiro
molecule is soluble in the less polar solvents whereas the ben-
zopyrylium salt precipitates. In the basic medium, we noted no
progress in the reaction for ten days carried out in toluene, even
with application of heat. However, we observed the progress of
The acid catalyzed reaction medium constituted glacial acetic
acid saturated with dry HCl gas. One would expect the formation
of bis-chalcone by this procedure without any by-products. Inter-
estingly in this method, we obtained a different type of product
that was not the desired bis-chalcone. The solution was deep vi-
olet in colour and so was the solid obtained. TLC showed pres-
ence of a single compound. Upon characterization, we concluded
that the product belonged to a different class of compounds called
the benzopyrylium salts. The structure of the compound isolated is
captured in Fig. 4.
This type of reaction was first reported by Borsche and Geyer
in 1913 along with the associated mechanism [17] and by other re-
search groups [18,19]. Benzopyrylium salts are well known for var-
ious applications in color photography, fluorescent dyes and dye
lasers [20]. These observations prompted us to verify the princi-
ple with the other ketone reactants under same reaction condi-
tions. In this regard, we chose cyclopentanone and cycloheptanone
to react with salicylaldehyde. The Table 1 below summarizes our
3

S. Kar, P. Rai, S.M. Chelli et al.
Journal of Molecular Structure 1240 (2021) 130598
Fig. 2. Purified products obtained from the Claisen-Schmidt reaction.
Fig. 3. Proposed mechanism of the intramolecular: (a) spiroketal formation and (b) benzopyrylium cation formation.
The UV-vis spectra of the spirobibenzopyrans showed one peak
between 240–243 nm and the second between 278–321 nm. These
absorptions can be attributed to the two substituted benzene rings.
On the other hand, the benzopyrylium salts show three distinc-
tive peaks, one between 250–260 nm, second in 290–320 nm
and the third between 400–530 nm. The peak between 400–
530 nm results from the benzopyrylium moiety and is respon-
sible for the colour observed from the pyrylium salts. IR spec-
troscopy of all the compounds demonstrated a few key com-
mon features. The absence of C=O stretch in the range 1760–
1660 cm-1 confirms the formation of the spiroketal and the
benzopyrylium salt while the C=O stretch in the range 1640–
1670 cm−1 was observed in the diarylidenecycloalkanones. The
Fig 4. Benzopyrylium salt obtained from the Claisen-Schmidt reaction in acidic
medium.
the reaction in dichloromethane when refluxed for seven days. We
detected the presence of the bis-chalcone through TLC. The reac-
tion was very slow but satisfyingly without the presence of by-
products. This confirms the effect of the basic medium to enable
the progress of the reaction only in polar solvents. The spiro by-
product is observed only when aqueous medium is involved.
We consequently were able to grow the single crystals for the
spiro molecule obtained from the cycloheptanone reactant that is
provided in the following Fig. 5.
C=C double bond stretch was seen at around 1635–1600 cm−1
.
The aromatic skeletal bands were noted in the range 1550–
1400 cm−1 and C-H OOP bend in the region 900–650 cm−1
.
Strong bands in the region 850–800 cm-1 affirm the tri-substituted
benzene rings. The C-O stretch was observed in the range of
1380–1100 cm−1
.
4

S. Kar, P. Rai, S.M. Chelli et al.
Journal of Molecular Structure 1240 (2021) 130598
Fig. 5. ORTEP diagram of the Spiro molecule from the cycloheptanone reactant.
In 1H NMR spectra, aromatic protons are observed at their
expected values with various degrees of shielding (δ 6.60–7.40).
The olefinic protons also appear in the same region as the aro-
matic protons. 13C NMR clearly identifies the quaternary carbons
by the weak signals. The spiro-carbon has a higher shift because
it is attached to two oxygen atoms at around δ 100–95. In case
of the benzopyrylium salts and the diarylidenecycloalkanones this
peak is absent. The carbon attached to the oxonium ion shows a
chemical shift value at δ 170–165 which is on the lower range
of carbonyl carbon shifts as observed in the diarylidenecycloalka-
nones ~δ 190 ppm. Mass spectrometry for all samples gave the ex-
pected molecular ion peaks.
(THF), acetonitrile (ACN), dichloromethane (DCM), and toluene. We
chose the reactants to be cycloheptanone (ketone) and salicylalde-
hyde. The reaction conditions were kept similar to as described by
Salama et. al. [16], i.e., ketone of 1 equivalent, aldehyde in 2 equiv-
alents and the catalyst of 3 equivalents. Since the reported reaction
completed in 2 h, we monitored all the reactions for 24 h.
At the set conditions, toluene and ACN as solvents did not show
any progress in the reaction, even upon reflux. With the catalysts,
ZnCl2, Ti{OCH(CH3)2}4, InCl3, and AlCl3 did not show any progress
in the reaction. The following Table 2 summarizes the yield from
various reaction conditions:
From the point of view of yield, both SiCl4 and TiCl4 have given
the best results. Accordingly, SnCl4 is less effective for the forma-
tion of the spirobibenzopyran. From time taken for the reaction to
complete, SiCl4 and TiCl4 showed completion between 4 to 6 h. In
this regard, although BF3.OEt2 yielded comparable spirobibenzopy-
ran in abs. EtOH, it took 24 h to completion. Hence, as effective
catalysts, we shortlisted SiCl4 and TiCl4 as ideal for further analy-
sis.
As revealed earlier, the synthesis of spirobibenzopyrans has
been reported in literature [15,16]. The reactions conditions involve
the application of Lewis acids, i.e., BF3 [15] and SiCl4 [16]. Follow-
ing the procedures mentioned by Salama et. al., [16] we were able
to synthesize novel spirobibenzopyrans and explore their biological
properties [21,22]. But the formation of spirobibenzopyrans in sim-
ilar reaction conditions as the Claisen-Schmidt condensation reac-
tion by the use of Lewis acid is fascinating as the product forma-
tion occurs in a single step. In order to further investigate the reac-
tion, we chose different Lewis acids as our catalyst and also chose
different solvents.
Among the solvents, it is clear that both abs. EtOH and dioxane
proved optimum and hence were chosen for further studies. In the
next step of the study, we chose to optimize the quantity of the
catalyst. It is known that a catalyst must be able to carry out the
reaction at a minimum quantity.
The following Lewis acids were chosen: SiCl4, TiCl4, SnCl4,
Ti{OCH(CH3)2}4, InCl3, AlCl3, BF3.OEt2, and ZnCl2. The solvents cho-
sen were absolute ethanol (abs. EtOH), dioxane, tetrahydrofuran
Here we chose to compare the yields at 3 equivalents, 2 equiv-
alents, 1 equivalent, 0.1 equivalents and 0.01 equivalents of both
5

S. Kar, P. Rai, S.M. Chelli et al.
Journal of Molecular Structure 1240 (2021) 130598
Table 2
Reaction conditions for the formation of the spirobibenzopyran CHP-SK at 3 equivalents of the catalyst.
Formation of CHP-SK (24 hrs)
SiCl4
SnCl4
TiCl4
BF3.OEt2
InCl3
AlCl3
ZnCl2
Ti{OCH(CH3)2}4
Abs. EtOH
Dioxane
THF
72.60%
68.00%
52.70%
-
25.60%
71.18%
69.00 %
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
10.00%
69.50%
5% (incomplete)
5% (incomplete)
47.90%
-
-
-
-
ACN
-
-
DCM
28%
10% (incomplete)
-
29% (incomplete)
-
Toluene
-
Table 3
Reaction conditions for the formation of the spirobibenzopyran CHP-SK at varying catalyst quantities.
CHP-SAL
(24 hrs)
SiCl4
3 eq
TiCl4
3 eq
2 eq
1 eq
0.1 eq
0.01 eq
2 eq
1 eq
87.45%
0.1 eq
0.01 eq
Abs. EtOH 72.60%
Dioxane 68.00%
76.23%
70.12%
79.50%
-
83.22%
-
Incomplete 71.18%
69.50%
79.63%
incomplete incomplete
-
incomplete incomplete incomplete incomplete
Table 4
•
A good yield of spirobibenzopyrans are obtained when a Lewis
acid catalyst is used instead of an alkali or acid in the Claisen-
Schmidt condensation reaction between a ketone and salicy-
laldehyde as a reactant.
Optimized conditions for the formation of the spirobibenzopyran CHX-SK and CP-
SK.
0.1 eq SiCl4
CHX-SK
1eq TiCl4
CHX-SK
Other Ketones (24 hrs)
•
•
Among the solvents, absolute ethanol is the best.
CP-SK
CP-SK
Among the Lewis acids, SiCl4 gives faster yields at 0.1 equiva-
lent whereas TiCl4 gives higher yields though at 1 equivalent.
Abs. EtOH
56%
43%
61%
30%
65.97%
43%
73%
30%
Prior yields (studied above)
We have thus illustrated the effect of Lewis acids and solvents
on the Claisen-Schmidt reaction yielding spirobibenzopyrans.
Returning to the unexpected formation of the spirobibenzopy-
rans and benzopyrylium salts in the original reaction conditions,
we aim to understand the role of entropy. In order to understand
that, we carried out in silico studies for each of these molecules.
Fig. 6 given below shows the structures along with their labels
used in the study. The global reactivity and thermal parameters
[23] have been tabulated in SD Table I in the supplementary sec-
tion.
SiCl4 and TiCl4 in the best solvents identified, i.e., abs. EtOH and
dioxane for the formation of CHP-SK. The observations are tabu-
lated below in Table 3.
Interestingly, we observed that the yield of the CHP-SK in-
creased as the quantity of the Lewis acid catalyst reduced. It can
be attributed to the lower amount of catalyst that increases its ef-
ficiency in the reaction. Among the solvents, we observe that abs.
EtOH is better reflected in the higher yields. SiCl4 gave the best
yield at 0.1 equivalents of the catalyst whereas TiCl4 gave the best
yield at 1 equivalent of the catalyst. Here we would like to draw
attention to the work-up procedure. In order to quench the cata-
lyst and obtain the desired product, the contents of the reaction
mixture are poured in ice-cold water and then the solid obtained
is filtered. We have observed that by-products were formed in case
of SiCl4 which from the literature was found to be Si(OEt)4 along
with the solid spiro compound. In order to obtain the spirobiben-
zopyran we have to further extract the desired product using DCM
or diethyl ether. This process leads to the loss of the spiro com-
pound thus reducing the yield. On the other hand, TiCl4 forms wa-
ter soluble TiO2 which makes the work-up procedure very easy
and we obtain the spiro product directly as a solid hence providing
a higher yield than SiCl4. We also observed that the reaction cat-
alyzed by SiCl4 completed faster, within 4 h, as compared to TiCl4
catalyzed reactions which took around 6 h.
In Table 5 given below, the entropy values of each of these
molecules have been tabulated.
The in silico findings confirm our hypothesis regarding the loss
of entropy in all the intramolecular cyclizations observed. The en-
tropy loss is highest for the cyclopentanone reactant from the bis-
chalcone to the spiro structure accounting for the lack of forma-
tion of the respective spiro molecule. Further, we observe that the
entropy loss between the spiro form and the benzopyrylium salt
form is almost half in case of both cyclopentanone and cyclohex-
anone reactants indicating that benzopyrylium salt forms readily in
their case. Interestingly, in the case of the cycloheptanone reactant,
the entropy loss for both spiro and benzopyrylium form is compa-
rable which reinforces our earlier observation that cycloheptanone
reactant favours the spiro product.
We have compared the bond angles and bond lengths obtained
from the single crystal XRD experiment and the Gaussian calcu-
lations in order to establish the soundness of the chosen theory
and basis set for the calculations. The two are summarized in SD
Table IIa and b in the supplementary section. The regression anal-
ysis of the comparison between the experimental XRD data and
the simulated Gaussian data returned a significant R2 of 0.9724
for the crystal structure (1) and 0.9671 for the associated crystal
(1A) in case of the bond lengths. Similarly, we recorded, an R2 of
0.9319 and 0.8371 for 1 and 1A, respectively, in case of the bond
angles. These high coefficients indicate the minimized confirmation
of the Gaussian structure to align with the synthesised XRD struc-
ture. Further, the theoretical and the experimental IR values com-
parison returned a R2 of 0.8099. This demonstrates the accuracy of
the chosen level of theory for computation and the soundness of
our computed values.
After optimizing the reaction conditions, we must apply the
same with the other ketones. Hence, we applied the same for the
cyclohexanone and cyclopentanone ketones giving the spirobiben-
zopyrans CHX-SK and CP-SK, respectively. The results are summa-
rized below in Table 4.
We observe that the yields of the respective spirobibenzopy-
rans have improved in the optimized conditions indicating that our
study of the catalyst and solvent effects have been successful. We
also for the first time conclude that the application of Lewis acid in
the Claisen-Schmidt condensation reaction yields spirobibenzopy-
rans rather than the expected bis-chalcones when the aldehyde
chosen is salicylaldehyde. The final conclusions from the study are
as follows:
6

S. Kar, P. Rai, S.M. Chelli et al.
Journal of Molecular Structure 1240 (2021) 130598
Fig. 6. Molecules used for the in silico studies.
Table 5
irreversible endothermic transition at 214 °C, where melting be-
gins. The endothermic peak represents the temperature at which
the melting terminates, which corresponds to its melting point at
215 °C. Further, it indicates that there is no phase transition before
melting. The sharpness of the peak shows the good degree of crys-
tallinity and purity of the sample. The TGA curve of this sample
indicates that the sample is stable up to 245 °C beyond which the
weight loss is not due to self-degradation of CHP-SK, but merely
due to evaporation after its melting. The exothermic peak at 297 °C
indicates that the sample undergoes decomposition at this temper-
ature.
Simulated entropy values obtained from the energy minimization studies.
Entropy loss ꢀS (cal/mol-kelvin) w.r.t.
Molecule
S (cal/mol-kelvin)
to the bis-chalcone
CP-SK
118.39
130.81
143.45
124.60
133.76
149.18
133.49
139.46
153.26
25.06
12.64
–
CP-PS
DA-CP
CHX-SK
CHX-PS
DA-CHX
CHP-SK
CHP-PS
DA-CHP
24.58
15.42
–
19.77
13.80
–
3.1. In vitro antibacterial activity
Thermal analysis of a material provides useful information re-
garding the thermal stability of that material [24]. Dried crystals
of the sample were selected for this purpose and the analysis was
carried out under nitrogen flow of 100 mL/min from 30 to 600 °C
at a heating rate of 10 °C/min in the SDT Q600 TGA/DTA analyzer.
The SD Fig. 1 in the supplementary section shows the TGA/DTA
curve for CHP-SK since we have reported the single crystal XRD of
the same. The DTA curve implies that the material undergoes an
The spirobibenzopyrans have previously not been explored for
any biological properties. Our group was the first to report their
anti-cancer properties [21,22]. In this study, for the first time we
report the anti-bacterial properties of the spirobibenzopyrans. The
spirobibenzopyrans and the benzopyrylium salts were screened for
their antibacterial potency against ESKAPE pathogen panel. The re-
sults are tabulated below in Table 6. Both the spirobibenzopyrans
and benzopyrylium salts were found to be inactive against E. coli
Table 6
Anti-bacterial susceptibilities of the synthesized spirobibenzopyrans and the benzopyrylium salts.
MIC (μg/ml)
Sample Code
E. coli ATCC 25,922
S. aureus ATCC 29,213
K. pneumoniae BAA 1705
A. baumannii BAA 1605
P. aeruginosa ATCC 27,853
CHP-SK
CHX-SK
TH-SK
>64
>64
>64
>64
>64
>64
>64
0.0156
>64
>64
>64
32
>64
>64
>64
>64
>64
>64
>64
64
>64
>64
>64
>64
>64
>64
>64
8
>64
>64
>64
>64
>64
>64
>64
1
CP-SK
3P-SK
>64
64
CP-PS
CHX-PS
Levofloxacin
>64
0.25
7

S. Kar, P. Rai, S.M. Chelli et al.
Journal of Molecular Structure 1240 (2021) 130598
(ATCC 25,922), K. pneumoniae (BAA 1705), A.baumannii (BAA 1605),
and P.aeruginosa (ATCC 27,853) strains. However, they were found
to have some activity against S. aureus (ATCC 29,213). The active
compounds CP-SK has a MIC of 32 μg/mL and CP-PS has a MIC of
64 μg/mL. This study was limited to the parent spirobibenzopyrans
and benzopyrylium salts. The results indicate that the derivatives
might possess better activity than the parent molecules. Further
studies are under progress in this regard.
Investigation, Formal analysis, Validation. Naveen Shivalinge-
gowda: Investigation, Formal analysis, Validation, Project ad-
ministration. Sidharth Chopra: Formal analysis, Validation, Su-
pervision, Project administration. Lokanath Neratur Krishnap-
pagowda: Formal analysis, Validation, Supervision. Siva Kumar
Belliraj: Formal analysis, Validation, Supervision, Project adminis-
tration. Nageswara Rao Golakoti: Formal analysis, Validation, Su-
pervision, Project administration.
Acknowledgments
4. Conclusions
The authors are grateful to Bhagawan Sri Sathya Sai Baba,
Founder Chancellor, SSSIHL, for His constant inspiration. The au-
thors are grateful to Institution of Excellence, Vijnana Bhavan,
University of Mysore, India for providing the single crystal X-ray
diffractometer facility. The authors would like to thank the JST
Sakura Science Program from the Govt. of Japan. We thank Dr. Ra-
jesh Babu, Department of Chemistry, SSSIHL for his valuable sug-
gestions. We also acknowledge the valuable suggestions made by
Dr. Srinivas Oruganti, Director at Dr. Reddy’s Institute of Life Sci-
ences and Dr. Srinivas Nanduri, Associate professor, NIPER for their
valuable insights. The authors also acknowledge Sri Nitesh Tamang,
Department of Chemistry, SSSIHL for his help.
In summary, this study reported unusual products formed from
the well-known Claisen-Schmidt reaction between a cycloalka-
none and salicylaldehyde. Using the standard reaction conditions,
in alkaline medium we obtained spiro molecule along with the
expected bis-chalcone. This study is the first report of isolating
spirobibenzopyrans using the Claisen-Schmidt reaction. In acidic
medium, we demonstrated the formation of benzopyrylium salt.
Firstly, we concluded that the 2-hydroxybenzaldehyde reactant is
essential to observe these deviations. Further, we evaluated the ef-
fect of polarity of solvent and ring size on the cycloalkanone re-
actant. For the cyclohexanone, we confirmed the influence of sal-
icylaldehyde and acidic medium to generate the alternative prod-
uct as benzopyrylium salt from the Claisen-Schmidt reaction irre-
spective of the solvent system, while the basic medium enabled
the progress of the reaction only in polar solvents. For the spiro
molecules, we concluded that the conformational changes in the
ring system must aid the entropy loss in the intramolecular cy-
clization reaction, which in case of cyclopentanone is not possible
due to high ring strain. In case of the benzopyrylium salts, we ob-
served that the stability offered by aromaticity is the driving factor.
The effect of Lewis acid instead of alkali and acid was evaluated.
We observed that SiCl4 and TiCl4 yielded spirobibenzopyrans with
absolute ethanol as the solvent. This is the first report of formation
of spirobibenzopyrans from TiCl4. The reaction conditions were op-
timized to get the best yield of spirobibenzopyrans.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130598.
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