Kwakhurin, a unique isoflavone with rejuvenating activity from "kwao keur": Further characterization by 2D-NMR spectrometry and synthesis of triisopropylkwakhurin

Article abstract of DOI:10.3987/com-04-10060

Kwakhurin (5), a characteristic isoflavone component of Pueraria mirifica which has been used in folk medicine as "kwao keur" for rejuvenating purpose, was structurally approached by 2D-NMR experiments. Furthermore, the synthesis of kwakhurin (5) was attempted starting from the methyl ketone (12). Thus, deoxybenzoin derivative (22) was prepared by Friedel-Crafts acylation after protection of the phenolic function with a triisopropyl group. Modification of the substituents, O-prenylation, prenyl 1,3-rearrangement, and O-methylation afforded kwakhurin triisopropyl ether (38).

Full text of DOI:10.3987/com-04-10060

HETEROCYCLES, Vol. 63, No. 6, 2004, pp. 1375 - 1392
Received, 3rd March, 2004, Accepted, 5th April, 2004, Published online, 9th April, 2004
KWAKHURIN, A UNIQUE ISOFLAVONE WITH REJUVENATING
ACTIVITY FROM “KWAO KEUR”: FURTHER CHARACTERIZATION
BY
2D-NMR
SPECTROMETRY
AND
SYNTHESIS
OF
TRIISOPROPYLKWAKHURIN
Misako Iwasaki,a Toshiko Watanabe,a* Tsutomu Ishikawa,a Sunee
Chansakaow,a, § Yoshihiro Higuchi,b and Satoshi Taharac
aGraduate School of Pharmaceutical Sciences, Chiba University, 1-33 Yayoi,
Inage, Chiba 263-8522, Japan; E-mail: toshiw@p.chiba-u.ac.jp
bCentral Research Laboratory, Shiratori Pharmaceutical Co. Ltd., 6-11-24
Tsudanuma, Narashino, Chiba 275-0016, Japan
cGraduate School of Agriculture, Hokkaido University, Kita 9, Nishi 9, Kita-ku,
Sapporo 060-0879, Japan
Abstract - Kwakhurin (5), a characteristic isoflavone component of Pueraria
mirifica which has been used in folk medicine as “kwao keur” for rejuvenating
purpose, was structurally approached by 2D-NMR experiments. Furthermore, the
synthesis of kwakhurin (5) was attempted starting from the methyl ketone (12).
Thus, deoxybenzoin derivative (22) was prepared by Friedel-Crafts acylation after
protection of the phenolic function with a triisopropyl group. Modification of the
substituents, O-prenylation, prenyl 1,3-rearrangement, and O-methylation
afforded kwakhurin triisopropyl ether (38).
INTRODUCTION
A medicinal plant “kwao keur” (known by other Thai native names: kwao krua, kwao khua, and kwao
kreu) has been locally considered to be a rejuvenating drug in Thailand and Burma and at present is
commercially available in some countries including Japan. Therefore, the relation between the chemical
constituents (Figure 1) and estrogenic activity has been widely studied.1 Recently, we2 reported that the
actual estrogenic component of “kwao keur” might be a new (+)-deoxymiroestrol (1), but not the known

(+)-miroestrol (2) because of spontaneous conversion of 1 into 2 in methanol solution. On the other hand,
kwakhurin (5) was at first isolated from the extract of “kwao keur” as compound PM-7 by Tahara,
Ingham and co-workers in 1986,3a and then named kwakhurin after structural elucidation.3b We also
isolated kwakhurin (5) as a minor component in the bioassay-guided isolation of phytoestrogens from the
same plant and it was found that it showed moderate activity, nearly equal to that of daizein (3), but less
than that of genistein (4).4
O
O
HO
R2
OH
H
Me
Me
H
H
R
H
R3
OH
R1
R1
R4
OH
O
HO
O
R2
H
R3
H
R4
H
(+)-deoxymiroestrol (1) : R=H
(+)-miroestrol (2) : R=OH
daizein (3)
H
genistein (4)
kwakhurin (5)
H
H
H
OH
H
3,3-dimethylallyl
OMe OH
Figure 1. Selected Chemical Constituents of Pueraria mirifica.
The source of “kwao keur” is the tuberous root of Pueraria mirifica (Leguminosae); however, the quality
of the commercial product is occasionally in doubt, because the aerial parts of P. mirifica are very similar
to those of other vine legume species. Therefore, it is necessary to determine whether the commercial
“kwao keur” is genuine or not. We focused on kwakhurin (5) as an index of quality standard because of
one characteristic component of P. mirifica. In this paper we present the precise structural assignment of
kwakhurin (5) using 2D-NMR spectral experiments and synthetic approaches to kwakhurin (5).
RESULTS AND DISCUSSION
The spectral data of HMQC and HMBC experiments of kwakhurin (5) are summarized in Table 1,
resulting in reasonable confirmation of the assignment of each signal reported previously.3b
The synthesis of isoflavones can be divided into three main synthetic pathways:5 1) the formylation of
deoxybenzoins, 2) the oxidative rearrangement of chalcones and flavanones, and 3) the arylation of a
preformed chromanone ring. We decided to adopt pathway 1, because the deoxybenzoin derivative (9)
could be easily obtained by acylation of the phenolic compound with arylacetyl halide (Scheme 1).
Retrosynthetically, the 6’-prenyl group in the B-ring of 5 is envisaged to arise via prenyl
1,3-rearrangement of 6. This intermediate originates from phenol (7), which is available by oxidative
conversion of a halogen or an acyl substituent in the B-ring of isoflavone (8).

1
13
Table 1. H- and C-NMR Spectral Data of Kwakhurin (5) (δ ppm).*
4"
5"
Me
Me
3"
8
A
5
8a
4a
HO
O
C
2"
2
1"
6'
OMe
6
4
kwakhurin (5)
B
4'
O
HO 2'
OH
3'
δH (600 MHz, CD3OD)
[reported: 500 MHz, acetone-d6]3b)
δC (150 MHz, CD3OD)
(reported: 60 MHz, acetone-d6)3b)
C#
2
3
4
155.3 (155.5)
120.6 (121.4)
7.74 (s) [7.84 (s)]
177.8 (176.6)
117.3 (118.7)
127.7 (128.3)
4a
5
8.02 (d, J=8.5) [8.02 (d, J=8.5)]
6
115.3 (115.4)
162.7 (163.0)
102.5 (102.4)
6.91 (dd, J=8.5, 2.2) [6.98 (dd, J=8.5, 2.4)]
7
8
6.84 (d, J=2.2) [6.91 (d, J=2.4)]
8a
1'
158.5 (159.0)
111.4 (**)
152.4 (**)
102.3 (103.1)
150.3 (**)
139.6 (**)
2'
3'
6.39 (s) [6.39 (s)]
4'
5'
6'
135.7 (**)
26.9 (27.5)
1"
3.11 (dd, J=14.7, 7.0) [3.09 (dd, J=14.7, 7.3)]
3.30 (dd, J=14.7, 7.0) [3.31 (dd, J=14.7, 6.1)]
2"
3"
4"
123.5 (124.8)
131.3 (130.8)
4.97 (br t, J=6.9) [5.01 (br t, J=6.7)]
25.4 (25.6)
17.5 (17.6)
61.2 (61.1)
1.51 (s) [1.49 (br s)]
1.38 (s) [1.39 (br s)]
3.75 (s) [3.71 (s)]
5"
OMe
*All assignments are based on 2D-NMR (HMQC and HMBC) spectral experiments. Peak
multiplicities in 1H-NMR spctrum are quoted in Hz.
**Five carbons (δ 111.5, 136.3, 140.1, 151.4 and 153.3 ppm) of B-ring were not assigned in
reference 3b.
The isoflavone skeleton (8) is constructed by formylation of 2-hydroxyphenyl phenymethyl ketone (9),
which is prepared from a protected resorcinol (10) by Friedel-Crafts acylation with arylacetyl chloride
(11). An isopropyl group was chosen for phenol protection6 of 2,4-dihydroxyacetophenone (12). Thus,
phenylacetic acid (15) was prepared in good yield by oxidative rearrangement of 13 with thallium
trinitrate (TTN) and perchloric acid in methanol7 followed by alkaline hydrolysis. Friedel-Crafts acylation
of diisopropylresorcinol (16) with 15 after conversion to acid chloride in the presence of tin chloride
(SnCl4) afforded four products including the desired phenyl phenylmethyl ketone (17) (45%).

8
A
5
prenyl 1,3-
rearrangement
HO
O
C
2
RO
O
O
6'
OMe
OH
6
O
B
4'
O
HO 2'
3'
RO
OR
kwakhurin (5)
6
RO
O
RO
RO
O
[O]
OH
OR
Y
Y = Br or
acyl group
O
O
RO
RO
7
OR
8
Friedel-
Crafts
acylation
RO
OH
X
C1-unit
Cl
OR
B
X
A
O
+
RO
OR
O
RO
OR
10
9
11
X= H or Br
Scheme 1. Retrosynthetic Analysis of Kwakhurin (5).
1
The H-NMR spectra of the side products showed that they were the γ-lactone (18) (13%), the
self-coupling product (19) (6%), and the over-coupling product (20) (7%), respectively. To avoid the
formation of these self-acylation products, the reactive C-5 position of the starting (15) was blocked by a
O
X
1
X
5
10%- aq. NaOH
TTN, HClO4
HO2C
Me
MeO2C
B
MeOH
MeOH
OPr-i
15: X = H (84%)
RO
OPr-i
i-PrO
OR
i-PrO
3
82%
i-PrBr
K2CO3
DMF
NBS
SiO2
CHCl3
12: R = H
14: X = H
20: X = Br
21: X = Br (78%)
13: R = i-Pr
85%
99%
i-PrO
OPr-i
X
A
X
B
O
O
B
OPr-i
+
O
OPr-i
1) (COCl)2
i-PrO
18: X = H (13%)
23: X = Br (18%)
17: X = H (45%)
22: X = Br (71%)
i-PrO
OPr-i
2)
A
16
i-PrO
HO2C
i-PrO
OPr-i
i-PrO
OPr-i
OPr-i
SnCl4 / CH2Cl2
O
+
A
B
B
+
B
B
O
O
i-PrO
OPr-i
OPr-i
i-PrO
(6%)
19
(7%)
20
Scheme 2

8
bromo group, which may be available as an oxygen synthon. Bromination of ester (14) with NBS-SiO2
followed by hydrolysis yielded 5-bromophenylacetic acid (21). Friedel-Crafts acylation of 16 with acyl
chloride from 21 smoothly proceeded to give deoxybenzoin (22) in 71% yield; however, concomitant
formation of γ-lactone (23) was inevitable even in low yield (18%) (Scheme 2).
Next, selective deprotection of tetraisopropoxy derivatives (17) and (22) was tried (Table 2). Previously,
we reported the successful use of magnesium iodide (MgI2) as a reagent for selective demethylation of an
aryl methoxy group in the ortho position to the acyl group.9 Thus, treatment of deoxybenzoin (17) with
MgI2 proceeded smoothly to give phenol (26) in good yield (entries 1 and 2), but a complex mixture was
formed in the case of the bromo derivative (22) (entry 3).
We have found that protic acids in AcOH act as efficient deisopropylating agents.10 The use of
concentrated HCl led to desired deprotection in moderate yield (entry 4). On the other hand, higher yield
(71%) was observed when concentrated H2SO4 was used (entry 5).
Table 2. Selective Deisopropylation of Deoxybenzoin Derivatives (17 and 22).
i-PrO
OPr-i
i-PrO
OH
MgI2 6H2O
or H+
X
X
O
O
OPr-i
17: X = H
22: X = Br
OPr-i
26: X = H
27: X = Br
i-PrO
i-PrO
Yield (%)
27
Condition
Reagent
Solvent
entry
X =
Time (h) 26
Temp.
H
H
MgI2 6H2O
MgI2 6H2O
benzene
toluene
1
2
70
72
reflux
reflux
70
20
Br
Br
Br
3
4
5
MgI2 6H2O
conc. HCl
toluene
AcOH
AcOH
reflux
60 oC
a complex mixture
5
22
4
46
71
50 oC
conc. H2SO4
For C-ring construction of an isoflavone skeleton, treatment of 27 with ethyl oxalyl chloride in pyridine11
provided a mixture of an expected ester (28) and a hydrolyzed product (29) in nearly equal amount, and
the former was completely converted to the latter by alkaline hydrolysis. Thermal decarboxylation of (29)
1
afforded isoflavone (30), in which C-2 proton was newly born at δ 7.91 ppm in the H-NMR spectrum.
The isoflavone (30) was also obtained by formylation with N,N-dimethylformamide dimethylacetal
(DMF-DMA)12 followed by cyclization with silica gel. The yield was quantitative; however, the use of

DMF- POCl313 in place of DMF-DMA resulted in the formation of a complex mixture (Scheme 3).
i-PrO
OH
i-PrO
O
1) DMF-DMA
Br
Br
2) SiO2-benzene
quant.
O
O
i-PrO
OPr-i
OPr-i
27
i-PrO
30
ClCOCO2Et
pyridine
86%
∆
36%
33%
i-PrO
i-PrO
O
O
CO2H
CO2Et
O
5%-aq. Na2CO3
Br
Br
EtOH
96%
O
OPr-i
i-PrO
OPr-i
i-PrO
28
29
Scheme 3
Since all trials for conversion of the bromo substituent of 30 to a phenolic function in isoflavone (31) via
metallation, boration or formylation failed, the bromo group was reductively removed with tributyltin
hydride to afford 33 in 97% yield. Although Vilsmeier formylation of 33 was unsuccessful, introduction
of an acetyl group under the conditions of Friedel-Crafts acylation with acetic anhydride in the presence
of SnCl4 smoothly proceeded to give acetyl compound (34). Baeyer-Viliger oxidation of 34 followed by
hydrolysis yielded the desired phenolic compound (31) (Scheme 4).
1) n-BuLi or Mg
2) B(OPr-i)3
i-PrO
O
3) AcOH, H2O2
30
1) n-BuLi
2) DMF
OH
[O]
O
i-PrO
i-PrO
O
OPr-i
Bu3SnH
AIBN
benezene
CHO
OPr-i
31
97%
O
i-PrO
K2CO3
MeOH
63%
32
POCl3-DMF
toluene
i-PrO
O
i-PrO
O
H2O2
HCOOH
Ac2O
SnCl4
O
Me
O
Ar
O
Me
OPr-i
O
i-PrO
CH2Cl2
92%
O
i-PrO
88%
OPr-i
i-PrO
OPr-i
33
34
35
Scheme 4

After prenylation of phenol (31) with prenyl bromide and potassium carbonate, a prenyl
1,3-rearrangement was examined. The reaction of prenyl ether (36) with montmorillonite-KSF clay14 in
benzene yielded the desired ether (37) in 31% yield along with phenol (31) (41%). In other solvent
systems such as THF and CH2Cl2, or under sonication conditions no improvement of the reaction was
observed. Conventional methylation of 37 afforded triisopropylkwakhurin (38) (Scheme 5).
1
In the H-NMR spectrum this compound showed very similar signal patterns to those of kwakhurin
triethyl ether prepared from natural kwakhurin (5) by Tahara et al.3b) We attempted deprotection of the
triisopropyl group in 38 under various conditions, but unfortunately no production of kwakhurin (5) was
observed.15 Further efforts are underway to complete the synthesis of kwakhurin (5) itself.
i-PrO
(CH3)2C=CHCH2Br
i-PrO
montmorillonite-
O
O
OH
O
K2CO3
KSF
+
31
(41%)
O
i-PrO
31
O
i-PrO
OPr-i
OPr-i
benzene
DMF
76%
(31%)
37
36
Me2SO4
K2CO3
i-PrO
O
OMe
OPr-i
kwakhurin (5)
DMF
87%
O
i-PrO
38
Scheme 5
CONCLUSION
The structure of kwakhurin (5) was further confirmed by the inspection of the 2D-NMR spectra.
Triisopropylkwakhurin (38) was synthesized from 12 in 3.4% overall yield. Although successful results
were not obtained in the final deprotection step, this method could provide a synthetic route to
biologically active isoflavones from easily prepared deoxybenzoin compounds.
EXPERIMENTAL
All melting points were measured on a melting-point hot stage MP-3S (Yanaco) and are uncorrected.
1
13
Infrared spectra were recorded on a JASCO FT/IR-300E spectrophotometer. H- and C-NMR spectra
were recorded in CDCl3 unless otherwise stated, on JEOL JNM GSX-400A, GSX-500A, ECP-400 and
ECP-600 spectrometers with tetramethylsilane (TMS) as internal reference. MS spectra were recorded on

a JEOL JMS-AM20 (EI), and JEOL JMS-AX-500 and JMS-HX 10A (FAB). For column chromatography,
silica gel (particle size: 100 µm) (FL-100D, Fuji Silysia) was used unless otherwise stated.
Isolation and Structural Determination of Kwakhurin (5)
Isolation of kwakhurin (5) from Pueraria mirifica was described in the previous report.4 The precise
structural assignment of 5 was obtained from 2D-NMR (HMQC and HMBC) spectral experiments (Table
1).
2,4-Diisopropoxyacetophenone (13). To a mixture of 2,4-dihydroxyacetophenone (12) (15.0 g, 99
mmol) and K2CO3 (109 g, 789 mmol) was added a solution of isopropyl bromide (32 mL, 341 mmol) in
o
DMF (150 mL). The whole mixture was sirred at 55 C for 3.3 h under argon atmosphere, poured into
water (750 mL), and extracted with AcOEt (400 mL + 200 mL x 2). The organic layer was washed with
water (250 mL x 3) and brine (250 mL), dried over MgSO4, and evaporated to dryness in vacuo to afford
a yellow oil (28.4 g), which was purified by distillation (bp 142-146 oC/9 mmHg) to give 13 as a colorless
1
oil (19.9 g, 85%). IR νmax cm-1 (neat): 1664 (C=O). H-NMR (400 MHz) δ : 1.35 [6H, d, J=6.0 Hz,
CH(CH3)2], 1.41 [6H, d, J=6.0 Hz, CH(CH3)2], 2.58 (3H, s, CH3), 4.61 [1H, sep, J=6.0 Hz, O-CH(CH3)2],
4.64 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 6.41 (1H, d, J=2.2 Hz, C3-H), 6.47 (1H, dd, J=8.8, 2.2 Hz, C5-H),
7.80 (1H, d, J=8.8 Hz, C6-H). HRFAB-MS m/z: 237.1471 (Calcd for C14H21O3: 237.1490). EI-MS m/z:
236 (M+, 100%).
Methyl 2,4-Diisopropoxyphenylacetate (14). To a solution of TTN (10.1 g, 22.7 mmol) in MeOH (17.2
mL) was added 60% aq. HClO4 (11.4 mL, 113 mmol) and a solution of acetophenone (13) (4.87 g, 20.6
mmol) in MeOH (84.5 mL), and the whole mixture was stirred at rt for 4.3 h under argon atmosphere.
The reaction mixture was poured into water (500 mL) and extracted with AcOEt (3 x 200 mL). The
organic layer was washed with water and brine, dried over MgSO4 and evaporated to dryness in vacuo to
yield a brown oil (5.67 g). The crude oil was purified by silica gel column chromatography
(hexane-AcOEt, 5:1) to give a pale yellow oil (14) (4.48 g, 82%). IR νmax (neat) cm-1: 1741 (C=O).
1H-NMR (500 MHz) δ: 1.29 [6H, d, J=6.1 Hz, CH-(CH3)2], 1.33 [6H, d, J=6.1 Hz, CH-(CH3)2], 3.52 (2H,
s, Ar-CH2-COOCH3), 3.67 (3H, s, COO-CH3), 4.49 and 4.51 [each 1H, sep, J=6.1 Hz, O-CH-(CH3)2],
6.41 (1H, dd, J=7.3, 2.4 Hz, C5-H), 6.42 (1H, d, J=2.4 Hz, C3-H), 7.04 (1H, d, J=7.3 Hz, C6-H).
2,4-Diisopropoxyphenylacetic Acid (15). To a solution of ester (14) (4.25 g, 16.0 mmol) in MeOH (85

mL) was added 10% aq. NaOH (85 mL), and the whole mixture was stirred at rt for 1.5 h. The reaction
mixture was poured into AcOEt (200 mL) and extracted with water (400 mL). The organic layer was
extracted with10% aq. NaOH (2 x 100 mL). The combined alkaline solution was acidified (pH ca.1) with
10% aq. HCl and extracted with AcOEt (2 x 200 mL + 100 mL). The organic layer was washed with
water and brine, dried over MgSO4, and evaporated to dryness in vacuo to afford pale yellow prisms (3.70
o
g). The crude solid was washed with hexane to yield colorless prisms (15) (3.38 g, 84%, mp 81-83 C),
which were further purified by recrystallization from hexane to give 15 as colorless prisms (mp 80.5-82
oC). IR νmax (Nujol) cm-1: 3400-2400 (OH), 1709 (C=O). 1H-NMR (500 MHz) δ: 1.31 and 1.33 [each 6H,
d, J= 6.1 Hz, CH-(CH3)2], 3.56 (2H, s, Ar-CH2-COOH), 4.51 and 4.53 [each 1H, sep, J= 6.1 Hz,
O-CH-(CH3)2], 6.42 (1H, dd, J= 8.2, 2.4 Hz, C5-H), 6.44 (1H, d, J= 2.4 Hz, C3-H), 7.06 (1H, d, J= 8.2 Hz,
C6-H). Anal. Calcd for C14H20O4: C, 66.64; H, 7.99. Found: C, 66.37; H, 7.93.
1,3-Diisopropoxybenzene16 (16). To a mixture of 1,3-dihydroxybenzene (10.0 g, 90.9 mmol), K2CO3
(75.3 g, 545 mmol) and DMF (100 mL) was added i-PrBr (25.5 mL, 272 mmol), and the whole mixture
was stirred at 55 oC for 12.5 h under argon atmosphere. The reaction mixture was poured into water (500
mL) and extracted with AcOEt (300 mL + 2 x 150 mL). The organic layer was washed with water (3 x
200 mL) and brine (200 mL), dried over MgSO4, and evaporated to dryness in vacuo to afford a yellow
o
o
oil (21.1 g). The crude oil was purified by distillation [bp 88-90 C /6 mmHg (lit.16 bp 89.5-95 C /3
1
mmHg)] to give 16 as a colorless oil (13.6 g, 77%). H-NMR (400 MHz) δ: 1.33 [12H, d, J=6.2 Hz,
CH(CH3)2], 4.52 [2H, sep, J=6.2 Hz, O-CH(CH3)2], 6.44 (1H, d, J=2.2 Hz, C2-H), 6.46 (2H, dd, J=7.9,
2.2 Hz, C4-H and C6-H), 7.14 (1H, t, J=7.9 Hz, C5-H).
Friedel-Crafts Acylation of 2,4-Diisopropoxybenzene (16) with 2,4-Diisopropoxyphenylacetic Acid
(15). A mixture of acid (15) (100 mg, 0.40 mmol) and (COCl)2 (0.1 mL, 1.18 mmol) was stirred at rt for 2
h. The excess of (COCl)2 was removed by evaporation in vacuo to yield 2,4-diisopropoxyphenylacetyl
chloride as a yellow viscous liquid. [IR νmax (neat) cm-1: 1803 (C=O)].
To a solution of the acyl chloride in CH2Cl2 (4.8 mL) was added a solution of 2,4-diisopropoxybenzene
(16) (76.6 mg, 0.39 mmol) in CH2Cl2 (4.8 mL) under ice-cooling, then added 1.0 M SnCl4 / CH2Cl2 (0.44
mL, 0.44 mmol) at -78 oC. The whole mixture was stirred at -78 oC for 18.5 h. The reaction mixture was
poured into water (30 mL) and extracted with AcOEt (3 x 70 mL). The organic layer was washed with sat.
aq. NaHCO3 and water, dried over K2CO3 and evaporated to dryness in vacuo to afford a yellow oil (151
mg). The crude product was purified by silica gel column chromatography (hexane-AcOEt, 20:1 to 2:1) to

give 17 (75.6 mg, 45%) as a pale yellow oil. Further elution gave recovery of starting
1,3-diisopropoxybenzene (16) (22.7 mg, 30%), lactone (18) (9.7 mg, 13%), dimer (19) (6.0 mg, 6.3%), and
trimer 20 (10.0 mg, 7.0%).
i) 2,4-Diisopropoxyphenyl 2’,4’-Diisopropoxybenzyl Ketone (17). A pale yellow oil. IR νmax (neat)
1
cm-1: 1672 (C=O). H-NMR (500 MHz) δ: 1.20, 1.32, 1.35 and 1.39 [each 6H, d, J=6.1 Hz,
O-CH-(CH3)2], 4.17 (2H, s, CO-CH2), 4.44, 4.50, 4.60 and 4.64 [each 1H, sep, J= 6.1 Hz, O-CH-(CH3)2],
6.40-6.43 (3H, m, C3,3' and 5'-H), 6.47 (1H, dd, J= 8.9, 2.1 Hz, C5-H), 7.02 (1H, d, J= 8.9 Hz, C6'-H), 7.73
(1H, d, J= 8.9 Hz, C6-H).
1
ii) 2,3-Dihydro-6-isopropoxybenzo[b]furan-2-one (18). A brown powder. H-NMR (500 MHz) δ: 1.34
[6H, d, J=6.1 Hz, CH(CH3)2], 3.67 (2H, s, -CO-CH2), 4.52 [1H, sep, J=6.1 Hz, O-CH(CH3)2], 6.65 (1H,
dd, J=8.2, 2.4 Hz, C5-H), 6.67 (1H, d, J=2.4 Hz, C7-H), 7.14 (1H, d, J=8.2 Hz, C4-H).
iii) 2,4-Diisopropoxy-5-[(2’,4’-diisopropoxyphenyl)acetyl]phenylacetic Acid (19). A pale yellow oil.
1H-NMR (600 MHz) δ: 1.20, 1.31, 1.35, 1.40 [each 6H, d, J=6.1 Hz, CH(CH3)2], 3.55 (2H, s,
ArCH2-COOH), 4.16 (2H, s, ArCH2COAr’), 4.44, 4.50, 4.60, 4.64 [each 1H, sep, J=6.1 Hz,
O-CH(CH3)2], 6.39-6.42 (3H, m, Ar-H), 7.01 (1H, d, J=7.4 Hz, C6’-H), 7.63 (1H, s, C6-H), no signal for
COOH.
iv) 2,4-Diisopropoxyphenyl 2’,4’-Diisopropoxy-5’-[(2”,4”-Diisopropoxyphenyl)acetyl]benzyl Ketone
1
(20). A pale yellow oil. H-NMR (500 MHz) δ: 1.22, 1.25, 1.32, 1.35, 1.39, 1.40 [each 6H, d, J=6.1 Hz,
CH(CH3)2], 4.16, 4.19 (each 2H, s, ArCH2COAr’), 4.41-4.68 [6H, m, O-CH(CH3)2], 6.39-6.50 (5H, m,
Ar-H), 6.99 (1H, d, J=8.0 Hz, C6”-H), 7.61 (1H, s, C6’-H), 7.70 (1H, d, J=8.8 Hz, C6-H).
Methyl 5-Bromo-2,4-diisopropoxyphenylacetate (20). To a solution of phenylacetate (14) (1.00 g, 3.76
mmol) in CHCl3 (30 mL) was added SiO2 (Micro Bead 3A, Fuji Silysia) (3.30 g) and NBS (0.669 g, 3.76
mmol), and the whole mixture was stirred at rt for 70 min. After filtration of SiO2 and washing with
CHCl3 (120 mL), the combined organic layer was washed with 10% aq. Na2S2O3 (60 mL), dried over
MgSO4 and evaporated to dryness in vacuo to afford 20 (1.29 g, 99%) as a labile yellow oil, which was
used for the next step without further purification. IR νmax (neat) cm-1: 1736 (C=O). 1H-NMR (500 MHz)
δ: 1.30 [6H, d, J=6.0 Hz, CH-(CH3)2], 1.37 [6H, d, J=6.0 Hz, CH-(CH3)2], 3.50 (2H, s, CO-CH2-), 3.68
(3H, s, O-CH3), 4.48 [2H, sep, J=6.0 Hz, O-CH-(CH3)2], 6.48 (1H, s, C3-H), 7.29 (1H, s, C6-H).
5-Bromo-2,4-diisopropoxyphenylacetic Acid (21). To a solution of bromophenylacetate (20) (1.29 g,
3.72 mmol) in MeOH (14 mL) was added 10% aq. NaOH (14 mL, 3.50 mmol), and the whole mixture

was stirred at rt for 3.5 h. After evaporation of MeOH the residue was dissolved in AcOEt (70 mL). The
organic layer was extracted with water (50 mL + 3 x 30 mL) and 10% aq. NaOH (3 x 30 mL). The
combined aqueous solution was acidified (pH 1) with conc. HCl and extracted with AcOEt (150 mL + 2 x
100 mL). The organic layer was washed with water and brine (each 100 mL), dried over MgSO4 and
evaporated to dryness in vacuo to afford pale yellow prisms (1.09 g), which were recrystallized from
o
hexane to give acid (21) (0.961 g, 78%) as colorless prisms (mp 98-100 C). IR νmax (Nujol) cm-1:
1
3100-2500 (OH), 1704 (C=O). H-NMR (400 MHz) δ: 1.32 [6H, d, J=6.0 Hz, CH-(CH3)2], 1.38 [6H, d,
J=6.2 Hz, CH-(CH3)2], 3.54 (2H, s, CH2-CO-), 4.48 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 4.50 [1H, sep,
J=6.2 Hz, O-CH (CH3)2], 6.49 (1H, s, C3-H), 7.32 (1H, s, C6-H), no signal of hydroxy proton was
observed. Anal. Calcd for C14H19O4Br: C, 50.77; H, 5.78. Found: C, 50.82; H, 5.90.
Friedel-Crafts Acylation of 1,3-Diisopropoxybenzene (16) with 5-Bromo-2,4-diisopropoxyphenyl-
acetic Acid (21). A mixture of bromoacid (21) (0.653 g, 1.97 mmol) and (COCl)2 (0.47 mL, 5.55 mmol)
was stirred at rt for 2.5 h. The excess of (COCl)2 was removed by evaporation in vacuo to afford
5-bromo-2,4-diisopropoxyphenylacetyl chloride as a pale green solid. [IR νmax (Nujol) cm-1: 1786 (C=O)].
To a solution of the acyl chloride in CH2Cl2 (3.7 mL) was added at -78 oC 1,3-diisopropoxybenzene (16)
(0.389 g, 2.00 mmol) followed by 1.0 M solution of SnCl4 in CH2Cl2 (8.1 mL, 8.1 mmol), and the whole
o
mixture was stirred at -78 C for 65 min. The reaction mixture was poured into water (200 mL) and
extracted with AcOEt (100 mL + 2 x 70 mL). The organic layer was washed with sat. aq. NaHCO3 and
water (each 70 mL), dried over K2CO3, and evaporated to dryness in vacuo to give a dark green oil (1.01
g). The crude product was purified by silica gel column chromatography (hexane-AcOEt, 20:1 to 2:1) to
afford 22 (0.707 g, 71%) and lactone (23) (0.0974 g, 18%).
i) 5’-Bromo-2’,4’-diisopropoxybenzyl 2,4-Diisopropoxyphenyl Ketone (22). A pale yellow oil. IR νmax
1
(neat) cm-1: 1671(C=O). H-NMR (400 MHz) δ: 1.22 [6H, d, J=6.0 Hz, O-CH-(CH3)2], 1.36 [6H, d,
J=6.0 Hz, O-CH-(CH3)2], 1.37 [6H, d, J=6.0 Hz, O-CH-(CH3)2], 1.40 [6H, d, J=6.0 Hz, O-CH-(CH3)2],
4.14 (2H, s, CO-CH2), 4.42, 4.46, 4.60, 4.65 [each 1H, sep, J=6.0 Hz, O-CH-(CH3)2], 6.42 (1H, d, J=2.2
Hz, C3-H), 6.47 (1H, dd, J=8.8, 2.2 Hz,C5-H), 6.49 (1H, s, C3’-H), 7.28 (1H, s, C6’-H), 7.74 (1H, d, J=8.8
Hz, C6-H).
1
ii) 5-Bromo-2,3-dihydro-6-isopropoxybenzo[b]furan-2-one (23). A yellow powder. H-NMR (400
MHz) δ: 1.40 [6H, d, J=6.0 Hz, CH(CH3)2], 3.693 (1H, d, J=1.1 Hz, C3-Ha), 3.694 (1H, d, J=1.1 Hz,
C3-Hb), 4.53 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 6.73 (1H, s, C7-H), 7.44 (1H, t, J=1.1 Hz, C4-H).

2-Hydroxy-4-isopropoxyphenyl 2,4-Diisopropoxybenzyl Ketone (26). To a solution of ketone (17)
(75.6 mg, 0.176 mmol) in toluene (4.5 mL) was added MgI2・6H2O (79.5 mg, 0.206 mmol). The whole
o
mixture was refluxed using Dean-Stark apparatus for 20 h (bath temperature 150 C). The reaction
mixture was poured into water (30 mL) and extracted with AcOEt (3 x 70 mL). The organic layer was
washed with water and brine, dried over MgSO4 and evaporated to dryness in vacuo to give a brown oil
(80.8 mg). The crude product was purified by silica gel column chromatography (hexane-AcOEt, 20:1) to
afford 2-hydroxy ketone (26) as a pale yellow oil (48.9 mg, 72%). IR νmax (neat) cm-1: 1629 (C=O).
1H-NMR (600 MHz) δ: 1.25 [6H, d, J= 6.0 Hz, CH-(CH3)2], 1.32 [6H, d, J= 6.0 Hz, CH-(CH3)2], 1.35
[6H, d, J= 6.0 Hz, CH-(CH3)2], 4.08 (2H, s, CO-CH2), 4.50 [2H, sep, J= 6.0 Hz, O-CH-(CH3)2], 4.59 [1H,
sep, J= 6.0 Hz, O-CH-(CH3)2], 6.30-6.48 (4H, m, C3, 5, 3' and 5'-H), 7.09 (1H, d, J= 8.5 Hz, C6'-H), 7.86 (1H,
d, J= 9.3 Hz, C6-H), 12.81 (1H, s, -OH).
5'-Bromo-2',4'-diisopropoxybenzyl 2-Hydroxy-4-isopropoxyphenyl Ketone (27). A mixture of
ketone (22) (0.325 g, 0.64 mmol), conc. H2SO4 (0.82 mL, 14.9 mmol) and AcOH (26 mL) was stirred at
50 oC for 4 h. The reaction mixture was poured into water (50mL) and extracted with AcOEt (3 x 30 mL).
The organic layer was washed with 5% aq. NaHCO3 (30 mL) and 2% aq. NaOH (3 x 30 mL + 10 mL),
dried over MgSO4, and evaporated to dryness in vacuo to afford a brown oil (0.348 g). The crude product
was purified by silica gel column chromatography (hexane-AcOEt, 30:1) to give pale yellow crystals
o
(0.213 g, 71%) which were recrystallized from hexane to afford 27 as colorless prisms (mp 89-90.5 C).
IR νmax (Nujol) cm-1: 1625 (C=O). 1H-NMR (400 MHz) δ: 1.25 [6H, d, J=6.8 Hz, CH(CH3)2], 1.36 [6H, d,
J=6.8 Hz, CH(CH3)2], 1.39 [6H, d, J=6.2 Hz, CH(CH3)2], 4.07 (2H, s, CO-CH2), 4.44-4.52 [2H, m,
O-CH-(CH3)2], 4.60 [1H, sep, J=6.2 Hz, O-CH-(CH3)2], 6.38-6.41 (2H, m, Ar-H), 6.49 (1H, s, C3’-H),
7.35 (1H, s, C6’-H), 7.81 (1H, d, J= 9.7 Hz, C6-H), 12.71 (1H, s, -OH). Anal. Calcd for C23H29O5Br: C,
59.36; H, 6.28. Found: C, 59.54; H, 6.27.
Ethyl 5'-Bromo-7,2',4'-triisopropoxyisoflavone-2-carboxylate (28). To a solution of ketone (27) (1.256
g, 2.70 mmol) in pyridine (12.8 mL) was added ClOCCO2Et (1.27 mL, 11.21 mmol) under ice-cooling.
The whole mixture was heated at 120 oC for 7.5 h. The reaction mixture was poured into water (200 mL)
and extracted with CHCl3 (150 mL + 2 x 70 mL). The organic layer was washed with 10% HCl (200 mL),
dried over MgSO4, and evaporated to dryness in vacuo to afford brown oil (2.093 g). The crude product
was separated by silica gel column chromatography (hexane-AcOEt, 20:1 to 2:1, then AcOEt) to afford
two fractions:

i) From the less polar fraction: a yellow solid (28) (0.525 g, 36%), which was recrystallized from
o
ether-hexane to give isoflavone (28) as pale yellow prisms (mp 140-141 C). IR νmax (Nujol) cm-1: 1733,
1
1648 (C=O). H-NMR (400 MHz) δ: 1.07 (3H, t, J=7.1 Hz, OCH2-CH3), 1.14 [3H, d, J=6.0 Hz,
CH(CH3)2], 1.22 [3H, d, J=6.0 Hz, CH(CH3)2], 1.40-1.42 [12H, m, CH(CH3)2], 4.11-4.23 (2H, m,
OCH2CH3), 4.35 [1H, sep, J=6.0 Hz, O-CH-(CH3)2], 4.53 [1H, sep, J=6.0 Hz, O-CH-(CH3)2], 4.67 [1H,
sep, J=6.1 Hz, O-CH(CH3)2], 6.53 (1H, s, C3’-H), 6.94-6.97 (2H, m, C6-H, C8-H), 7.38 (1H, s, C6’-H),
8.11 (1H, d, J=8.6 Hz, C5-H). Anal. Calcd for C27H31O7Br: C, 59.24; H, 5.71. Found: C, 59.12; H, 5.68.
ii) From the polar fraction: a brown solid (29) (0.469 g, 33%), which was identical with the sample
described below.
5'-Bromo-7,2',4'-triisopropoxyisoflavone-2-carboxylic Acid (29). To a solution of ester (28) (0.171 g,
0.31 mmol) in EtOH (6 mL) was added 5% aq. Na2CO3 (6.64 mL, 3.13 mmol), and the whole mixture
o
o
was heated at 50 C for 4.5 h and 70 C for 1.5 h. The reaction mixture was dissolved in ether (80 mL)
and extracted with water (50 mL) and 10% aq. NaOH (40 mL + 2 x 20 mL). The combined aqueous layer
was acidified (pH 1) with conc. HCl and extracted with AcOEt (100 mL + 2 x 70 mL). The orgnic layer
was washed with water and brine (each 70 mL), dried over MgSO4, and evaporated to dryness in vacuo to
afford acid (29) (0.157 g, 96%) as pale yellow prisms. This compound was used for the next step without
further purification. A part of the sample was recrystallized from ether-hexane to give pale yellow prisms
o
1
(mp 216-220.5 C). IR νmax (Nujol) cm-1: 1742, 1625 (C=O). H-NMR (500 MHz) δ: 1.16 [3H, d, J=6.1
Hz, CH(CH3)2], 1.24 [3H, d, J=6.1 Hz, CH(CH3)2], 1.35-1.43 [12H, m, CH(CH3)2], 4.39 [1H, sep, J=5.8
Hz, O-CH(CH3)2], 4.54 [1H, sep, J=5.8 Hz, O-CH(CH3)2], 4.67 [1H, sep, J=6.1 Hz, O-CH(CH3)2], 6.53
(1H, s, C3’-H), 6.94 (1H, d, J=2.1 Hz, C8-H), 6.97 (1H, dd, J=8.9, 2.1 Hz, C6-H), 7.40 (1H, s, C6’-H), 8.11
(1H, d, J=8.9 Hz, C5-H), no signal for COOH. Anal. Calcd for C25H27O7Br: C, 57.81; H, 5.24. Found: C,
57.79; H, 5.27.
5'-Bromo-7,2',4'-triisopropoxyisoflavone (30)
i) Decarboxylation of acid 29. Acid (29) (0.201 g, 0.39 mmol) was heated at 230 oC for 0.5 h. The crude
brown oil (0.185 g) was purified by silica gel column chromatography (hexane-AcOEt, 20:1 to 2:1) to
o
afford pale yellow prisms (30) (0.158 g, 86%, mp 116.5-121.5 C). A part of the sample was
o
recrystallized from cyclohexane-hexane to give 30 as pale yellow prisms (mp 119.5-120 C). IR νmax
(Nujol) cm-1: 1638 (C=O). 1H-NMR (600 MHz) δ: 1.25 [6H, d, J=6.0Hz, CH(CH3)2], 1.41 and 1.42 [each
6H, d, J=6.0 Hz, CH(CH3)2], 4.41 [1H, sep, J=6.0 Hz, CH(CH3)2], 4.53 [1H, sep, J=6.0 Hz, O-CH(CH3)2],

4.67 [1H, sep, J=4.4 Hz, O-CH(CH3)2], 6.57 (1H, s, C3'-H), 6.83 (1H, d, J=2.2 Hz, C8-H), 6.94 (1H, dd,
J=9.0, 2.2 Hz, C6-H), 7.54 (1H, s, C6'-H), 7.91 (1H, s, C2-H), 8.17 (1H, d, J=9.0 Hz, C5-H). Anal. Calcd
for C24H27O5Br: C, 60.64; H, 5.72. Found: C, 60.68; H, 5.71.
ii) One pot synthesis from 27. To a solution of ketone (27) (3.00 g, 6.45 mmol) in dry benzene (135 mL)
o
was added Me2CH(OMe)2 (1.7 mL, 12.8 mmol), and the whole mixture was heated at 100 C for 3 h
under argon atmosphere. After disappearance of the starting ketone (27) on TLC, silica gel (FL100D,
15.00 g) was added to the reaction mixture, and the whole was stirred at rt for 2.3 h. The insoluble
material was removed by filtration and washed with AcOEt. The combined filtrate was washed with 3.3%
aq. HCl (2 x 60 mL), water (60 mL), and brine (60 mL), dried over MgSO4, and evaporated to dryness in
o
vacuo to afford 30 (3.07 g, 100%) as a yellow powder (mp 113-119 C). This compound was identical
with the sample prepared from 29 described above.
7,2’,4’-Triisopropoxyisoflavone (33). To a solution of 5’-bromoisoflavone (30) (0.443 g, 0.93 mmol)
and AIBN (0.019 g, 0.12 mmol) in dry toluene (5.3 mL) was added Bu3SnH (0.43 mL, 1.60 mmol) and
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o
the whole mixture was heated at 115 C for 5 h and at 140 C for 1.5 h. For completing the reaction
additional reagent was added as follows: Bu3SnH (0.08 mL, 0.30 mmol) and a trace amount of AIBN in
toluene (0.2 mL), 3.5 h, then Bu3SnH (0.25 mL, 0.93 mmol), AIBN in toluene (2 mL), 1 h. The reaction
mixture was diluted with pentane (10 mL) and the soluble fraction was evaporated to dryness in vacuo to
give a yellow oil (1.406 g). The crude product was purified by column chromatography (hexane, benzene,
benzene-AcOEt, 20:1 to 2:1) to yield a pale yellow oil (33) (0.358 g, 97%). IR νmax cm-1 (neat):
1
1641(C=O). H-NMR (400 MHz) δ: 1.25 [6H, d, J=6.0 Hz, CH(CH3)2], 1.36 [6H, d, J=6.0 Hz,
CH(CH3)2], 1.40 [6H, d, J=6.0 Hz, CH(CH3)2], 4.45 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 4.56 [1H, sep,
J=6.0 Hz, O-CH(CH3)2], 4.66 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 6.52 (1H, s, C3'-H), 6.53 (1H, d, J=8.0
Hz, C5'-H), 6.83 (1H, d, J=2.4 Hz, C8-H), 6.93 (1H, dd, J=8.8, 2.4 Hz, C6-H), 7.30 (1H, d, J=8.0 Hz,
C6'-H), 7.93 (1H, s, C2-H), 8.18 (1H, d, J=8.8 Hz, C5-H). HRFAB-MS m/z: 397.2024 (Calcd for
C24H29O5: 397.2015). EI-MS m/z: 396 (M+, 100%).
5'-Acetyl-7,2',4'-triisopropoxyisoflavone (34). To a solution of isoflavone (33) (0.468 g, 1.18 mmol) in
CH2Cl2 (1.4 mL) was added Ac2O (0.33 mL, 3.5 mmol) followed by a solution of SnCl4 (0.41 mL, 3.50
mmol) in CH2Cl2 (0.7 mL) at -78 oC, and the whole mixture was stirred at -40 oC for 3 h and at -25 oC for
1.5 h. The reaction mixture was poured into water (20 mL) and extracted with CH2Cl2 (30 mL, 2 x 10
mL). The organic layer was washed with water (15 mL) and brine (15 mL), dried over MgSO4, and

evaporated to dryness in vacuo to afford a yellow solid (0.477 g, 92%). Recrystallization from ether gave
o
acetyl compound (34) (0.330 g, 64%) as pale a yellow powder (mp 138-140 C). IR νmax cm-1 (KBr):
1
1657, 1639 (C=O). H-NMR (400 MHz) δ: 1.30 [6H, d, J=6.0 Hz, CH(CH3)2], 1.41 [6H, d, J=6.0 Hz,
CH(CH3)2], 1.45 [6H, d, J=6.0 Hz, CH(CH3)2], 2.60 (3H, s, CO-CH3), 4.56 [1H, sep, J=6.0 Hz,
O-CH(CH3)2], 4.67 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 4.69 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 6.48 (1H,
s, C3'-H), 6.83 (1H, d, J=2.4 Hz, C8-H), 6.93 (1H, dd, J=9.0, 2.4 Hz, C6-H), 7.78 (1H, s, C2- or C6'-H),
7.83 (1H, s, C6'- or C2-H), 8.15 (1H, d, J=9.0 Hz, C5-H). EI-MS m/z: 438 (M+, 29%), 297(100%). Anal.
Calcd for C26H30O6: C, 71.21; H, 6.90. Found: C, 71.36; H, 7.00.
5'-Hydroxy-7,2',4'-triisopropoxyisoflavone (31)
i) Baeyer-Villiger oxidation. A mixture of 98% HCOOH (0.20 mL, 5.19 mmol) and 30% H2O2 (0.14 mL,
1.23 mmol) was stirred at 0 oC for 1 h. A solution of 5'-acetyisoflavone (34) (0.093 g, 0.21 mmol) in 98%
HCOOH (0.4 mL) was added to the above solution under ice-cooling, and the whole mixture was stirred
at 0 oC for 4 h. The reaction mixture was poured into a solution of Na2SO3 (0.32 g) in water (8 mL) and
extracted with ether (3 x 10 mL). The organic layer was washed with water (2 x 10 mL + 2 x 30 mL) and
brine (10 mL), dried over MgSO4, and evaporated to dryness in vacuo to give acetate (35) (0.085 g, 88%)
as a yellow oil.
ii) Hydrolysis of acetate (35). To a solution of 5’-acetoxyisoflavone (35) (14.4 mg, 0.032 mmol) in
anhydrous MeOH (1.8 mL) was added K2CO3 (17.5 mg, 0.13 mmol) and the whole mixture was stirred at
rt for 3.3 h. The reaction mixture was poured into water (10 mL), acidified (pH 1) with 10% aq. HCl (5
mL) and extracted with AcOEt (3 x 10 mL). The organic layer was washed with water (10 mL) and brine
(10 mL), dried over MgSO4, and evaporated to dryness in vacuo to afford phenol (31) (8.2 mg, 63%) as a
o
pale yellow powder (mp128-136 C). Recrystallization from ether gave 31 as colorless prisms (mp
135.5-137 oC). IR νmax cm-1 (Nujol): 3195 (OH), 1637 (C=O). 1H-NMR (400 MHz) δ: 1.18 [6H, d, J=6.1
Hz, CH(CH3)2], 1.39 [6H, d, J=6.1 Hz,-CH(CH3)2], 1.41 [6H, d, J=6.1 Hz, CH(CH3)2], 4.19 [1H, sep,
J=6.1 Hz, O-CH(CH3)2], 4.56 [1H, sep, J=6.1 Hz, O-CH(CH3)2], 4.67 [1H, sep, J=6.1 Hz, O-CH(CH3)2],
5.36 (1H, s, OH), 6.58 (1H, s, C3'-H), 6.83 (1H, d, J=2.2 Hz, C8-H), 6.94 (1H, dd, J=8.8, 2.4 Hz, C6-H),
6.98 (1H, s, C6'-H), 7.93 (1H, s, C2-H), 8.18 (1H, d, J=8.8 Hz, C5-H). EI-MS m/z: 412 (M+, 58%), 328
(100%). Anal. Calcd for C24H28O6: C, 69.89; H, 6.84. Found: C, 69.73; H, 6.77.
5'-(3-Methyl-2-butenyloxy)-7,2',4'-triisopropoxyisoflavone (36). To a mixture of 5'-hydroxyisoflavone
(31) (0.199 g, 0.48 mmol), K2CO3 (0.173 g, 1.25 mmol) and acetone (5 mL) was added

4-bromo-2-methyl-2-butene (0.17 mL, 1.42 mmol), and the whole mixture was stirred at rt for 17 h under
argon atmosphere. The reaction mixture was poured into water (10 mL) and extracted with ether (20 mL
+ 2 x 10 mL). The organic layer was washed with 2% aq. NaOH (10 mL) and water (10 mL), dried over
K2CO3, and evaporated to dryness in vacuo to afford a brown oil (0.239 g). Recrystallization from
pentane gave a pale yellow powder (36) (0.177 g, 76%, mp 48-52 oC). 1H-NMR (400 MHz) δ: 1.20 [6H, d,
J=6.0 Hz, CH(CH3)2], 1.36 [6H, d, J=6.0 Hz, CH(CH3)2], 1.41 [6H, d, J=6.0 Hz, CH(CH3)2], 1.70 (3H, s,
C4”H3), 1.77 (3H, d, J=0.7 Hz, C5”H3), 4.26 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 4.51 (2H, d, J=6.0 Hz,
C1''H2), 4.52 [1H, sep, J=6.0 Hz, O-CH(CH3)], 4.67 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 5.49-5.53 (1H, m,
C2”-H), 6.61 (1H, s, C3'-H), 6.84 (1H, d, J=2.4 Hz, C8-H), 6.95 (1H, dd, J=8.9, 2.4 Hz, C6-H), 7.04 (1H, s,
C6'-H), 7.99 (1H, s, C2-H), 8.19 (1H, d, J=9.0 Hz, C5-H). EI-MS m/z: 480 (M+, 24%), 327 (100%).
3'-Hydroxy-2'-(3-methyl-2-butenyl)-7,4',6'-triisopropoxyisoflavone (37). A mixture of isoflavone (36)
(0.526 g, 1.09 mmol), montmorillonite KSF clay (0.527 g) and dry benzene (11 mL) was stirred at rt for
9.5 days under argon atmosphere. The catalyst was filtered off and washed with AcOEt and MeOH. The
combined organic solution was evaporated to dryness in vacuo to give a yellow oil (0.549 g). The crude
product was purified by silica gel column chromatography (hexane, benzene, benzene-AcOEt, 20:1 to
2:1) to give a colorless powder (37) (0.165 g, 31%) together with starting ether (36) (0.056 g, 11%) and
5'-hydroxyisoflavone (31) (0.161g, 41%). 1H-NMR (400 MHz) δ: 1.06 [3H, d, J=6.1 Hz, CH(CH3)2], 1.16
[3H, d, J=5.8 Hz, CH(CH3)2], 1.35-1.45 [18H, m, CH(CH3)2, C4”H3 and C5”H3], 3.04 (1H, dd, J=14.3, 7.9
Hz, C1”H), 3.39 (1H, dd, J=14.3, 6.1 Hz, C1”H), 4.13 [1H, sep, J=6.1 Hz, O-CH(CH3)2], 4.55 [1H, sep,
J=6.1 Hz, O-CH(CH3)2], 4.68 [1H, sep, J=6.1 Hz, O-CH(CH3)2], 5.08 (1H, br t, J=6.5 Hz, C2”-H), 5.50
(1H, s, OH), 6.46 (1H, s, C5'-H), 6.84 (1H, d, J=2.1 Hz, C8-H), 6.94 (1H, dd, J=8.9, 2.1 Hz, C6-H), 7.74
(1H, s, C2-H), 8.17 (1H, d, J=8.9 Hz, C5-H). FAB-MS m/z: 481 ([M+H]+).
3'-Methoxy-2'-(3-methyl-2-butenyl)-7,4',6'-triisopropoxyisoflavone (38). To
a
mixture of
3'-hydroxyisoflavone (37) (24.6 mg, 0.051 mmol) and K2CO3 (141.7 mg, 1.03 mmol) was added a
solution of Me2SO4 (0.05 mL, 0.53 mmol) in DMF (0.25 mL), and the whole mixture was stirred at rt for
4 h and heated at 50 oC for 18 h. The reaction mixture was poured into 5% aq. NH3 (1 mL) and stirred at
rt for 15 min. After dilution with water (10 mL) the reaction mixture was extracted with AcOEt (3 x 10
mL). The orgnic layer was washed with water (15 mL + 2 x 10 mL) and brine (10 mL), dried over MgSO4,
and evaporated to dryness in vacuo to give oil (28.3 mg). The crude product was purified by preparative
TLC (hexane-AcOEt, 7:1) to afford a pale yellow oil (38) (22.0 mg, 87%). IR νmax cm-1 (neat): 1647

1
(C=O). H-NMR (400 MHz) δ: 1.12 [3H, d, J=6.0 Hz, CH(CH3)2], 1.18 [3H, d, J=6.0 Hz, CH(CH3)2],
1.38 [6H, d, J=5.2 Hz, CH(CH3)2], 1.41 (3H, s, C4”-H or C5”-H), 1.41 [6H, d, J=7.6 Hz, CH(CH3)2], 1.55
(3H, s, C4”-H or C5”-H), 3.04 (1H, dd, J=14.5, 7.5 Hz, C1”-H), 3.35 (1H, dd, J=14.5, 6.0 Hz, C1”-H), 3.79
(3H, s, OMe), 4.26 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 4.54 [1H, sep, J=6.0 Hz, O-CH(CH3)2], 4.67 [1H,
sep, J=6.0 Hz, O-CH(CH3)2], 5.01 (1H, br t, J=6.0 Hz, C2”-H), 6.45 (1H, s, C5'-H), 6.83 (1H, d, J=2.4 Hz,
C8-H), 6.93 (1H, dd, J=9.0, 2.4 Hz, C6-H), 7.62 (1H, s, C2-H), 8.16 (1H, d, J=9.0 Hz, C5-H). HRFAB-MS
m/z: 494.2625 (Calcd for C30H38O6: 494.2669). EI-MS m/z: 494 (M+, 100%).
REFERENCES AND NOTES
§ Present address: Faculty of Pharmacy, Chiang Mai University, Chiang Mai, 50200, Thailand.
1. J. L. Ingham, S. Tahara, and G. S. Pope, “Pueraria,” ed. by W. M. Keung, Chapter 6, pp. 97-118,
Taylor & Francis, London and New York, 2002; J. Cain, Nature, 1960, 188, 774; J. L. Ingham, K. R.
Markham, and S. Z. Dziedzic, Phytochemistry, 1986, 25, 1772; H. E. M. Jones and G. S. Pope, J.
Endocrinol., 1960, 20, 229; H. E. M. Jones, H. B. Waynforth, and G. S. Pope, J. Endocrinol., 1961,
22, 293; H. E. M. Jones and G. S. Pope, J. Endocrinol., 1961, 22, 303.
2. S. Chansakaow, T. Ishikawa, H. Seki, K. Sekine, M. Okada, and C. Chaichantipyuth, J. Nat. Prod.,
2000, 63, 173.
3. a) J. L. Ingham, S. Tahara, and S. Z. Dziedzic, Z. Naturforsch., 1986, 41C, 403; b) S. Tahara, J.
L. Ingham, and S. Z. Dziedzic, Naturforsch., 1987, 42C, 510.
4. S. Chansakaow, T. Ishikawa, K. Sekine, M. Okada, Y. Higuchi, M. Kudo, and C. Chaichantipyuth,
Planta Med., 2000, 66, 572.
5. D. M. X. Donnelly and G. M. Boland, Nat. Prod. Rep., 1995, 321; P. M. Dewick, in “The
Flavonoids,” ed. by J. B. Harborne, Chapman and Hall, London, 1994, pp. 117-238.
6. T. W. Greene and P. G. Wuts, “Protective Groups in Organic Synthesis,” 3rd Edition, John Wiley &
Sons, Inc., New York, 1999.
7. A. McKillop, B. P. Swann, and E. C. Taylor, J. Am. Chem. Soc., 1973, 95, 3340.
8. H. Konishi, K. Aritomi, T. Okano, and J. Kiji, Bull. Chem. Soc. Jpn., 1989, 62, 591.
9. T. Tanaka, T. Kumamoto, and T. Ishikawa, Tetrahedron: Asymm., 2000, 11, 4633.
10. H. Ishii, I.-S. Chen, and T. Ishikawa, J. Chem. Soc., Perkin Trans. 1, 1987, 671.
11. W. Baker, J. Chadderton, J. B. Harborne, and W. D. Ollis, J. Chem. Soc., 1953, 1852.
12. A. Pelter and S. Foot, Synthesis, 1976, 326.
13. Y. Zaou, W. Zhang, W. Wan, Z. Chen, and D. Wang, Zhongguo Yiyao Gongye Zazhi, 2001, 32, 9

(Chem. Abstr., 2001, 136, 19994).
14. a) W. G. Dauben, J. M. Cogen, and V. Behar, Tetrahedron Lett., 1990, 31, 3241; b) E. J. Corey and L.
I. Wu, J. Am. Chem. Soc., 1993, 115, 9327.
15. Trials for deisopropylation of 38: i) With 0.1 M BCl3 / CH2Cl2: HCl-adduct (39). A pale pink powder.
1H-NMR (400 MHz) δ: 1.42 (3H, s, -CH3), 1.48 (3H, s, -CH3), 1.79 (1H, m, C2”-H), 1.88 (1H, m,
C2”-H), 2.63 (1H, m, C1”-H), 2.73 (1H, m, C1”-H), 3.80 (3H, s, -OCH3), 6.42 (1H, s, C3’-H), 6.85 (1H,
s, C8-H), 6.92 (1H, m, C6-H), 7.82 (1H, s, C2-H), 8.09 (1H, d, J=8.8 Hz, C5-H); ii) With 0.1 M BCl3 /
1
CH2Cl2 and 2-methyl-2-butene: partially deprotected product (40). A pale yellow powder. H-NMR
(400 MHz) δ: 1.39-1.43 [12H, m, CH(CH3)2], 1.49 [3H, s, C=C(CH3)2], 1.58 [3H, s, C=C(CH3)2],
3.16 (1H, dd, J=15.4, 7.1 Hz, C1”-Ha), 3.36 (1H, dd, J=15.4, 4.2 Hz, C1”-Hb), 4.56 [1H, sep, J=5.9 Hz,
O-CH(CH3)2], 4.70 [1H, sep, J=5.9 Hz, O-CH(CH3)2], 5.10 (1H, dd, J=7.1, 4.2 Hz, C2”-H), 6.55 (1H,
s, C5'-H), 6.88 (1H, d, J=2.4 Hz, C8-H), 7.00 (1H, dd, J=9.0, 2.4 Hz, C6-H), 7.87 (1H, s, C2-H), 8.21
1
(1H, d, J=9.0 Hz, C5-H); iii) With MeSO3H: chroman (41). A yellow powder. H-NMR (400 MHz,
CDCl3 + CD3OD) δ: 1.28 (3H, s, CH3), 1.30 (3H, s, CH3), 1.68 (2H, t, J=6.7 Hz, C3-H), 2.46 (2H, t,
J=6.7 Hz, C4-H), 6.39 (1H, s, C7-H), 6.80 (1H, d, J=2.2 Hz, C8-H), 6.87 (1H, dd, J=8.8, 2.2 Hz, C6-H),
7.78 (1H, s, C2-H), 8.03 (1H, d, J=8.8 Hz, C5-H). EI-MS m/z: 354 (M+, 49%), 137 (100%).
1
It was noteworthy that in H-NMR spectra these products (39) and (41) were identical with the
compounds obtained from kwakhurin (5) with hydrochloric acid by Tahara and Ingham.3b)
Cl
i-PrO
O
O
HO
O
O
HO
O
O
OMe
O
OMe
OH
HO
OPr-i
HO
OH
HO
39
40
41
16. H. J. Gardner and R. J. Stevens, J. Am. Chem. Soc., 1947, 69, 3086.