Oxidative rearrangement of pentaalkoxychalcones with phenyl-iodine(III) bis(trifluoroacetate) (PIFA): Synthesis of (±)-10-bromopterocarpin and (±)-pterocarpin

Article abstract of DOI:10.1039/a803561j

The oxidative rearrangement of pentaalkoxychalcones using the hypervalent iodine compound, phenyliodine(III) bis(trifluoroacetate) (PIFA), has been examined. Treatment of 2,2′-bis(benzyloxy)-4′-methoxy-4,5-methylenedioxychalcone with PIFA gives a rearrangement product in low yield, but 2,2′-bis(benzyloxy)-3-bromo-4′-methoxy-4,5-methylenedioxychalcone yields the rearrangement product, which leads to (±)-bromopterocarpin and (±)-pterocarpin.

Full text of DOI:10.1039/a803561j

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Oxidative rearrangement of pentaalkoxychalcones with phenyl-
iodine(III) bis(trifluoroacetate) (PIFA): synthesis of ( )-10-bromo-
pterocarpin and ( )-pterocarpin
Yasuyoshi Miki,* Rie Fujita and Ko-ichi Matsushita
Faculty of Pharmaceutical Sciences, Kinki University, 3-4-1 Kowakae, Higashi-Osaka 577-8502,
Japan
The oxidative rearrangement of pentaalkoxychalcones using the hypervalent iodine compound,
phenyliodine(III) bis(trifluoroacetate) (PIFA), has been examined. Treatment of 2,2Ј-bis(benzyloxy)-4Ј-
methoxy-4,5-methylenedioxychalcone with PIFA gives a rearrangement product in low yield, but 2,2Ј-
bis(benzyloxy)-3-bromo-4Ј-methoxy-4,5-methylenedioxychalcone yields the rearrangement product,
which leads to ( )-bromopterocarpin and ( )-pterocarpin.
Pterocarpans 1,1 ( )-homopterocarpin 1a and ( )-pterocarpin
1c, are known as phytoalexins which are produced when plants
are attacked biologically. Moreover, Engler et al. have shown
that several pterocarpans inhibit HIV-1 reverse transcriptase
and the cytopathic effect of HIV-1 in cell culture.2 Pterocarpans
have been synthesized by several methods,2–5 but the most effi-
cient method for their synthesis is the rearrangement of chal-
cones by thallium nitrate,3,4 which is a highly toxic compound.
Moriarty et al.6 have reported that treatment of 4-methoxy-
or 4,4Ј-dimethoxy-chalcones with [hydroxy(tosyloxy)iodo]-
benzene gives rearrangement products which could not be
converted to pterocarpans because the chalcones had no 2,2Ј-
dihydroxy substituent on the benzene ring. Recently, we have
developed a synthesis of ( )-homopterocarpin 1a by the
rearrangement of 2,2Ј-bis(benzyloxy)-4,4Ј-dimethoxychalcone
with phenyliodine() bis(trifluoroacetate) (PIFA).7 In this
paper we report the application of PIFA to the rearrangement
of pentaalkoxychalcones, and the conversion of the rearrange-
ment product to ( )-10-bromopterocarpin 1b and ( )-ptero-
carpin 1c.
Table 1 Rearrangement reactions of chalcone 2b with PIFA in MeOH
PIFA
(Equiv.)
CF3COOH
(Equiv.)
Yield of
3b (%)
Entry
R1
t/h
b
1
2
3
Br
Br
Br
1.2
6.0
4.8
5.0
—
5.0
24a
9
7
—
35
56
a Rt in CH(OMe)3. b Many compounds seen by TLC.
of 1-(2-benzyloxy-4-methoxyphenyl)ethanone9 and 2-benzyl-
oxy-3-bromo-4,5-methylenedioxybenzaldehyde in the presence
of NaOEt. Treatment of 2b with PIFA in CH(OMe)3 gave a
complex mixture, but in hot MeOH afforded the rearrangement
product 3b in 35% yield (Table 1, entries 1 and 2). A better
result was obtained by treating 2b in hot MeOH in the presence
of trifluoroacetic acid and 3b was isolated in 56% yield (entry 3)
(Scheme 1 and Table 1).10
O
OCH2Ph
MeO
O
CH(OMe)2
MeO
OCH2Ph
i
O
O
MeO
O
R1
OCH2Ph
O
R1
R2
O
PhCH2O
R1
O
2
3
a; R1 = H, b; R1 = Br
R3
Scheme 1 Reagents and conditions: i, PIFA, MeOH, reflux
1a; R1 = R3 = H, R2 = OMe
1b: R1, R2 = OCH2O, R3 = Br
1c; R1, R2 = OCH2O, R3 = H
Compound 3a was converted into the isoflavone 4a by treat-
ment with BF3ؒEt2O (20 equiv.) and Me2S (55 equiv.) in 58%
yield, but 3b gave a mixture of the 3Ј-bromoisoflavone 4b and
7-bromobenzofuran 5 in 52 and 19% yields, respectively (Table
2, entries 1 and 2). However, 4b was obtained in 61% yield as
the sole product by using BF3ؒEt2O (3 equiv.) and Me2S (3
equiv.) (entry 4) (Scheme 2 and Table 2).
Results and discussion
An initial attempt to obtain the rearrangement product 3a from
the chalcone 2a3 with PIFA in CH(OMe)3 in the presence of
trifluoroacetic acid7 was unsuccessful. However, treatment of
2a with PIFA in MeOH for two weeks gave 3a in 7% yield and
the starting material 2a was recovered in 42% yield. Many
attempts to attain 3a under various conditions were less than
satisfactory. The ease of rearrangement of 2,2Ј,4,4Ј,5-penta-
alkoxychalcone 3a is dependent on the electron density of the
benzene ring compared with 2,2Ј,4,4Ј-tetraalkoxychalcone7 and
2,2Ј-bis(benzyloxy)-4,4Ј-dimethoxychalcone. Consequently, we
introduced an electron-withdrawing bromo group on the ben-
zene ring of 2a, which could be easily converted to many other
functional groups.8
The isoflavone 4b could be converted to anhydropisatin 711
by catalytic hydrogenation with PtO2, followed by acid cycliz-
ation and reduction with 10% Pd–C and ammonium formate in
52% overall yield. However, reduction of the double bond of
7 to give ( )-pterocarpin 1c under several catalytic reduction
conditions7 could not be achieved (Scheme 3).
Next, we attempted to obtain ( )-pterocarpin by using
Mori’s method.12 Sodium borohydride reduction of 4b afforded
the diol 8, which was treated with BF3ؒEt2O to give ( )-10-
bromopterocarpin 1b in 40% yield. Reduction of the bromo
group of 1b was performed by using 10% Pd–C and ammonium
formate to yield ( )-pterocarpin 1c13 in 81% yield (Scheme 4).
The bromo-substituted chalcone 2b was obtained by reaction
J. Chem. Soc., Perkin Trans. 1, 1998
2533

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Table 2 Conversion of 3 into isoflavones 4 and benzofuran 5
MeO
O
O
Yield (%)
O
O
BF3ؒEt2O
Me2S
Entry
R1
(Equiv.)
(Equiv.)
4
5
HO
1
2
3
4
H
20
20
3
55
55
7.5
3
58
52
35
61
—
19
4
Br
Br
Br
Br
4b
i
3
—
MeO
O
OCH2Ph
CH(OMe)2
MeO
O
O
O
O
OH
HO
O
Br
PhCH2O
8
R1
3
ii
MeO
O
O
MeO
O
O
O
O
O
O
HO
Br
1b
iii
R1
a; R1 = H, b; R1 = Br
4 Isoflavone
MeO
O
+
O
MeO
OH
O
O
O
Br
1c
O
Scheme 4 Reagents and conditions: i, NaBH4, EtOH–THF; ii, BF3ؒ
OEt2, CH2Cl2, 40%; iii, 10% Pd–C, HCOONH4, MeOH, reflux, 10 h,
81%
O
O
5 Benzofuran
Scheme 2 Reagents and conditions: i, BF3ؒEt2O, Me2S in CH2Cl2, rt
2-Benzyloxy-3-bromo-4,5-methylenedioxybenzaldehyde
To a mixture of 2-benzoyl-4,5-methylenedioxybenzaldehyde14
(2.05 g, 8 mmol) and pyridinium hydrobromide perbromide
(5.12 g, 16 mmol) in CH2Cl2 (20 cm3) was added trifluoroacetic
acid (3.1 cm3, 40 mmol) and the mixture was stirred for 1 day
at room temperature. The reaction mixture was diluted with
CH2Cl2 (120 cm3), washed with water, and dried over Na2SO4,
then concentrated under reduced pressure. The residue was
purified by column chromatography on silica gel (n-hexane–
AcOEt = 10:1) to yield 3-bromo-2-hydroxy-4,5-methylenedi-
oxybenzaldehyde (1.44 g, 74%).
MeO
O
MeO
i
O
O
O
O
O
O
O
HO
4b
Br
Br
6
ii
MeO
O
O
MeO
O
O
A mixture of 3-bromo-2-hydroxy-4,5-methylenedioxybenz-
aldehyde (0.98 g, 4 mmol), benzyl chloride (0.25 cm3, 4.4
mmol), Adgen 464 (Aldrich) (0.3 cm3) in 10% KOH (6.4 cm3)
and CH2Cl2 (4 cm3) was stirred for 1 day at room temperature.
The mixture was diluted with CH2Cl2 (80 cm3), washed with
water, and dried over Na2SO4, then concentrated under reduced
pressure. The residue was purified by column chromatography
on silica gel (n-hexane–AcOEt = 10:1) to yield 2-benzyloxy-3-
bromo-4,5-methylenedioxybenzaldehyde (1.20 g, 90%), mp
120–123 ЊC (from EtOH) (Found: C, 53.52; H, 3.42.
C15H11O4Br requires C, 53.76; H, 3.31%); νmax(Nujol)/cmϪ1
1681; δH 5.08 (2H, s, CH2Ph), 6.14 (2H, s, OCH2O), 7.19 (1H, s,
6-H), 7.34–7.46 (5H, m, ArH), 9.96 (1H, s, CHO).
O
O
O
O
Anhydropisatin 7
1c
Scheme 3 Reagents and conditions: i, 1) PtO2, H2, acetone, 2) conc.
HCl, MeOH, 68%; ii, 10% Pd–C, HCOONH4, MeOH, reflux, 10 h, 76%
Experimental
Melting points were determined on a Yanagimoto micromelting
1
point apparatus and are uncorrected. The H NMR spectra
were determined on a JEOL JNM-GSX 270 spectrometer using
tetramethylsilane as an internal standard and CDCl3 as solvent.
The IR spectra were recorded with a JASCO FT/IR-7000
spectrophotometer. The high-resolution MS were recorded on a
JEOL JMS-HX100 spectrometer. Column chromatography
was performed on E. Merck silica gel 60 (70–230 mesh or 230–
400 mesh). Tetrahydrofuran (THF) was distilled from sodium
and benzophenone prior to use. Dichloromethane (CH2Cl2)
was distilled from calcium hydride prior to use.
2,2Ј-Bis(benzyloxy)-3-bromo-4Ј-methoxy-4,5-methylenedioxy-
chalcone 2b
A
solution of 1-(2-benzyloxy-4-methoxyphenyl)ethanone9
(1.51 g, 4.5 mmol), 2-benzyloxy-3-bromo-4,5-methylenedioxy-
benzaldehyde (1.51 g, 4.5 mmol), and sodium (207 mg, 9 mmol)
in EtOH (27 cm3) was refluxed for 1 h. The mixture was acid-
ified with 2  HCl and extracted with CH2Cl2. The organic
extracts were washed with water, dried over Na2SO4 and con-
2534
J. Chem. Soc., Perkin Trans. 1, 1998

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centrated. The residue was purified by column chromatography
(n-hexane–AcOEt = 10:1) to afford 2,2Ј-bis(benzyloxy)-3-
bromo-4Ј-methoxy-4,5-methylenedioxychalcone 2b (2.15 g,
83%), mp 129–130 ЊC (from EtOH) (Found: C, 64.70; H, 4.47.
C31H25O6Br requires C, 64.93; H, 4.39%); νmax(Nujol)/cmϪ1
1643; δH 3.87 (3H, s, OMe), 4.83 (2H, s, CH2), 5.08 (2H, s, CH2),
6.08 (2H, s, OCH2O), 6.56 (1H, d, J 2, 3Ј-H), 6.59 (1H, dd, J 9
3-(2-Hydroxy-4,5-methylenedioxyphenyl)-7-methoxyiso-
flavone 4a. Compound 4a was prepared in 58% yield according
to the general procedure from compound 3a (70 mg, 0.13
mmol), Me2S (0.44 ml, 6.6 mmol) and BF3ؒOEt2 (0.30 ml, 2.4
mmol) in CH2Cl2 (1 cm3), as a solid, mp 197–199 ЊC (from
MeOH) (lit.,3 mp 203–204 ЊC; lit.,14 mp 197–198 ЊC), νmax
-
(Nujol)/cmϪ1 1607; δH 3.94 (3H, s, OMe), 5.95 (2H, s, OCH2O),
6.61 (1H, s, 3Ј-H or 6Ј-H), 6.64 (1H, s, 6Ј-H or 3Ј-H), 6.92 (1H,
d, J 2.5, 8-H), 7.07 (1H, dd, J 9 and 2.5, 6-H), 8.04 (1H, s, 2-H),
8.26 (1H, d, J 9, 5-H), 8.76 (1H, s, OH).
and 2, 5Ј-H), 6.63 (1H, s, 6-H), 7.28–7.50 (11H, m, COCH᎐
᎐
CH and ArH), 7.81 (1H, d, J 9, 6Ј-H), 7.91 (1H, d, J 16,
COCH᎐CH).
᎐
3-(3-Bromo-2-hydroxy-4,5-methylenedioxyphenyl)-7-methoxy-
isoflavone 4b and 7-bromo-3-(2-hydroxy-4-methoxyphenyl-
carbonyl)-5,6-methylenedioxybenzofuran 5. Compounds 4b and
5 were prepared in 52 and 19% yields, respectively, according to
the general procedure from compound 3b (2.54 g, 4.0 mmol),
Me2S (16 ml, 220 mmol) and BF3ؒOEt2 (10 ml, 80 mmol) in
CH2Cl2 (24 cm3).
Compound 4b, mp 216–219 ЊC (from AcOEt) (Found: C,
52.11; H, 3.00. C17H11O6Br requires C, 52.20; H, 2.83%);
νmax(Nujol)/cmϪ1 1613; δH 3.95 (3H, s, OMe), 6.04 (2H, s,
OCH2O), 6.60 (1H, s, 6Ј-H), 6.93 (1H, d, J 2, 8-H), 7.08 (1H,
dd, J 9 and 2, 6-H), 8.04 (1H, s, 2-H), 8.26 (1H, d, J 9, 5-H),
9.24 (1H, s, OH).
Compound 5, mp 212–214 ЊC (from AcOEt) (Found: C,
52.02; H, 2.95. C17H11O6Br requires C, 52.20; H, 2.83%);
νmax(Nujol)/cmϪ1 1622; δH 3.88 (3H, s, OMe), 6.12 (2H, s,
OCH2O), 6.50 (1H, dd, J 9 and 2.5, 5Ј-H), 6.53 (1H, d, J 2.5,
3Ј-H), 7.34 (1H, s, 7-H), 7.76 (1H, d, J 9, 6Ј-H), 8.05 (1H, s,
2-H), 12.50 (1H, s, OH).
1-(2-Benzyloxy-4-methoxyphenyl)-2-(2-benzyloxy-4,5-methylene-
dioxyphenyl)-3,3-dimethoxypropan-1-one 3a
A suspension of 2,2Ј-bis(benzyloxy)-4Ј-methoxy-4,5-methylene-
dioxychalcone 2a3 (45 mg, 0.1 mmol), PIFA (52 mg, 0.12
mmol), and zinc chloride in THF solution (2 ; 40 µl, 0.02
mmol) in MeOH (6 cm3) was stirred for 2 weeks at room tem-
perature. The reaction mixture was neutralized with saturated
aqueous NaHCO3 and extracted with CH2Cl2. The combined
extracts were washed with water and dried over Na2SO4, then
concentrated under reduced pressure. The residue was purified
by column chromatography on silica gel (CH2Cl2–AcOEt =
200:1) to give 2a (21 mg, 42%) and 1-(2-benzyloxy-4-meth-
oxyphenyl)-2-(2-benzyloxy-4,5-methylenedioxyphenyl)-3,3-di-
methoxypropan-1-one 3a3 (4 mg, 7%) as an oil; νmax(Nujol)/
cmϪ1 1670; δH 3.13 (1H, s, OMe), 3.38 (1H, s, OMe), 3.74 (3H, s,
OMe), 4.80 (1H, d, J 12, PhCHH), 4.88 (1H, d, J 12, PhCHH),
4.92 (1H, d, J 12, PhCHH), 4.96 (1H, d, J 12, PhCHH), 5.13
(1H, d, J 8, COCH), 5.63 [1H, d, J 8, CH(OMe)2], 5.85 (1H, d,
J 1.5, OCHHO), 5.87 (1H, d, J 1.5, OCHHO), 6.31–6.38 (2H,
m, 3Љ-H and 5Љ-H), 6.44 (1H, s, 3Ј-H), 6.99 (1H, s, 6Ј-H), 7.22–
7.33 (10H, m, ArH), 7.60 (1H, d, J 8, 6Љ-H).
10-Bromoanhydropisatin 6
A suspension of 4b (391 mg, 1.0 mmol) and PtO2 (60 mg) in
acetone (30 cm3) was stirred for 1 day under hydrogen. The
catalyst was removed by filtration through Celite and the filtrate
was evaporated off. A suspension of the residue and a few drops
of conc. HCl in MeOH (15 cm3) was refluxed for 2 h. Water was
added to the reaction mixture and the mixture was extracted
with CH2Cl2. The combined extracts were washed with water
and dried over Na2SO4, then concentrated under reduced pres-
sure. The residue was purified by column chromatography
on silica gel (CH2Cl2–n-hexane = 1:2) to yield 10-bromo-3-
methoxy-8,9-methylenedioxypterocarpene 6 (256 mg, 68%),
mp 177–180 ЊC (from AcOEt) (Found: C, 54.29; H, 3.09.
C17H11O5Br requires C, 54.43; H, 2.96%); δH 3.81 (3H, s, OMe),
5.50 (2H, s, 6-H), 6.07 (2H, s, OCH2O), 6.50 (1H, d, J 2, 4-H),
6.55 (1H, dd, J 8 and 2, 2-H), 6.68 (1H, s, 7-H), 7.47 (1H, d, J 8,
1-H).
2-(2-Benzyloxy-3-bromo-4,5-methylenedioxyphenyl)-1-(2-
benzyloxy-4-methoxyphenyl)-3,3-dimethoxypropan-1-one 3b
A suspension of 2,2Ј-bis(benzyloxy)-3-bromo-4Ј-methoxy-4,5-
methylenedioxychalcone 2b (688 mg, 1.2 mmol), PIFA (619 mg,
1.44 mmol), and trifluoroacetic acid (0.93 cm3, 1.2 mmol) in
MeOH (90 cm3) was refluxed for 7 h. While the mixture was
refluxed, PIFA (619 mg, 1.44 mmol) was added to the mixture
at 2 h intervals. Finally, PIFA (619 mg, 1.44 mmol), and tri-
fluoroacetic acid (0.93 cm3, 1.2 mmol) were added, then the
mixture was refluxed for 1 h. The reaction mixture was neutral-
ized with saturated aqueous NaHCO3 and extracted with
CH2Cl2. The combined extracts were washed with water and
dried over Na2SO4, then concentrated under reduced pressure.
The residue was purified by column chromatography on silica
gel (n-hexane–AcOEt = 1:2) to give 2-(2-benzyloxy-3-bromo-
4,5-methylenedioxyphenyl)-1-(2-benzyloxy-4-methoxyphenyl)-
3,3-dimethoxypropan-1-one 3b (422 mg, 56%), mp 127 ЊC
(from n-hexane–AcOEt) (Found: C, 62.37; H, 4.98. C33H31-
O8Br requires C, 62.37; H, 4.92%); νmax(Nujol)/cmϪ1 1669;
δH 3.17 (1H, s, OMe), 3.36 (1H, s, OMe), 3.73 (3H, s, OMe),
4.69 (1H, d, J 11, PhCHH), 4.82 (1H, d, J 12.5, PhCHH),
4.91 (1H, d, J 12.5, PhCHH), 4.98 (1H, d, J 11, PhCHH),
5.06 (1H, d, J 8, COCH), 5.63 [1H, d, J 8, CH(OMe)2],
5.99 (1H, d, J 1.5, OCHHO), 6.01 (1H, d, J 1.5, OCHHO),
6.31 (1H, d, J 2, 3Љ-H), 6.40 (1H, dd, J 9 and 2, 5Љ-H), 6.44
(1H, s, 3Ј-H), 6.97 (1H, s, 6Ј-H), 7.17–7.48 (10H, m, ArH), 7.63
(1H, d, J 9, 6Љ-H).
Anhydropisatin 7
A suspension of 6 (38 mg, 0.1 mmol), HCOONH4 (38 mg, 0.6
mmol) and 10% Pd–C (8 mg) in MeOH (2 cm3) was refluxed for
6 h. The catalyst was removed by filtration through Celite and
the filtrate was evaporated off. The residue was purified by PLC
on silica gel (CH2Cl2–n-hexane = 1:2) to yield 3-methoxy-8,9-
methylenedioxypterocarpene 7 (18 mg, 76%), mp 182–183 ЊC
(from EtOH) (lit.,11 mp 183–185 ЊC); δH 3.81 (3H, s, OMe), 5.52
(2H, s, 6-H), 5.99 (2H, s, OCH2O), 6.50 (1H, d, J 2.5, 4-H), 6.53
(1H, dd, J 8 and 2.5, 2-H), 6.73 (1H, s, 7-H or 10-H), 7.02 (1H,
s, 10-H or 7-H), 7.37 (1H, d, J 8, 1-H).
( )-10-Bromopterocarpin 1b
General procedure: preparation of isoflavones 4 and benzofuran 5
A solution of 3,3-dimethoxypropan-1-one 3, Me2S and BF3ؒ
OEt2 in CH2Cl2 was stirred for 1 day at room temperature.
Water was added to the reaction mixture and the mixture was
extracted with CH2Cl2. The combined extracts were washed
with water and dried over Na2SO4, then concentrated under
reduced pressure. The residue was purified by column chrom-
atography on silica gel (CHCl3) to give isoflavones 4 and benzo-
furan 5.
A solution of NaBH4 (113 mg, 3 mmol) in EtOH (16 cm3) was
added to a solution of 4b (117 mg, 0.3 mmol) in THF (16 cm3)
and the mixture was stirred for 2 d. Saturated aqueous NH4Cl
was added to the reaction mixture, which was extracted with
CH2Cl2. The extracts were washed with saturated aqueous
NaHCO3 and saturated aqueous NaCl, dried over Na2SO4, and
concentrated to give a residue. To the residue in CH2Cl2 (30
cm3) was added BF3ؒOEt2 (0.03 cm3) in CH2Cl2 (20 cm3) at
Ϫ78 ЊC and the mixture was stirred for 2 h. Saturated aqueous
J. Chem. Soc., Perkin Trans. 1, 1998
2535

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2 T. A. Engler, K. O. Lynch Jr., J. P. Reddy and G. S. Gregory, Bioorg.
Med. Chem. Lett., 1993, 3, 1229; T. A. Engler, W. Chai and K. O.
LaTessa, J. Org. Chem., 1996, 61, 9297.
NaHCO3 was added to the reaction mixture, which was
extracted with CH2Cl2. The extracts were washed with satur-
ated aqueous NaHCO3 and saturated aqueous NaCl, dried over
Na2SO4, and concentrated to give a residue, which was purified
by column chromatography on silica gel (CH2Cl2–n-hexane =
2:1) to yield ( )-10-bromopterocarpin 1b (45 mg, 40%), mp
190–191 ЊC (from CHCl3–MeOH) (Found: C, 54.18; H, 3.67.
C17H13O5Br requires C, 54.13; H, 3.47%); δH 3.58 (1H, ddd,
J 11, 7 and 4.5, 6a-H), 3.71 (1H, t, J 11, 6-H), 3.79 (3H, s,
OMe), 4.23 (1H, dd, J 11 and 4.5, 6-H), 5.59 (1H, d, J 7,
11a-H), 5.98 (1H, d, J 1.5, OCHHO), 6.01 (1H, d, J 1.5,
OCHHO), 6.46 (1H, d, J 2.5, 4-H), 6.65 (1H, dd, J 8.5 and 2.5,
2-H), 6.67 (1H, s, 7-H), 7.47 (1H, d, J 8.5, 1-H) (Found: M,
375.9963. Calc. for C17H13O5Br: M, 375.9947).
3 L. Farkas, À. Gottsegen and M. Nógrádi, J. Chem. Soc., Perkin
Trans. 1, 1974, 305.
4 K. Thakkar and M. Cushman, J. Org. Chem., 1995, 60, 6499;
T. Horie, K. Shibata, K. Yamashita, K. Fujii, M. Tsukayama and
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5 R. A. Lichtenfels, A. L. Coelho and P. R. R. Costa, J. Chem. Soc.,
Perkin Trans. 1, 1995, 949.
6 R. M. Moriarty, J. S. Khosrowshahi and O. Prakash, Tetrahedron
Lett., 1985, 26, 2961.
7 Y. Miki, S. Kobayashi, N. Ogawa and H. Hachiken, Synlett, 1994,
1001 and references cited therein.
8 J. A. Ferreira, J. W. Nel, E. V. Brandt, B. C. B. Bezuidenhoudt and
D. Ferreira, J. Chem. Soc., Perkin Trans. 1, 1995, 1049.
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10 The chalcone 2a was treated with TTN [Tl(NO3)ؒ3H2O] in MeOH
for 1 h to give the rearrangement product 3a in 83% yield. Treatment
of the chalcone 2b with TTN in MeOH gave a complex mixture but
in MeOH–CH(OMe)3 for 20 h 2b yielded rearrangement product 3b
in 46% yield.
( )-Pterocarpin 1c
A suspension of 1b (33 mg, 0.09 mmol), HCOONH4 (34 mg,
0.54 mmol) and 10% Pd–C (7 mg) in MeOH (2 cm3) was
refluxed for 6 h. The catalyst was removed by filtration through
Celite and the filtrate was evaporated off. The residue was puri-
fied by recrystallization (EtOH) to yield ( )-pterocarpin 1c (22
mg, 81%), mp 182–183 ЊC (from EtOH) (lit.,13 mp 190–192 ЊC;
lit.,14 mp 184–185 ЊC); δH(CDCl3–CD3OD = 10:1) 3.45–3.74
(2H, m, 6-H and 6a-H), 3.80 (3H, s, OMe), 4.25 (1H, dd, J 11
and 5, 6-H), 5.51 (1H, d, J 7, 11a-H), 5.90 (1H, d, J 1.5,
OCHHO), 5.93 (1H, d, J 1.5, OCHHO), 6.43 (1H, s, 10-H),
6.47 (1H, d, J 2.5, 4-H), 6.64 (1H, dd, J 8.5 and 2.5, 2-H), 6.76
(1H, s, 7-H), 7.40 (1H, d, J 8.5, 1-H) (Found: M, 298.0817.
Calc. for C17H14O5: M, 298.0841).
11 Y. Ozaki, K. Mochida and S.-W. Kim, J. Chem. Soc., Chem.
Commun., 1988, 374; T. A. Engler, J. P. Reddy, K. D. Combrink and
D. V. Velde, J. Org. Chem., 1990, 55, 1248.
12 K. Mori and H. Kishida, Liebigs Ann. Chem., 1988, 721.
13 T. A Engler, J. P. Reddy, K. D. Combrink and Q. V. Velde, J. Org.
Chem., 1990, 55, 1254.
14 M. Uchiyama and M. Matsui, Agric. Biol. Chem., 1967, 31, 1490.
References
Paper 8/03561J
Received 28th April 1998
Accepted 12th June 1998
1 W. Friedrichsen, in Comprehensive Heterocyclic Chemistry, ed. A. R.
Katritzky and C. W. Rees, Pergamon, New York, 1984, vol. 4, p. 998.
2536
J. Chem. Soc., Perkin Trans. 1, 1998