An efficient synthesis of the phytoestrogen 8-prenylnaringenin from isoxanthohumol with magnesium iodide etherate

Article abstract of DOI:10.1016/j.tet.2008.07.072

Xanthohumol was isolated from supercritical carbon dioxide-spent hop and transformed into isoxanthohumol. The demethylation of isoxanthohumol with the best yield 93% occurred when MgI₂ etherate in anhydrous THF was applied. Salts such as MgBr

Full text of DOI:10.1016/j.tet.2008.07.072

Tetrahedron 64 (2008) 9544–9547
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An efficient synthesis of the phytoestrogen 8-prenylnaringenin from
isoxanthohumol with magnesium iodide etherate
*
_
Miros1aw Anio1 , Katarzyna Szyma n´ ska, Anna Zo1nierczyk
Department of Chemistry, Wrocław University of Environmental and Life Sciences, Norwida 25, 50-375 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 May 2008
Received in revised form 30 June 2008
Accepted 17 July 2008
Available online 23 July 2008
Xanthohumol was isolated from supercritical carbon dioxide–spent hop and transformed into iso-
xanthohumol. The demethylation of isoxanthohumol with the best yield 93% occurred when MgI
etherate in anhydrous THF was applied. Salts such as MgBr , MgCl , CaI , Mg(OAc) , Mg(OMe) were
2
2
2
2
2
2
also investigated. A convenient method for the xanthohumol isolation from supercritical carbon dioxide–
spent hop is also described.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Xanthohumol
Isoxanthohumol
8-Prenylnaringenin
Phytoestrogen
Demethylation
Magnesium iodide etherate
1
. Introduction
was demethylation of 2 to 3 with Lewis acids such as AlBr
MeAlCl in the presence of collidine (up to 30% yield); ZnBr
ZnBr /CuI Yb (SO /KI or CuI, Sm(OTf) /KI, CeCl /LiI (product not
detected or low yield); and Sc(OTf) /KI (92% yield). In an alternative
route, the two hydroxyl groups of were protected with
using chlorotriisopropylsilane and this product was treated with
3
, BBr
3
or
2
2
, CuI,
The female flowers of hops (Humulus lupulus L.) are used in the
brewing industry to add flavor and bitterness to beer. The flavor of
beer arises mainly from essential oils and the bitterness from
iso-alpha-acids. Hops consist of many prenylated chalcones and
flavanones.1 Among them, xanthohumol (1) has received much
2
2
4
)
3
3
3
3
2
AlBr
3
and the silyl groups deprotected with (n-Bu)
4
NF (73% overall
2
,3
attention in recent years as an anti-cancer and antioxidant agent.
-Prenylnaringenin (3) is one of the strongest phytoestrogen
yield of 3).
8
4
known in nature. This compound and its precursors, xanthohumol
1) and isoxanthohumol (2), may also have anti-breast cancer ac-
OH
(
5
,6
tivity. Xanthohumol (1) is readily accessible from carbon dioxide-
extracted hops7 where its content ranges up to 1% of dry matter.
Isoxanthohumol (2) is also present in this source in low concen-
tration, but it can be easily obtained from 1 by dissolving in 1%
NaOH and acidification of the reaction mixture (Scheme 1). The
content of 8-prenylnaringenin (3) in hops is 10–100 times lower
than the content of 1 and it is hardly accessible from this source.
Because of the interesting biological properties and the increase of
the commercial importance of 3, several methods for its synthesis
have been proposed. The methods included synthesis from nar-
Me
O
Me
O
O
O
8
,9
O
H
1. MeONa/ MeOH
2. H+
HO
O
HO
84%
OH
1
2
1
0–15,16
ingenin (up to 45% overall yield),
phloroacetophenone with
In the last of the above-men-
Scheme 1. Cyclization of xanthohumol (1) to isoxanthohumol (2).
low yield17 or xanthohumol (1).
9,18
In this paper we report a convenient method of demethylation
of isoxanthohumol (2) using magnesium iodide. Magnesium iodide
etherate is the reagent for the demethylation of a methoxy group
especially at the ortho-position to the carbonyl group. Such an
arrangement is present in 2. This complex was previously applied
tioned studies, 1 was isomerized to 2 using 1% NaOH. The next step
19
*
Corresponding author. Tel.: þ48 71 3205 252; fax: þ48 71 3284 124.
E-mail address: miroslaw.aniol@wnoz.up.wroc.pl (M. Anio1).
0
040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.07.072

M. Anioł et al. / Tetrahedron 64 (2008) 9544–9547
9545
in the regioselective demethylation of 5-acetyl-4,6-dimethoxy-
Table 1
2
0
Demethylation of xanthohumol (1) and isoxanthohumol (2)
2
-isopropenyl-2,3-dihydrobenzofuran
and substituted 2,6-
21
Yielda (%)
Recycled 1 or 2b (%)
0 (0)c
0
81.7
73.2
60.3
76.0
0
dimethoxybenzaldehydes.
Entry
Substrate
Metal salt
MgI
MgBr
MgCl
CaI
Mg(OAc)2
Mg(OMe)
MgI
t (h)
1
2
3
4
5
6
7
2
2
2
2
2
2
1
2
12
12
12
72
48
48
48
88.1 (93.0)c
38.6
2
2
. Results and discussion
The crucial step in the synthesis of 8-prenylnaringenin (3) from
d
2
3.9
2
11.0
0, Many prod.e
0
xanthohumol (1) is the demethylation of isoxanthohumol in-
termediate product (2). For the first time, we have used magnesium
and calcium salts for the demethylation of 2. Magnesium iodide
proved to be a powerful, convenient, and inexpensive reagent, in
contrast to the scandium salts.
2
Many prod.e
2
a
Isolated yield, the reaction was carried out with 50 mg of 1 or 2.
Yield based on HPLC analysis.
The reaction was carried out with 400 mg of 2.
b
c
d
6.2 mg of the mixture of mainly two compounds with the same R
f
on TLC was
The starting compound, xanthohumol (1), was isolated from
supercritical carbon dioxide–spent hop by the extraction with ac-
etone, purification on silica gel, and subsequent crystallization.
isolated, yield based on HPLC analysis of this mixture.
e
Complicated product mixture.
9
Treatment with 1% NaOH as described in the literature, acidifica-
ꢀ
Mg(OAc)
Mg(OMe)
2
was used as a donor of weak nucleophilic group AcO .
tion and purification on silica gel gave isoxanthohumol (2) with
ꢀ
was applied as a donor of strong nucleophile MeO (also
8
4% yield (Scheme 1). Scheme 2 shows the route of the demethy-
2
21
a strong base, which reacts with the hydroxyl groups of iso-
xanthohumol). If the reaction could occur, dimethyl ether should be
formed, which as a gas could be eliminated from the reaction
medium. It would promote the reaction. Our attempt at applying
the procedure to xanthohumol demethylation was unsuccessful
and we observed on HPLC and TLC many compounds in the reaction
mixture. It can be connected with the fact that the carbonyl group
and hydroxyl group form a strong intramolecular hydrogen bond in
xanthohumol (1), Scheme 1, and it is not able to create a complex
with magnesium iodide similar to that shown in Scheme 2.
In conclusion, we have developed an efficient and simple
method for the synthesis of 8-prenylnaringenin. Further work to-
ward the demethylation of isoxanthohumol derivatives using
magnesium iodide is underway in our laboratory. Work in progress
will be reported in due course.
lation of 2, based on a previously proposed mechanism. Iso-
xanthohumol (2) forms a complex with MgX . The next step is the
2
subsequent thermal elimination of volatile MeX to form the mag-
nesium salt of 8-prenylnaringenin, which is converted in acidic
conditions into 3.
The results of the demethylation of substrates 1 and 2 are pre-
sented in Table 1. As a reaction medium we have chosen THF, which
improved the solubility of the reagents. In the case of Et
2
O the re-
action occurred very slowly because the reagents, isoxanthohumol
(2) and 8-prenylnaringenin, had low solubility.
We have found that the best yields, 88% and 93%, were obtained
using magnesium iodide etherate under strictly anhydrous condi-
tions. In the case of MgBr and MgCl , the reaction was worse and
9% and 4% yields were obtained, respectively. Although we did not
2
2
3
find in literature the detailed mechanism of this reaction, it seems
that nucleophilic attack of an anion of halogen on the methyl group
is involved in the demethylation reaction. Such a mechanism has
been proposed for the catalytic dealkylation of aryl ethers at the
3. Experimental
2
2
position ortho to the carbonyl group by LiCl. Taking this into
consideration, MgI works the best because the iodide anion is
a more powerful nucleophile than bromide or chloride anions. As
shown in Table 1, CaI was less effective than MgI in promoting the
3.1. General experimental information
2
Spent hop originating from supercritical carbon dioxide
extraction of hop variety Marynka, extracted at the beginning of
December 2005, was obtained from Production Plant of Hop
Extracts of Fertilizer Institute, Pu1awy, Poland. Moisture content of
spent hop was 5.34%. All the remaining reagents were purchased
from Fluka. Analytical thin-layer chromatography was carried out
2
2
demethylation reaction and was able to produce 8-prenylnar-
ingenin with 11% yield after 72 h of reaction. In this case, 73% of
substrate was recovered. Mg(OAc) and Mg(OMe) were not useful
2 2
and we did not detect 8-prenynaringenin after the reaction.
X
X
X
Mg
Mg
Me
Me
O
O
O
O
O
O
O
O
MgX2
- MeX
HO
p-C H OH
HO
O
p-C H OH
HO
p-C H OH
6 4
6
4
6
4
aq NH Cl
4
2
pH~5
H
O
O
O
HO
p-C6H4OH
3
Scheme 2. Demethylation of isoxanthohumol with magnesium halogenates.

9546
M. Anioł et al. / Tetrahedron 64 (2008) 9544–9547
on DC-Alufolien Kieselgel 60 F254 silica gel (0.2 mm; Merck) with
chloroform/methanol (96:4) as the developing solvent. Visualiza-
3.4.1. With MgI
A solution of I
2
2
2
(3 equiv, 859 mg, 3.39 mmol) in anhydrous Et O
tion was effected with a solution of 10 g Ce(SO
4
)
2
and 20 g phos-
(30 mL) and Mg (6 equiv, 165 mg, 6.78 mmol) in a round-bottomed
flask protected from light were stirred at room temperature until
the reaction mixture turned colorless (3 h). The resulting mixture of
magnesium iodide etherate was separated from unreacted Mg and
transferred via syringe into the two-neck flask (250 mL) equipped
phomolybdic acid in 1 L of 10% H SO , followed by heating.
2
4
Preparative column chromatography was accomplished with the
same silica gel (Kiesel 60, 230–400 mesh; Merck). H NMR spectra
were recorded on an NMR Bruker Avance II instrument at 600 MHz
1
with acetone-d
6
as solvent and TMS as an internal standard. IR
2
with condenser under N and containing isoxanthohumol (1 equiv,
spectra in KBr were recorded on a Mattson IR 300 spectrometer.
HPLC/UV analyses were performed on a Waters HPLC system
equipped with 2690 separations module and 996 photodiode array
detector fitted. A reverse phase C-18 column (Waters, Spherisorb
400 mg, 1.13 mmol) in anhydrous THF (80 mL). The reaction mix-
ture was stirred and refluxed for 12 h and next the solvent was
evaporated under reduced pressure to obtain 10 mL of residue.
Saturated solution of NH
mixture was extracted with CH
tracts were dried over Na SO and the solvent was removed under
reduced pressure to give 483 mg of crude product. After purifica-
tion by column chromatography on silica gel (CHCl /MeOH, 98:1)
8-prenylnaringenin (357 mg, 93%) was obtained as a light-yellow
4
Cl (100 mL) was added and the whole
5
m
m ODS2, 4.6ꢁ250) was used. The mobile phase consisted of two
elements: A – MeCN and B – 1% HCOOH in H O. The flow-rate was
set at 1 mL/min and the gradient elution was performed: 0–1 min,
2
Cl
2
(3ꢁ50 mL). The combined ex-
2
2
4
5
5
0% A; 1–16 min, 50–100% A; 16–21 min, 100% A; 21–23 min, 100–
3
0% A; 23–28 min, 50% A. The column temperature was maintained
ꢂ
1
at 28 C. The samples placed in carrousels were thermostated at
solid, HPLC: t ¼7.8 min.;
R
l
max¼292.8 nm and 335.6 nm. H NMR
ꢂ
10 C. Then 2–20
mL of filtered metanolic extracts were injected.
(acetone-d ) d (ppm): 1.60 (s, 3H); 1.61 (s, 3H); 2.76 (dd, 1H,
6
The content of isoxanthohumol and 8-prenylnaringenin was de-
termined at 290 nm or 368 nm for xanthohumol, via external
standard calibration. Moisture content of spent hop was de-
J¼17.0 Hz, J¼3.1 Hz); 3.14 (dd,1H, J¼17.0 Hz, J¼12.7 Hz); 3.22 (d, 2H,
J¼7.3 Hz); 5.19 (t, 1H, J¼7.3 Hz); 5.45 (dd, 1H, J¼12.7 Hz, J¼3.1 Hz);
6.03 (s, 1H); 6.90 (d, 2H, J¼8.6 Hz); 7.41 (d, 2H, J¼8.6 Hz); 8.50 (s,
1H); 9.55 (s, 1H); 12.14 (s, 1H). IR (KBr): 3361, 1636, 1608, 1519, 1437,
ꢂ
termined by loss on drying at 105 C for 1 h in a preheated oven.
ꢀ1
Identity and purity of isolated compounds were confirmed by TLC,
1360, 1171, 1076, 830 cm .
1
HPLC, IR and H NMR.
3
.4.2. With MgBr
The demethylation was carried out similarly as described for
MgI but Br instead of I was used. Isolated yield: 39%.
2
3
.2. Extraction and isolation of xanthohumol (1) from
2
2
2
the spent hop
3
.4.3. With MgCl
The demethylation was carried out similarly as described for
MgI but 3 equiv of anhydrous MgCl was stirred with Et O, THF,
2
Spent hop (1 kg) and 4 L of acetone were shaken at room tem-
perature for 24 h. The acetone extract was filtered and the residue
of spent hop was washed several times with acetone (totally 1 L).
The combined acetone extract was filtered and evaporated. The
residue (30.3 g) was dissolved in 300 mL acetone, 120 g of silica gel
were added with mixing, and solvent was evaporated. The sus-
pension of silica gel, with absorbed spent hop extract in chloroform,
2
2
2
and substrate for 1 h and then refluxed. Mixture of two compounds.
Yield of 3 based on HPLC: 3.9%.
3
.4.4. With CaI
The demethylation was carried out similarly as described for
MgI but Ca instead of Mg was used. Isolated yield: 11%.
2
was subjected to silica gel flash chromatography using CHCl
3
/
2
MeOH (98:2) eluent. Fractions containing xanthohumol were col-
lected and evaporated to give a dark green residue (6.61 g). CH
2
Cl
2
ꢂ
3.4.5. With Mg(OAc)
The demethylation was carried out similarly as described for
2
MgCl but 3 equiv of anhydrous Mg(OAc) or solid Mg(OMe) ,
2 2
or Mg(OMe)
(
25 mL) was added and refluxed for 20 min. After 24 h at 6 C
crystals were filtered and washed three times with 3 mL of CH
2 2
Cl .
2
2
CHCl (30 mL) was added and next methanol was added dropwise,
3
prepared from Mg and MeOH, was used.
until the residue was dissolved. The solvent was evaporated under
reduced pressure (mainly methanol) until the crystalline product
started to appear. After 2 h of crystallization the rest of the solvent
was evaporated slowly and the residue was washed carefully three
3.5. Demethylation of xanthohumol (1)
times with 3 mL of CH
2
Cl
2
. Crystallization and washing were re-
The demethylation was carried out in the same way as described
peated twice to obtain 2.04 g of xanthohumol as a yellow-orange
2
for demethylation of 2 with MgI .
crystalline product. TLC: brown spot after visualization. HPLC:
1
t
R
¼10.5 min;
l
max¼366.4 nm. H NMR spectroscopic data were in
Acknowledgements
12
agreement with those reported for xanthohumol.
.3. Preparation of isoxanthohumol (2)
This compound was obtained from 2 g of 1 by dissolving in 1%
Financial support for this work was provided by the Ministry of
Sciences and Higher Education in Poland (project N N312 279634,
years 2008–2011).
3
9
NaOH and acidification of the reaction mixture with 50% H
The dry, crude product was subjected to flash chromatography
chloroform/methanol, 97:3) to yield 2 (1.68 g, 83.8%), HPLC:
2
SO
4
.
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2
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2
9,23
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1
1
1