Conversion of naringenin and hesperetin by heterogeneous catalytic Baeyer-Villiger reaction into lactones exhibiting apoptotic activity

Article abstract of DOI:10.1016/S0040-4039(03)01139-0

Naringenin and hesperetin were converted into the corresponding lactones by hydrogen peroxide activated by poly(4-vinylpiridine)-supported methyltrioxorhenium in t-butanol, an environmentally friendly catalytic system. The products showed a marked apoptotic activity in the genetic tests.

Full text of DOI:10.1016/S0040-4039(03)01139-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4823–4825
Conversion of naringenin and hesperetin by heterogeneous
catalytic Baeyer–Villiger reaction into lactones exhibiting
apoptotic activity
Roberta Bernini,a,* Enrico Mincione,a Manuela Cortese,a Raffaele Saladino,a Giampiero Gualandib
and Maria Cristina Belfioreb
aOrganic Chemistry, Dipartimento ABAC, Universita` degli Studi della Tuscia, Via S. Camillo De Lellis, 01100 Viterbo, Italy
bGenetic of Microorganisms, Dipartimento ABAC, Universita` degli Studi della Tuscia, Via S. Camillo De Lellis, 01100 Viterbo,
Italy
Received 10 April 2003; revised 29 April 2003; accepted 3 May 2003
Abstract—Naringenin and hesperetin were converted into the corresponding lactones by hydrogen peroxide activated by
poly(4-vinylpiridine)-supported methyltrioxorhenium in t-butanol, an environmentally friendly catalytic system. The products
showed a marked apoptotic activity in the genetic tests. © 2003 Elsevier Science Ltd. All rights reserved.
Among naturally occurring flavonoids, naringenin and
hesperetin are very common in some edible fruits and
vegetables as aglycons and glycosides. Naringenin is
most abundant in grapefruit,1 hesperetin in oranges,
tangerines and lemons.2 They occupy an important
position in perfumery, cosmetic and in pharmaceutical
formulations and hypocholesterolemic,3 antiestrogenic,4
hypolipidemic,5 antihypertensive,6 antiflammatory7
activities have been reported.
solvent, using the MTO supported on poly(4-
vinylpyridine) cross-linked with divinylbenzene10
(Scheme 1, pathway b). We chose this matrix because
pyridine beside being an acid buffer is also a good
ligand of MTO.11 Different reaction conditions as well
as supports were experimented. As reported in Table 1,
high conversions and satisfactory yields of lactones 9
and 10 were obtained without quinone formation. The
same 5-methoxyflavanone 1 and 7-methoxyflavanones 2
gave the corresponding lactones 7 and 8. The demethyl-
ation previously observed in homogeneous catalysis
did not occur now.8 Shorter reaction times, higher
conversions of the substrates (90 and 88%) and better
yields of 9 and 10 (80 and 72%) were observed with
MTO supported on the poly(4-vinylpyridine) 25%
cross-linked (PVP-25%/MTO, Table 1: entries 3 and 8),
evidencing the influence of the morphologic characteris-
tics of the polymeric support on the efficiency of the
catalyst.
In a previous paper, we reported the Baeyer–Villiger
rearrangement of some flavanones derivatives by the
Methyltrioxorhenium
(MTO)/Hydrogen
Peroxide
(H2O2) system in homogeneous acidic conditions to get
the corresponding lactones.8 However, the methylated
naringenin 3 and hesperetin 4 in these conditions
reacted to give the quinones 5 and 6 as the only
reaction products (Scheme 1, pathway a) as a conse-
quence of the opening of the formed lactones to the
corresponding phenols further oxidized to quinones by
MTO. With the aim to develop an efficient and useful
method to prepare these lactones which are reported to
have many biological activities,9 now we set up the
conditions to operate the Baeyer–Villiger reaction in
heterogeneous catalytic conditions, as well in a neutral
In a typical experiment, to a suspension of 150 mg of
the appropriate resin, prepared as reported,10 in 5.0 ml
of t-butanol, warmed at 80°C, were added the
flavanone (0.3 mmol) and hydrogen peroxide H2O2
(50% water solution). The reaction was monitored by
thin layer chromatography. At the end of the experi-
ment, the suspension was filtered. The recovered cata-
lyst was washed with ethyl acetate and after drying
under high vacuum it was efficiently used for three
further runs of oxidation. After the removal of the
Keywords: naringenin; hesperetin; Baeyer–Villiger reaction; heteroge-
neous catalysis.
* Corresponding author. Fax:+39(0)761357230; e-mail: berninir@
unitus.it
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01139-0

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R. Bernini et al. / Tetrahedron Letters 44 (2003) 4823–4825
Scheme 1. Oxidation of the methylated flavanones 3 and 4 to the corresponding quinones 5 and 6 by homogeneous catalytic
system (pathway a) and conversion of 1–4 into the corresponding lactones by the heterogeneous MTO/H2O2 catalytic system
(pathway b).
Table 1. Conversion of methylated naringenin 3 and hesperetin 4 by supported-MTO/H2O2 in t-butanol at 80°C; conver-
sions and yields were calculated after chromatographic separation of the compounds
Entry
Substrate
Catalyst
Lactone
H2O2 (equiv.)
Time (h)
Conversion (%)
Yield (%)
1
3
3
3
3
3
4
4
4
4
4
–
9
9
9
9
10
10
10
6
24
12
8
6
6
24
8
6
8
6
–
–
2a
3b
4c
5d
6
PVP-2%/MTO
PVP-25%/MTO
PVPN-2%/MTO
PVPN-25%/MTO
–
PVP-2%/MTO
PVP-25%/MTO
PVPN-2%/MTO
PVPN-25%/MTO
80
90
82
98
–
76
90
78
98
71
80
46
42
–
58
72
47
35
9
6
10
10
10
10
10
10
10
8
10
6
7a
8b
9c
10d
a PVP-2%/MTO: poly(4-vinylpyridine) 2% cross-linked with divinylbenzene.
b PVP-25%/MTO: poly(4-vinylpyridine) 25% cross-linked with divinylbenzene.
c PVPN-2%/MTO: poly(4-vinylpyridine-N-oxide) 2% cross-linked with divinylbenzene.
d PVPN-25%/MTO: poly(4-vinylpyridine-N-oxide) 25% cross-linked with divinylbenzene.
solvent, the mixture reaction was purified by flash-chro-
matography. The lactones were characterized by 1H and
13C NMR, IR spectroscopy and mass-spectroscopy (EI)
analyses.
Methylation of the hydroxyl group increased the apop-
togenic efficiency, again showing the methylated form of
naringenin 3 to be more apoptotic than the corresponding
methylated form of hesperetin 4. A further increase of the
apoptosis was obtained with the lactone derivatives, the
lactone 9 being the most active. Noteworthy, at the doses
tested, these compounds show also a selective toxicity
because they don’t cause cellular necrosis.‡ These results
increase the interest for further researches on a possible
role of these compounds in the tumoral therapies.
We then submitted the natural naringenin and hesperetin,
their relative methylated compounds 3 and 4, and the
corresponding lactones 9 and 10 to a toxicological
examination in order to evaluate a possible apoptogenic
activity. These assays were operated on a E2 human
lymphoma cell line which is very resistant to apoptosis.13
Both the natural naringenin and hesperetin did not induce
significantlyapoptosisoncellsline, butthenaringeninwas
shown be more apoptogenic than the hesperetin (Fig. 1).†
To the best of our knowledge, no references are
reported on the utilization of this heterogeneous cata-
lytic system in the Baeyer–Villiger reaction. Further
† All compounds were tested in the same range of mmolar concentra-
tions.
‡ Trypan blue test (data not shown).

R. Bernini et al. / Tetrahedron Letters 44 (2003) 4823–4825
4825
Figure 1. Apoptotic induction and estimation. Cells were seeded at conc. 3×105/ml and the compounds were added for 18 h.
DMSO never excedeed 0.5% v/v. For morphological analysis of apoptosis 2–6×105 cells were fixed in 4% (v/v) paraformeldehyde
and stained with a solution (0.2 mg/ml) of 4,6-diamino-2-phenylindole (DAPI). Apoptosis was quantified by scoring cells with
condensed and fragmented nuclei, according to Ghibelli;12 >500 cells in random fields were scored using fluorescence microscopy.
work is therefore in progress to extend this efficient and
environmentally friendly method to other classes of
organic compounds with particular regard to polyfunc-
tionalized and structurally complex natural compounds.
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Acknowledgements
We are grateful to Consorzio INCA and Italian 5%
MURST-CNR ‘Progetto Reflui Oleari’ for their finan-
cial support.
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