PhI(OAc)2-BF3-OEt2mediated domino imine activation, intramolecular C-C bond formation and β-elimination: New approach for the synthesis of fluorenones, xanthones and phenanthridines

Article abstract of DOI:10.1039/c4ra06159d

PhI(OAc)₂-BF₃-OEt₂mediated domino synthesis of biologically important fluorenones, xanthones and phenanthridines has been developed. The reaction proceeds through imine activation, intramolecular C-C bond formation and β-elimination. This journal is

Full text of DOI:10.1039/c4ra06159d

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COMMUNICATION
PhI(OAc) –BF –OEt mediated domino imine
2
3
2
activation, intramolecular C–C bond formation and
b-elimination: new approach for the synthesis of
fluorenones, xanthones and phenanthridines†
Cite this: RSC Adv., 2014, 4, 40964
Received 24th June 2014
Accepted 19th August 2014
Satinath Sarkar and Narender Tadigoppula*
DOI: 10.1039/c4ra06159d
www.rsc.org/advances
PhI(OAc)
2
–BF
3
–OEt
2
mediated domino synthesis of biologically synthesis of uorenones, xanthones and phenanthridines via a
important fluorenones, xanthones and phenanthridines has been hypervalent iodine-Lewis acid mediated C–C bond forming
developed. The reaction proceeds through imine activation, intra- process.
molecular C–C bond formation and b-elimination.
Fluorenones are an important structural moiety found in
various natural products exhibiting promising biological activ-
15
ities. These compounds also show optical and electrical
The construction of carbon–carbon bonds is the prime art in
organic synthesis. For this reason, carbon–carbon bond form-
ing reactions are enormous and the development of new
methodologies is a subject of constant interest. Over the last
decade, palladium, copper, iron and nickel-catalyzed cross
coupling reactions have become fundamental methodologies in
16
properties. There are several methods for the synthesis of
17

uorenones, e.g. oxidation of uorenes, Friedel–Cras ring
18
closure of biarylcarboxylic acids and its derivatives, palladium
catalyzed cyclo carbonylation of o-halobiaryls, remote metal-
ation of 2-biphenylcarboxamides and 2-biphenyloxazolines,
19
20
21
imidoyl palladium migration involving C–H bond activation,
1
organic synthesis for C–C bond formation. The discovery of
22
decarboxylation of o-carboxyarylketones, palladium catalyzed
metal free efficient processes for C–C bond formation will also
be of great signicance because such procedures prevent the
need for disposing the metal catalysts from the reaction resi-
dues and eliminating traces of metals from the nal
compounds. Among metal free reagents, hypervalent iodines
have received great attention from synthetic organic chemists
for C–C bond forming reactions, leading to the synthesis of
23
C–H functionalization of arylaldoxime ethers with arenes,
palladium catalyzed annulation of arynes with 2-haloar-
24
25
26
enecarboxaldehydes, and metal or metal free intra-
molecular cyclization of biaryl-2-carbaldehydes. Recently, Pd
27
catalyzed dehydrogenative cyclization, nitrile directed dual
28
C–H bond activation, cross dehydrogenative coupling via base-
29
promoted homolytic aromatic substitution (BHAS), radical
cyclizations of arylboronic acids and triuoroborates, and
2
carbocyclic and heterocyclic molecules. The immense interest
30
in hypervalent iodines is because of easy availability, mild Lewis
acidity and their strong oxidizing properties. The hypervalent
iodine and Lewis acid combination was highly employed in
quaternary ammonium salt-promoted intramolecular dehydro-
genative arylation of aldehydes have been applied for the
synthesis of uorenones.
31
3
various organic transformations, such as oxidation of alcohols,
Michael reactions, C2-acyloxy glycosylation, C–H iodination,
Xanthones are naturally occurring molecules, which exhibit
4
5
6
32
excellent pharmaceutical properties. Jackson et al. synthesized
7
8
oxidative coupling of aromatic substrates, C–H amidation, a-
xanthones from functionalized diaryl ethers via Friedel–Cras
9
acetoxylation of ketones, allylic amination of allylsilanes and
allylstannanes, a-halogenation of dicarbonyl compounds,
oxidation of sillyl ether in alcohol or water, heterocyclic
suldes, and ligand exchange. As a part of our ongoing
33
reaction. Snieckus reported LDA-mediated conversion of 2-
10
11
carbamoyl diaryl ethers to xanthone derivatives via an anionic
12
34
Friedel–Cras process.
Frahm presented
a
series of
13
3
substituted xanthones synthesized from 2-aryloxybenzoic acids
synthetic programme on the carbocyclic and heterocyclic scaf-
35
in the presence of PPA. Larock et al. reported a synthesis of
14
folds, we intended on developing a new method for the
xanthones via tandem coupling of arynes with salicylates and
also developed a synthesis of xanthones via aryl to imidoyl
21,36
palladium migration involving C–H bond activation.
Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute,
Lucknow-226-031, India. E-mail: t_narendra@cdri.res.in; tnarender@rediffmail. Recently, copper(II)-catalyzed aza-Friedel–Cras reaction of o-
com; Fax: +91-522-2771941; Tel: +91-522-2772450
37
phenoxyl-N-tosyl-benzaldimine, rhodium catalyzed dehydro-
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra06159d
38
genative cross coupling of 2-aryloxybenzaldehydes, copper-
1
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a
Table 1 Optimization of reaction conditions for the formation of imine group of 2, and their combination might assist the C–C
fluorenone, 3
bond formation and b-elimination to give ketimine. The
hydrolysis of the resultant ketimine might produce uorenone
21
3
(Table 1). The coordination of the hypervalent iodine to the
imine moiety is the key step for this hypervalent iodine-Lewis
acid mediated C–C bond forming reaction or domino process.
To explore the idea, we have selected 2 as a model substrate for
the screening of experimental conditions for the formation of

uorenone 3 with various hypervalent iodines, additives (HFIP
b
Entry
Hypervalent iodine
PhIO
Additive
Solvent
Yield (%)
and Lewis acid BF –OEt ), and solvents, and the results are
3
2
summarized in Table 1.
It was observed that 0.375 mmol PhI(OAc) and 0.375 mmol
boron triuoride diethyl ether complex in dry DCE at 80 C in
1
2
3
4
5
6
7
—
—
—
—
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCE
DCE
DCE
DCE
DCE
—
—
—
—
—
—
—
20
32
52
61
62
65
41
PhI(OAc)
PhI(OTf)
2
2
ꢀ
2
PhI(OH)(OTs)
—
—
2
4 h followed by hydrolysis afforded 3 in good yield (entry 11,
Table 1). No signicant improvement was observed in the
formation of 3 while increasing the quantity of PhI(OAc) and
HFIP
BF
3
–OEt
2
2
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
2
2
2
2
2
2
2
2
HFIP
c
8
BF
3
BF
3
BF
3
BF
3
BF
3
BF
3
BF
3
–OEt
–OEt
–OEt
–OEt
–OEt
–OEt
–OEt
2
2
2
2
2
2
2
boron triuoride diethyl ether complex (entry 12). Major
increment was not observed in the formation of 3 by increasing
the time or the concentrations of PhI(OAc) or boron triuoride
c
9
d
1
1
1
1
1
0
1
2
3
4
2
e
f
diethyl ether complex (entry 13). Further, 0.375 mmol PhI(OAc)2
g
h
and 0.375 mmol boron triuoride diethyl ether complex in dry
ꢀ
MeCN
acetonitrile at 80 C in 36 h, followed by hydrolysis, also did not
furnish 3 in good yield (entry 14). It is noteworthy to mention
here that the domino process was not feasible with other
mmol additive, unless otherwise noted. Isolated yield of the aldimine, which was prepared from 2-(naphthalen-2-yl)
a
0
.25 mmol 2 was dissolved in a solvent (1.0 mL) and added in the same
solvent (3 mL) containing 0.275 mmol hypervalent iodine and 0.275
b
c
product, 3 with respect to 1. 1N HCl (2 mL) was added and run for
4
added and run for 5 h. 0.375 mmol PhI(OAC)
OEt
HCl (3 mL) was added and run for 6 h. 0.5 mmol PhI(OAC)
benzaldehyde 1 and aniline at room temperature in 2 h
(ref. 53) under the same reaction conditions.
With the above optimized conditions (entry 11, Table 1) the
metal free domino process was explored with various aldimines
12–19. The structures of the uorenones 20–27 obtained
d
ꢀ
h. The reaction was conducted at 80 C and 1N HCl (2 mL) was
e
2
and 0.375 mmol BF
were employed and the reaction was performed at 80 C and 1N
3
–
ꢀ
2
f
2
and 0.5
mmol BF
3
–OEt
2
were employed and the reaction was performed at 80
ꢀ
g
C and 1N HCl (3 mL) was added and run for 6 h. 0.75 mmol
PhI(OAC)
2
and 0.75 mmol BF
3
–OEt
2
were employed and the reaction through domino process, reaction times and isolated yields are
ꢀ
was performed at 80 C for 36 h and 1N HCl (2 mL) was added and
run for 6 h. 0.75 mmol PhI(OAC)
employed and the reaction was performed at 80 C for 36 h and 1N
HCl (2 mL) was added and run for 5 h.
presented in Scheme 1.
h
2
and 0.75 mmol BF
3
–OEt
2
were
ꢀ
Better yields were observed in the formation of uorenones 3
and 20 because of the rich electronic nature of 2 and 12. The
39
catalyzed ortho-acylation of phenols with aryl aldehydes, and
cross dehydrogenative coupling via base-promoted homolytic
29
aromatic substitution (BHAS), have been employed for the
synthesis of xanthones.
Phenanthridine core containing molecules such as
4
0
41
42
43
ethidium, trispheridine, bicolorine, and decarine are
44
found in nature and are biologically important. Microwave
assisted [2 + 2 + 2] cyclotrimerisation, photolysis, benzyne
mediated cyclization, radical cyclization, TFA catalyzed C–C
4
5
46
47
48
49
and C–N bond formation, Pd catalyzed oxidative C–H bond
5
0
51
activation, hypervalent iodine mediated process and transi-
52
tion metal free intramolecular direct C–H bond arylation have
been utilized for the synthesis of phenanthridines.
In order to synthesize uorenones, we initiated our work
with the synthesis of 2-(naphthalen-2-yl)benzaldehyde 1 using a
Suzuki coupling reaction between 2-bromobenzaldehyde and 2-
naphthylboronic acid and consequently 1 was converted into
key intermediate aldimine 2 by treating with benzyl amine
53
(
Table 1). We anticipated that hypervalent iodine in presence
Scheme 1 Synthesis of fluorenones 3 and 20–27, reaction times and
isolated yields are given in parenthesis.
of a Lewis acid could recognize and coordinate through the
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Scheme
2 Synthesis of heterocycle based fluorenone (30) and
anthranone (33), reaction times and isolated yields are given in
parenthesis.
Scheme 4 Synthesis of phenanthridines 49–51, reaction times and
isolated yields are given in parenthesis.
The plausible reaction mechanism for the formation of u-
orenones (3, 20–27 and 30), xanthones (40–42) and phenan-
thridines (49–51) is described in Scheme 5. The rst step
appears to be the coordination of iodosobenzene diacetate with
aldimine and nucleophilic attack of the acetate ion to the
5
polarized imine carbon to provide intermediate (I). In the
Scheme 3 Synthesis of Xanthones 40–42, reaction times and isolated
yields are given in parenthesis.
generality and versatility of this process was proved by pursuing
the synthesis of uorenones 21–26 from aldimines 13–18. The
isolated yield of product 27 was low because of the aldimine 19
having the unreactive or electronically weak phenyl ring.
Applicability of this method was proved with the synthesis of
heterocycle based uorenone (30) and anthranone (33)
Scheme 2.
Aer the successful synthesis of uorenones, we explored
the synthesis of various xanthones using the optimized condi-
54
tions (entry 11, Table 1). 2-(p-tolyloxy)benzaldehyde 34 was
prepared using aromatic nucleophilic substitution reaction and
converted into aldimine (37, Scheme 3) by treating with benzyl
53
amine. The domino process was carried out on 37 (entry 11,
Table 1), which resulted in the formation of xanthone 40. Aer
getting these successful results the domino process was
employed on other aldimines 38 and 39, which gave the
respective xanthones 41 and 42 in reasonable yields (Scheme 3).
The applicability of this domino process was further proved
with the synthesis of phenanthridines. To synthesize phenan-
0
8
thridines 49–51, 4 -chlorobiphenyl-2-amine 43 was synthesized
using Suzuki coupling and converted into aldimine (46,
53
Scheme 4) by treating with benzaldehyde. The domino process
was applied on 46 under the above optimized experimental
conditions (entry 11, Table 1), which provided phenanthridine
49. Encouraged by these results the domino process was
employed with other aldimines 47 and 48 to produce xanthones
50 and 51 (Scheme 4).
Scheme 5 Plausible reaction mechanism for the formation of fluo-
renones, xanthones and phenanthridines.
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+1
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(I) might have afforded (II) in which I readily converted to I .
In the third step C–C bond formation and cleavage of C–OAc
bond in the presence of BF –OEt , might have taken place to
3
2
55
3 2
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In conclusion, we have developed a new method for the
synthesis of biologically important uorenones, xanthones and
phenanthridines using the combination of a hypervalent iodine
and a Lewis acid. The formation of the uorenones, xanthones
and phenanthridines appears to be through hypervalent iodine-
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Acknowledgements
Authors are thankful to CSIR and DST (New Delhi) for nancial
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