Ultraviolet-light-induced aerobic oxidation of benzylic C(sp3)-H of alkylarenes under catalyst- and additive-free conditions

Article abstract of DOI:10.1016/j.tet.2021.131947

A mild and efficient system has been discovered for the synthesis of α-aryl carbonyl compounds via oxidation of benzylic C–H to C[dbnd]O bonds. This ultraviolet-light-mediated oxygenation reaction exhibited excellent substrate scope including various xanthenes, thioxanthenes and 9, 10-dihydroacridines and afforded the corresponding ketones with good to excellent yields under catalyst- and additive-free conditions at room temperature.

Full text of DOI:10.1016/j.tet.2021.131947

Tetrahedron 82 (2021) 131947
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Ultraviolet-light-induced aerobic oxidation of benzylic C(sp3)-H of
alkylarenes under catalyst- and additive-free conditions
, b,
Jiacheng Zhou a, Meichao Li a, Tianci Li a, Chunmei Li a **, Xinquan Hu a, Liqun Jin a,
,
*
Nan Sun a, Baoxiang Hu a, Zhenlu Shen a
a College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310032, China
b School of Chemistry and Chemical Engineering, Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University,
Shaoxing, 312000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A mild and efficient system has been discovered for the synthesis of
a-aryl carbonyl compounds via
Received 1 November 2020
Received in revised form
28 December 2020
Accepted 11 January 2021
Available online 14 January 2021
oxidation of benzylic CeH to C]O bonds. This ultraviolet-light-mediated oxygenation reaction exhibited
excellent substrate scope including various xanthenes, thioxanthenes and 9, 10-dihydroacridines and
afforded the corresponding ketones with good to excellent yields under catalyst- and additive-free
conditions at room temperature.
© 2021 Elsevier Ltd. All rights reserved.
Keywords:
Xanthenes
Thioxanthenes
Acridines
Ultraviolet light
Benzylic C(sp3)-H
1. Introduction
importantly, it is safe because the by-product of oxidation is water
[3]. However, the molecular oxygen, which generally exists in the
The development of green and sustainable methods for the
oxidation of benzylic CeH to C]O bond for the synthesis of -aryl
stable triplet ground state, has low activity towards benzylic C(sp3)-
H bonds [4]. As a result, numerous catalyst systems based on
transition metals or metal-free systems, which can activate the
CeH bonds or the molecular oxygen, have been devised to enable
aerobic oxidation of benzylic C(sp3)-H bonds, for example, palla-
dium, copper, manganese, cesium, iridium, iron, and gold com-
plexes [3c,4a-b,5-8], DDQ/TBN, NHPI/TBN, NH4I/AcOH, and TEMPO
derivatives [9,10].
In addition, in recent years, another kind of attractive catalyst
system based on light-induced photocatalysis, involving photo-
sensitive molecules, transition metal complexes, and organic
dyes, has been designed and applied in the reaction [11]. For
example, various copper [11c,12] and iron-based [13] photocatalytic
reactions of the aerobic oxidation of CeH bonds has been estab-
lished. Despite these advances, most of the above-mentioned pro-
cesses suffer from some shortcomings such as use of metal catalyst,
additives, hazardous reagents, tedious work-up processes, and the
requirement of special apparatus, which adversely affect the eco-
nomics as well as the ecofriendly nature of the reaction.
a
carbonyl compounds under milder conditions continues to attract
broad interest in modern synthetic chemistry [1]. Not only such
strategies are consistent with the criterion of environmental and
economic demands, but also these CeH bonds are very abundant in
numerous organic compounds.
Consequently, considerable efforts have been devoted to
develop more efficient synthetic methodologies or to improve the
existing synthetic process to access this transformation by using
various traditional toxic or corrosive metal-, peracid-, or oxoacid-
based oxidants [2], which would cause serious environmental
problems. Compared with the above oxidants, there is no doubt
that molecular oxygen, on behalf of green and sustainable chem-
istry, as oxidant for this transformation is more ideal because it is
abundant, inexpensive, clean, readily available, nontoxic, and more
* Corresponding author.
** Corresponding author. College of Chemical Engineering, Zhejiang University of
Technology, Hangzhou, 310032, China.
At the same time, xanthones, thioxanthones, and acridones have
been widely utilized in many areas [14]. Xanthones, as
a
E-mail addresses: lchm@usx.edu.cn (C. Li), zhenlushen@zjut.edu.cn (Z. Shen).
https://doi.org/10.1016/j.tet.2021.131947
0040-4020/© 2021 Elsevier Ltd. All rights reserved.

J. Zhou, M. Li, T. Li et al.
Tetrahedron 82 (2021) 131947
Table 1
heterotricyclic compounds, exhibit a variety of biological activities
and have been used to treat some diseases such as hypertension,
convulsions, thrombosis, tumor and Alzheimer [15]. Thioxanthones
are widely used in coating, microelectronics, photoresist and pho-
toinitiator [16], and acridones play an important role in medicines
for their significant antitumor activity [17]. In addition, they are
also used as the raw fluorescent materials [18]. In view of their
interesting properties, it remains highly desirable to develop
ecologically benign strategies for synthesis of these ketone com-
pounds, requiring milder conditions as well as avoiding the need
for toxic metal catalysts or additives.
Optimization of reaction conditions for product 2a.a
Entry
Light source
Solvent
T (h)
Yield(%)b
Herein, we report a new strategy for photoinduced aerobic
oxidation of the benzylic C (sp3)-H for synthesis of xanthones,
thioxanthones and acridones from xanthenes, thioxanthenes and
9,10-dihydroacridines, respectively. The reactions gave corre-
sponding ketones with good yields under mild conditions. To the
best of our knowledge, the ultraviolet-induced methodology of
synthesis of these compounds under catalyst- and additive-free
conditions at room temperature has not been reported so far. It is
noteworthy to mention that, in 2017, one benzophenone compound
was involved when Wang and co-workers reported a photoinduced
oxidative formylation of N,N-dimethylanilines [19]. Then, Jiang also
disclosed visible light-induced selective aerobic oxidative trans-
position of vinyl halides using a tetrahalogenoferrate(III) complex
catalyst [13a]. In addition, recently, we also noted that Wang and
co-worker developed a strategy for selective oxidation of benzylic
CeH to C]O bonds at 140 ꢀC under 0.5 MPa of oxygen [1b]. By
comparison, the use of ultraviolet light under catalyst- and
additive-free at room temperature makes this reaction quite sim-
ple, mild, sustainable and practical.
1
2
3
4
5
6
7
8
365e370 nm
380e385 nm
400e405 nm
420e425 nm
450e455 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
In dark
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
DCE
toluene
DMF
DMSO
dioxane
H2O
DME
MeCN
THF
DMSO
DMSO
DMSO
DCE
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
24
24
9
23
11
7
6
60
20
23
54
29
4
28
35
27
N.R.
24
94
88
9
10
11
12
13
14
15c
16d
17
18
380e385 nm
380e385 nm
a
Reaction conditions: xanthene (1a, 0.2 mmol), solvent (2 mL), at room tem-
perature under light irradiation in air.
b
Internal standard yield of gas chromatography, N.R. ¼ no reaction.
c
Under N2 atmosphere.
d
80 ꢀC (oil bath).
2. Results and discussion
circumstance. This data showed that the reaction under ultraviolet
irradiation was faster than that on elevated temperature (entry 16).
After mastering the optimized conditions, to explore the gen-
erality of the ultraviolet-induced strategy, some xanthene de-
rivatives were examined (Table 2). A variety of xanthenes were
employed under the optimal reaction conditions. Xanthenes with
an electron-donating group (2-tert-butyl, 2-methyl, 3-methyl, 2-
methoxy, or 4-methoxy) reacted with O2 and gave the desired
products 2b-2f in 80e98% yields. Xanthenes bearing electron-
withdrawing groups, such as 2-F, 2-Cl, 2-Br, or 2-MeO2C, under-
went smooth transformations to give the products 2g-2i and 2k in
isolated yields of 90e99%. In addition, disubstituted xanthenes
could proceed smoothly to provide the corresponding products 2l
and 2m in 48% and 88% yields, respectively. Moreover, two of
benzo-fused xanthenes 1n and 1o still displayed high reactivity and
led to the desired products 2n and 2o in 95% and 94% yields.
Furthermore, the benzo-fused xanthenes with an electron-
donating group or electron-withdrawing group (9-methyl, 10-
methyl, 11-methoxy, 9-Cl, 2-Br) were also oxidized to afford the
target products 2p-2t in 75%e99% yields. Moreover, to improve the
yield of 2q, the reaction temperature was elevated to 40 ꢀC, how-
ever, the yield was reduced to 61%. Compared to room temperature,
the amount of by-products was significantly increased which
influenced the formation of the desired product. The 14H-dibenzo
[a,h]xanthene 1u and 14H-dibenzo[a,j]xanthene 1v were well
tolerated and gave the expected products 2u and 2v in 62% and 68%
yields, respectively. At the same time, we have also observed that
when the R2 substituent was NO2 group, the desired product was
obtained with only 41% yield, probably because of the strong
electron-withdrawing inductive effect of nitro group, which greatly
influences the formation of the desired product. Subsequently, 9H-
xanthen-2-amine was employed for the oxidation reaction.
Firstly, the reaction conditions were optimized by using 9H-
xanthene (1a) as a model substrate (Table 1). When the model
reaction was carried out in the molecular oxygen with the illumi-
nation of LEDs (365-370 nm) in ethyl acetate at room temperature
for 12 h, the target product 9H-xanthen-9-one (2a) was detected in
9% yield by internal standard method of gas chromatography (entry
1).
illumination of LEDs (365e370 nm) in ethyl acetate at room
temperature for 12 h, the target product 9H-xanthen-9-one (2a)
was detected in 9% yield by internal standard method of gas
chromatography (entry 1). The optimization process revealed that
product 2a was detected in 23%, 11%, 7% and 6% under the irradi-
ation of LEDs at 380e385 nm, 400e405 nm, 420e425 nm and
450e455 nm, respectively (entries 2e5). The substrate exhibited
higher reactivity under irradiation of LED (380e385 nm). It was
found that the ultravioletevisible absorption intensity of the
starting material 1a was stronger at 380e385 nm than that at
visible ranges. The effects of various solvents on this reaction were
evaluated. The results indicated that using DCE or DMSO as a sol-
vent is obviously better than other solvents for the reactions after
12 h. When DCE and DMSO were used as the solvents, the reaction
gave desired product with 60% and 54% yields, respectively (entries
6 and 9). However, when the reaction time was prolonged to 24 h,
DMSO as the solvent gave slightly higher yield (entries 17 and 18).
The poor yields (4e35%) of product 2a were obtained, when the
reactions were performed in other solvents including toluene, DMF,
dioxane, H2O, DME, MeCN and THF (entries 7 and 8, 10e14).
However, it was not clear why the reaction has a strong depen-
dence on the solvent. Furthermore, the result showed that none of
the desired product was observed in the absence of the molecular
oxygen (entry 15). In addition, when the model reaction was per-
formed in oil bath at 80 ꢀC, 24% yield of 2a was isolated in dark
2

J. Zhou, M. Li, T. Li et al.
Tetrahedron 82 (2021) 131947
Table 2
,b
Synthesis of xanthones.a
a
Reaction conditions: xanthenes (1, 0.2 mmol), DMSO (2 mL), LEDs (380e385 nm), at room temperature in air.
b
Isolated yield.c at 40 ꢀC.
However, no product was obtained, instead of only the material was
observed, even after longer reaction times (40 h).
In addition, an obvious loss of the yield was observed when the
reaction time was prolonged to 12 h (entries 7 and 8). For N-
methyl-9,10-dihydroacridine compound, the reaction media DCE
afforded slightly higher yield (56%) than DMSO. In addition, we also
noted that the substrates thioxanthene and acridine exhibit better
reaction activity than xanthene.
Under the above optimized conditions LEDs (380e385 nm) as
the light source, DCE as the solvent, and at room temperature in
air), the substrate scope of this reaction was examined by using
various thioxanthenes and acridines. The results are summarized in
Table 4. Thioxanthenes, both bearing electron-donating groups and
Then, we were interested in whether the system would work for
synthesis of other heterotricyclic compounds, such as thio-
xanthenes and 9,10-dihydroacridines while modifying the condi-
tions slightly. At first, only 18% yield of target product was obtained
when 9H-thioxanthene was used under the above optimized con-
ditions. To improve the yield, a study on the effect of the solvent
was undertaken. As depicted in Table 3, DCE was the optimal sol-
vent under the irradiation of LEDs (380e385 nm) and gave the yield
up to 83% yield (entry 3).
3

J. Zhou, M. Li, T. Li et al.
Tetrahedron 82 (2021) 131947
Table 3
To explore the reaction mechanism, the control experiment and
the isotopic tracer experiment were carried out (Schemes 1 and 2).
No desired product was found when stoichiometric radical-
trapping reagent 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)
were added to the photoinduced reaction of 1a. It was suggested
that this transformation might proceed via a radical pathway. On
the other hand, in order to find the source of oxygen in product,
stoichiometric water labelled with 18O were added to the isotopic
tracer experiment and analyzed by MS. The result indicated that
90% oxygen of xanthone obtained from O2 in air.
Optimization of reaction conditions for product 4a.a
Entry
Light source
Solvent
T (h)
Yield (%)b
On the basis of these experiments and reported literature [20],
the possible reaction mechanism is proposed (Scheme 3). Ground-
state triplet oxygen 3O2 was irradiated by ultraviolet light
(380e385 nm), which formed to singlet oxygen 1O2 with a high
reactivity, then reacted with xanthene 1a and generated a free-
radical cation I and O2·- through a single electron transfer (SET)
process. Then the proton transfer between I and O2$- formed a
superoxide radical HOO· and radical II, which combined to form
intermediates III. Finally, the intramolecular dehydration of III
produced the product 2a.
1
2
3
4
5
6
7
8
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
380e385 nm
DMSO
DCM
DCE
DMF
DME
EtOAc
DCE
3
3
3
3
3
3
6
12
18
30
83
N.D.
N.D.
N.D.
72
DCE
60
a
Reaction conditions: thioxanthene (3a, 0.2 mmol), solvent (2 mL), at room
temperature under light irradiation in air.
b
Internal standard yield of Gas Chromatography, N.D. ¼ not detected.
electron-withdrawing groups, were well tolerated and gave cor-
responding thioxanthones 4b-4e in 70e83% yields. N-methyl-9,10-
dihydroacridines with a functional group, including 4-methoxy or
2-Cl, could proceed smoothly to provide the desired acridones 6b
and 6c in 54% and 72% yields, respectively. In addition, N-benzyl-
9,10-dihydroacridine could be also converted into the corre-
sponding products in 88% yield in the reaction conditions.
Scheme 1. Control experiment for mechanism exploration.
Table 4
,b
Synthesis of thioxanthenes and 9,10-dihydroacridines.a
a
Reaction conditions: thioxanthenes or acridines (3 or 5, 0.2 mmol), DCE (2 mL), LEDs (380e385 nm), at room temperature in air.
Isolated yield.
b
4

J. Zhou, M. Li, T. Li et al.
Tetrahedron 82 (2021) 131947
2H), 7.37e7.26 (m, 2H); 13C NMR (126 MHz, CDCl3)
d 177.3, 156.3,
134.9, 126.8, 124.0, 122.0, 118.1. MS (EI), m/z 196.01 [Mþ, 100%].
4.2. 2-(tert-Butyl)-9H-xanthen-9-one (2b)
White solid. m.p.: 113e116 ꢀC.1H NMR (500 MHz, CDCl3):
d
8.36e8.34 (m, 1H), 8.33 (d, J ¼ 2.5 Hz, 1H), 7.79e7.77 (m, 1H),
Scheme 2. The isotopic tracer experiment.
7.71e7.68 (m, 1H), 7.47 (d, J ¼ 8.4 Hz, 1H), 7.42 (d, J ¼ 8.8 Hz, 1H),
7.36 (t, J ¼ 7.7 Hz, 1H), 1.40 (s, 9H); 13C NMR (126 MHz, CDCl3):
d
177.6, 156.3, 154.5, 147.2, 134.7, 132.9, 126.9, 123.8, 122.6, 122.0,
121.3, 118.0, 117.7, 34.9, 31.5. MS (EI), m/z 237.10 [Mþ, 100%].
4.3. 2-Methyl-9H-xanthen-9-one (2c)
White solid. m.p.: 119e122 ꢀC. 1H NMR (500 MHz, CDCl3):
d 8.34
(d, J ¼ 7.9Hz, 1H), 8.12 (s, 1H), 7.72e7.69 (m, 1H), 7.53 (d, J ¼ 8.5, 1H),
7.48e7.46 (m,1H), 7.39 (d, J ¼ 8.8Hz,1H), 7.36 (d, J ¼ 7.5Hz,1H), 2.47
(s, 3H); 13C NMR (126 MHz, CDCl3):
d 177.4, 156.3, 154.6, 136.2,
134.8, 133.8, 126.9, 126.2, 123.8, 122.0, 121.6, 118.1, 117.9, 21.0. MS
(EI), m/z 209.97 [Mþ, 100%].
Scheme 3. The possible reaction mechanism.
4.4. 3-Methyl-9H-xanthen-9-one (2d)
White solid. m.p.: 95e97 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.32 (d,
3. Conclusion
J ¼ 8.1Hz, 1H), 8.20 (d, J ¼ 8.3Hz, 1H), 7.71e7.67 (m, 1H), 7.45 (d,
J ¼ 8.5Hz, 1H), 7.36 (t, J ¼ 7.5Hz, 1H), 7.17 (d, J ¼ 8.1Hz, 1H), 2.49 (s,
In summary, a new method for ultraviolet light induced oxida-
tion of benzylic C(sp3)-H was developed. A series of alkylarenes,
including xanthenes, thioxanthenes, and 9,10-dihydroacridines
were converted to the corresponding carbonyl compounds in
good yields. The procedure provides a simple and mild route to
form ketones under catalyst-, and additive-free at room tempera-
ture. In addition, the probable reaction mechanism clearly
explained the reaction process and the role of oxygen.
3H); 13C NMR (126 MHz, CDCl3)
d 177.1, 156.4, 156.3, 146.4, 134.7,
126.8, 126.6, 125.6, 123.9, 122.1, 119.8, 118.0, 117.8, 22.1.MS(EI), m/z
210.08 [Mþ, 100%].
4.5. 2-Methoxy-9H-xanthen-9-one (2e)
White solid.m.p.: 127e129 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.33
(d, J ¼ 8.2 Hz, 1H), 7.71e7.67 (m, 1H), 7.46 (d, J ¼ 8.7 Hz, 1H), 7.41 (d,
J ¼ 9.2 Hz, 1H), 7.37e7.34 (m, 1H), 7.32e7.29 (m, 1H), 3.90 (s, 3H);
13C NMR (126 MHz, CDCl3)
d 177.2, 156.2, 156.1, 151.1, 134.7, 126.8,
4. Experimental section
125.0, 123.8, 122.2, 121.4, 119.5, 118.1, 106.0, 56.0. MS (EI), m/z
226.07 [Mþ, 100%].
Unless otherwise stated, all the reactions were performed at
room temperature. All reactions were detected by GC. GC analyses
were conducted on an Agilent GC7890A system with a flame
ionization detector (FID) and a SE-54 capillary column. GC-MS was
performed on Thermo Trace ISQ instrument with TG 5MS capillary
column. Purification of reaction products was carried out by col-
umn chromatography on silica gel. 1H NMR (500 MHz) and 13C NMR
(126 MHz) spectra were obtained on a Bruker Avance III spec-
trometer. CDCl3 was used as the solvent with tetramethylsilane
(TMS) as the internal standard. Photoinduced reactions were per-
formed with 6W LEDs. Xanthenes (1b-1v) [21a], thioxanthenes (3a-
3e) [21b] and 9,10-dihydriacridines (5b-5d) [21c] were prepared in
our laboratory according to reported procedures. Other reagents
and solvents were purchased from commercial supplier and used
without any further treatment.
4.6. 4-Methoxy-9H-xanthen-9-one (2f)
White solid. m.p.: 174e175 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.34
(d, J ¼ 8.0 Hz, 1H), 7.90 (d, J ¼ 8.1 Hz, 1H), 7.74e7.71 (m, 1H), 7.61 (d,
J ¼ 8.4 Hz, 1H), 7.40e7.37 (m, 1H), 7.30 (t, J ¼ 8.0 Hz, 1H), 7.23 (d,
J ¼ 8.0 Hz, 1H), 4.03 (s, 3H); 13C NMR (126 MHz, CDCl3)
d 177.3,
156.1, 148.8,146.7, 134.9,126.8,124.2, 123.6,122.9,121.8, 118.4,117.8,
115.5, 56.6. MS (EI), m/z 225.96 [Mþ, 100%].
4.7. 2-Fluoro-9H-xanthen-9-one (2g)
White solid. m.p.: 153e155 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.31
(d, J ¼ 7.9 Hz, 1H), 7.97e7.95 (m, 1H), 7.75e7.72 (m, 1H), 7.50e7.47
(m, 2H), 7.46e7.42 (m, 1H), 7.38 (t, J ¼ 7.5 Hz, 1H); 13C NMR
4.1. Typical procedure for the preparation of 9H-Xanthen-9-one
(2a)
(126 MHz, CDCl3)
d
176.7, 158.8 (d, J ¼ 245.8 Hz), 156.3, 152.5, 135.2,
126.8, 124.3, 123.1 (d, J ¼ 25.3 Hz), 122.8 (d, J ¼ 7.1 Hz), 121.2, 120.1
(d, J ¼ 7.8 Hz), 118.1, 111.6 (d, J ¼ 23.6 Hz). MS (EI), m/z 214.03 [Mþ,
100%].
A 15-mL Schlenk tube equipped with a magnetic stirrer bar was
charged with xanthene (1a; 36.4 mg, 0.2 mmol) and DMSO (2 mL).
The Schlenk tube was placed in a dark box and illuminated with 6W
blue LED (380e385 nm). The mixture was stirred vigorously at
room temperature in air until the reaction was complete (GC). The
mixture was then concentrated on a rotary evaporator, and the
residue was purified by column chromatography (silica gel,
PEeEtOAc) to 2a: white solid. m.p.: 174e176 ꢀC. 1H NMR (500 MHz,
4.8. 2-Chloro-9H-xanthen-9-one (2h)
White solid. m.p.: 166e169 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.31
(d, J ¼ 8.0 Hz, 1H), 8.28 (d, J ¼ 2.6 Hz, 1H), 7.75e7.72 (m, 1H),
7.66e7.64 (m, 1H), 7.48 (d, J ¼ 8.5 Hz, 1H), 7.45 (d, J ¼ 8.9 Hz, 1H),
7.39 (t, J ¼ 7.5 Hz, 1H); 13C NMR (126 MHz, CDCl3)
d 176.3, 156.2,
CDCl3)
d
8.33e8.31 (m, 2H), 7.72e7.68 (m, 2H), 7.46 (d, J ¼ 8.5 Hz,
154.6, 135.3, 135.0, 129.9, 126.9, 126.2, 124.4, 122.8, 121.6, 119.9,
5

J. Zhou, M. Li, T. Li et al.
Tetrahedron 82 (2021) 131947
118.2. MS (EI), m/z 229.99 [Mþ, 100%].
4.16. 9-Methyl-7H-benzo[c]xanthen-7-one (2p)
White solid. m.p.: 162e165 ꢀC.1H NMR (500 MHz, CDCl3)
d
8.64
4.9. 2-Bromo-9H-xanthen-9-one (2i)
(d, J ¼ 8.1 Hz, 1H), 8.26 (d, J ¼ 8.8 Hz, 1H), 8.17 (s, 1H), 7.91 (d,
J ¼ 7.7 Hz, 1H), 7.72e7.65 (m, 3H), 7.56 (s, 1H), 2.49 (s, 3H); 13C NMR
White solid. m.p.: 149e152 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.43
(126 MHz, CDCl3)
d 177.1, 154.2, 153.8, 136.6, 135.7, 134.4, 129.6,
(s, 1H), 8.31 (d, J ¼ 7.9 Hz, 1H), 7.78 (d, J ¼ 8.9 Hz, 1H), 7.74 (t,
J ¼ 7.8 Hz, 1H), 7.48 (d, J ¼ 8.3 Hz, 1H), 7.40e7.37 (m, 2H); 13C NMR
128.2, 126.9, 126.0, 124.3, 123.9, 123.0, 122.2, 121.7, 118.0, 117.7, 21.1.
MS (EI), m/z 259.95 [Mþ, 100%].
(126 MHz, CDCl3)
d 176.1, 156.2, 155.1, 137.8, 135.3, 129.4, 126.9,
124.4, 123.2, 121.7, 120.1, 118.2, 117.2. MS (EI), m/z 275.97 [Mþ, 95%],
273.85 [100%].
4.17. 10-Methyl-7H-benzo[c]xanthen-7-one (2q)
White solid. m.p.: 166e169 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.59
4.10. 2-Nitro-9H-xanthen-9-one (2j)
(d, J ¼ 8.0 Hz, 1H), 8.25 (d, J ¼ 3.1 Hz, 1H), 8.23 (d, J ¼ 3.7 Hz, 1H),
Brown solid. m.p.: 201e203 ꢀC.1H NMR (500 MHz, CDCl3)
d 9.19
7.88 (d, J ¼ 7.7 Hz, 1H), 7.70e7.63 (m, 3H), 7.42 (s, 1H), 7.21 (d,
J ¼ 8.1 Hz, 1H), 2.52 (s, 3H); 13C NMR (126 MHz, CDCl3)
d 176.8,
(d, J ¼ 2.9 Hz, 1H), 8.55e8.53 (m, 1H), 8.33 (d, J ¼ 8.0 Hz, 1H),
7.83e7.79 (m, 1H), 7.64 (d, J ¼ 9.2 Hz, 1H), 7.55 (d, J ¼ 8.5 Hz, 1H),
156.0, 153.6, 145.9, 136.6, 129.5, 128.2, 126.9, 126.4, 126.0, 124.2,
123.9, 122.9, 121.7, 120.3, 117.9, 117.7, 22.1. MS (EI), m/z 259.99 [Mþ,
100%].
7.46 (t, J ¼ 7.7 Hz, 1H); 13C NMR (126 MHz, CDCl3)
d 175.8, 159.3,
156.0, 144.0, 136.0, 129.1, 127.1, 125.4, 123.7, 121.8, 121.5, 119.8, 118.3.
MS (EI), m/z 240.92 [Mþ, 100%].
4.18. 11-Methoxy-7H-benzo[c]xanthen-7-one (2r)
4.11. Methyl 9-oxo-9H-xanthene-2-carboxylate (2k)
White solid. m.p.: 275e277 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.77
White solid. m.p.: 217e219 ꢀC.1H NMR (500 MHz, CDCl3)
d 9.02
(d, J ¼ 7.1 Hz,1H), 8.28 (d, J ¼ 8.8 Hz,1H), 7.97 (d, J ¼ 8.1 Hz,1H), 7.94
(d, J ¼ 2.2 Hz, 1H), 8.38e8.34 (m, 2H), 7.78e7.74 (m, 1H), 7.55e7.51
(d, J ¼ 7.0 Hz, 1H), 7.74 (m, 3H), 7.37 (t, J ¼ 8.0 Hz, 1H), 7.29 (d,
(m, 2H), 7.43 (t, J ¼ 7.6 Hz, 1H), 3.97 (s, 3H); 13C NMR (126 MHz,
J ¼ 7.9 Hz, 1H), 4.11 (s, 3H); 13C NMR (126 MHz, CDCl3)
d 177.1, 153.6,
CDCl3)
d 176.7, 166.0, 158.9, 156.1, 135.5, 135.4, 129.5, 127.0, 126.2,
149.3,146.4,136.7,129.7,128.1,127.1,124.6,124.3, 124.1, 123.5,123.4,
124.7, 122.0, 121.6, 118.6, 118.2, 52.5. MS (EI), m/z 254.06 [Mþ, 100%].
121.6, 117.6, 117.5, 115.2, 56.7. MS (EI), m/z 275.92 [Mþ, 100%].
4.12. 2-Methoxy-6-methyl-9H-xanthen-9-one (2l)
4.19. 9-Chloro-7H-benzo[c]xanthen-7-one (2s)
White solid. m.p.:137e139 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.21
Light yellow solid. m.p.: 224e226 ꢀC.1H NMR (500 MHz, CDCl3)
(d, J ¼ 8.1 Hz, 1H), 7.69 (d, J ¼ 3.1 Hz, 1H), 7.40 (d, J ¼ 9.1 Hz, 1H),
d
8.64 (d, J ¼ 8.2 Hz, 1H), 8.35 (d, J ¼ 2.6 Hz, 1H), 8.24 (d, J ¼ 8.7 Hz,
1H), 7.94 (d, J ¼ 7.7 Hz, 1H), 7.76e7.69 (m, 4H), 7.65 (d, J ¼ 8.9 Hz,
1H); 13C NMR (126 MHz, CDCl3)
175.9, 154.2, 153.8, 136.8, 134.6,
7.31e7.28 (m, 1H), 7.25 (s, 1H), 7.17 (d, J ¼ 8.1 Hz, 1H), 3.91 (s, 3H),
2.49 (s, 3H); 13C NMR (126 MHz, CDCl3)
d 177.0, 156.4, 156.0, 151.1,
d
146.2, 126.6, 125.4, 124.7, 122.3, 119.4, 119.2, 117.8, 106.0, 56.0, 22.1.
130.5, 130.0, 128.3, 127.3, 126.1, 124.6, 124.1, 123.5, 123.0, 121.5,
MS (EI), m/z 239.96 [Mþ, 100%].
120.0, 117.5. MS (EI), m/z 279.86 [Mþ, 100%].
4.13. 2-Chloro-6-methyl-9H-xanthen-9-one (2m)
4.20. 2-Bromo-7H-benzo[c]xanthen-7-one (2t)
White solid. m.p.: 154e157 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.27
White solid. m.p.: 218e221 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.70
(d, J ¼ 2.6 Hz, 1H), 8.19 (d, J ¼ 8.1 Hz, 1H), 7.64e7.62 (m, 1H), 7.42 (d,
(s, 1H), 8.36 (d, J ¼ 7.9 Hz, 1H), 8.22 (d, J ¼ 8.7 Hz, 1H), 7.79e7.71 (m,
J ¼ 8.9 Hz, 1H), 7.27 (s, 1H), 7.20 (d, J ¼ 8.2 Hz, 1H), 2.51 (s, 3H); 13
C
3H), 7.66e7.64 (m, 2H), 7.44 (t, J ¼ 7.5 Hz, 1H); 13C NMR (126 MHz,
NMR (126 MHz, CDCl3)
d 176.0, 156.3, 154.6, 147.0, 134.8, 129.7,
CDCl3)
d 176.6, 155.7, 152.5, 135.0, 134.7, 132.9, 129.8, 126.7, 125.4,
126.7, 126.1, 126.0, 122.9, 119.8, 119.4, 117.9, 22.2. MS (EI), m/z 243.90
125.3, 124.8, 123.8, 122.5, 122.2, 121.2, 118.3, 118.2. MS (EI), m/z
[Mþ, 100%].
324.00[Mþ, 85%], 189.01 [100%].
4.14. 7H-Benzo[c]xanthen-7-one (2n)
4.21. 14H-Dibenzo[a,h]xanthen-14-one (2u)
White solid. m.p.: 159e161 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.64
White solid.m.p.: 222e225 ꢀC.1H NMR (500 MHz, CDCl3)
d 10.18
(d, J ¼ 8.1 Hz, 1H), 8.39 (d, J ¼ 8.0 Hz, 1H), 8.25 (d, J ¼ 8.7 Hz, 1H),
(d, J ¼ 8.5 Hz,1H), 8.70e8.68 (m, 1H), 8.39 (d, J ¼ 8.7 Hz,1H), 8.19 (d,
7.90 (d, J ¼ 7.9 Hz, 1H), 7.78e7.74 (m, 1H), 7.72e7.65 (m, 4H),
7.45e7.42 (m, 1H); 13C NMR (126 MHz, CDCl3)
d 177.0, 155.9, 153.8,
J ¼ 9.1 Hz, 1H), 7.97e7.92 (m, 2H), 7.82e7.79 (m, 2H), 7.76e7.69 (m,
3H), 7.63 (t, J ¼ 7.5 Hz,1H); 13C NMR (126 MHz, CDCl3)
d 178.4,157.2,
136.7, 134.4, 129.7, 128.2, 127.0, 126.7, 124.5, 124.2, 124.1, 123.0,
122.6, 121.6, 118.2, 117.7. MS (EI), m/z 246.08 [Mþ, 100%].
152.1,136.4,136.2,131.3,130.5,129.6,129.5,128.5,128.3,127.3,127.1,
126.5, 124.6, 124.0, 122.8, 121.8, 119.7, 118.1, 115.5. MS (EI), m/z
296.14 [Mþ, 100%].
4.15. 12H-Benzo[a]xanthen-12-one (2o)
White solid. m.p.: 152e154 ꢀC.1H NMR (500 MHz, CDCl3)
d
10.07
4.22. 14H-Dibenzo[a,j]xanthen-14-one (2v)
(d, J ¼ 8.8 Hz, 1H), 8.42 (d, J ¼ 7.9 Hz, 1H), 8.07 (d, J ¼ 9.0 Hz, 1H),
7.86 (d, J ¼ 8.0 Hz, 1H), 7.76 (t, J ¼ 7.8 Hz, 1H), 7.70 (t, J ¼ 7.9 Hz, 1H),
7.58 (t, J ¼ 7.4 Hz,1H), 7.51 (s,1H), 7.49 (s,1H), 7.42 (t, J ¼ 7.5 Hz,1H);
Light yellow solid. m.p.: 189e192 ꢀC.1H NMR (500 MHz, CDCl3)
d
10.16 (d, J ¼ 8.7 Hz, 2H), 8.03 (s, 2H), 7.85 (s, 2H), 7.79 (t, J ¼ 7.7 Hz,
13C NMR (126 MHz, CDCl3)
d
178.5, 157.7, 154.7, 136.7, 134.0, 131.2,
2H), 7.59 (t, J ¼ 7.32 Hz, 2H), 7.51 (s, 2H); 13C NMR (126 MHz, CDCl3)
130.2, 129.6, 128.4, 127.1, 126.8, 126.2, 124.4, 123.7, 118.1, 117.6, 114.6.
d 180.3, 155.8, 135.9, 131.1, 130.6, 129.3, 128.5, 127.1, 126.2, 117.6,
MS (EI), m/z 246.10 [Mþ, 100%].
116.5. MS (EI), m/z 296.02 [Mþ, 100%].
6

J. Zhou, M. Li, T. Li et al.
Tetrahedron 82 (2021) 131947
4.23. 9H-Thioxanthen-9-one (4a)
d
176.9, 142.3, 140.8, 134.1, 133.8, 127.7, 127.2, 126.7, 123.1, 122.3,
121.6, 116.7, 114.9, 33.8. MS (EI), m/z 243.05 [Mþ, 100%].
Yellow solid. m.p.: 211e214 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.61
(d, J ¼ 8.1 Hz, 2H), 7.62e7.59 (m, 2H), 7.57e7.55 (m, 2H), 7.48 (t,
4.31. 10-Benzylacridin-9(10H)-one (6d)
J ¼ 7.5 Hz, 2H); 13C NMR (126 MHz, CDCl3)
d 180.1, 137.4, 132.4,
130.0, 129.4, 126.4, 126.1. MS (EI), m/z 212.07 [Mþ, 100%].
Yellow solid. m.p.: 172e174 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.59
(d, J ¼ 8.1 Hz, 2H), 7.62 (t, J ¼ 7.9 Hz, 2H), 7.37e7.28 (m, 7H), 7.21 (d,
J ¼ 7.5 Hz, 2H), 5.60 (s, 2H); 13C NMR (126 MHz, CDCl3)
d 178.4,
4.24. 2-Isopropyl-9H-thioxanthen-9-one (4b)
142.7, 135.6, 134.2, 129.4, 127.9, 127.9, 125.8, 122.7, 121.8, 115.3, 50.9.
Yellow solid. m.p.: 75e79 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.62 (d,
MS (EI), m/z 284.99 [Mþ, 100%].
J ¼ 8.2 Hz, 1H), 8.49 (s, 1H), 7.60e7.55 (m, 2H), 7.51 (s, 2H),
7.48e7.46 (m, 1H), 3.09e3.03 (m, 1H), 1.33 (d, J ¼ 6.9 Hz, 6H); 13C
Declaration of competing interest
NMR (126 MHz, CDCl3)
d 180.2, 147.5, 137.5, 134.6, 132.2, 131.5,
130.0, 129.4, 129.3, 127.3, 126.2, 126.1, 126.1, 34.1, 24.0.MS (EI), m/z
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
254.04 [Mþ, 55%], 239.07 [100%].
4.25. 2,4-Diethyl-9H-thioxanthen-9-one (4c)
Acknowledgments
Yellow solid. m.p.: 72e75 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.59 (d,
J ¼ 8.0 Hz, 1H), 8.36 (s, 1H), 7.60e7.57 (m, 1H), 7.47e7.44 (m, 1H),
This work was financially supported by the National Natural
Science Foundation of China (21776260, 21773211 and 21773210).
7.36 (s, 1H), 2.90e2.86 (m, 2H), 2.79e2.74 (m, 2H), 1.38 (t, J ¼ 7.6 Hz,
3H),1.31 (t, J ¼ 7.6 Hz, 3H); 13C NMR (126 MHz, CDCl3)
d 180.8,142.2,
139.9,137.1,133.3,132.1,132.1,129.8,129.7,128.9,126.5,126.4,126.2,
Appendix A. Supplementary data
28.7, 26.2, 15.6, 13.8.MS (EI), m/z 268.14 [Mþ, 100%].
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2021.131947.
4.26. 2-Chloro-9H-thioxanthen-9-one (4d)
Yellow solid. m.p.: 151e154 ꢀC. 1H NMR (500 MHz, CDCl3)
d 8.59
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(d, J ¼ 8.7 Hz, 2H), 7.51e7.47 (m, 2H); 13C NMR (126 MHz, CDCl3)
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d 8.44 (d,
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Yellow solid. m.p.:170e173 ꢀC.1H NMR (500 MHz, CDCl3)
d 8.41
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