[RuCl2(p-cymene)2]2 catalyzed cross dehydrogenative coupling (CDC) toward xanthone and fluorenone analogs through intramolecular C-H bond functionalization reaction

Article abstract of DOI:10.1016/j.tetlet.2014.08.062

A synthetic approach toward xanthone and fluorenone derivatives through ruthenium catalyzed intra-molecular C-H bond functionalization using an external oxidant has been developed. In the presence of [RuCl₂(p-cymene)₂]₂, a variety of substituted ortho-aryloxy/aryl benzaldehydes underwent cross dehydrogenative coupling to afford the corresponding analogs in moderate to good yields.

Full text of DOI:10.1016/j.tetlet.2014.08.062

Tetrahedron Letters 55 (2014) 5759–5763
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Tetrahedron Letters
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[RuCl2(p-cymene)2]2 catalyzed cross dehydrogenative coupling (CDC)
toward xanthone and fluorenone analogs through intramolecular
C–H bond functionalization reaction
Sudipta Kumar Manna a, Srinivas Lavanya Kumar Manda a, Gautam Panda a,b,
⇑
a Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute, BS 10/1, Jankipuram Extension, Sitapur Road, Lucknow 226031, UP, India
b Academy of Scientific and Innovative Research, New Delhi 110001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A synthetic approach toward xanthone and fluorenone derivatives through ruthenium catalyzed intra-
molecular C–H bond functionalization using an external oxidant has been developed. In the presence
of [RuCl2(p-cymene)2]2, a variety of substituted ortho-aryloxy/aryl benzaldehydes underwent cross dehy-
drogenative coupling to afford the corresponding analogs in moderate to good yields.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 19 June 2014
Revised 12 August 2014
Accepted 14 August 2014
Available online 21 August 2014
Keywords:
Xanthones
Fluorenones
Cross-dehydrogenative coupling
ortho-C–H bond functionalization
‘Xanthones’ derived from the Greek ‘xanthos,’ are secondary
metabolites produced by many bacteria, fungi. They are typically
polysubstituted and occur as either fully aromatic, dihydro, hexa-
hydro or dimer, and heterodimer derivatives. The wide variety of
naturally occurring and manmade xanthone and thioxanthone
scaffolds exhibit anti-inflammatory and anticancer activity.1,2
Some plant extracts are directly used in traditional medicines.3
Because of their distinct biological activity within an especially
broad spectrum of disease states, they excited the synthetic as well
as medicinal chemists with a correspondingly diverse range of
bio-architectures as ‘privileged structures’.4 In natural product
chemistry, the xanthone subunit has great significance such as
psorospermin and mangiferin (Fig. 1) due to its excellent biological
and pharmacological activities.1a,5a Thus, construction of such
substructures has always been synthetically attractive.5b–f
Austocystins A–F (Fig. 1) were isolated from Aspergillus ustus,
alongside the known mycotoxins versicolorin C and averufin.6
The isolation of varixanthone (Fig. 1), alongside tajixanthone
hydrate and several nonxanthone products is reported from a mar-
ine derived strain of Emericella variecolor (a fungus which is the
‘perfect state’ of Aspergillus variecolor).7 Varixanthone was found
to be inactive in cytotoxicity tests against several cell lines,
including human lung and colon carcinoma; however, it was active
against Gram-positive and negative bacteria, including Escherichia
coli, Proteus sp., Bacillus subtilis, and Staphylococcus aureus at lower
concentration than the structurally related compounds terrein,
shamixanthone, and tajixanthone hydrate.7 Isokibdelone A rham-
noside 4 was isolated from Kibdelosporangium sp. (MST-108465).8
The isolation resulted in a study of the cytotoxic, antibacterial,
and nematocidal properties of this particular genus.9
Among all the C–C bond formation reactions, transition metal
catalyzed cross-coupling of two prefunctionalized substrates ranks
as one of the most powerful approaches for C–C bond formation.10
Recently, C–C bond formation through C–H bond functionalization
has been of further interest.11 Further development along this line
has resulted in a new synthetic strategy commonly known as ‘cross
dehydrogenative coupling12 (CDC).’ This does not require prefunc-
tionalization of the reacting partners and hence is an even more
atom economic approach.13 C–H arylation through cross-coupling
of two C2sp–C2sp electrophiles has been a useful entry of this reaction.
Various approaches to the synthesis of xanthone scaffold14 have
been developed, mostly through Friedel–Crafts reactions using
functionalized diaryl ethers. In the beginning, aluminum chloride
and oxalyl chloride were used to obtain xanthone analogs from
diaryl ethers in dichloromethane at room temperature as reported
by Jackson et al.15 Afterward, Snieckus group reported xanthone
derivatives from 2-carbamoyl diaryl ethers by using LDA through
complex induced proximity effect.16 Subsequently, Frahm also
synthesized a series of substituted xanthone derivatives from
⇑
Corresponding author. Tel.: +91 522 2772450, 2772550x4661, 4662; fax: +91
522 2771941.
E-mail
addresses:
gautam.panda@gmail.com,
gautam_panda@cdri.res.in
(G. Panda).
http://dx.doi.org/10.1016/j.tetlet.2014.08.062
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

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S. K. Manna et al. / Tetrahedron Letters 55 (2014) 5759–5763
R2
O
R3
R2
O
O
R3
R4
R4
R1
O
O
O
H
O
H
O
Me
HO
Me
R1 = -Cl, R2 = R3 = -OMe,
R4 = -H, Austocystin A (1a)
2
3
4
R1 = -H, R2 = R3 = R4 = -OH,
Austocystin B (1c)
R = R = -OH, R = -H,
R2 = -OH, R3 = -OMe, R4 = -H, Austocystin C (1d)
R2 = R3 = R4 = -OH, Austocystin D (1e)
Austocystin F (1b)
O
H
R2 = R4 = -OH, R3 = -OMe, Austocystin E (1f)
Me
OH
OH
O
O
O
HO
O
O
OH
OH
HO
HO
OH
OMe
OH
O
Psorospermin(2)
Mangiferin (3)
OH
O
Me
H2C
HO
Me
HO
HO
OH
OH
O
OH
O
OMe
O
OH
O
O
O
O
Me
O
Me
HO
Me
O
O
O
N
Me
Cl
H
Pr
Varixanthone (5)
Isokibdelone A rhamnoside (4)
Figure 1. Representative biologically important xanthone scaffold containing natural products.
2-aryloxybenzoic acids with PPA.17 Recently, Lu constructed cop-
per(II) catalyzed nonsubstituted xanthone via aza-Friedel–Crafts
reaction of ortho-phenoxyl N-tosyl benzaldimine.18 Alternatively,
Larock used the ortho-arylated imino group of the aryl ether and
the imidoyl C–H bonds were activated through aryl to imidoyl pal-
ladium migration toward xanthones subsequently.19 In addition, a
cascade reaction, that is, electrocyclization of cyclobutenedione
derivatives to corresponding xanthone scaffolds through benzann-
ulation intermediate has been reported by Liebeskind.20a Studer
also reported base-promoted homolytic aromatic substitutions to
xanthones and fluorenones from ortho-formyl biphenyl ethers
and ortho-formyl biphenyls respectively by using FeCp2 as radical
initiator.20b In addition, CDC between arylated aldehyde C–H bonds
and aromatic C–H bonds has presented a dominant tool to generate
diaryl ketones.21 Recently, Li et al. reported an unprecedented
rearrangement of 2-aryloxybenzaldehydes to 2-hydroxy-benz-
ophenones by cleavage of both aldehyde C–H bonds and aryloxy
C–O bonds in the presence of [RhCl(CO)2]2.22
To the best of our knowledge, synthesis of xanthone and fluore-
none derivatives through [RuCl2(p-cymene)2]2 catalyzed ortho-C–H
bond functionalization strategy from ortho-aryloxy/aryl benzalde-
hydes has not been explored. The potential bioactivity of xanthone
and fluorenone derivatives along with the quest for bioactive het-
erocycles and their bioevaluation made us interested23 in the metal
catalyzed ortho-C–H bond functionalization for the synthesis of
xanthone and fluorenone scaffolds.
R1
O
X
Y
Ref.
23g
CHO
R
R
R
X
6a-e
R = -Me, -H
X = -O, -CH2
R
R
1 = -OMe, -SMe, -Me,
Y = -S, -O
CHO
F
CHO
Ref.
R2
R2
R1
23b&i
Y
7a-k
R2 = -OMe
CHO
Br
CHO
Ref.
19
R3
R3
R4
R3 = -H, -OMe
8a-b
R4 = -OMe, -SMe
Scheme 1. Synthesis of substituted ortho-aryloxy/aryl benzaldehydes (6a–e, 7a–k,
and 8a–b).
temperatures (Table 1) for optimization of the desired reaction
conditions.
When 5 mol % of RuCl3Á3H2O was used as catalyst in the pres-
ence of TBPB (2 equiv) and acetonitrile, there was no reaction
(entry 1, Table 1). Similar results were obtained when ligand as
PPh3 and solvent as acetonitrile were used at 110 °C in 15 h (entry
2, Table 1). When RuCl3Á3H2O (5 mol %) and oxidant TBPB were
used, it provided the product 9a in trace amount in the presence
of 1,4 dioxane and acetonitrile (entries 3 and 4, Table 1). These
results indicate both oxidant and ligand are required. Pleasantly,
we obtained the desired xanthone derivative 9a in 41% yield after
17 h stirring with dry acetonitrile at 110 °C (entry 5, Table 1). This
observation encouraged us to find the optimal catalytic conditions
for the ortho C–H bond functionalization. Upon using reaction con-
ditions similar to entry 5, Table 1, but catalyst loading increased
from 5 mol % to 10 mol % and TBHP as oxidant, we observed the
First, the syntheses of ortho-aryloxy benzaldehydes were
undertaken. All of them were prepared from different substituted
chromanones, tetralones, and ortho-halogenated benzaldehydes
through Vilsmeier–Haack–Arnold,24 Michael addition,25 Suzuki
coupling26 etc. in reasonably high yields at room temperature
(Scheme 1). With the substituted ortho-aryloxy/aryl benzalde-
hydes (6a–e, 7a–k and 8a–b) in hand, the stage was set for crucial
catalytic C–C bond formation toward CDC.
At first we used 4-(3-methoxy-phenylthio)-2,2-dimethyl-2H-
chromene-3-carbaldehyde 6a as a model substrate with different
Ru-metal sources, ligands, oxidants, solvents, and bases at varied

S. K. Manna et al. / Tetrahedron Letters 55 (2014) 5759–5763
5761
Table 1
Optimization studies for the synthesis of xanthone derivatives
O
Me Me
O
Me Me
CHO
Metal Catalyst
O
Ligand, Base
Solvent, Temp.
S
6a
OMe
S
9a
OMe
Entry
Catalysts
Oxidants
Ligands
Solvents
T (°C)
Yielda (%)
1
2
3
4
5
6
7
8
RuCl3Á3H2O (5 mol %)
RuCl3Á3H2O (5 mol %)
RuCl3Á3H2O (5 mol %)
RuCl3Á3H2O (5 mol %)
RuCl3Á3H2O (5 mol %)
RuCl3Á3H2O (10 mol %)
RuCl3Á3H2O (10 mol %)
RuCl3Á3H2O (10 mol %)
RuCl3Á3H2O (10 mol %)
[RuCl2(p-cymene)2]2 (5 mol %)
[RuCl2(p-cymene)2]2 (10 mol %)
[RuCl2(p-cymene)2]2 (10 mol %)
[RuCl2(p-cymene)2]2 (10 mol %)
[RuCl2(p-cymene)2]2 (10 mol %)
[RuCl2(p-cymene)2]2 (10 mol %)
—
TBPB
—
TBPB
TBPB
TBPB
TBHP (decane)
TBHP (decane)
TBHP (decane)
DDQ
TBHP (decane)
TBHP (decane)
TBHP (decane)
TBHP (decane)
TBHP (decane)
TBHP (decane)
TBHP (decane)
DDQ
—
Toluene
Toluene
Toluene
110
110
110
110
110
110
110
110
110
110
110
120
130
110
110
110
120
ND
ND
PPh3
PPh3
PPh3
PPh3
PPh3
PCy3
PPh3
PPh3
PPh3
PPh3
—
Trace
Trace
41 (46)b
55 (49)c
Trace
ND
1,4-Dioxane
Acetonitrile
Acetonitrile
Acetonitrile
DCE
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
DMF
9
34
48
64
10
11
12
13
14
15
16
17
64 (64)d
56
—
—
—
—
Trace
33
ND
DMSO
Acetonitrile
Acetonitrile
[RuCl2(p-cymene)2]2 (10 mol %)
—
Trace
a
b
c
Isolated yield after preparative thin layer column chromatography.
When RuCl3Á3H2O (10 mol %) was used.
When reaction was carried out at 130 °C up to 36 h.
d
15 mol % of [RuCl2(p-cymene)2]2, 15 mol % of PPh3. All reactions were carried out at 15–30 h. TBPB = tert-butyl perbenzoate, TBHP = tert-butyl hydrogen peroxide (ꢀ5.5 M
in decane), DDQ = dichloro-dicyano-benzoquinone. ND = no detection.
Table 2
Synthesis of xanthone and fluorenone derivatives (9a–r)
O
Y
R
R
R
R
10 mol%
CHO
[RuCl2(p-cymene)2]2
X
X
R1
R1
Acetonitrile, TBHP
120 oC, 15-25 h
Y
n
n
n = 0, 1
R = -H, -Me
R
1 = -Me, -OMe, -SMe
X = -O, -CH2
Y = -O, -S
Entry
1
Substrates
Me Me
O
Products
Me Me
O
Entry
Substrates
Products
Me
O
Me
O
S
CHO
S
Me
CHO
Me
OMe
OMe
O
O
OMe
9k1, 34%
+
6a, 71%
7f, 81%
9a, 67%
11
Me Me
O
Me Me
O
CHO
OMe
O
Me
Me
O
2
O
O
O
9k2, 38%
O
6b, 65%
9b, 64%
CHO
OMe
OMe
Me Me
O
O
Me Me
O
CHO
O
OMe
S
3
4
12
13
Me
S
7g, 66%
O
Me
Me
6c, 68%
Me
9l, 58%
O
9c, 71%
Me Me
Me Me
O
CHO
O
CHO
O
OMe
OMe
OMe
OMe
OMe
O
O
OMe
O
7h, 76%
O
OM e
9m, 61%
6d, 72%
9d, 75%
(continued on next page)

5762
S. K. Manna et al. / Tetrahedron Letters 55 (2014) 5759–5763
Table 2 (continued)
Entry
Substrates
Products
Entry
Substrates
Products
CHO
O
S
OMe
O
OMe
CHO
S
OMe
5
OMe
O
O
6e, 77%
OMe
OMe 9n1, 33%
7i, 63%
9e, 67%
14
CHO
O
O
O
OMe
6
7
8
9
O
OMe
7a, 64%
O
OMe
Me
9f, 69%
OMe 9n2, 41%
CHO
O
Me
O
Me
CHO
Me
S
OMe
OMe
O
O
S
OMe
OMe
7b, 73%
OMe
OMe
7j, 81%
9o1, 27%
9g, 71%
O
15
CHO
O
Me
Me
O
O
7c, 77%
O
9h, 57%
OMe
9o2, 53%
CHO
OMe
OMe
O
MeO
MeO
CHO
OMe
O
MeO
MeO
OMe
OMe
OMe
O
O
16
O
9p, 41%
O
O
7d, 80%
7k, 77%
9i, 78%
O
CHO
O
CHO
OMe
OMe
Me
17
18
O
Me
Me
7e, 81%
8a, 71%
(1:1.5)
9q, 67%
Me
9j, 67%
10
CHO
O
MeS
OMe
MeS
OMe
8b, 63%
9r, 51%
yield increased to 55% (entry 6, Table 1). Changing the solvent,
ligand, and oxidant, did not further improve the product yields
(entries 7–9, Table 1). So, we changed the Ru source to [RuCl2(p-
cymene)2]2. Further, the yield of 9a increased from 48% to 64%
when the substrate was stirred with TBHP (2 equiv) and PPh3
(15 mol %) and catalyst, [RuCl2(p-cymene)2]2 loading from
5 mol % to 10 mol % in acetonitrile at 110 °C under argon atmo-
sphere (entries 10 and 11, Table 1). Similar results were obtained
in the absence of ligand at 110 °C in 20 h (entry 12, Table 1). This
result indicates that there is no need of any external ligand. Subse-
quently, various other solvents, bases, and catalytic combinations
were screened, which are summarized in Table 1 (entries 13–17).
It was noticed that the increase of reaction time and catalyst load-
ing to 30 h and 15 mol %, respectively, in dry acetonitrile, had no
effect on the yield of the product 9a (entry 12d). However, in the
absence of catalyst, reactants remained unchanged during the
course of the reaction (entry 16). In summary, the optimal condi-
tion in acetonitrile includes [RuCl2(p-cymene)2]2 (10 mol %) and
TBHP (2 equiv) at 120 °C for 18–24 h under an argon atmosphere
(see Table 1).
To this end, after having successfully optimized conditions, we
further extended this reaction to
a variety of substituted
O
O
O
tBuOH
TBHP
IV
Ru
II
Ru
H
R2
H
H
R1
R2
O
TBHP
tBuOH
O
O
II
Ru
II
Ru
O
H
R1
O
O
H
IV
Ru
II
Ru
R1
R2
O
O
O
H
R2
R1
Figure 2. Proposed mechanistic pathway for the synthesis of xanthone and fluorenone derivatives.

S. K. Manna et al. / Tetrahedron Letters 55 (2014) 5759–5763
5763
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ortho-aryloxy/aryl benzaldehydes (6a–e, 7a–k, and 8a–b).
Reactions with electron withdrawing groups resulted in no
reaction or gave trace amount of products. Mostly, electron rich
substituents reacted smoothly. Acylation of meta substituted
ortho-aryloxy benzaldehydes resulted in a mixture of regioisomers
(7f, 7i, and 7k). This robust C–H bond functionalization can be suc-
cessfully applied for synthesizing xanthones (7a, 7c–f, and 7h–k),
thioxanthones (7b and 7g), and fluorenones (8a–b). Interestingly,
chromene based carbaldehydes were successfully cyclized to xan-
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the final molecules were characterized by 1H, 13C, ESI-MS, HRMS,
and IR analysis. The final molecules (9a and 9m) were confirmed
through DEPT, HSQC, HMBC, and COSY spectroscopic analysis
(see Supporting information). The reaction occurred through oxi-
dative addition by using TBHP (cyclic seven membered transition
states for xanthone and six membered for fluorenone) and fol-
lowed by reductive elimination to afford the desired scaffolds.
With these observations, a plausible mechanism is proposed in
Figure 2.
In summary, we have developed a straight forward route to
construct biologically active xanthone and fluorenone analogs in
moderate to good yields. This transformation is based on ortho
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Acknowledgments
This research project was partly supported by Department of
Science & Technology (DST), New Delhi, India. S.K.M. and S.L.K.M.
thank the Council of Scientific and Industrial Research (CSIR), India
for research fellowships. We would like to thank referees for their
valuable suggestions which helped us to improve the quality of
presentation. Instrumental facilities from SAIF, CDRI are acknowl-
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