First, atropo-enantioselective total synthesis of the axially chiral phenylanthraquinone natural products knipholone and 6′-O-methylknipholone

Article abstract of DOI:10.1002/1521-3773(20010504)40:9<1687::AID-ANIE16870>3.0.CO;2-6

The first stereoselective access to an axially chiral arylanthraquinone, the antimalarial natural product knipholone (3), was achieved by application of the "lactone concept". Key steps were the intramolecular coupling of the bromoester 1, to give the configurationally unstable biaryl lactone 2, and the enantioselective ring cleavage to form 3.

Full text of DOI:10.1002/1521-3773(20010504)40:9<1687::AID-ANIE16870>3.0.CO;2-6

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Our data demonstrate that a significant enhancement in the
performance of CEC over capillary HPLC lies in the different
dimension of the perfusion mechanism. The intraparticle
tortuosity factor tintra plays a key role in achieving that goal in
both cases, and may also have a sensitive influence on pore
migration for charged solutes. Further development in CEC
particle technology should focus on the minimum pore size of
the through-pore network which still allows a significant
intraparticle EOF at decent buffer concentrations, while
keeping the surface-to-volume ratio of the pore space
attractive for the separation (gigapores, which are used in
pressure-driven flows, are not required for electroosmotic
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hierarchically structured media of micrometer dimension,
which leaves molecular diffusion as the ultimate limitation to
performance.[40, 41] Thus, the perfusive EOF field translates to
an even higher separation efficiency than can currently be
achieved in CEC, increased mass sensitivity in on-line
coupling schemes (such as nano-ESI-MS), and the possibility
of using pressurized CEC for higher analysis speed and flow
stability, without significant increase in dispersion.
Ä
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First, Atropo-Enantioselective Total Synthesis
of the Axially Chiral Phenylanthraquinone
Natural Products Knipholone and
6'-O-Methylknipholone**
Received: October 30, 2000 [Z16012]
[1] N. B. Afeyan, N. F. Gordon, I. Mazsaroff, L. Varady, S. P. Fulton, Y. B.
Yang, F. E. Regnier, J. Chromatogr. 1990, 519, 1 ± 29.
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Gerhard Bringmann* and Dirk Menche
Dedicated to Professor Wolfgang Kiefer
on the occasion of his 60th birthday
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1992, 2, 62 ± 72.
Among the more than one hundred anthraquinone natural
products with biaryl axes,[1] phenylanthraquinones,[2±4] like
knipholone (1a)[2] and 6'-O-methylknipholone (1b),[3] occupy
a special position: Since they are constitutionally unsymmet-
ric, they are most likely formed biosynthetically by a directed,
enzymatic biaryl coupling and not merely by a ªchemicalº
dimerization of the corresponding monoanthraquinones. A
further hint at such an enzymatic origin of 1a and 1b is the
fact that they are optically active and are thus axially chiral.
First isolated by Dagne and Steglich in 1984,[2] knipholone
(1a) and related phenylanthrachinones have been found in
numerous African plant species of the genera Bulbine,
Bulbinella, and Kniphofia (all Asphodelaceae),[4] which are
widely used in folk medicine.[2, 5]
Â
[5] D. D. Frey, E. Schweinheim, Cs. Horvath, Biotechnol. Prog. 1993, 9,
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[9] J. J. Meyers, A. I. Liapis, J. Chromatogr. A 1998, 827, 197 ± 213.
[10] J. F. Pfeiffer, J. C. Chen, J. T. Hsu, Am. Inst. Chem. Eng. J. 1996, 42,
932 ± 939.
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800 ± 814.
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Electrophoresis 1997, 18, 2162 ± 2174.
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Wiley, New York, 1994.
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837, 3 ± 15.
[16] R. Stol, W. Th. Kok, H. Poppe, J. Chromatogr. A 1999, 853, 45 ± 54.
[*] Prof. Dr. G. Bringmann, Dipl.-Chem. D. Menche
Institut für Organische Chemie
Â
[17] E. Wen, R. Asiaie, Cs. Horvath, J. Chromatogr. A 1999, 855, 349 ± 366.
Universität Würzburg
[18] A. I. Liapis, B. A. Grimes, J. Chromatogr. A 2000, 877, 181 ± 215.
[19] P. T. Vallano, V. T. Remcho, Anal. Chem. 2000, 72, 4255 ± 4265.
[20] R. Stol, H. Poppe, W. Th. Kok, J. Chromatogr. A 2000, 887, 199 ± 208.
[21] B. A. Grimes, J. J. Meyers, A. I. Liapis, J. Chromatogr. A 2000, 890,
61 ± 72.
[22] C. L. Rice, R. Whitehead, J. Phys. Chem. 1965, 69, 4017 ± 4024.
[23] R. J. Gross, J. F. Osterle, J. Chem. Phys. 1968, 49, 228 ± 234.
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Am Hubland, 97074 Würzburg (Germany)
Fax : (‡49)931-888-4755
E-mail: bringman@chemie.uni-wuerzburg.de
[**] Novel Concepts in Directed Biaryl Synthesis, Part 93. This work was
supported by the Deutsche Forschungsgemeinschaft (grant: SFB 347)
and by the Fonds der Chemischen Industrie. We thank Prof. B. M.
Abegaz und Dr. M. Bezabih for an authentic sample of 6'-O-
methylknipholone and for valuable discussions. We thank J. Kraus
for helpful suggestions and V. Barthel for technical support. Part 92:
G. Bringmann, J. Hinrichs, K. Peters, E.-M. Peters, J. Org. Chem. 2001,
66, 629 ± 632.
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Ï
[27] P. T. Callaghan, J. Stepisnik, Adv. Magn. Opt. Reson. 1996, 19, 325 ±
388.
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Only recently the axial configuration of knipholone (1a)
has been determined to be M, by quantum chemical CD
calculations.[6] Due to its interesting structure (including the
rotationally hindered biaryl axis) and its good antimalarial
activity with only little cytotoxicity,[7] knipholone (1a) con-
stitutes an attractive synthetic target, as do related phenyl-
anthraquinone natural products. In this paper, we report on
the atroposelective total synthesis of knipholone (1a) and the
likewise naturally occurring 6'-O-methyl ether 1b and, thus,
on the first stereoselective approach to axially chiral anthra-
quinone natural products.[8]
For the directed construction of the biaryl axis we chose the
ªlactone methodº as developed in our group.[9] Envisaged key
steps of the resulting synthetic strategy (Scheme 1) were the
intramolecular biaryl coupling of the bromoester 4 to give the
biaryl lactone 3, the atropo-enantioselective cleavage of 3 to
provide the ªopenº biaryl 2, and the regioselective introduc-
tion of an acetyl group on C-3'.
Scheme 2. Synthesis of the anthraquinone building block 10. a) NBS,
(PhCO2)2, CCl4, reflux, 6 h; b) CaCO3, dioxane/H2O, 1208C, 7 h, 71%
(over two steps); c) iPrI, Cs2CO3, acetone, reflux, 4 d, 72%; d) Br2,
NaOAc, CHCl3/CCl4, reflux, 2 h, 32%; e) Ac2O, pyridine, 508C, 3 h, 94%;
f) Br2 (4 equiv), NaOAc, CHCl3/CCl4, 708C, 3 h, 89%; g) KOH, MeOH,
708C, 2 h, 91%; h) MnO2, CH2Cl2, RT, 6 h, 93%; i) NaClO2, amidosul-
furic acid, NaOAc, dioxane/H2O/HOAc, RT, 97%. NBS ˆ N-bromosuccin-
imide.
of 5 by O-isopropylation and subsequent bromination of 7 to
give 8 (Scheme 2).
Due to the necessary resolution of regioisomeric bromina-
tion products, the sensitivity of the side chain to oxidation
under the bromination conditions, and the low yields that
were obtained, it seemed, however, far more convenient to
introduce two bromo-substituents, that is, at C-4 and C-5,
straightaway after protection of the free hydroxy group of 7 by
O-acetylation. The bromination then proceeded smoothly to
give 9 in 89% yield. The second bromine at C-5, which was
functionally not required, did not interfere with any of the
following reaction steps (see below). From 9, the anthraqui-
none carboxylic acid 10 could easily be prepared by hydrolysis
of the ester and stepwise oxidation (Scheme 2).
Thus, the building block 10 was available in a suitably
activated and protected form and in sufficient quantities. For
the ªlowerº half of the molecule, which in its free form is also
a natural product (xanthoxylin),[14] the precursor was, how-
ever, not a trioxyacetophenone, but the acetyl-free phenolic
compound 11 (Scheme 3). According to preliminary inves-
tigations it seemed more favorable not to have the acetyl
group (as present in 1a and 1b) in the molecule from the
beginning, but to introduce it only after the scheduled
subsequent reduction steps.
Scheme 1. Retrosynthetic analysis of knipholone (1a) and its 6'-O-methyl
ether 1b.
The ªupperº molecular halves of the target molecules 1a
and 1b correspond to the natural product chrysophanol (6).
For the synthesis of a suitably functionalized anthraquinone
building block (Scheme 2), one can start directly from the
likewise natural[10] aloe-emodin (5), whose side chain is
already oxygenated. This can be conveniently obtained in
large quantities from purchasable aloin[11] or, as we describe
here, from chrysophanol (6), which is commercially available
or can easily be prepared by total synthesis.[12]
Radical side-chain bromination of 6 with subsequent
bromine/hydroxy group exchange gave aloe-emodin (5) in a
smooth reaction; compound 5 can be now conveniently
prepared on a multi-gram scale. A selective bromination of
the diacetate of 5 in the 4-position, as required, has already
been described in the literature.[13] However, this actually
takes place exclusively at C-2 under the given conditions, so
that a different synthetic strategy had to be pursued here. The
desired activation of the 4-position was finally successful, as
could be unambiguously shown by NMR spectroscopy
(HMBC, NOE), after steric blocking of the ªnorthern partº
The ester 12, synthesized from 10 and 11, was submitted to
the conditions of a Pd-catalyzed intramolecular biaryl cou-
pling.[9] The concern that the ªlowerº carbonyl group of the
quinoid system (C-10) might prevent or, at least, hamper the
ring closure to form the lactone 13 proved to be unfounded: In
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bridged biaryl 13 was configurationally unstable; this fortu-
nate circumstance permitted the subsequent ring-cleavage
reactions to be performed highly efficiently, according to the
principle of a dynamic kinetic resolution. The oxazaboro-
lidine-assisted borane reduction, which has also been success-
fully used on previous occasions, proved to be the most
effective method for this purpose.[9, 15] By means of this so-
called CBS reaction,[16] the lactone in the rapidly intercon-
verting mixture of the atropo-enantiomers (P)-13>(M)-13
could be cleaved selectively; such that depending on the
oxazaborolidine enantiomer used, (S)-14 or (R)-14, either the
desired atropisomer (P)-15 or, optionally, the undesired
ªwrongº enantiomer (M)-15 could be obtained in high
atropisomeric excesses (up to 96% ee) through an atropo-
enantiodivergent reaction.[17] The optical purity of the product
(for example, of the required P isomer) could be further
enhanced (to >99% ee) by a simple crystallization step. The
small quantities of the (almost) racemic mother liquor thus
obtained (14% ee) canÐif desiredÐbe recycled into the
process, by oxidation (MnO2, NaClO2) and cyclization (DCC,
DMAP) back to the configurationally unstable lactone (71%
chemical yield).
With the phenylanthraquinone framework now constructed
enantioselectively, including the correct configuration at the
biaryl axis, the benzylic oxygen function of 15 was reductively
eliminated by hydroxy/bromine group exchange and hydro-
genolytic removal of the bromine substituent. For this, the
reaction conditions could be controlled in such a way that,
within a one-pot reaction, the additional, no longer required
bromine functionality at C-5 was simultaneously removed.
After cleavage of the O-isopropyl protective groups with
titanium tetrachloride, the likewise TiCl4-mediated, strictly
regioselective introduction of the acetyl group at C-3' was
achieved by the use of acetic anhydride. The 6-O-methylkni-
pholone (1b) thus obtained proved to be chromatographically
and spectroscopically identical with authentic 1b from
Bulbine capitata.[3] This first synthesis of a phenylanthraqui-
none natural product confirms, in all details, the structure as
established by Abegaz and co-workers.[3]
For the synthesis of knipholone (1a) itself, it was still
necessary to cleave the methoxy group at C-6'. While
nucleophilic reagents (such as LiPH2[18]) almost exclusively
attacked the ªwrongº ether functionality at C-4', a selective
O-demethylation of the 6'-oxygen functionality was achieved
with AlBr3. The (‡)-knipholone (1a) thus obtained was
identical in all respects to an authentic sample from Bulbine
frutescens[6] and to the data[2] published for 1a by Dagne and
Steglich.
As demonstrated for the atropo-enantiodivergent cleavage
of 13 to form (P)- or (M)-15, the strategy for the first synthesis
of knipholone (1a) and 6'-O-methylknipholone (1b) present-
ed here can, in principle, likewise be adopted for the synthesis
of the other respective atropo-enantiomeric product (ent-1a
or ent-1b). Analogously, the concept should be easily
applicable to the synthesis of the other phenylanthraquinones
and -anthrones described in the literature,[2±4] and, further-
more, to the preparation of unnatural analogues with possibly
even better antimalarial activities. This work is presently
under investigation.
Scheme 3. Atropo-enantioselective coupling of the molecular halves of
knipholone (1a). a) DCC, DMAP, CH2Cl2/DMF, 08C !RT, 1 h, 90%;
b) Pd(OAc)2 (60 mol%), PPh3 (1.2 equiv), NaOPiv (2 equiv), DMA,
1308C, 4.5 h, 68%; c) (S)-14 (3 equiv), BH3 (4 equiv), THF, 08C, 1 h,
81%, 96% ee (after crystallization from dichloromethane/diethyl ether/
n-hexane >99% ee); c') analogous procedure with (R)-14; d) (CBrCl2)2
(1.5 equiv), polymer-bound PPh3 (3 equiv), CH2Cl2, RT, 10 min; e) Pd/C
(10 mol%), H2, MeOH, 48% (over 2 steps); f) TiCl4 (6 equiv), CH2Cl2,
208C !RT, 2 h, 89%; g) Ac2O, TiCl4, 208C !RT, 1 h, 82%; h) AlBr3
(5 equiv), chlorobenzene, 808C, 2 h, 41%. DCC ˆ N,N'-dicyclohexylcarbo-
diimide, DMAP ˆ 4-dimethylaminopyridine, DMF ˆ N,N-dimethylform-
amide, DMA ˆ N,N-dimethylacetamide, NaOPiv ˆ sodium pivalate.
the presence of Pd(OAc)2, 12 was smoothly coupled intra-
molecularly to give 13. The bromine atom at C-5, which was
not required for the cyclization, remained unaffected.
Through the following conversions (see below) it became
obvious thatÐdespite the oxygen functionality at C-10Ðthe
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1993, 34, 1440 ± 1441; b) F. van Staden, S. E. Drewes, Phytochemistry
1994, 35, 685 ± 686; c) A. Yenesew, E. Dagne, M. Müller, W. Steglich,
Phytochemistry 1994, 37, 525 ± 528; d) E. Dagne, A. Yenesew, Pure
Appl. Chem. 1994, 66, 2395 ± 2398; e) B.-E. van Wyk, A. Yenesew, E.
Dagne, Biochem. Syst. Ecol. 1995, 23, 277 ± 281; f) G. Alemayehu, A.
Hailu, B. M. Abegaz, Phytochemistry 1996, 42, 1423 ± 1425; g) M.
Bezabih, B. M. Abegaz, Phytochemistry 1998, 48, 1071 ± 1073.
[5] a) J. M. Watt, M. G. Breyer-Brandwijk, The Medicinal and Poisonous
Plants of Southern and Eastern Africa, 2nd ed., Livingstone, Edin-
burgh, 1962, pp. 568 ± 574, p. 695, p. 707; b) M. Gelfand, S. Mavi, R. B.
Drummond, B. Ndemera, The Traditional Medical Practitioner in
Zimbabwe, Mambo Press, Gweru, 1993, p. 89.
Experimental Section
Compound 13: A mixture of 12 (1.00 g, 1.51 mmol), palladium(ii) acetate
(203 mg, 910 mmol), triphenylphosphane (475 mg, 1.81 mmol), and sodium
pivalate (375 mg, 3.02 mmol) were dried in vacuo (10 2 mbar) for 1.5 h at
608C, and then dry N,N-dimethylacetamide (40 mL) was added. The
orange-colored suspension was degassed three times and heated under
argon for 4.5 h at 1308C. After cooling to room temperature, the dark
brown suspension was diluted with ethyl acetate, washed sequentially with
2n HCl and saturated aqueous NaCl solution, dried (Mg2SO4), and
concentrated in vacuo. After flash chromatography on silica gel (dichloro-
methane/ethyl acetate 100:2), 13 (597 mg, 1.03 mmol, 68%) was obtained
[6] G. Bringmann, J. Kraus, D. Menche, K. Messer, Tetrahedron 1999, 55,
7563 ± 7572.
[7] G. Bringmann, D. Menche, M. Bezabih, B. M. Abegaz, R. Kaminsky,
Planta Med. 1999, 65, 757 ± 758.
[8] For the preparation of the bridged biarylic anthraquinone pradimi-
cinon in enantiomerically pure form, albeit by racemate resolution,
see: M. Kitamura, K. Ohmori, T. Kawase, K. Suzuki, Angew. Chem.
1999, 111, 1308 ± 1311; Angew. Chem. Int. Ed. 1999, 38, 1229 ± 1232.
[9] For a recent review, see: G. Bringmann, M. Breuning, S. Tasler,
Synthesis 1999, 525 ± 558.
as a red solid, which was crystallized from dichloromethane/petroleum
1
ether. M.p. 1388C; IR (KBr): nÄ ˆ 2950, 1710, 1660, 1190, 1100 cm
;
1H NMR (200 MHz, CDCl3): d ˆ 1.40 ± 1.56 (m, 12H; CH(CH3)2), 3.67 (s,
3H; OCH3), 3.87 (s, 3H; OCH3), 4.64 (sept, J ˆ 6.1 Hz, 1H; CH(CH3)2),
4.77 (sept, J ˆ 6.1 Hz, 1H; CH(CH3)2), 6.35 (d, J ˆ 2.3 Hz, 1H; H-3' or
H-5'), 6.53 (d, J ˆ 2.3 Hz, 1H; H-3' or H-5'), 7.12 (d, J ˆ 9.2 Hz, 1H; H-7),
7.80 (d, J ˆ 8.9 Hz, 1H; H-6), 7.94 (s, 1H; H-2); 13C NMR (63 MHz, CDCl3):
d ˆ 19.23 (CH(CH3)2), 21.88 (CH(CH3)2), 22.36 (CH(CH3)2), 55.75
(OCH3), 57.69 (OCH3), 73.11 (CH(CH3)2), 73.30 (CH(CH3)2), 93.79,
96,37, 102.53, 110.18, 116.67, 121.30, 123.62, 125.60, 128.02, 128.73, 133.02,
ˆ
[10] A. Tschirch, G. Pedersen, Arch. Pharm. 1898, 236, 200 ± 212.
[11] M. Rychener, W. Steiger, Pharm. Acta Helv. 1989, 64, 8 ± 15.
[12] A. T. Khan, B. Blessing, R. R. Schmidt, Synthesis 1994, 255 ± 257.
[13] J. Alexander, A. V. Bhatia, L. A. Mitscher, S. Omoto, T. Suzuki, J.
Org. Chem. 1980, 45, 20 ± 24.
137.89, 139.21, 152.44, 155.61, 155.86, 156.85, 160.41, 161.75, 180.07 (C O),
‡
ˆ
184.03 (C O); MS (70 eV): m/z (%): 582/580 (5/5) [M ], 509/507 (9/6)
‡
‡
‡
‡
[M
C4H9O ], 467/465 (15/11) [M
C8H19 ], 183 (100); elemental
analysis (%): calcd for C29H25O8Br (581.42): C 59.91, H 4.33; found: C
59.66, H 4.25.
[14] J. Stenhouse, Liebigs Ann. Chem. 1854, 89, 251 ± 262.
[15] G. Bringmann, T. Hartung, Tetrahedron 1993, 49, 7891 ± 7902.
[16] For a review on enantioselective reductions with the oxazaboro-
lidine ± borane reagent, see: E. J. Corey, C. J. Helal, Angew. Chem.
1998, 110, 2092 ± 2118; Angew. Chem. Int. Ed. 1998, 37, 1986 ± 2012.
[17] a) Enantiomeric ratios were determined by HPLC analysis on chiral
phase: Daicel Chiralcel OD-H, 25 cm, 4.6 mm diameter, detection at
280 nm, flow: 0.5 mLmin 1, eluent: n-hexane/2-propanol 65:35, re-
tention times: 29 min for (P)-15 and 46 min for (M)-15; b) Attribution
of the axial configuration of the alcohol 15 was done by CD
spectroscopy and by correlation with the stereochemically known[6]
final product 1a.
Compound (P)-15: The solvent was removed in vacuo from a solution of
(S)-14 (1.0m in toluene, 360 mL, 360 mmol) and the residue was dissolved
under argon in dry THF (1 mL). This solution was treated with a solution of
the BH3 ± THF complex (1.0m in THF, 480 mL) and stirred for 30 min at
room temperature. After dropwise addition of a solution of the lactone 13
(69.8 mg, 120 mmol) in dry tetrahydrofuran (1 mL) at 08C the solution was
stirred for 1 h at this temperature, then water (1 mL) and 2n HCl (1 mL)
were added and the aqueous phase was thoroughly extracted with ethyl
acetate. The combined organic phases were dried (MgSO4) and the solvents
were removed in vacuo. After flash chromatography of the residue on silica
gel (dichloromethane/ethyl acetate 7:3), (P)-15 (56.9 mg, 97.2 mmol, 81%)
was obtained as a yellow solid (96% ee). Crystallization from dichloro-
methane/diethyl ether/n-hexane yielded yellow crystals (45.6 mg,
[18] R. E. Ireland, D. M. Walba, Organic Syntheses Collective Volume, Vol.
6, Wiley, New York, 1988, p. 567 ± 570.
77.9 mmol, 65%; > 99% ee). M.p. 122 ± 1248C; [a]D20
ˆ
28 (c ˆ 0.01 in
methanol); IR (KBr): nÄ ˆ 3120 (br., OH), 2950, 1660, 1570, 1190,
1
1090 cm
;
1H NMR (400 MHz, CDCl3): d ˆ 1.37 ± 1.44 (m, 12H;
CH(CH3)2), 3.59 (s, 3H; OCH3), 3.76 (s, 3H; OCH3), 4.33 (d, J ˆ 13.7,
1H; CHHOH), 4.42 (d, J ˆ 13.7, 1H; CHHOH), 4.55 (sept, J ˆ 5.8 Hz, 1H;
CH(CH3)2), 4.67 (sept, J ˆ 5.8 Hz, 1H; CH(CH3)2), 6.12 (d, J ˆ 1.8 Hz, 1H;
H-3' or H-5'), 6.20 (d, J ˆ 1.8 Hz, 1H; H-3' or H-5'), 6.97 (d, J ˆ 8.8 Hz, 1H;
H-7), 7.43 (s, 1H; H-2), 7.60 (d, J ˆ 8.8 Hz, 1H; H-6); 13C NMR (63 MHz,
CDCl3): d ˆ 21.88 (CH(CH3)2), 21.98 (CH(CH3)2), 55.06 (OCH3), 55.57
(OCH3), 62.76 (CH2OH), 72.73 (CH(CH3)2), 73.46 (CH(CH3)2), 91.32,
94.05, 105.66, 109.82, 118.94, 121.31, 122.43, 124.61, 128.01, 135.62, 137.44,
High-Temperature Synthesis of an Open-
Framework Compound, Na2Cs2Cu3(P2O7)2Cl2
(CU-4), by Molten-Salt Methods**
ˆ
138.41, 147.96, 154.94, 155.78, 155.84, 157.21, 160.73, 182.40 (C O), 186.23
Qun Huang, Shiou-Jyh Hwu,* and Xunhua Mo
‡
‡
‡
ˆ
(C O); MS (70 eV): m/z (%): 586/584 (54/54) [M ], 543/541 (44/39) [M
C3H6 ], 495/493 (20/19) [M
‡
‡
‡
CH4O2 ], 453/451 (100/100) [M
C7H17O2 ]; elemental analysis (%): calcd for C29H25O8Br (585.45): C
59.50, H 4.99; found: C 59.59, H 4.74.
‡
Porous materials have stimulated much interest for their
applications in catalysis, ion-exchange, separation, sensor, and
molecular recognition.[1] Transition metal containing micro-
porous (TMCM) solids have attracted particular attention
because of their unique functions, such as redox catalysis,[2]
Received: November 23, 2000 [Z16167]
[1] For a recent overview on naturally occurring biaryl compounds, see:
G. Bringmann, C. Günther, M. Ochse, O. Schupp, S. Tasler in Progress
in the Chemistry of Organic Natural Products, Vol. 82 (Eds.: W. Herz,
H. Falk, G. W. Kirby, R. E. Moore, C. Tamm), Springer, New York,
2001, in press.
[2] E. Dagne, W. Steglich, Phytochemistry 1984, 23, 1729 ± 1731.
[3] M. Bezabih, S. Motlhagodi, B. M. Abegaz, Phytochemistry 1997, 46,
1063 ± 1067.
[*] Prof. Dr. S.-J. Hwu, Dr. Q. Huang, X. Mo
Department of Chemistry
Clemson University
Clemson, SC 29634-0973 (USA)
Fax : (‡1)864-656-6613
E-mail: shwu@clemson.edu
[**] Financial support for this research (DMR-9612148, DMR-0077321
and EPS-9977797) and the single-crystal X-ray diffractometer (CHE-
9207230) from the National Science Foundation is gratefully acknowl-
edged.
[4] A total of six axially chiral phenylanthraquinones have so far been
described, which differ in the methylation pattern of the acetylphloro-
glucinol unit and the oxidation state of the chrysophanol part
(quinone or anthrone): a) E. Dagne, A. Yenesew, Phytochemistry
1690
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4009-1690 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 9