Synthesis of xanthones, thioxanthones and Acridones by a metal-free photocatalytic oxidation using visible light and molecular oxygen

Article abstract of DOI:10.3390/molecules26040974

9H-Xanthenes, 9H-thioxanthenes and 9,10-dihydroacridines can be easily oxidized to the corresponding xanthones, thioxanthones and acridones, respectively, by a simple photo-oxidation procedure carried out using molecular oxygen as oxidant under the irradiation of visible blue light and in the presence of riboflavin tetraacetate as a metal-free photocatalyst. The obtained yields are high or quantitative.

Full text of DOI:10.3390/molecules26040974

molecules
Article
Synthesis of Xanthones, Thioxanthones and Acridones by a
Metal-Free Photocatalytic Oxidation Using Visible Light and
Molecular Oxygen
Alejandro Torregrosa-Chinillach and Rafael Chinchilla *
Department of Organic Chemistry, Faculty of Sciences, and Institute of Organic Synthesis (ISO),
University of Alicante, Apdo. 99, 03080 Alicante, Spain; alex.torregrosa@ua.es
* Correspondence: chinchilla@ua.es; Tel.: +34-965903822
Abstract: 9H-Xanthenes, 9H-thioxanthenes and 9,10-dihydroacridines can be easily oxidized to the
corresponding xanthones, thioxanthones and acridones, respectively, by a simple photo-oxidation
procedure carried out using molecular oxygen as oxidant under the irradiation of visible blue light
and in the presence of riboflavin tetraacetate as a metal-free photocatalyst. The obtained yields are
high or quantitative.
Keywords: photo-oxidation; photocatalysis; xanthones; thioxanthones; acridones
1. Introduction
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
In the last years, photocatalysis has emerged enormously as a powerful tool in syn-
thetic organic chemistry when promoting electron transfer-involved chemical transforma-
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Torregrosa-Chinillach, A.;
Chinchilla, R. Synthesis of Xanthones,
Thioxanthones and Acridones by a
Metal-Free Photocatalytic Oxidation
Using Visible Light and Molecular
Oxygen. Molecules 2021, 26, 974.
https://doi.org/10.3390/
tions [
1
–
5
]. In this context, the use of visible light results particularly attractive due to its
lack of absorbance by organic compounds [6,7
]. This reduces side reactions, frequently
associated with classical photochemical reactions carried out with high energy UV light
absorbed directly by the molecule. When less energetic and innocuous visible light is
employed, the addition of catalytic amounts of a sensitizer as photocatalyst is required, but
the reaction setup results simple and is not too different from that of typical thermal chem-
istry, except for the light source. Among the most frequent and successful transformations
carried out under these conditions are photoredox reactions [8–17].
molecules26040974
The visible light-induced photocatalytic oxidation of benzylic bonds [18
when the final oxidant is the cheap and environmentally friendly molecular oxygen [20,21]
results in a convenient and direct technique to generate the corresponding carbonyl systems.
This concept can be applied to the preparation of interesting structural compounds such
as xanthones, thioxanthones and acridones by oxidation of benzylic C-H bonds of appro-
,19], especially
Academic Editor: Wanhong Ma
Received: 30 December 2020
Accepted: 9 February 2021
Published: 12 February 2021
,
priate precursors. Thus, xanthones (9H-xanthen-9-ones) are interesting to prepare [22
as their structural motif is present in compounds showing a multitude of biological ac-
tivities, including -glucosidase inhibition, as well as anticancer, anti-Alzheimer and
anti-inflammatory properties [24 27]. Besides, thioxanthones have also shown interesting
,23],
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
α
–
antitumor activities [28]. Moreover, acridones [acridin-9(10H)-ones] are important moieties
present in antiviral and antitumor agents [29–31].
Xanthone has been obtained by photocatalytic oxidation of the benzylic methylene
position in 9H-xanthene. Thus, ruthenium complexes as catalysts have been used, the
Copyright:
© 2021 by the authors.
oxidant being oxygen [32] or iodate salts [33,34]. Iron complexes have also been employed
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
as catalysts under aerobic conditions [35 37], as well as a copper complex [38] or cop-
–
per(II) chloride [39]. All these procedures, although effective, employ a metal-containing
catalyst, something not too environmentally desirable. Therefore, procedures involving
metal-free conditions have been explored. Thus, functionalized carbon-dots have been
employed as catalysts for the photo-oxidation of 9H-xanthene, using tert-butyl hydroper-
oxide as oxidant [40], as well as graphitic carbon nitride, oxygen being the oxidant in this
Molecules 2021, 26, 974. https://doi.org/10.3390/molecules26040974
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 974
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case [41]. Xanthones have also been prepared by visible-light aerobic oxidation of different
9H-xanthenes, a reaction promoted by the combination of 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ), tert-butyl nitrite and acetic acid [42]. In addition, 9H-xanthene has
been photo-oxidized to xanthone using 9-mesityl-10-methylacridinium tetrafluoroborate
dye as catalyst and Selectfluor as oxidant [43]. Moreover, a flavin-derivative has been
shown to photocatalyze this oxidation reaction under aerobic conditions, although in the
presence of trifluoroacetic acid [44].
The photocatalytic oxidation of the methylene position in N-substituted 9,10-dihydro-
acridines (acridanes) leading to acridones has been less frequently explored. Thus, irra-
diation of N-methylacridane in the presence of a manganese complex and oxygen gave
the corresponding N-methylacridone [45], a transformation which has also been carried
out with air but using a ruthenium complex as photocatalyst [46]. In addition, the former
combination of DDQ, tert-butyl nitrite and acetic acid has also been used for the visible
light-induced oxidation of 9,10-dihydroacridines to acridones [42].
We report now a very simple and environmentally benign preparation of xanthones,
thioxanthones and acridones carried out by irradiation of 9H-xanthenes, 9H-thioxanthenes
and 9,10-dihydroacridines, respectively, with visible blue light in the presence of oxygen
and using a low-costing metal-free photocatalyst.
2. Results and Discussion
Different organic dyes
1–7 (Figure 1), employed previously in other photoredox
reactions [13 47 48], were explored as photocatalysts in the model photo-oxidation of 9H-
,
,
xanthene (9a) to xanthone (10a), under a molecular oxygen atmosphere (1 atm, balloon)
and irradiation with visible blue light during 8 h (Table 1).
Figure 1. Photocatalysts employed in this study.

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Table 1. Screening and optimization of the reaction conditions for the photo-oxidation of
9H-xanthene.
10a Conversion
(%) a
11 Conversion
(%) a
Entry
Photocatalyst
Solvent
1
2
3
4
5
6
7
8
9
10
11
12
13 b
14 c
15 d
16
1
2
3
4
5
6
7
8
7
7
7
7
7
7
7
-
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
13
65
74
7
10
11
100
6
47
31
56
100
55
73
2
4
0
15
2
0
3
0
0
53
13
44
0
24
26
0
MeOH
CH2Cl2
ClCH2CH2Cl
MeCN
MeCN
MeCN
MeCN
8
0
a
Relative to the starting material and determined by proton nuclear magnetic resonance (1H NMR) (300 MHz).
b
c
The reaction was carried out under air in an open flask. The reaction was carried out without oxygen balloon,
d
after just sparging the solution and closing the reaction flask. The reaction was performed in the absence of
light.
When fluorescein sodium salt (1) was used as photocatalyst (5 mol%) in acetonitrile
as solvent, the desired oxidation product 10a was obtained, although in a low conversion
(Table 1, entry 1). Moreover, the proton nuclear magnetic resonance (1H NMR) analysis
of the reaction crude also revealed the presence of a small amount of bixanthene 11 as an
undesirable coupling byproduct (H9 singlet at 4.18 ppm [49]) (Table 1, entry 1). Changing
the photocatalyst to 20,70-dichlorofluorescein (
2
) increased noticeably the conversion to
xanthone 10a, with no bixanthene 11 being observed (Table 1, entry 2). The conversion
to xanthone 10a resulted furtherly increased to 74% using eosin Y sodium salt ( ) as
3
photocatalyst, although, again, the presence of the bixanthene byproduct 11 was detected
(Table 1, entry 3). However, the reaction performed poorly when photocatalyzed by rose
bengal sodium salt (4) (Table 1, entry 4), similarly to when methylene blue (5) was employed
(Table 1, entry 5).
Riboflavin (
pound, and hence it can absorb visible light with a maximum in the blue range, increasing
its redox power upon excitation [50 54]. Riboflavin performed poorly under the former
reaction conditions (Table 1, entry 6), but the use of riboflavin tetraacetate ( ) as photo-
6), also known as vitamin B2, is a cheap naturally occurring yellow com-
–
7
catalyst gave full conversion to the desired xanthone 10a, with no traces of bixanthene
1
11 being observed in H NMR (Table 1, entry 7). Perhaps the higher solubility of
7
in
organic solvents plays a role in its activity compared to
6. Moreover, we also explored if
a metal-containing photocatalyst could be used, employing the very known visible light-
0
absorbing complex Ru(bpy)3Cl2 (bpy = 2,2 -bipyridyl) (
8) (Figure 1) [12
,
55,
56], although its
performance resulted very poor under these conditions (Table 1, entry 8).

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Then, we explored the use of other solvents using riboflavin tetraacetate (
7) as the
optimum photocatalyst. Thus, the use of N,N’-dimethylformamide (DMF) gave a similar
mixture of xanthone 10a and the bixanthene byproduct 11 (Table 1, entry 9), whereas the
use of methanol gave both compounds in lower conversions (Table 1, entry 10). In addition,
when dichloromethane was used as solvent, a mixture of 10a and 11 was obtained (Table 1,
entry 11), but, when another chlorinated solvent such as 1,2-dichloroethane was used, the
starting xanthene 9a was fully converted to the oxidation product 10a (Table 1, entry 12).
With these results in hand, we chose acetonitrile as the solvent, attempting to avoid the use
of a chlorinated solvent.
We also carried out the reaction under air in an open flask using the best reaction
conditions (7 as photocatalyst and MeCN as solvent), but a conversion of only 55% to 10a
was achieved with a noticeable 24% conversion to bixanthene 11 (Table 1, entry 13). In
addition, the reaction was carried out after just sparging molecular oxygen into the solution
and closing the reaction flask, avoiding the use of the balloon, but a 73% conversion to 10a
and 26% for 11 was detected (Table 1, entry 14). Moreover, the reaction was carried out in
the absence of any light, giving almost no reaction (Table 1, entry 15). Finally, the oxidation
was attempted in the absence of photocatalyst 7, with only 8% conversion to 10a being
obtained (Table 1, entry 16). We also used a lower photocatalyst loading (2 mol%), but a
remaining 22% of starting 9a was observed.
Once we found the most appropriate reaction conditions for the photo-oxidation
reaction, we extended the procedure to other 9H-xanthenes and related systems, observing
in all cases full conversion to the desired oxidated derivative (Figure 2). Thus, when
the methyl- or methoxy-substituted 9H-xanthenes 9b and 9c were photo-oxidized, the
corresponding xanthones 10b and 10c were obtained in high isolated yields (Figure 2).
In addition, when 9H-xanthenes with substituents such as fluoro, chloro or bromo were
employed in the reaction, the corresponding halogenated xanthones 10d, 10e and 10f,
respectively, were also isolated in high yields (Figure 2). Moreover, benzocondensed
9H-xanthenes were also employed in the oxidation reaction, affording the corresponding
xanthones 10g and 10h (Figure 2). Furthermore, 9H-thioxanthene was also explored for the
photo-oxidation reaction, affording quantitatively the final thioxanthone 10i (Figure 2).
We also used this photo-oxidation reaction with N-substituted 9,10-dihydroacridines
(acridanes), obtaining excellent results of the corresponding acridones (Figure 2). Thus,
when N-methyl-9,10-dihydroacridine was employed, acridone 10j was isolated in a 91%
yield, whereas N-phenyl-9,10-dihydroacridine afforded the oxidation derivative 10k in 93%
yield. In addition, the presence of different substituents on the N-phenyl ring in the last
acridane, such as methyl, methoxy or fluoro, allowed isolating the corresponding acridones
10l, 10m and 10n, respectively, in high yields. Moreover, when a benzoyl N-substituent was
present in the starting 9,10-dihydroacridine, the N-benzoylated acridone 10o was isolated
in 82% yield. Furthermore, the presence of an acid-sensitive N-substituent, such as the
tert-butoxycarbonyl (Boc) group was tolerated under these neutral oxidation conditions,
allowing to isolate the N-Boc-acridone 10p in 89% yield.
This photo-oxidation reaction probably takes place similarly to other proposed aerobic
photocatalytic oxidation of benzylic systems [57], although particularized to the use of
riboflavin as photocatalyst [44]. Thus, it is known that the riboflavin nucleus is excited in
the presence of visible light [50–54] and then would be able to oxidize the benzyl system to
a cation radical, generating a riboflavin radical anion after a single-electron transfer (SET)
process. This riboflavin radical anion can convert oxygen to a superoxide radical anion
which reduces the benzyl cation radical to a benzyl radical able to react with oxygen to
generate a benzyl hydroperoxide which would decompose to the carbonyl system. Singlet
oxygen is probably involved in this process, as it is known that it can be produced by flavin
sensitization [58].

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Figure 2. Photocatalyzed oxidation of 9H-xanthenes, 9H-thioxanthene and 9,10-dihydroacridines. Reaction time and yield,
after isolation by flash chromatography, detailed for each compound.
We also explored this photo-oxidation reaction using the simple N-unsubstituted 9,10-
dihydroacridine 9q as starting material, but, instead of the corresponding N-unsubstituted
acridone, fully aromatized acridine (12) was isolated after 8 h in a 96% yield (Scheme 1).

Molecules 2021, 26, 974
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This photocatalytic oxidative dehydrogenation of acridine has been reported previously [42
]
and used as a procedure for the aromatization of N-heterocycles [59,60].
Scheme 1. Photo-oxidation of 9,10-dihydroacridine.
3. Materials and Methods
3.1. General Information
All reagents and solvents employed were of the best grade available and were used
without further purification. Riboflavin tetraacetate ( ) was obtained by acetylation of
riboflavin ( ) as described [61]. Substituted 9H-xanthenes 9b were prepared as re-
ported [62]. Starting dihydroacridines 9j 63], 9k 64], 9o 65] and 9p 66] were ob-
7
6
–h
[
–
n
[
[
[
tained by N-substitution of 9,10-dihydroacridine (9q) following literature procedures.
9,10-Dihydroacridine (9q) was prepared by reduction of acridine as described [67]. The
1H and 13C spectra were recorded at room temperature on a Bruker Oxford AV300 at
300 MHz and 75 MHz, on a Bruker Oxford AV400 at 400 MHz and 101 MHz, or a Bruker
Avance DRX500 at 500 MHz and 125 MHz, respectively (Bruker, Billerica, MA, USA),
using CDCl3 as solvent and TMS as the internal standard. Reactions were carried out
in borosilicate vials (length: 4 cm, diameter: 1.4 cm, volume: 4 mL) and irradiated in a
EvoluchemTM PhotoRedOx Box Duo (HepatoChem Inc., Beverly, MA, USA) equipped with
two EvoluchemTM CREE XPE 450–455 nm 18W LEDs (HCK1012-01-002) (HepatoChem
Inc., Beverly, MA, USA).
3.2. General Procedure for the Photocatalytic Oxidation Reaction.
A mixture of
9 (0.2 mmol) and 7 (0.01 mmol, 5.4 mg) was dissolved in acetonitrile
(0.4 mL), and the solution was bubbled with O2 for 2 min. The flask was closed and
stirred at room temperature under an O2 atmosphere (balloon) with irradiation by blue
LEDs during the indicated reaction time (Figure 2). After that, H2O (2 mL) was added,
and the reaction product was extracted with AcOEt (3
×
2 mL). The combined organic
phases were dried over MgSO4, filtered and evaporated under reduced pressure (15 Torr)
to get the crude product, which was purified by flash column chromatography on silica
gel (n-hexane/AcOEt gradients). The adducts 10 and 12 were identified by comparison of
their NMR data with those of the literature (Supplementary Materials, NMR spectra).
9H-Xanthen-9-one (10a) [68]. White solid (39 mg, 99%); 1H NMR (400 MHz): δH = 8.34
(dd, J = 8.0, 1.5 Hz, 2H), 7.76–7.67 (m, 2H), 7.48 (d, J = 8.6 Hz, 2H), 7.41–7.33 (m, 2H) ppm;
13C NMR (101 MHz): δC = 177.3, 156.3, 134.9, 126.8, 124.0, 122.0, 118.1 ppm.
2-Methyl-9H-xanthen-9-one (10b) [68]. White solid (41 mg, 97%); 1H NMR (300 MHz):
δH = 8.34 (dd, J = 8.0, 1.7 Hz, 1H), 8.16–8.08 (m, 1H), 7.75–7.66 (m, 1H), 7.56–7.44 (m, 2H),
7.42–7.32 (m, 2H), 2.47 (s, 3H) ppm; 13C NMR (101 MHz): δC = 177.4, 156.3, 154.5, 136.2,
134.7, 133.8, 126.8, 126.1, 123.8, 121.9, 121.6, 118.1, 117.9, 21.0 ppm.
2-Methoxy-9H-xanthen-9-one (10c) [68]. White solid (43 mg, 96%); 1H NMR (300 MHz):
δH = 8.35 (dd, J = 8.0, 1.6 Hz, 1H), 7.76–7.67 (m, 2H), 7.52–7.29 (m, 4H), 3.92 (s, 3H) ppm;
13C NMR (75 MHz): δC = 177.2, 156.2, 156.1, 151.1, 134.7, 126.8, 125.0, 123.8, 122.2, 121.4,
119.5, 118.1, 105.9, 56.1 ppm.
2-Fluoro-9H-xanthen-9-one (10d) [68]. White solid (40 mg, 93%); 1H NMR (300 MHz):
δH = 8.33 (dd, J = 8.0, 1.6 Hz, 1H), 8.02–7.93 (m, 1H), 7.80–7.70 (m, 1H), 7.55–7.35 (m, 4H)
ppm; 13C NMR (101 MHz): δC = 176.7 (d, J = 2.2 Hz), 158.9 (d, J = 245.3 Hz), 156.3, 152.5 (d,
J = 1.6 Hz), 135.2, 126.8, 124.3, 123.1 (d, J = 25.3 Hz), 122.8 (d, J = 7.1 Hz), 121.2, 120.1 (d,
J = 7.8 Hz), 118.1, 111.6 (d, J = 23.5 Hz) ppm.

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2-Chloro-9H-xanthen-9-one (10e) [69]. White solid (43 mg, 94%); 1H NMR (300 MHz):
δH = 8.40–8.23 (m, 2H), 7.81–7.61 (m, 2H), 7.55–7.33 (m, 3H) ppm; 13C NMR (75 MHz):
δC = 176.3, 156.2, 154.6, 135.3, 135.1, 129.9, 126.9, 126.2, 124.4, 122.8, 121.6, 119.9, 118.2 ppm.
2-Bromo-9H-xanthen-9-one (10f) [68]. White solid (52 mg, 94%); 1H NMR (400 MHz):
δH = 8.45–8.41 (m, 1H), 8.31 (dd, J = 8.0, 1.8 Hz, 1H), 7.82–7.70 (m, 2H), 7.51–7.45 (m, 1H),
7.43–7.35 (m, 2H) ppm; 13C NMR (101 MHz): δC = 176.1, 156.1, 155.0, 137.8, 135.3, 129.3,
126.9, 124.4, 123.2, 121.6, 120.1, 118.2, 117.2 ppm.
7H-Benzo[c]xanthen-7-one (10g) [68]. White solid (42 mg, 85%); 1H NMR (300 MHz):
δH = 8.70–8.63 (m, 1H), 8.44–8.37 (m, 1H), 8.27 (d, J = 8.7 Hz, 1H), 7.95–7.89 (m, 1H),
7.81–7.64 (m, 5H), 7.48–7.41 (m, 1H) ppm; 13C NMR (101 MHz): δC = 177.0, 155.9, 153.8,
136.7, 134.5, 129.7, 128.2, 127.0, 126.7, 124.6, 124.2, 124.2, 123.0, 122.6, 121.6, 118.2, 117.7
ppm.
1
9-Methyl-7H-benzo[c]xanthen-7-one (10h) [70]. White solid (45 mg, 86%); H NMR
(300 MHz): δH = 8.68–8.60 (m, 1H), 8.26 (d, J = 8.8 Hz, 1H), 8.19–8.14 (m, 1H), 7.95–7.87
(m, 1H), 7.75–7.64 (m, 3H), 7.56 (d, J = 1.3 Hz, 2H), 2.49 (s, 3H) ppm; 13C NMR (101 MHz):
δC = 177.1, 154.1, 153.8, 136.6, 135.7, 134.4, 129.6, 128.2, 126.9, 126.0, 124.3, 123.9, 123.0, 122.2,
121.7, 117.9, 117.7, 21.1 ppm.
1
9H-Thioxanthen-9-one (10i) [68]. White solid (42 mg, 99%); H NMR (300 MHz):
δH = 8.63 (ddd, J = 8.1, 1.5, 0.6 Hz, 2H), 7.67–7.56 (m, 4H), 7.53–7.46 (m, 2H) ppm; 13C NMR
(101 MHz): δC = 180.1, 137.4, 132.4, 130.0, 129.4, 126.5, 126.1 ppm.
10-Methylacridin-9(10H)-one (10j) [68]. Yellow solid (38 mg, 91%); 1H NMR (300 MHz):
δH = 8.55 (dd, J = 8.0, 1.6 Hz, 2H), 7.75–7.65 (m, 2H), 7.53–7.46 (m, 2H), 7.31–7.24 (m, 2H),
3.87 (s, 3H) ppm; 13C NMR (101 MHz): δC = 178.1, 142.7, 134.1, 128.0, 122.5, 121.5, 114.9,
33.9 ppm.
10-Phenylacridin-9(10H)-one (10k) [71]. Yellow solid (50 mg, 93%); 1H NMR (500 MHz):
δH = 8.60 (dd, J = 8.0, 1.6 Hz, 2H), 7.74–7.69 (m, 2H), 7.68–7.64 (m, 1H), 7.53–7.48 (m, 2H),
7.40–7.36 (m, 2H), 7.31–7.27 (m, 2H), 6.76 (d, J = 8.6 Hz, 2H) ppm; 13C NMR (125 MHz):
δC = 178.3, 143.3, 139.1, 133.4, 131.2, 130.2, 129.8, 127.4, 121.9, 121.7, 116.9 ppm.
10-(p-Tolyl) acridin-9(10H)-one (10l) [71]. Yellow solid (53 mg, 94%); 1H NMR (500 MHz):
δH = 8.59 (dd, J = 8.0, 1.5 Hz, 2H), 7.52–7.47 (m, 4H), 7.29–7.23 (m, 4H), 6.80 (d, J = 8.6 Hz,
2H), 2.54 (s, 3H) ppm; 13C NMR (125 MHz): δC = 178.3, 143.5, 139.8, 136.5, 133.3, 131.8,
129.9, 127.4, 122.0, 121.6, 117.0, 21.5 ppm.
1
10-(4-Methoxyphenyl) acridin-9(10H)-one (10m) [71]. Yellow solid (57 mg, 95%); H
NMR (300 MHz): δH = 8.58 (dd, J = 8.1, 1.4 Hz, 2H), 7.55–7.46 (m, 2H), 7.31–7.24 (m, 4H),
7.22–7.16 (m, 2H), 6.82 (d, J = 8.6 Hz, 2H), 3.95 (s, 3H) ppm; 13C NMR (125 MHz): δC = 178.3,
160.3, 143.7, 133.4, 131.6, 131.2, 127.4, 122.1, 121.6, 117.0, 116.3, 55.8 ppm.
10-(4-Fluorophenyl) acridin-9(10H)-one (10n) [71]. Yellow solid (52 mg, 91%); 1H NMR
(300 MHz): δH = 8.59 (dd, J = 8.0, 1.7 Hz, 2H), 7.56–7.48 (m, 2H), 7.45–7.34 (m, 4H), 7.33–
7.26 (m, 2H), 6.75 (d, J = 8.4 Hz, 2H) ppm; 13C NMR (101 MHz): δC = 178.3, 163.0 (d,
J = 250.4 Hz), 143.3, 135.0 (d, J = 3.4 Hz), 133.6, 132.1 (d, J = 8.7 Hz), 127.6, 122.0, 121.9, 118.4
(d, J = 22.8 Hz), 116.7 ppm.
10-Benzoylacridin-9(10H)-one (10o) [65]. Yellow solid (49 mg, 82%); 1H NMR (500 MHz):
δH = 8.57 (dd, J = 8.1, 1.3 Hz, 2H), 7.85–7.80 (m, 2H), 7.69–7.63 (m, 1H), 7.58–7.53 (m, 2H),
7.48–7.43 (m, 2H), 7.36–7.31 (m, 2H), 7.19 (d, J = 8.5 Hz, 2H) ppm; 13C NMR (101 MHz):
δC = 178.2, 172.7, 139.9, 135.7, 133.9, 132.4, 131.5, 129.8, 127.9, 122.9, 121.7, 116.6 ppm.
Tert-butyl 9-oxoacridine-10(9H)-carboxylate (10p) [66]. Yellow solid (52 mg, 89%); 1H
NMR (300 MHz): δH = 8.48–8.41 (m, 2H), 7.71–7.63 (m, 2H), 7.61–7.55 (m, 2H), 7.39–7.31
(m, 2H), 1.69 (s, 9H) ppm; 13C NMR (75 MHz): δC = 178.7, 152.0, 139.6, 133.5, 127.4, 123.4,
123.0, 117.7, 86.4, 27.9 ppm.
Acridine (12) [72]. Yellow solid (34 mg, 96%); 1H NMR (300 MHz): δH = 8.76 (s, 1H),
8.27 (d, J = 8.8 Hz, 2H), 7.99 (d, J = 8.5 Hz, 2H), 7.83–7.74 (m, 2H), 7.57–7.49 (m, 2H) ppm;
13C NMR (101 MHz): δC = 148.3, 137.2, 131.1, 128.7, 128.4, 126.6, 126.1 ppm.

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4. Conclusions
Riboflavin tetraacetate acts as a convenient photocatalyst in the visible light-promoted
benzylic oxidation of 9H-xanthenes, 9H-thioxanthenes and 9,10-dihydroacridines to xan-
thones, thioxanthones and acridones, respectively, using molecular oxygen as the oxidant
in acetonitrile as solvent. This easy oxidation procedure allows the isolation of these inter-
esting products in high to quantitative yields, using a methodology that avoids the use of
metal-containing catalysts or chemical oxidants and working under neutral conditions.
Supplementary Materials: The following are available online, Figures S1–S34 NMR spectra, Figure S35
spectrum of the LED lamp, and Figures S36–S43 UV–visible absorption spectra of the photocatalysts.
Author Contributions: Conceptualization, methodology, project administration, supervision and
writing original draft, R.C.; and investigation and methodology, A.T.-C. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded by the Spanish Ministerio de Economía, Industria y Competitivi-
dad (PGC2018-096616-B-I00 and CTQ201788171-P) and the University of Alicante (VIGROB-173).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available in insert article or
supplementary material here.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds 10a–j and 10m–p are available from the authors.
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