pH dependent structural interconversion of 2-(2-hydroxy-benzylidene)-cyclohexan-1-one: Crystal structures and spectroscopic investigation

Article abstract of DOI:10.1016/j.molstruc.2017.02.003

The behavior of 2-(2-hydroxy-benzylidene)-cyclohexan-1-one (BC) in the acidic and basic media was investigated. In acidic conditions a new dimeric structure 4′a-methoxy-2,2′,3,3′,4′,4′a,9′,9′a-octahydro-1H, 1′H-4,9′-bixanthene (XTC) was isolated. The stru

Full text of DOI:10.1016/j.molstruc.2017.02.003

Accepted Manuscript
pH dependent structural interconversion of 2-(2-hydroxy-benzylidene)-cyclohexan-1-
one: Crystal structures and spectroscopic investigation
Ana-Maria Pană, Iulia Păuşescu, Sergiu Shova, Valentin Badea, Ramona Tudose,
Mihaela Silion, Otilia Costişor, Liliana Cseh
PII:
S0022-2860(17)30135-7
10.1016/j.molstruc.2017.02.003
MOLSTR 23397
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 16 December 2016
Revised Date: 31 January 2017
Accepted Date: 1 February 2017
Please cite this article as: A.-M. Pană, I. Păuşescu, S. Shova, V. Badea, R. Tudose, M. Silion, O.
Costişor, L. Cseh, pH dependent structural interconversion of 2-(2-hydroxy-benzylidene)-cyclohexan-1-
one: Crystal structures and spectroscopic investigation, Journal of Molecular Structure (2017), doi:
10.1016/j.molstruc.2017.02.003.
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ACCEPTED MANUSCRIPT
pH Dependent structural interconversion of
2-(2-hydroxy-benzylidene)-cyclohexan-1-one: crystal structures and
spectroscopic investigation
Ana-Maria Panăa, Iulia Păuşescua,b, Sergiu Shovac, Valentin Badeab*, Ramona Tudosea,
Mihaela Silionc, Otilia Costişora, Liliana Csehb*
aInstitute of Chemistry Timisoara of Romanian Academy, 24 M. Viteazul Bvd, 300223,
Timisoara, Romania, lili_cseh@yahoo.com
bUniversity Politehnica Timisoara, Faculty of Industrial Chemistry and Environmental
Engineering, 6 V. Parvan Bvd, 300223, Timisoara, Romania, valentinus.badea@gmail.com
cPetru Poni Institute of Macromolecular Chemistry, Inorganic Chemistry, 41A Aleea Gr.
Ghica Voda, 700487, Iasi, Romania
Abstract
The behavior of 2-(2-hydroxy-benzylidene)-cyclohexan-1-one (BC) in the acidic and basic
media was investigated. In acidic conditions a new dimeric structure 4'a-methoxy-
2,2',3,3',4',4'a,9',9'a-octahydro-1H,1'H-4,9'-bixanthene (XTC) was isolated. The structures of
BC and XTC were confirmed by single crystal X-ray diffraction and NMR. In basic
conditions, unprotonated specie (BC-) was characterized by NMR, UV-Vis and fluorescent
spectroscopy; BC could function as pH sensor.
Keywords: xanthene derivative; crystallographic structure; pH sensor; deuteration.
1. Introduction
Di-substituted cyclohexanone derivatives, curcumin analogues, have been studied so
far due to their toxic and cytotoxic activities [1-7]. Recently, our research group has opened a
new direction of research and application of xanthylium derivatives on photochemistry at
molecular level [8,9]. All practical applications of photochromic compounds are based on the
chemical transformations they can be submitted to by the actions of different stimuli, namely,
light, pH, temperature, etc.[10-16]. The study of curcumin analogues based on cyclohexanone
are done most on symmetrical structures; from the point of view of photochromic
investigation, the complexity of asymmetrical systems (see Fig. 1) due to the transient species
* lili_cseh@yahoo.com, valentinus.badea@gmail.com
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from the network of the chemical reaction open the way to a diversity of applications in opto-
electronics and biomedicine.
Fig. 1. Structure of asymmetrical di-substituted benzylidene cyclohexanone derivatives
2-(2-Hydroxy-benzylidene)-cyclohexan-1-one (BC) is a very useful intermediate for
the synthesis of a variety of functional systems (photochromic, dyes, therapeutics) [17-22].
The presence of phenolic group, the conjugated double bond system and the active methylene
group from the alpha position of ketone moiety turn this compound into attractive candidate
for various types of chemical reactions [23]. Herein we present the BC behavior in acidic and
basic conditions and the characterization of the isolated compounds.
2. Experimental
2.1. Instrumentation
The NMR spectra were recorded on a Bruker AVANCE III 500 MHz spectrometer (500 MHz
13
for 1H and 125 MHz for C) at 298 K. NMR assignments were done on the basis of 1D and
2D spectra. The HPLC-MS results were obtained using an Agilent 6500 Series Accurate-Mass
Quadrupole Time-of-Flight (Q-TOF) LC/MS. The sample was separated on an Zorbax SB-
C18 reverse phase column (4.6 mm x 250 mm, 5 ꢀm particle size) using a 20 ꢀL injection
volume, solvent flow 0.6 mL/min and isocratic elution. The mobile phase consisted of 100%
acetonitrile filtered and degassed under vacuum before use. The detector UV-VIS DAD was
monitored at 282 nm. After the chromatographic detector, the system was fitted with a flow
splitter, which directed 10% of the flow directly to the electrospray ion source (ESI) mass
spectrometer. The ESI-Q/TOF MS conditions were set as follows: electrospray ionisation
(positive ion mode), drying gas (N2) flow rate 7.0 L/min; drying gas temperature 325ºC;
nebulizer pressure 15 psig, capillary voltage 4000 V; fragmentation voltage 200 V. The full-
scan mass spectra of the investigated compounds were acquired in the m/z range of 100–1000.
The mass scale was calibrated using the standard calibration procedure and compounds
provided by the manufacturer. Data were collected and processed using MassHunter
Workstation Software Data Acquisition for 6200/6500 Series, version B.01.03. The samples
were dissolved in acetonitrile and then diluted up to a concentration of 100 ꢀg/mL. Only
positive the ions were analysed, because, in ESI-MS, the formation of the molecular ion
occurs in positive mode of operating.
UV-Vis absorption spectra were recorded on Cary 60 UV-Vis spectrophotometer.
Fluorescence spectra were recorded on Perkin Elmer LS-55 spectrometer on basic aqueous
solutions using λex = 364 nm and λem = 443 nm, the excitation slit was set at 10 and the
emission slit at 5 and the scanning speed was 100 nm/min; for the acetonitrile solution of
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XTC λex = 362 nm and λem = 439 nm with 15 and 5 slits respectively. The fluorescent spectra
were recorded using front-face illumination of XTC on front surface powder accessory with
1% attenuation and 15/7.5 slits, and λex = 397 nm and λex = 443 nm. The pH of solutions was
measured with a Wissenschaftlich-Techniche Werkstatten 330I pH/ion meter. The pH of the
solutions was adjusted by addition of NaOH or universal buffer based on sodium citrate [24].
X-ray diffraction: Intensity data were collected at 298 K with Oxford Diffraction SuperNova
diffractometer using hi-flux micro-focus Nova Cu Ka radiation. The crystal was placed at 47
mm from the CCD detector and 566 frames were measured each for 10/50 s over 1_ scan
width. Data were processed with the CrysAlis PRO software [25]. The structure was solved by
direct methods using Olex2 [26] software with the SHELXS27 structure solution program and
refined by full-matrix least-squares based on F2 with SHELXL-97 [27]. Atomic
displacements for non-hydrogen atoms were refined using an anisotropic model. C-bound
hydrogen atoms were placed at idealized positions. Positions of hydrogen atoms for the O-H
groups were located on electron density difference map and refined accounting for the
hydrogen bonds parameters. The O–H distances were constrained to 0.86 Å. All hydrogen
atoms were refined using riding model with Uiso(H) = 1.2 eq of their carrier atoms. Crystal
data and further details of data collection and structure refinement are given in Table 1.
CCDC 1456381 for BC and 1506218 for XTC contains the supplementary crystallographic
data for this contribution. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
deposit@ccdc.ca.ac.uk).
Table 1
Crystallographic data, details of data collection and structure refinement parameters for BC
and XTC.
BC
XTC
Empirical formula
Formula weight
C26H28O4
404.48
100
monoclinic
P21/n
17.070(2)
7.2073(9)
17.140(3)
90
C27H28O3
400.49
Temperature/K
Crystal system
Space group
a/Å
b/Å
c/Å
220
triclinic
P-1
8.0572(8)
11.6405(9)
11.7201(9)
85.318(6)
83.526(8)
74.506(8)
1051.00(16)
2
α
/°
β/°
104.201(17)
90
γ
/°
V/Å3
2044.3(5)
4
1.314
Z
Dcalc/mg/mm3
1.266
/mm-1
0.699
0.081
µ
Crystal size/mm3
0.092 × 0.072 × 0.028 0.25 × 0.1 × 0.08
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θmin, θ max(°)
13.39 to 117.834
4853
2817 [Rint = 0.0626]
2817/2/273
1.070
3.502 to 50.054
Reflections collected
Independent reflections
Data/restraints/parameters
GOFc
13371
3711 [Rint = 0.0811]
3711/0/272
1.045
a
0.0808
0.0581
R1 (I
>
2σ(I)
b
wR2 (all data)
Largest diff. peak/hole/e Å-3
0.1847
0.22/-0.26
0.1255
0.19/-0.21
b
2
2 2
c
2
aR1 = Σ||Fo| − |Fc||/Σ|Fo|. wR2 = {Σ[w(Fo −Fc2)2]/Σ[w(Fo ) ]}1/2. GOF = {Σ[w(Fo −Fc2)2]/(n−p)}1/2,
where n is the number of reflections and p is the total number of parameters refined
2.2. Materials and methods:
Salicylaldehyde, 99%, cyclohexanone, 99% were of analytic grade and were purchased from
Sigma-Aldrich. Other reagents, sodium hydroxide (powder), chloroform, petroleum ether,
acetonitrile, perchloric acid 70% and methanol were purchased from Merck and were used
without further purification.
2.3. Synthesis of compounds:
2-(2-hydroxy-benzylidene)-cyclohexan-1-one (see Scheme 1) was obtained following the
literature [3] with a slight modification. A solution of salicylaldehyde (5.2 mL, 0.05 mol) in
4% aqueous NaOH (150 mL) was added dropwise to a mixture of 4% aqueous NaOH (50
mL) and cyclohexanone (25.9 mL, 0.25 mol) under stirring. The reaction mixture was stirred
at room temperature for 40h and then neutralized with HCl 6N (40 mL). The HCl solution
was added dropwise to avoid heating. The product was extracted into CHCl3 (100 ml),
washed with water (4 X 100 ml) and dried over Na2SO4. The chloroform was evaporated and
the resulting dark brown oil was extracted with hot petroleum ether. Upon cooling a yellow
precipitate was filtered off, while from the mother liquor yellow crystals were formed. (η =
24%, m.p. = 126-128 ºC).
Scheme 1. Synthesis of BC
FT-IR (KBr) cm-1: 3370 (br,s), 2937 (w), 2871 (w), 1655 (m), 1561 (s), 1249 (s), 1145 (m),
750 (m).
1H-NMR (500 MHz, CD3OD, δ ppm): 7.65 (s, 1H, H7), 7.27 - 7.16 (m, 2H, H9,11), 6.87 - 6.82
(m, 2H, H10,12), 2.78 (t, 2H, H2, J = 6.7 Hz), 2.51 (t, 2H, H5, J = 6.7 Hz), 1.97 - 1.89 (m, 2H,
H3), 1.79 - 1.72 (m, 2H, H1)
13C-NMR (125 MHz, CD3OD, δ ppm): 204.7 (C6), 158.2 (C13), 137.3 (C4), 133.2 (C7),
131.5 (C9, C11), 123.9 (C8), 120.3 (C10), 116.6 (C12), 41.4 (C5), 30.2 (C2), 25.1 (C1), 24.8
(C3).
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4'a-Methoxy-2,2',3,3',4',4'a,9',9'a-octahydro-1H,1'H-4,9'-bixanthene (XTC) – Fig. 2. 50 mg
of BC were dissolved into 20 mL of methanol, then 5 mL of HClO4 70 % w solution were
added while stirring. The solution immediately turned red and after 2 hours of stirring at room
temperature, a white precipitate was obtained and isolated by filtration. Suitable single
crystals for X-ray diffraction were obtained in about 24 hours by slow evaporation of a
methanol/chloroform solution of XTC. m.p. = 156-158ºC
Fig. 2. Structure of XTC
1H-NMR (500 MHz, CDCl3, δ ppm): 7.1 (m, 1H, H24), 7.07 (m, 1H, H26), 7.03 (m, 1H, H9),
6.90 (m, 1H, H7), 6.81 - 6.83 (m, 4H, H8,10, 23,25, ), 6.05 (s, 1H, H5), 4.41 (d, 1H, H14, J = 11.7
Hz), 3.25 (s, 3H, H21), 2.45 (m, 2H, H3), 2.37, 1.46 m (m, 2H, Hx), 1.93, 1.85 (m, 2H, H1),
1.79 (m, 1H, H15), 1.77, 1.27 (m, 2H, Hx), 1.75, 1.42 (m, 2H, Hx), 1.68, 1.57 (m, 2H, H2),
1.68, 1.45 (m, 2H, Hx).
13C-NMR (125 MHz, CDCl3, δ ppm): 153.2 (C11), 152.7 (C22), 147.5 (C12), 131.1(C4),
128.5 (C26), 128.0 (C9), 127.1 (C24), 126.6 (C27), 125.2 (C7), 122.3 (C6), 121.7 (C23),
120.9 (C25),117.6 (C5), 116.8 (C8), 114.6 (C10), 112.2 (C13), 99.5 (C20), 48.0 (C21), 41.6
(C15), 35.7 (C14), 31.4 (C18), 30.7 (C3), 27.2 (C16), 25.7 (Cx), 24.5 (C1), 22.6 (Cx), 21.8
C2).
ESI-MS: calcd. for C27H28O3 m/z = 400.2038 [M+], found m/z = 400.1974 [M+]
Sodium 2-((2-oxocyclohexylidene)methyl)phenolate (BC-Na+) 20 mg of BC were dissolved
into 1 mL of methanol then 40ꢀL of NaOH 40% wt. solution were added. Similar conditions
were used for deuterated solvents and the structure for the product is presented in Fig. 3.
Fig. 3. Stucture of BC-Na+ in NaOD and methanol-d4
1H-NMR (500 MHz, 1 ml CD3OD + 40ꢀL NaOD, δ ppm): 8.01 (s, 1H, H7), 7.14 (dd, 1H,
H9, J = 7.7, 1.8 Hz), 7.02-6.99 (m, 1H, H11), 6.63 (dd, 1H, H12, J = 8.3, 1.1 Hz), 6.40-6.37 (m,
1H, H10), 2.82-2.80 (m, 2H, H5), 1.90-1.87 (m, 2H, H3), 1.76-1.72 (m, 2H, H4).
13C-NMR (CDCl3, δ ppm): 205.3 (C1), 170.7 (C13), 138.9 (C7), 132.8 (C6), 132.3 (C11),
131.4 (C9), 126.1 (C8), 121.7 (C12), 113.9 (C10), 40.5 (weak – C2), 30.5 (C5), 25.3 (C3),
24.5 (C4).
ESI-MS: calcd. for C13H13O2 m/z = 201.0921 [M-], found m/z = 201.0386 [M-]
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3. Results and discussions
According to X-ray crystallography both BC and XTC compounds have a molecular crystal
structure, as shown in Fig. 4 and 5, respectively. The asymmetric part of the unit cell of BC is
consisted of two crystallographic independent but chemically equivalent neutral entities,
denoted as molecule A and B. As expected, the geometric parameters of two molecules are
very close, which is evidenced by the values of bond distances and angles summarized in
Table 1S and 2S (see Supplementary Information).
Fig. 4. View of the asymmetric part in the crystal structure of BC. Thermal ellipsoids are
drawn at 50% probability level.
Fig. 5. X-ray molecular structure of XTC. Thermal ellipsoids are drawn at 40% probability
level.
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The interaction between the independent molecules A and B in the crystal is realized via two
intermolecular H-bonds, where O1A-H and O1B-H act as donor towards O2B and O2A as
acceptor of protons, respectively. It determines the formation of the supramolecular zig-zag
like chains running along 101 direction. A view of one-dimensional supramolecular chain is
shown in Fig. 6.
Fig. 6. One–dimensional H-bonded chain in the crystal structure of BC.
H-bonds parameters: O1B−HꢁꢁꢁO2A [O1B−H 0.86 Å, HꢁꢁꢁO2A 1.97 Å, O1BꢁꢁꢁO2A (-0.5 – x,
0.5 – y, -0.5 + z) 2.774(4) Å, O1B−HꢁꢁꢁO2A 155.4°], O1A−HꢁꢁꢁO2B [O1A−H 0.86 Å,
HꢁꢁꢁO2B 1.90 Å, O1AꢁꢁꢁO2B 2.736(4) Å, O1A−HꢁꢁꢁO2B 164.9°].
The BC in acidic condition in the first step turns to a red solution due to the formation of
cationic form BC+ and after two hours it converts to a white solid product (XTC). BC+ is the
unstable form and in the presence of perchloric acid turns into metastable mixture of HX and
BC+. These two unstable species react with the formation of dimeric structure HTX, stabilized
by addition of a nucleophile (methoxy group). Kanitz and co-workers [28] previously reported
the formation of a different dimer structure resulting from dimerization of two HX molecules
in the acidic conditions. The mechanism of the reaction for XTC form is depicted in Scheme
2.
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Scheme 2. The proposed mechanism for the formation of XTC
The XTC structure identified by use of single crystal X-ray diffraction is confirmed by NMR
spectroscopy but, the attribution signals to each proton of 1H-NMR spectrum is very complex.
In the low field they are two sets of signals attributed to the absorption by the aromatic
1
protons: 7.12-7.01 ppm corresponding to 3H, 6.91- 6.79 ppm corresponding to 5H. The H-
13C 2D HSQC NMR spectrum (correlates 13C with directly attached protons) indicated the
exact correlation, as a cross peak, between each proton and its corresponding carbon- first set:
H from 7.10 ppm with C from 127.1 ppm; H from 7.07 ppm with C from 128.5 ppm; H from
7.03 ppm with C from 128.0 ppm; - second set: H from 6.90 ppm with C from 125.2 ppm; H
from 6.86 ppm with C from 120.9 ppm; H from 6.85 ppm with C from 116.8 ppm; H from
6.83 ppm with C from 121.7 ppm; H from 6.81 ppm with C from 114.6 ppm. Based on 1H-13C
2D HMBC NMR spectrum (correlates long range coupling 1H-13C over two and three bond) it
was obtained the next correlations: H from 7.1 ppm with C from 152.7 and 128.5 ppm,
respectively; H from 7.07 ppm with C from 152.7, 127,1 and 35.7 ppm, respectively, H from
7.03 ppm with C from 153.2 and 125.2 ppm, respectively; H from 6.90 ppm with C from
153.2, 128.0 and 117.6 ppm, respectively; H from 6.86 ppm with C from 125.6 and 116.8
ppm, respectively; H from 7.85 ppm with C from 153.2, 128.0, 122.3 and 114.6 ppm,
respectively; H from 6.83 ppm with C from 152.7, 125.6 and 120.9 ppm, respectively; H
from 7.81 ppm with C from 153.2 and 122.3 ppm, respectively. The correlation of the above
results had led to the establishment of chemical shift for each atom of carbon and hydrogen
(see Experimental section and Supplementary Information).
HPLC analysis revealed the presence of only one peak at about 11 min, for which was
extracted the mass spectrum in the positive ionization mode. The ESI-MS spectrum (see Fig.
S19) illustrates the presence of the molecular ion of the XTC at m/z = 400.1974. In addition,
the base peak at m/z = 369.1804 corresponds to XTC structure without methoxy group, the
fragmentation being produced during the ionization process.
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BC in the basic condition turned into unprotonated form (BC-) (see Scheme 3). In the
presence of deuterated solvent, a hydrogen-deuterium exchange reaction takes place; the
hydrogen atoms in alpha position to the keto group were exchanged with deuterium atoms
(see Fig. 3). This is a well-known method for synthesis of many deuterated aldehydes and
ketones with a suitable alpha position to the carbonyl group [29]. Confirmation of deuterated
BC- specie was proved by NMR experiments. Thus, comparative H-NMR spectra of BC and
BC- reveals the shifts of all proton signals and the absence of the protons signal from alpha
position to keto group (triplet from 2.51 ppm). COSY-NMR spectrum of BC- shows faint
signals of correlation between proton from position no. 2 and 3, while HSQC-NMT displayed
weak correlation signals of C from 40.5 ppm and H from 2.51 ppm.
BC is a chalcone derivative that is expected to have photochromic behavior upon pH variation
and light excitation, similar to those registered for 2,6-bis-(2-hydroxybenzylidene)-
cyclohexan-1-one [8,9]. As a consequence, experiments were carried out in order to establish
color change during pH shift from very acidic to very basic environment and identify the
chemical species responsible for this behavior (see Scheme 3). We have seen beforehand that
in very acidic media, BC turns into the irreversible dimeric structure identified as XTC, so we
have focused in studying the photochromic behavior only in basic environment.
pH
Scheme 3. BC behavior along pH variation (only stable species are displayed)
The UV-Vis spectra of BC in aqueous solutions of basic pH are presented in Fig. 7. The
samples in basic pH exhibit yellow color and present a broad absorption band at 394 nm. At
pH = 13.26 we identify the most intense absorption band, with ε = 3077.6 L mol-1 cm-1. This
band, corresponding to the deprotonated form Ct- registers a drastic decay along pH drop,
expressed by a logarithmic function. Fig. 8 shows the linear dependence between ln A (394
nm) and pH on the range 6-14, which could recommend BC to be used as pH sensor in basic
environment.
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Fig. 7. UV-Vis spectra of 2-(2-hydroxy-benzylidene)-cyclohexan-1-one at basic pH values
(2x10-4 mol/L in aq. solution)
0
6
7
8
9
10
11
12
13
14
-0.5
-1
-1.5
-2
-2.5
-3
y = 0.5845x - 8.247
R2 = 0.9216
-3.5
-4
-4.5
-5
pH
Fig. 8. Linear variation of ln A (394 nm) versus pH in basic environment
The excitation and emission spectra of 2-(2-hydroxy-benzylidene)-cyclohexan-1-one in
different pH solutions, ranging from 6.96 to 13.75 are presented in Fig. 9. The compound
exhibits fluorescent properties in mild basic conditions. The fluorescence intensity decreases
with the increase of pH, while at pH values above 11 the fluorescence is quenched.
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Fig. 9. Superimposed excitation (left) and emission (right) spectra of 2-(2-hydroxy-
benzylidene)-cyclohexan-1-one at different pH values (2x10-4 mol/L in aqueous solution)
.
Fig. 10. Fluorescent behaviour on XTC solid sample: left on powder and right on
monocrystals; top - samples before irradiation and bottom after irradiation with UV light
The maximum emission intensity was obtained at pH = 7.28 and the corresponding Stokes
shift is ꢂλ = 69 nm.
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XTC displays fluorescent behavior in solution and even in solid state (see Fig. 10). By
irradiation at λ = 365 nm using the laboratory UV lamp it shows obvious fluorescent
properties, which is confirmed by fluorescence spectroscopy (see Fig. 11: a) on solid, b) in
solution)
Fig. 11. Fluorescence behaviour of XTC on solid state - left, in acetonitile (10-5 mol L-1) –
right.
4. Conclusions
2-(2-hydroxy-benzylidene)-cyclohexan-1-one (BC) was synthesized in good yield and its X-
ray diffraction structure was assessed. BC shows pH dependent photochromic behavior and it
could be used as a pH sensor on basic pH range (pH = 6 ÷14). In very acid environment it
turns into a dimeric form HTX stabilized by addition of methoxy group – XTC form. The
structure of XTC was identified by single crystal X-ray diffraction and confirmed by NMR
and mass spectrometry. Good fluorescence properties were found for the BC on the 6 -11 pH
range and for the XTC in solid state and in solution.
Acknowledgements
The authors aknowledge the support to the Romanian Academy, Project 4.1.
We thank Phys. Racu Andrei from National Institute for Research and Development in
Electrochemistry and Condensed Matter, Timisoara for useful desscusions regarding
fluorescent investigations.
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Highlights
-
Synthesis and characterization of 2-(2-hydroxy-benzylidene)-cyclohexan-1-
one (BC) and 4'a-methoxy-2,2',3,3',4',4'a,9',9'a-octahydro-1H,1'H-4,9'-
bixanthene (XTC);
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Crystal structures of BC and XTC;
UV-Vis and fluorescence spectroscopic investigation;
Mechanism formation of XTC from BC;
Deuteration of α-position to keto group.