Pyrrolidine and iodine catalyzed domino aldol-Michael-dehydrogenative synthesis of flavones

Article abstract of DOI:10.1016/j.tetlet.2014.04.051

A one pot synthesis of flavones is established from 2′- hydroxyacetophenones and substituted aromatic aldehydes. The method uses domino aldol-Michael-oxidation reaction catalyzed by pyrrolidine as a base and iodine as an oxidant in dimethyl sulfoxide.

Full text of DOI:10.1016/j.tetlet.2014.04.051

Accepted Manuscript
Pyrrolidine and iodine catalyzed domino aldol-Michael-dehydrogenative syn-
thesis of flavones
Mayuri M. Naik, Santosh G. Tilve, Vijayendra P. Kamat
PII:
S0040-4039(14)00663-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.04.051
TETL 44513
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 March 2014
11 April 2014
15 April 2014
Please cite this article as: Naik, M.M., Tilve, S.G., Kamat, V.P., Pyrrolidine and iodine catalyzed domino aldol-
Michael-dehydrogenative synthesis of flavones, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.
2014.04.051
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Graphical Abstract
Pyrrolidine and iodine catalyzed domino
aldol-Michael-dehydrogenative synthesis of
flavones
Leave this area blank for abstract info.
Mayuri M. Naik, Santosh G. Tilve∗ and Vijayendra P. Kamat
0.5 eq. pyrrolidine
0.05 eq. iodine
R'
R
OH
O
R
O
O
R'
DMSO, 150 0C
60-88%
OHC
1
2
3

1
Tetrahedron Letters
journal homepage: www.els evier.com
Pyrrolidine and iodine catalyzed domino aldol-Michael-dehydrogenative synthesis of
flavones
Mayuri M. Naik, Santosh G. Tilve∗ and Vijayendra P. Kamat
Department of chemistry, Goa University, Taleigao Plateau, Goa 403 206, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A one pot synthesis of flavones is established from 2’-hydroxyacetophenones and substituted
aromatic aldehydes. The method uses domino aldol-Michael-oxidation reaction catalyzed by
pyrrolidine as a base and iodine as an oxidant in dimethyl sulfoxide.
2009 Elsevier Ltd. All rights reserved.
Available online
Keywords:
Flavones
Pyrrolidine
Iodine
Domino
Aldol
Flavones or 2-phenylchromones are naturally occurring
oxygen containing heterocyclic compounds belonging to the
flavonoid family present in fruits, vegetables, grains, bark, roots,
stems, flowers, tea and wine.1 Owing to their broad range of
biological activities, continuous investigation has led to the
isolation of over 4000 chemically unique flavonoids from plants.2
Multifarious biological activities exhibited by flavones include
anti-inflammatory, anti-viral, anti-estrogenic, anticancer,
antioxidant, leishmanicidal, ovipositor stimulant phytoalexins,
anti-HIV, antimutagenic, antiallergic, etc.3-4 Some flavonoids are
known to show modulatory properties of enzymes such as
activation of sirtuins5 and inhibition of monoamine oxidase
(MAO).6 Some of the well known naturally occurring potent
bioactive flavones are shown in Figure 1. As a consequence of
these vital properties researchers constantly study these
interesting flavonoids and come up with new strategies to
synthesize them.
rearrangement,11 Allan-Robinson12 and Auwers synthesis.13 Most
of the reported synthesis makes use of chalcones which on
oxidation using numerous oxidizing agents such as molecular
I214, DDQ, Ph-S-S-Ph, I2-DMSO,15 I2-SiO216, I2-Al2O317, NH4I18,
InBr3 and InCl319 give flavones. Microwave irradiation technique
is also used to obtain flavones.20 Similarly oxidation of
flavanones to flavones is well known in literature.21 Recently
various reports have emerged using diverse Palladium catalysts,22
however in many cases competitive side reactions leading to
aurones as side products are detected. Ionic liquids are used to
deliver the target molecule either by dehydrative cyclization of
1,3-(diaryl) diketones or using CuI catalyst.23 Besides this,
various other methods have appeared in literature for dehydrative
cyclization of 1,3-diketones to produce flavones.23a-b,24 Some of
the catalysts employed to furnish flavones comprises of FeCl3-
piperidine25 and DMAP.26 Intramolecular Wittig reaction has also
been reported.27 A convenient one pot method from hydrolysis of
flavylium salt obtained from condensation of 2’-
hydroxyacetophenone and aryl aldehydes using perchloric acid is
also known.28
1
3
5
6
2
4
7
6
Apigenin (R , R R , R =H, R , R , R =OH)
,
R
7
7
R
antibacterial activity.
5
5
2
R
R
1
3
2
4
6
7
4
3
Luteolin (R , R , R =H R , R , R , R =OH)
8
R
R
O
O
Recently flavanone synthesis is reported using aniline and
catalytic amount of iodine from aryl aldehydes and 2’-
hydroxyacetophenone.29 Also it is well known that 2’-
hydroxychalcone get cyclized to flavone using iodine and DMSO
as a solvent.15 In view of this we conjectured that it should be
possible to devise synthesis of flavone directly from aryl
aldehyde and 2’-hydroxyacetophenone. We speculated that a
secondary amine could give chalcone followed by Michael to
form flavanone which could then get oxidized with iodine in
DMSO to render flavone (Scheme 1). However, the crucial
anti-cancer.
1
2
3
4
5
6
7
1
Nobiletin (R =H R , R , R , R , R , R =OMe)
9
R
anti-inflammatory.
1
2
4
7
3
5
6
Kaempferol (R , R , R , R =OH R , R , R =H)
10
anti-oxidant.
Fig 1. Naturally occurring biologically active flavones
A variety of methods have been developed for flavone
traditionally used being Baker-Venkataraman
s—yn—the—sis,
∗ Corresponding author. Tel.: 0832-6519317; fax: 091-832-2452886; Email: stilve@unigoa.ac.in

2
Tetrahedron
OMe
OMe
reaction for the catalytic sequence to be successful was the
OH
O
0.5 eq. base
OMe
OMe
O
O
requirement of regeneration of pyrrolidine and iodine via
oxidation of HI formed from dissociation of pyrrolidinium
iodide.
OHC
0.1 eq. iodine
3a
DMSO, reflux, 2h
2a
1a
O
R'
2-(3,4-dimethoxyphenyl)-4H-chromen-4-one
R
O
R
R'
H
Scheme 2. Reaction of 2’-hydroxyacetophenone with 3,4-
OH
3
O
dimethoxybenzaldehyde
O
1
2
C4H9N + HI
C4H10NI
2 HI
Ref. 33
I2 + H2
N
H
The amount of pyrrolidine was standardized by investigating
the reaction in absence of iodine which furnished 2-(3,4-
dimethoxyphenyl)chroman-4-one 3a’ exclusively. Varying
concentrations of pyrrolidine like 0.1, 0.2, 0.3, 0.5, 1.0 and 1.5
equivalence were tried which showed 0.5 equivalence of
pyrrolidine to be the optimum concentration as the reaction got
completed in minimum time of 15 minutes.
H
O
-C4H10NI
R'
R
R'
N
OH
4
R
O
O
H
I
1
HN
Aldol
OMe
OMe
R
7
O
O
R'
I2
-C4H10NI
OH
H
I
O
N
R'
HN
R
O
O
Fig. 2. Flavanone 3a’ formed in absence of iodine
5
I
I
H
With 0.5 equivalence of pyrrolidine we proceeded with
temperature studies in DMSO solvent at room temperature, 60,
N
H
N
H
o
100 and 150 oC which revealed 150 C to be the optimum
R'
HO
O
R
temperature for flavone formation providing maximum yield of
80%. Other solvents tried were ethanol, methanol, toluene,
xylene and tetrahydrofuran showed no required product
formation even after refluxing for 24 hours. Similarly iodine
concentration was varied from 1 mol% to 100 mol% which
displayed 5 mol% of iodine to be optimum concentration as it
delivered flavone in highest yield of 88%. In the absence of
iodine no flavone formation was observed even after prolonged
heating.
R
O
N
H
R'
Michael
O
6
3'
Scheme 1. Probable mechanism for the formation of flavone 3 via chalcone 6
and flavanone 3’
We
commenced
our
work
by
choosing
2’-
hydroxyacetophenone 1a and 3,4-dimethoxybenzaldeyde 2a as
model substrates in presence of different bases (0.5 equivalence)
and iodine (10 mol%) catalyst as an oxidant in DMSO solvent to
deliver flavone under reflux for 2h (Scheme 2). Various bases
such as pyrrolidine, L-proline, piperidine, N-methylaniline and
morpholine were screened individually. To our delight, required
flavone 3a was formed in 75% yield when pyrrolidine was
employed as base catalyst. L-proline and piperidine were found
to diminish the yields to 36 and 22% respectively. On pursuing
with other bases viz N-methylaniline and morpholine no product
formation was observed.
0.5 eq. pyrrolidine
R'
R
OH
O
R
O
O
R'
0.05 eq. iodine
OHC
DMSO, 150 0
C
3
2
1
R=H 1a, OCH3 1b,
1c
OC2H5 , OCH2Ph
OCH2CH=CH2 1e
1d
,
Scheme 3. Standardized reaction condition for flavone formation
Table 1. Derivatives of flavones 3a-r using 2’-hydroxyacetophenones 1a-e and substituted aromatic aldehydes 2a-m under optimized reaction condition
Substr
ate (1)
Substrate
(2)
Time
(h)
Producta
(3)
Substr
ate (1)
Substrate
(2)
Time
(h)
Producta
(3)
OMe
OMe
OMe
OMe
OMe
OMe
7
1a
OMe
OMe
OMe
OMe
O
O
10
24
1a
1a
OHC
OHC
O
O
3c (78%)
3a (88%)
2c
2a
2b
OMe
OMe
9
1a
O
O
OHC
OHC
O
O
3d (85%)
2d
3b (82%)

3
F
F
O
9
6
8
1a
1a
1a
1a
O
S
S
OHC
OHC
O
O
2l
2e
3l (75%)
3e (82%)
CHO
Cl
Cl
12
S
O
O
S
OHC
2m
O
2f
O
3m (72%)
3f (84%)
OMe
OM
e
8
1b
Br
Br
MeO
O
13
1a
OHC
O
OHC
O
2b
O
3n (81%)
2g
3g (80%)
10
12
1b
1c
MeO
O
OHC
O
6
1a
1a
Br
O
2d
OHC
OHC
Br
3o (88%)
O
2h
2i
3h (70%)
O
O
OHC
12
O
NO2
NO2
2d
O
3p (78%)
O
3i (62%)
O
12
8
1d
1e
O
Ph
O
O
O
O
24
1a
OHC
O
2d
OHC
O
3q (60%)
O
2j
3j (74%)
O
O
O
OHC
O
1a
9
O
O
2d
3r (70%)
OHC
2k
O
a isolated yields
3k (80%)
After exploring various parameters we obtained the ideal
reaction condition shown in Scheme 3. Subsequently, we set
different aromatic aldehydes to the optimized reaction condition
in order to explore the generality of our methodology (Table 1).
Electron rich aromatic aldehydes 2a-2c furnished desired
flavones 3a-3c in good yields. Benzaldehyde too smoothly
formed required product 3d. Halogenated aromatic aldehydes
were well tolerated to provide 3e-3g flavones in good yields
which are good scaffolds for further functionalization. Aromatic
aldehydes with m-substituted bromo as well as strong electron
withdrawing nitro group resulted in flavone 3h and 3i formation
but with slightly declined yields. Thus our methodology could be
applied to both electron rich as well as electron deficient
aromatic aldehydes which are well tolerated under the reaction
condition as the yields were unchanged to the electronic effects.
Furthermore, 3,4-methylenedioxy benzaldehyde smoothly
favoured the formation of desired flavone 3j in satisfactory yield.
Reports have shown that the biological activity of flavones is
enhanced when 5 or 6 membered heterocyclic group is attached
at its C-2 position.30 Motivated from this we subjected different
heterocyclic aromatic aldehydes to the reaction condition to
achieve the desired flavone products 3k-3m in good yields. After
scanning numerous aromatic aldehydes, substituted 2’-
hydroxyacetophenones 1b-1e were put forth for determining
substrate scope. 4-Methoxy-2’-hydroxyacetophenone 1b was
reacted with benzaldehydes 2b and 2d to provide flavones 3n-3o
in good yields. Similarly 4-ethoxy-2’-hydroxyacetophenone 1c
and 4-benzyloxy-2’-hydroxyacetophenone 1d reacted under
standardized condition to furnish respective flavones 3p and 3q
in reasonable yields. One of the reports had shown deprotection
of 2’-allyloxychalcone leading to flavone in I2-DMSO.31
Interestingly, we got the desired flavone 3r from 4-allyloxy-2’-
hydroxyacetophenone 1e without the cleavage of allyloxy group.
The reaction protocol was also successfully scaled up to 5g of
starting aryl aldehyde 2a to get consistent yield of desired
flavone 3a. The present method is an alternative to the reported
one pot method (ref 28a) avoiding the use of explosive perchloric
acid.
In conclusion, one pot synthesis of flavones is described using

4
Tetrahedron
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pyrrolidine and iodine catalysts in DMSO solvent. Several
advantages of this methodology including inexpensive catalysts,
good substrate generality, lack of metal catalysts and products in
high yields with no side reactions make it a useful synthetic
approach to flavones. Also, this method avoids the step of
isolation of chalcone or flavanone intermediates and then
subjecting them to further oxidation.
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Acknowledgments
M. M. N. thanks CSIR, New Delhi, for awarding Senior
Research Fellowship. Authors also acknowledge CSIR and DST,
New Delhi for financial assistance.
References and notes
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Hydroxyacetophenone (1 mmol) and substituted aromatic
aldehyde (1 mmol) were mixed together along with pyrrolidine
(0.5 mmol) and iodine (0.05 mmol) in DMSO solvent (10 mL).
The resulting mixture was then heated at 150 oC for the given
time. After completion of reaction (monitored by TLC) the
reaction mass was allowed to cool and diluted with ethyl acetate
(20 mL). Resulting solution was then washed with water and
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with the corresponding flavanone and flavone.
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