Total Synthesis and Absolute Stereochemistry of the Natural Atropisomer of the Biflavone 4′4?,7″-Tetra-O-methylcupressuflavone

Article abstract of DOI:10.1021/jo970670w

The total synthesis of a natural atropisomer, 4′,4?,7,7″-tetra-O-methylcupressuflavone (1), has been achieved. The key intermediate, 3,3′-diacetyl-4,4′,6,6′-tetramethoxy-2,2′-biphenyldiol (5), was synthesized by the solid state phenol coupling reaction, and the racemate 5 was enantioresolved as bis(camphanate) esters. The absolute configuration of bis(camphanate) ester (-)-8b was determined to be (aR) by X-ray analysis. The ester (aR)-(-)-8b was converted to the natural biflavone [CD(+)362.0]-(-)-1, leading to the (aR) absolute configuration of 1. This conclusion is consistent with our previous theoretical determination of the absolute stereochemistry of biflavone 1 by the molecular orbital calculation of CD spectra.

Full text of DOI:10.1021/jo970670w

7222
J . Org. Chem. 1997, 62, 7222-7227
Tota l Syn th esis a n d Absolu te Ster eoch em istr y of th e Na tu r a l
Atr op isom er of th e Bifla von e
4′,4′′′,7,7′′-Tetr a -O-m eth ylcu p r essu fla von e
Hong-Yu Li, Tatsuo Nehira, Mami Hagiwara, and Nobuyuki Harada*
Institute for Chemical Reaction Science, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-77, J apan
Received April 15, 1997X
The total synthesis of a natural atropisomer, 4′,4′′′,7,7′′-tetra-O-methylcupressuflavone (1), has
been achieved. The key intermediate, 3,3′-diacetyl-4,4′,6,6′-tetramethoxy-2,2′-biphenyldiol (5), was
synthesized by the solid state phenol coupling reaction, and the racemate 5 was enantioresolved
as bis(camphanate) esters. The absolute configuration of bis(camphanate) ester (-)-8b was
determined to be (aR) by X-ray analysis. The ester (aR)-(-)-8b was converted to the natural
biflavone [CD(+)362.0]-(-)-1, leading to the (aR) absolute configuration of 1. This conclusion is
consistent with our previous theoretical determination of the absolute stereochemistry of biflavone
1 by the molecular orbital calculation of CD spectra.
Ch a r t 1a
In tr od u ction
The CD exciton chirality method has been extensively
used for determining the absolute configurations of
natural products and synthetic chiral compounds.1 On
the other hand, to determine the absolute stereochem-
istry of chiral compounds having a twisted π-electron
system, we have used a theoretical calculation to obtain
the CD spectra by the π-electron self-consistent field/
configuration interaction/ dipole velocity molecular or-
bital method (SCF-CI-DV MO method).2-4 For ex-
ample, we have succeeded in determining the absolute
configuration of dihydroazulene derivatives isolated from
liverworts,5 halenaquinols isolated from tropical marine
sponges,6,7 spiro troponoids,8 cyanin dyes,9 ternaphtha-
lenes,10 and unique chiral olefins.11-13 The reliability of
a
The sign of optical rotation was taken from the [R]D value of
the natural sample in MeOH.
X Abstract published in Advance ACS Abstracts, October 1, 1997.
(1) Harada, N.; Nakanishi, K. Circular Dichroic SpectroscopysExciton
Coupling in Organic Stereochemistry; University Science Books: Mill
Valley, CA, and Oxford University Press: Oxford, 1983.
(2) Moscowitz, A. Tetrahedron 1961, 13, 48.
(3) Kemp, C.; Mason, S. F. Tetrahedron 1966, 22, 629.
(4) Harada, N. In Circular Dichroism, Principles and Applications;
Nakanishi, K., Berova, N., Woody, R. W., Eds., VCH Publishers: New
York, 1994; p 335.
this theoretical CD method has been verified by synthetic
studies of model compounds, by stereoselective total
syntheses, and in some instances by X-ray crystallog-
raphy.5-13
Previously, we have determined the absolute stereo-
chemistry of a chiral biflavone isolated from plants,
4′,4′′′,7,7′′-tetra-O-methylcupressuflavone ((-)-1), by the
molecular orbital calculation of CD spectra (Chart 1).14
Chiral biflavone 1 is composed of two flavone units 2
connected at 8-8′′ positions and can exist as an enanti-
omer devoid of centers of chirality, i.e., a natural atrop-
isomer.15 The CD spectrum of 1 exhibits intense Cotton
effects reflecting its twisted π-electron framework. The
sign and intensity of CD Cotton effects of 1 were
satisfactorily reproduced by the theoretical calculation,
when the (aR)-enantiomer was chosen. Therefore, we
reported that the naturally occurring 4′,4′′′,7,7′′-tetra-O-
methylcupressuflavone (1) has the (aR) absolute stere-
ochemistry.14 Recently, Lin et al. reported the determi-
nation of the absolute configuration of compound 1 by
X-ray crystallography.16 However, there is considerable
confusion in their paper. Very recently, they reported
(5) Harada, N.; Kohori, J .; Uda, H.; Nakanishi,; K. Takeda, R. J .
Am. Chem. Soc. 1985, 107, 423. Harada, N.; Kohori, J .; Uda, H.;
Toriumi, K. J . Org. Chem. 1989, 54, 1820.
(6) Kobayashi, M.; Shimizu, N.; Kitagawa, I.; Kyogoku, Y.; Harada,
N.; Uda, H. Tetrahedron Lett. 1985, 26, 3833. Harada, N.; Sugioka,
T.; Ando, Y.; Uda, H.; Kuriki, T. J . Am. Chem. Soc. 1988, 110, 8483.
Harada, N.; Uda, H.; Kobayashi, M.; Shimizu, N.; Kitagawa, I. J . Am.
Chem. Soc. 1989, 111, 5668. Harada, N.; Sugioka, T.; Uda, H.; Kuriki,
T. J . Org. Chem. 1990, 55, 3158. Harada, N.; Sugioka, T.; Uda, H.;
Kuriki, T.; Kobayashi, M.; Kitagawa, I. J . Org. Chem. 1994, 59, 6606.
Harada, N.; Sugioka, T.; Soutome, T.; Hiyoshi, N.; Uda, H.; Kuriki, T.
Tetrahedron: Asymmetry 1995, 6, 375.
(7) Harada, N.; Sugioka, T. In Studies in Natural Products Chem-
istry, Vol. 17; Atta-ur-Rahman, Ed.; Elsevier Science Publishers: 1995;
p 33.
(8) Harada, N.; Uda, H,; Nozoe, T.; Okamoto, Y.; Wakabayashi, H.;
Ishikawa, S. J . Am. Chem. Soc. 1987, 109, 1661.
(9) Gargiulo, D.; Derguini, F.; Berova, N.; Nakanishi, K.; Harada,
N. J . Am. Chem. Soc. 1991, 113, 7046. Berova, N.; Gargiulo, D.;
Derguini, F.; Nakanishi, K.; Harada, N. J . Am. Chem. Soc. 1993, 115,
4769.
(10) Harada, N.; Hiyoshi, N.; Vassilev, V. P.; Hayashi, T. Chirality,
in press. Harada, N.; Vassilev, V. P.; Hiyoshi, N. Enantiomer 1997, 2,
123.
(11) Harada, N.; Saito, A.; Koumura, N.; Uda, H.; de Lange, B.;
J ager, W. F.; Wynberg, H.; Feringa, B. L. J . Am. Chem. Soc. 1997,
119, 7241.
(12) Harada, N.; Saito, A.; Koumura, N.; Roe, D. C.; J ager, W. F.;
Zijlstra, R. W. J .; de Lange, B.; Feringa, B. L. J . Am. Chem. Soc. 1997,
119, 7249.
(13) Harada, N.; Koumura, N.; Feringa, B. L. J . Am. Chem. Soc.
1997, 119, 7256.
(14) Harada, N.; Ono, H.; Uda, H.; Parveen, M.; Khan, N. U.-D.;
Achari, B.; Dutta, P. K. J . Am. Chem. Soc. 1992, 114, 7687.
(15) Ilyas, M.; Usmani, J . N.; Bhatnagar, S. P.; Ilyas, M.; Rahman,
W.; Pelter, A. Tetrahedron Lett. 1968, 5515. Rahman, W.; Ilyas, M.;
Okigawa, M.; Kawano, N. Chem. Pharm. Bull. 1982, 30, 1491.
S0022-3263(97)00670-1 CCC: $14.00 © 1997 American Chemical Society

4′,4′′′,7,7′′-Tetra-O-methylcupressuflavone
J . Org. Chem., Vol. 62, No. 21, 1997 7223
Ta ble 1. Solven t Dep en d en ce of [r]D a n d CD Da ta of Bifla von e 4′,4′′′,7,7′′-Tetr a -O-m eth ylcu p r essu fla von e
([CD(+)362.0]-(a R)-(-)-1)a
solvent
[R]D
CD data
natural sample
MeOH
EtOH
synthetic sample
MeOH
-25.3° (c 0.3)b
λext 362.0 nm (∆ꢀ +25.6), 326.2 (-54.4), 267.5 (+21.3)b
-11.3° (c 0.261)
(lit. +1° (c 0.2))c
+43.9° (c 0.259)
λext 362.6 nm (∆ꢀ +22.0), 326.4 (-51.3), 268.6 (+19.4),
226.8 (+19.4), 216.6 (-4.5), 207.4 (+11.6)
EtOH
λext 363.0 nm (∆ꢀ +25.5), 327.0 (-55.3), 269.2 (+21.3),
226.4 (+9.1), 216.8 (-2.6), 208.0 (+11.8)
(lit. +77° (c 0.2))c
+77.0° (c 0.257)
(lit. λext 360 nm (∆ꢀ +28.1), 324 (-66.4), 267 (+24.8))c
λext 361.0 nm (∆ꢀ +31.5), 326.2 (-64.0), 269.8 (+24.7)
CHCl3
a
b
The sign of optical rotation was taken from the [R]D value of the natural sample in MeOH. From ref 14. c From ref 16.
Sch em e 1a
the total synthesis of biflavone 117 and came to the same
absolute configuration as our theoretically determined
one.
In this paper, we report the total synthesis of chiral
biflavone 1. Although there were several papers report-
ing the synthesis of racemic 4′,4′′′,7,7′′-tetra-O-methyl-
cupressuflavone (1),18 the total synthesis of chiral 1 had
not been reported when we began. We have succeeded
in the total synthesis of natural biflavone and established
its (aR) absolute stereochemistry.
a
(a) (CH3)2SO4, K2CO3/acetone; (b) FeCl3, 50 °C, 12 h; (c) FeCl3/
Resu lts a n d Discu ssion
silica gel, 43-45 °C, 6 d.
Design a tion of th e En a n tiom er 1 by Its CD Da ta .
In general, enantiomers are designated by the sign of
optical rotation, [R]D. For example, the designation (aR)-
(-)-1 means that the enantiomer 1 with negative rotation
has the (aR) absolute configuration. However, it is also
well-known that [R]D values are dependent on the con-
centration and solvent used. In some extreme cases, the
sign of [R]D is inverted by changing solvents. As shown
in Table 1, 4′,4′′′,7,7′′-tetra-O-methylcupressuflavone (1)
falls in this extreme case. The [R]D of the natural product
1 was first measured in methanol to show a negative
value.14 Later it was reported by Lin et al. that the [R]D
of 1 takes positive values when measured in MeOH and
EtOH.16 Therefore, the use of [R]D value may cause
confusion.
We have proposed to use CD data to designate enan-
tiomers in addition to [R]D values.19 For example, the
designation [CD(+)362.0]-(aR)-1 means that the enanti-
omer 1 showing a positive CD Cotton effect at 362.0 nm
has the (aR) absolute configuration. This method is very
useful in the case of (-)-1, because the CD spectrum of 1
always exhibits a positive CD Cotton effect around 362.0
nm irrespective of the change of solvent as shown in
Table 1.20 Therefore, the natural biflavone is defined as
[CD(+)362.0]-(aR)-1. To keep the consistency with the
previous [R]D data,14 we adopt both designation methods
in this paper; the natural product 1 is thus defined as
[CD(+)362.0]-(-)-4′,4′′′,7,7′′-tetra-O-methylcupressufla-
vone.
It would be also useful to include P and M designations
for these enantiomers. The (aR) enantiomer has M
helicity and the (aS) isomer has P helicity.
Syn th esis of (()-3,3′-Diacetyl-4,4′,6,6′-tetr am eth oxy-
2,2′-bip h en yld iol (5) by th e Solid Sta te P h en ol
Cou p lin g Rea ction . As a chiral synthetic building
block for the total synthesis of (-)-1, we selected
3,3′-diacetyl-4,4′,6,6′-tetramethoxy-2,2′-biphenyldiol (5)
(Scheme 1). To synthesize biphenyldiol 5, phloroac-
etophenone 321 was selectively methylated with dimethyl
sulfate and K2CO3 in acetone giving 2,4-dimethylphlo-
roacetophenone 422 in 92% yield. Subsequently, the
phenol coupling reaction of 4 using typical oxidants, i.e.,
FeCl3, or (t-Bu)2O2, etc., in various solvents was com-
pletely unsuccessful.
We followed the solid state reaction conditions reported
by Toda23 and found that when a mixture of phenol 4
and FeCl3 (1.0 equiv) was heated at 50 °C for 12 h, the
desired coupling product 5 was obtained in 27% yield
(Scheme 1). On the other hand, when phenol 4 was
heated with excess FeCl3 (5.2 equiv) at 50 °C for 5.5 h,
the byproduct 6 was obtained in 33% yield, but no desired
coupling product 5 was obtained. We explored the
reaction condition further and found that the solid state
phenol coupling reaction in the presence of FeCl3/silica
gel24 brought the best yield of biphenyldiol 5; a mixture
of phenol 4 and 2.8 equiv of FeCl3/silica gel (1:2) was
heated at 43-45 °C for 6 d to give the desired coupling
product (()-5 in 81% yield (mp 211-212 °C).
(16) Zhang, F.-J .; Lin, G.-Q.; Huang, Q.-C. J . Org. Chem. 1995, 60,
6427. See also: Additions and Corrections in J . Org. Chem. 1996, 61,
5700.
(17) Lin, G.-Q.; Zhong, M. Tetrahedron Lett. 1997, 38, 1087.
(18) (a) Murti, V. V. S.; Raman, P. V.; Seshadri, T. R. Tetrahedron
1967, 23, 397. (b) Ahmad, S.; Razaq, S. Tetrahedron Lett. 1971, 48,
4633. (c) Ahmad, S.; Razaq, S. Tetrahedron 1976, 32, 503. (d)
Moriyama, S.; Okigawa, M.; Kawano, M. J . Chem. Soc., Perkin Trans.
1 1974, 2132. (e) Parthasarathy, M. R.; Gupta, S. Indian J . Chem.
1984, 23B, 227.
(19) Harada, N.; Iwabuchi, J .; Yokota, Y.; Uda, H.; Okamoto, Y.;
Yuki, H.; Kawada, Y. J . Chem. Soc., Perkin. Trans. 1 1985, 1845.
Harada, N. Enantiomer 1996, 1, 81.
(20) See also the case where CD spectra can be quite solvent
dependent: Cobb, D. I.; Lewis, D. A.; Armstrong, R. N. J . Org. Chem.
1983, 48, 4139.
(21) Gulati, K. C.; Seth, S. R.; Venkataraman, K. In Organic
Syntheses; Blatt, A. H., Ed.; J ohn Wiley & Sons, Inc.: New York, 1966;
Collect. Vol. II, p 522.
(22) Nakazawa, K. Chem. Pharm. Bull. 1962, 10, 1032.
(23) Toda, F.; Tanaka, K.; Iwata, S. J . Org. Chem. 1989, 54, 3007.
(24) Keinan, K.; Mazur, Y. J . Org. Chem. 1978, 43, 1020. J empty,
T. C.; Miller, L. L.; Mazur, Y. J . Org. Chem. 1980, 45, 751.

7224 J . Org. Chem., Vol. 62, No. 21, 1997
Li et al.
Sch em e 2a
a
(a) DMAP/pyridine, reflux.
En a n tior esolu tion of (()-3,3′-Dia cetyl-4,4′,6,6′-tet-
r a m eth oxy-2,2′-bip h en yld iol a s Bis(ca m p h a n a te)
Ester s 8 a n d X-r a y Cr ysta llogr a p h ic An a lysis.25
Resolution of (()-5 using recently developed chiral dichlo-
rophthalic acid method10,26,27 was unsuccessful because
of the steric hindrance around the phenol groups of 5.
However, application of the camphanate method to 5
(Scheme 2) using (1S)-(-)-camphanic acid chloride28 and
4-(dimethylamino)pyridine in pyridine, refluxed for 3
days, yielded a diastereomeric mixture of esters 8a and
8b. The mixture was separated by HPLC on silica gel
(CH3CN/CHCl3 1:19): separation factor R ) 1.18; resolu-
tion factor Rs ) 1.62. The diastereomeric mixture (ca.
700 mg) was separated to give the first eluted ester (+)-
F igu r e 1. ORTEP drawing of diester (aR)-(-)-8b. The atoms
are drawn as 50% probability.
Sch em e 3a
8a (42%, mp 255-257 °C, [R]22 +10.0° (c 0.78, CHCl3))
D
followed by (-)-8b (46%, mp 208-209 °C, [R]22 -6.3° (c
D
1.02, CHCl3)).
Recrystallization of diester 8b from propyl acetate
provided a colorless prism (0.35 × 0.33 × 0.32 mm)
suitable for X-ray diffraction: monoclinic; space group
P21. The absolute stereochemistry of the biphenyl moiety
of diester (-)-8b was determined as (aR) by the internal
reference method using the known absolute configuration
of camphanate ester moiety (Figure 1).30
(25) See the preliminary report of enantioresolution and X-ray
analysis: Harada, N.; Li, H.-Y.; Nehira, T.; Hagiwara, M. Enantiomer,
in press. The paper reports the X-ray analysis of ester (-)-8b using
the crystal obtained by recrystallization from hexane/EtOAc. The final
R-value, however, remained at a higher level because of the existence
of EtOAc as crystalline solvent.
(26) (a) Harada, N.; Nehira, T.; Soutome, T.; Hiyoshi, N.; Kido, F.
Enantiomer 1996, 1, 35. (b) Harada, N.; Koumura, N.; Robillard, M.
Enantiomer 1997, 2, 303. (c) Nemoto, H.; Miyata, J .; Fukumoto, K.;
Ehara, H.; Harada, N.; Uekusa, H.; Ohashi, Y. Enantiomer 1997, 2,
127.
(27) See also: (a) Harada, N.; Soutome, T.; Murai, S.; Uda, H.
Tetrahedron: Asymmetry 1993, 4, 1755. (b) Harada, N.; Hattori, T.;
Suzuki, T.; Okamura, A.; Ono, H.; Miyano, S.; Uda, H. Tetrahedron:
Asymmetry 1993, 4, 1789. (c) Harada, N.; Soutome, T.; Nehira, T.; Uda,
H.; Oi, S.; Okamura, A.; Miyano, S. J . Am. Chem. Soc. 1993, 115, 7547.
(d) Hattori, T.; Harada, N.; Oi, S.; Abe, H.; Miyano, S. Tetrahedron:
Asymmetry 1995, 6, 1043. (e) Hagiwara, H.; Okamoto, T.; Harada, N.;
Uda, H. Tetrahedron 1995, 51, 9891.
a
(a) 6 M HCl/EtOH, 78 °C, 18 h; (b) 4-anisaldehyde, KOH/
EtOH; (c) I2, H2SO4, DMSO, 80 °C, 20 h; (d) BCl3/CH2Cl2, 0 °C;
(e) (S)-(+)-MTPA chloride, N(Et)3, CH2Cl2.
Tot a l Syn t h esis of [CD(+)362.0]-(-)-4′,4′′′,7,7′′-
Tetr a -O-m eth ylcu p r essu fla von e (1). The total syn-
thesis of the natural enantiomer of biflavone 1 started
from chiral biphenyldiol (aR)-5. To recover chiral biphen-
yldiol (aR)-5, diester (-)-8b was hydrolyzed (Scheme 3).
When diester 8b was treated with aqueous LiOH in
EtOH, deacetyl products were obtained instead of 5.
Therefore, diester 8b was hydrolyzed under acidic condi-
tions; a mixture of diester (-)-8b, aqueous 6 M HCl, and
(28) Prepared from (-)-camphanic acid which was derived from
(1R,4R)-(+)-camphor.29 (-)-Camphanic acid: [R]22 -19.5° (c 1.0, 1,4-
dioxane). Methyl ester of (-)-camphanic acid: [RD]22 -22.4° (c 6.65,
D
CHCl3).
(29) Meyer, W. L.; Lobo, A. P.; McCarty, R. N. J . Org. Chem. 1967,
32, 1754.
(30) The authors have deposited atomic coordinates for this struc-
ture with the Cambridge Crystallographic Data Center. The coordi-
nates can be obtained, on request, from the Director, Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge, CB2 1EZ,
U.K.

4′,4′′′,7,7′′-Tetra-O-methylcupressuflavone
J . Org. Chem., Vol. 62, No. 21, 1997 7225
EtOH was heated at 78 °C for 18 h affording optically
active biphenyldiol (aR)-5 in 64% total yield, whose CD
spectrum showed a negative CD Cotton effect at 301.2
nm. Although we had expected to obtain enantiopure
biphenyldiol 5, it was found that chiral biphenol 5
obtained partially racemized during the acid-catalyzed
hydrolysis (79% ee). The enantiomeric excess of biphen-
1
yldiol 5 was determined by H NMR spectroscopy using
a chiral shift reagent, europium tris[3-((trifluorometh-
yl)hydroxymethylene)-(+)-camphorate], Eu(tfc)3 as de-
scribed in the Experimental Section.
Biphenyldiol (aR)-(-)-5 was next converted to bichal-
cone (aR)-(-)-9 (Scheme 3);18c a mixture of (aR)-(-)-5,
10% aqueous KOH, EtOH, and 4-anisaldehyde was
heated at 70-75 °C overnight giving (aR)-(-)-9 in 65%
yield. Bichalcone (aR)-(-)-9 was cyclized with iodine and
catalytic amount of H2SO4 in DMSO31 to afford 4′,4′′′,5,
5′′,7,7′′-hexa-O-methylcupressuflavone (aR)-(+)-10 in 52%
yield. Finally, hexa-O-methylcupressuflavone (aR)-(+)-
10 was selectively demethylated with BCl3 in CH2Cl2
giving the desired natural product (aR)-(-)-1 in 80% yield
(78% ee). During the purification of the synthetic sample,
we found that the racemic biflavone 1 is barely soluble
in MeOH. Therefore, biflavone (aR)-1 with 78% ee was
recrystallized from methanol, providing optically pure
biflavone (aR)-1. Its optical purity was confirmed by the
1H NMR spectrum after conversion to (S)-MTPA ester
(aR)-11, as described in Experimental Section. The
physical data of 100% ee biflavone 1 are mp 147-149 °C
F igu r e 2. CD and UV spectra of the synthetic sample of
biflavone, 4′,4′′′,7,7′′-tetra-O-methylcupressuflavone ([CD(+)-
362.0]-(aR)-(-)-1) in EtOH.
(MeOH), [R]22 -11.3° (c 0.261, MeOH), and CD (EtOH)
D
λext 363.0 nm (∆ꢀ +25.5).
CD Sp ectr a a n d Absolu te Ster eoch em istr y of (-)-
4′,4′′′,7,7′′-Tetr a -O-m eth ylcu p r essu fla von e (1). The
CD and UV spectra of synthetic biflavone (aR)-(-)-1 are
illustrated in Figure 2, and the CD curve is identical with
that of the natural sample 1 shown in Figure 3. There-
fore, the absolute stereochemistry of the natural atrop-
isomer, [CD(+)362.0]-(-)-4′,4′′′,7,7′′-tetra-O-methylcu-
pressuflavone (1), was determined to be (aR). This
conclusion is consistent with our previous theoretical
determination of the absolute stereochemistry of bifla-
vone 1 by the molecular orbital calculation of CD spectra.
Exp er im en ta l Section
Gen er a l P r oced u r es. Melting points are uncorrected. IR
spectra were obtained as KBr disks. 1H and 13C NMR spectra
were recorded on a 400 MHz/100 MHz or a 600 MHz/150 MHz
spectrometer, equipped with the Nalorac inverse probe. All
NMR data are reported in ppm (δ) downfield from TMS or
residual signals (CDCl3, δH/C 7.24/77.0) were used as internal
standards. MS spectra were obtained by the electron ioniza-
tion procedure (70 eV), unless otherwise noted. HPLC separa-
tion was carried out using a glass column (22 mm φ × 10 cm
or 25 mm φ × 40 cm) of silica gel. The purities of the title
compounds were shown to be g 95% by 1H NMR, TLC, HPLC,
and/or elemental analysis.
3,3′-Dia cet yl-4,4′,6,6′-t et r a m et h oxy-2,2′-b ip h en yld iol
((()-5). The silica gel bound ferric chloride was made accord-
ing to ref 12; silica gel (7.24 g) was dried in an oven at 150 °C
for 11 h. To a mixture of FeCl3‚6H2O (3.52 g) in dried diethyl
ether (190 mL) and ethanol (10 mL) was added the dried silica
gel. The solvents were removed to give a yellow solid, which
was dried at 85 °C in vacuo (66-133 Pa). The silica gel bound
ferric chloride (silica gel/FeCl3 ) 7:2) was obtained as a dark
brown solid. The oxidative coupling reaction was carried
out by adding silica gel bound ferric chloride (1.4 g) to a
solution of 2,4-dimethylphloroacetophenone (4, 146 mg) in
F igu r e 3. CD and UV spectra of the natural sample of
biflavone, 4′,4′′′,7,7′′-tetra-O-methylcupressuflavone ([CD(+)-
362.0]-(aR)-(-)-1) in EtOH.14
CH2Cl2 (10 mL). After well mixing by sonication and rotation,
the solvent was removed at reduced pressure to give a dark
solid, which was kept at 43-45 °C for 6 days. Water (1 mL)
and CH2Cl2 (30 mL) were added, and the mixture was
sonicated and filtered. The precipitate was washed with
CHCl3, and the combined organic layer was evaporated to
afford a red crude product (213 mg). This was purified by silica
(31) Zhang, F.-J .; Li, Y.-L. Chin. J . Chem. 1994, 12, 73.

7226 J . Org. Chem., Vol. 62, No. 21, 1997
Li et al.
gel chromatography eluting with hexane/EtOAC (changing the
ratio from 5:1, 3:1, 1:1, to 1:3) to yield biphenyldiol (()-5 (119
mg, 81%) as colorless small plates: mp 211-212 °C (H2O/
crystal stability, no indication of standard reflection decay
during data collection; total reflections scanned, 3888; unique
data Fo > 3σ(Fo), 3552. The skeletal structure was solved by
the direct method and successive Fourier syntheses. All
hydrogen atoms were found by the difference Fourier synthe-
ses. Absorption correction and full matrix least-squares
refinement of positional and thermal parameters led to the
final convergence with R ) 0.0346 and Rw ) 0.0409. The
absolute stereochemistry of biphenyldiol ester (-)-8b was
determined to be (aR) as shown in Figure 1 by the internal
reference method using the known absolute configuration of
the camphanate ester part.
DMF); IR (KBr) νmax 2949, 1618, 1590, 1405 cm-1 1H NMR
;
(600 MHz, CDCl3) δ 2.61 (6 H, s), 3.80 (6 H, s), 3.92 (6 H, s),
6.06 (2 H, s), 13.98 (2 H, s); 13C NMR (150 MHz, CDCl3) δ
33.2, 55.1, 55.4, 86.4, 102.4, 106.1, 163.3, 163.9, 164.0, 203.3;
MS m/ z 390 (M+, relative intensity 100), 375 (68), 333 (30);
high-resolution mass spectrum (HRMS) calcd for C20H22O8
390.13146, found 390.13119. Anal. Calcd for C20H22O8: C,
61.53; H, 5.68. Found: C, 61.63; H, 5.57.
The reaction yield for a larger scale (570 mg) was 74%.
3,3′-Diacetyl-4,4′,6,6′-tetr am eth oxy-2,2′-biph en yldiol Bis-
(ca m p h a n a te) Ester s (8a a n d 8b) a n d HP LC Sep a r a tion .
To a mixture of biphenyldiol (()-5 (300 mg, 0.77 mmol) and
(-)-camphanic acid chloride28 (1.67 g, 6.42 mmol) in dry
pyridine (20 mL) was added 4-(dimethylamino)pyridine (DMAP,
30 mg, 0.25 mmol). After being gently refluxed for 3 d under
argon, the reaction mixture was extracted with EtOAc. The
organic layer was washed with water, aqueous CuSO4, water,
and brine. The EtOAc layer was dried over MgSO4 and
evaporated in vacuo to give a crude product of bis(camphanate)
esters (ca. 700 mg). To separate the diastereomeric esters,
preparative HPLC was performed with a prepacked Kusano
glass column of silica gel (25 mm φ × 40 cm): UV/vis detector
at 272 nm; RI detector; flow rate, 14 mL/min, 64 mg/each
injection. The diastereomeric mixture was base-line separated
eluting with MeCN/CHCl3 (5:95): separation factor R ) 1.18,
resolution factor Rs ) 1.62. As the first-eluted fraction,
camphanate ester (aS)-(+)-8a (242 mg, 42%, retention time
19.5 min) was obtained: mp 255-257 °C (fine needles from
3,3′-Dia cet yl-4,4′,6,6′-t et r a m et h oxy-2,2′-b ip h en yld iol
([CD(-)301.2]-(a R)-(-)-5). A mixture of bis(camphanate)
ester (aR)-(-)-8b (135 mg, 0.18 mmol), aqueous HCl (6 M, 42
mL), and EtOH (49 mL) was heated under argon at 78 °C for
18 h. After the solvent and HCl were evaporated under
reduced pressure, the residue was subjected to silica gel
column chromatography eluting with hexane/EtOAc (changing
the ratio from 3:1, 1:1, to 2:3) to afford optically active
biphenyldiol (aR)-(-)-5 (33.2 mg, 47%) and a mixture of the
starting material and monocamphanate (57.8 mg). The re-
covery was recycled for the acid-catalyzed hydrolysis to give
(aR)-(-)-5 (12.1 mg, 17%: total yield, 64%): mp 218-220 °C
(fine needles from CHCl3); IR (KBr) νmax 1618, 1591, 1281, 1133
cm-1; 1H NMR (600 MHz, CDCl3) δ 2.61 (2 H, s), 3.81 (6 H, s),
3.92 (6 H, s), 6.06 (2 H, s), 13.97 (2 H, s); 13C NMR (150 MHz,
CDCl3) δ 31.1, 55.4, 55.8, 86.3, 102.4, 106.1, 163.3, 163.9, 164.0,
203.3; [R]22D -18.5° (c 2.45, CHCl3); UV (EtOH) λmax 338.0 nm
(sh, ꢀ 6300), 290.6 (34 200), 238.0 (23 100); CD (EtOH) λext
301.4 nm (∆ꢀ -15.2), 281.2 (+14.5), 247.8 (-7.6), 231.4 (-11.7),
211.6 (+44.9); HRMS calcd for C20H22O8 390.13144, found
390.13162. Anal. Calcd for C20H22O8: C, 61.53; H, 5.68.
Found: C, 61.24; H, 5.66.
1
EtOH); IR (KBr) νmax 1786, 1609, 1215, 1104 cm-1; H NMR
(600 MHz, CDCl3) δ 0.90 (6 H, s), 1.00 (6 H, s), 1.05 (6 H, s),
1.46 (2 H, ddd, J ) 13.6, 9.2, 4.2 Hz), 1.52 (2 H, ddd, J ) 13.2,
9.2, 3.7 Hz), 1.83 (2 H, ddd, J ) 13.2, 10.8, 4.2 Hz), 2.22 (2 H,
ddd, J ) 13.6, 10.8, 3.7 Hz), 2.49 (6 H, s), 3.77 (6 H, s), 3.93 (6
H, s), 6.45 (2 H, s); 13C NMR (150 MHz, CDCl3) δ 9.6, 16.0,
16.1, 28.9, 30.3, 31.7, 54.5, 55.0, 56.1, 56.2, 91.0, 93.2, 107.9,
117.4, 146.6, 158.9, 160.4, 164.8, 178.1, 199.4; [R]22D +10.0° (c
0.78, CHCl3); MS m/ z 750 (M+, relative intensity 100), 735
(57), 357 (28); HRMS calcd for C40H46O14 750.28870, found
750.28845.
The enantiomeric excess of biphenyldiol (aR)-(-)-5 was
determined to be 79% ee by the 1H NMR spectrum of a mixture
of (aR)-(-)-5 (1.5 mg) and europium tris[3-((trifluoromethyl)-
hydroxymethylene)-(+)-camphorate] (Eu(tfc)3, 6.9 mg, 2 equiv)
in CDCl3: 1H NMR (400 MHz, CDCl3) δ 2.6714 ppm (0.625 H,
s, corresponding to (aS)-enantiomer), 2.6836 (5.375 H, s,
corresponding to (aR)-enantiomer), 3.84 (6 H, s), 3.96 (6 H, s),
6.10 (2 H, s), 14.12 (2 H, s).
From the second-eluted fraction, camphanate ester (aR)-
(-)-8b (263 mg, 46%, retention time 23.4 min) was obtained:
mp 208-209 °C (prisms from PrOAc); IR (KBr) νmax 1785, 1604,
Op tica lly Active Bich a lcon e (a R)-(-)-9. To a solution
of optically active biphenyldiol (aR)-(-)-5 (79% ee, 33.2 mg,
0.0851 mmol) in a mixture of 10% aqueous KOH (3 mL) and
EtOH (2 mL) was added a solution of 4-anisaldehyde (226 mg,
1.66 mmol) in EtOH (1 mL) under ice-cooling. The reaction
mixture was kept under argon at 70 °C overnight and then at
75 °C for 3 h. After ice-cold 2 M HCl was added, the mixture
was extracted with EtOAc for three times. The combined
organic layer was washed with brine. Evaporation of the
solvent and crystallization from MeOH gave optically active
bichalcone (aR)-(-)-9 (34.7 mg, 65%) as small red needles: mp
1
1215, 1103 cm-1; H NMR (600 MHz, CDCl3) δ 0.55 (6 H, s),
0.94 (6 H, s), 1.02 (6 H, s), 1.61 (2 H, ddd, J ) 13.0, 9.2, 4.4
Hz), 1.82 (2 H, ddd, J ) 13.0, 10.6, 4.4 Hz), 2.09 (2 H, ddd, J
) 13.6, 9.2, 4.4 Hz), 2.23 (2 H, ddd, J ) 13.6, 10.6, 4.4 Hz),
2.47 (6 H, s), 3.83 (6 H, s), 3.91 (6 H, s), 6.44 (2 H, s); 13C NMR
(150 MHz, CDCl3) δ 9.5, 15.2, 16.2, 28.8, 30.9, 31.8, 54.1, 54.9,
56.1, 56.2, 90.8, 93.2, 108.1, 116.5, 147.2, 159.5, 160.7, 164.9,
178.2, 198.7; [R]22D -6.3° (c 1.02, CHCl3); UV (EtOH) λmax 295.0
nm (ꢀ 8700), 257.0 (16 900), 236.6 (29 800); CD (EtOH) λext
315.0 nm (∆ꢀ -6.0), 293.0 (+3.1), 274.4 (-7.8), 258.4 (+17.3),
240.8 (-11.5), 220.6 (-15.7); MS m/ z 750 (M+, relative
intensity 100), 735 (51), 357 (31); HRMS calcd for C40H46O14
750.28870, found 750.28910.
158-160 °C; IR (KBr) νmax 1624, 1560, 1218 cm-1
;
1H NMR
(600 MHz, CDCl3) δ 3.85 (12 H, s), 4.01 (6 H, s), 6.14 (2 H, s),
6.92 (4 H, d, J ) 8.3 Hz), 7.56 (4 H, d, J ) 8.3 Hz), 7.75 (2 H,
d, J ) 15.4 Hz), 7.80 (2 H, d, J ) 15.4 Hz), 14.21 (2 H s); 13C
NMR (150 MHz, CDCl3) δ 55.4, 55.8, 55.9, 86.9, 102.9, 106.7,
114.3, 128.5, 128.7, 130.0, 141.9, 161.2, 162.8, 163.8, 164.6,
193.0; [R]22D -85.2° (c 0.23, CHCl3); UV (EtOH) λmax 370.2 nm
(ꢀ 37 000), 224.6 (49 000); CD (EtOH) λext 397.0 nm (∆ꢀ -4.8),
308.0 (+2.1), 233.0 (-9.6), 212.2 (+21.0); HRMS calcd for
X-r a y Cr yst a llogr a p h y of 3,3′-Dia cet yl-4,4′,6,6′-t et r a -
m eth oxy-2,2′-bip h en yld iol Bis(ca m p h a n a te) Ester ((-)-
(8b)). Single crystals suitable for X-ray analysis were obtained
as colorless prisms by recrystallization from PrOAc: mp 208-
209 °C. A single crystal (dimension 0.35 × 0.33 × 0.32 mm)
was selected for data collection and mounted on a Mac Science
MXC18 automated four-circle diffractometer. The crystal was
found to be monoclinic, and the unit cell parameters and
orientation matrix were obtained. Data collection was carried
out by using a 2θ-θ scan: formula, C40H46O14; Mr ) 750.28870;
space group P21 (#4); a ) 14.172 9 (3), b ) 14.834 (3), c )
10.153 (2) Å, â ) 110.24 (2)°; V ) 2002.7 (7) Å3; Z ) 2; Dx )
1.245 g cm-3; Dm ) 1.241 g cm-3 by flotation using a CCl4-
hexane solution; radiation, Cu KR (1.54178 Å); monochroma-
C36H34O10 626.21516, found 626.21520. Anal. Calcd for C36
H34O10: C, 69.00; H, 5.57. Found: C, 68.85; H, 5.91.
-
4′,4′′′,5,5′′,7,7′′-Hexa -O-m et h ylcu p r essu fla von e ((a R)-
(+)-10). To a solution of (aR)-(-)-9 (25.6 mg, 0.0409 mmol)
and iodine (3.6 mg, 0.0142 mmol) in DMSO (0.6 mL) with
stirring was added concentrated H2SO4 (1 drop). After the
reaction mixture was kept at 85 °C for 20 min, additional
iodine (7.1 mg, 0.0279 mmol) and DMSO (0.4 mL) were added.
The mixture was stirred at 80 °C overnight and poured into
an ice-cold solution of Na2S2O3 and KOH in water. The
suspension was filtered, and the filtrate was subjected to
preparative TLC on silica gel with developing system of EtOH/
EtOAc (5:1) to give (aR)-(+)-10 (13.2 mg, 52%) as a colorless
solid: mp 155-156 °C (from CHCl3); IR (KBr) νmax 1638, 1339
tor, graphite crystal; linear absorption coefficient, 7.01 cm-1
;
temperature, 20 °C; scan speed, 14.0°/min; scan range, 1.64°
+ 0.2° tan θ; 2θ scan limits, 2-130°; standard reflections, 3
per 100 reflections; indices, (-9,-1,4), (9,1,-4), (7,-3,3);

4′,4′′′,7,7′′-Tetra-O-methylcupressuflavone
J . Org. Chem., Vol. 62, No. 21, 1997 7227
cm-1; 1H NMR (600 MHz, CDCl3) δ 3.67 (6 H, s), 3.78 (6 H, s),
4.13 (6 H, s), 6.59 (4 H, s), 6.77 (4 H, br d, J ) 9.0 Hz), 7.29 (4
H, br d, J ) 9.0 Hz); 13C NMR (150 MHz, CDCl3) δ 55.4, 56.1,
56.5, 91.6, 102.0, 106.7, 108.9, 114.3, 123.4, 127.1, 156.5, 160.6,
(S)-MTP A Ester s 11 of Ra cem ic Bifla von e (()-1. To a
solution of racemic biflavone (()-1 (0.9 mg, 0.0015 mmol) in
dry CH2Cl2 (0.5 mL) was added dry triethylamine (2 drops,
distilled from KOH) under argon. After the solution was
stirred for 10 min at room temperature, (S)-(+)-R-methoxy-R-
(trifluoromethyl)phenylacetyl chloride (MTPA-Cl, three drops)
was added. The reaction was kept at room temperature
overnight and then evaporated to dryness. The product was
purified by preparative TLC on silica gel developed with
hexane:EtOAc 2:1 to give (S)-MTPA diester of (()-biflavone
(11, 1.4 mg, 94%). The TLC confirmed that all biflavone (()-1
was converted to diester 11: 1H NMR (400 MHz, CDCl3) δ 3.77
(6 H, s), 3.80 (12 H, s), 3.81 (6 H, s), 3.90 (6 H, s), 3.91 (6 H,
s), 6.57 (2 H, s), 6.59 (2 H, s), 6.69 (2 H, s), 6.72 (2 H, s), 6.86
(4 H, d, J ) 9.1 Hz), 6.88 (4 H, d, J ) 9.1 Hz), 7.28 (4 H, d, J
) 9.1 Hz), 7.30 (4 H, d, J ) 9.1 Hz), 7.42 (2 H, m), 7.52 (8 H,
m), 7.57 (2 H, m), 7.91 (8 H, m).
(S)-MTP A Ester (a R)-11 of Op tica lly P u r e Bifla von e
[CD(+)362.0]-(a R)-(-)-1. Optically pure biflavone [CD(+)-
362.0]-(aR)-(-)-1 (0.6 mg, 0.001 mmol) was similarly converted
to (S)-MTPA diester (aR)-11 (0.9 mg, 91%): 1H NMR (400
MHz, CDCl3) δ 3.80 (6 H, s), 3.90 (6 H, s), 6.59 (2 H, s), 6.69
(2 H, s), 6.88 (4 H, d, J ) 9.1 Hz), 7.30 (4 H, d, J ) 9.1 Hz),
7.57 (2 H, m), 7.91 (8 H, m). The appearance of a peak at δ
6.69 ppm and complete loss of a peak at δ 6.72 ppm clearly
indicated that the biflavone (aR)-(-)-1 synthesized here was
enantiopure.
161.2, 161.7, 161.9, 178.2; [R]22 +23.4° (c 0.51, EtOH); UV
D
(EtOH) λmax 320.0 nm (ꢀ 40 100), 268.0 (49 900), 224.0 (62 700);
CD (EtOH) λext 349.2 nm (∆ꢀ +22.3), 312.4 (-42.1), 262.0
(+16.9); MS m/ z 622 (M+, relative intensity 100), 375 (68).
Anal. Calcd for C36H30O10: C, 69.45; H, 4.86. Found: C, 69.51;
H, 5.06.
4′,4′′′,7,7′′-Tetr a-O-m eth ylcu pr essu flavon e ([CD(+)362.0]-
(a R)-(-)-1). To an ice-cold solution of (aR)-(+)-10 (12.7 mg,
0.0204 mmol) in dry CH2Cl2 (1 mL) was added a solution of
BCl3 in CH2Cl2 (1 M, 50 mL, 0.05 mmol) under argon. The
reaction mixture was stirred for 1 h, and then MeOH was
added. After being stirred at 70 °C for 3 h, the mixture was
evaporated in vacuo. The crude product obtained was sub-
jected to preparative TLC developed with hexane/EtOAc (1:1)
to yield the final biflavone (aR)-1 in optically active form (9.7
mg, 80%, 78% ee): UV (EtOH) λmax 324.4 nm (ꢀ 40 000), 273.4
(40 600), 225.6 (51 400); CD (EtOH) λext 362.8 nm (∆ꢀ +19.8),
326.6 (-41.5), 301.6 (-12.6), 269.2 (+16.6), 225.8 (+6.7), 207.8
(+8.4). It was interesting to note that the racemate of
biflavone 1 is hardly soluble in MeOH. Therefore when the
optically active biflavone (aR)-1 with 78% ee was recrystallized
from methanol, optically pure biflavone (aR)-1 was obtained
from the mother liquor. Physical data of 100% ee biflavone
[CD(+)362.0]-(aR)-(-)-1: mp 147-149 °C (MeOH); IR (KBr)
Ack n ow led gm en t. This work was supported in part
by grants from the Ministry of Education, Science,
Sport, and Culture, J apan (General (A) No. 07408032,
Priority Areas Nos. 02250103, 03236101, 04220101, and
06240204, Developmental Research No. 05554017, and
International J oint Research No. 02044014 to N.H.;
Encouragement of Young Scientists No. 08780536 to
H.L.).
ν
max 2925, 1652, 1590 cm-1; 1H NMR (600 MHz, CDCl3) δ 3.80
(6 H, s), 3.82 (6 H, s), 6.59 (2 H, s), 6.60 (2 H, s), 6.86 (4 H, d,
J ) 9.0 Hz), 7.43 (4 H, br d, J ) 9.0 Hz), 13.22 (2 H s); 13C
NMR (150 MHz, CDCl3) δ 55.5, 56.2, 95.3, 103.5, 114.5, 127.6,
154.6, 162.57, 162.61, 163.3, 163.9, 182.9; [R]22 -11.3° (c
D
0.261, MeOH), see Table 1 for the solvent effect of [R]D value;
UV (EtOH) λmax 359.0 nm (sh, ꢀ 22 000), 325.2 (40 800), 273.0
(42 200), 225.2 (55 200); UV (MeOH) λmax 325.0 nm (ꢀ 37 100),
272.4 (37 900), 225.2 (47 400); UV (CHCl3) λmax 327.4 nm (ꢀ
47 500), 274.4 (49 400); CD (EtOH) λext 363.0 nm (∆ꢀ +25.5),
327.0 (-55.3), 300.8 (sh, -15.8), 269.2 (+21.3), 226.4 (+9.1),
208.0 (+11.8), see Table 1 for the solvent dependence of CD
data; MS m/ z 594 (M+, relative intensity 100), 595 (M+ + 1,
49), 135 (28); HRMS calcd for C34H26O10 594.15258, found
594.15195.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedure for the synthesis of 4, as well as 1H and 13C NMR
spectra of key compounds (aS)-(+)-8a , (aR)-(-)-8b , and
[CD(+)362.0]-(aR)-(-)-1 (4 pages). This material is contained
in libraries on microfiche, immediately follows this article in
the microfilm version of the journal, and can be ordered from
the ACS; see any current masthead page for ordering
information.
The optical purity of biflavone (aR)-(-)-1 obtained as
crystals was 25% ee: CD (CHCl3) λext 362.6 nm (∆ꢀ +6.6), 326.4
(-14.2), 276.0 (+5.0).
J O970670W