Total synthesis and biological evaluation of (+)- and (-)-bisanthraquinone antibiotic BE-43472B and related compounds

Article abstract of DOI:10.1021/ja9073694

The bisanthraquinone antibiotic BE-43472B [(+)-1] was isolated by Rowley and co-workers from a streptomycete strain found in a blue-green algae associated with the ascidian Ecteinascidia turbinata and has shown promising antibacterial activity against cli

Full text of DOI:10.1021/ja9073694

Published on Web 09/24/2009
Total Synthesis and Biological Evaluation of (+)- and
(-)-Bisanthraquinone Antibiotic BE-43472B and Related
Compounds
K. C. Nicolaou,* Jochen Becker, Yee Hwee Lim, Alexandre Lemire,
Thomas Neubauer, and Ana Montero
Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research
Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, and Department of
Chemistry and Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe,
La Jolla, California 92093
Received May 14, 2009; E-mail: kcn@scripps.edu
Abstract: The bisanthraquinone antibiotic BE-43472B [(+)-1] was isolated by Rowley and co-workers from
a streptomycete strain found in a blue-green algae associated with the ascidian Ecteinascidia turbinata
and has shown promising antibacterial activity against clinically derived isolates of methicillin-susceptible,
methicillin-resistant, and tetracyclin-resistant Staphylococcus aureus (MSSA, MRSA, and TRSA, respectively)
and vancomycin-resistant Enterococcus faecalis (VRE). Described herein is the first total synthesis of both
enantiomers of this bisanthraquinone antibiotic, the determination of its absolute configuration, and the
biological evaluation of these and related compounds. The developed synthesis relies on a highly efficient
cascade sequence involving an intermolecular Diels-Alder reaction between diene (R)-61 and dienophile
55, followed by an intramolecular nucleophilic aromatic ipso substitution. Late-stage transformations included
a remarkable photochemical R,ꢀ-epoxyketone rearrangement [80 f (+)-1]. Interestingly, the unnatural
enantiomer [(-)-1] of antibiotic BE-43472B exhibited antibacterial properties comparable to those of the
natural enantiomer [(+)-1].
Introduction
Bisanthraquinones 1 and 3 (Figure 1) were reported by
Rowley and co-workers as two naturally occurring antibiotics
with biological activities against drug-resistant bacteria.1,2
Possessing striking molecular architectures, these compounds
were isolated by these researchers from Streptomyces strain no.
N1-78-1, which was isolated from cultured cells of an unidenti-
fied unicellular blue-green algae (URI strain no. N36-11-10)
extracted from the ascidian Ecteinascidia turbinata, collected
from La Parguera, Puerto Rico. Interestingly, the gross structures
of these compounds, and their monomethylated siblings 2 and
4 (Figure 1), were previously reported in a Japanese patent as
antitumor agents.3 Isolated from Streptomyces sp. A43472, these
compounds were designated in this patent as BE-43472A (3),
BE-43472B (1), BE-43472C (4), and BE-43472D (2). Although
the Rowley group was able to assign the complete relative
stereochemistry of the bisanthraquinone antibiotics BE-43472B
(1) and BE-43472A (3) on the basis of NMR spectroscopic
analysis, and to point out their likely identities to those isolated
by the Japanese workers, they left their absolute configuration
unassigned.1,2
Figure 1. Bisanthraquinone antibiotics BE-43472A-D (1-4).
These bisanthraquinone antibiotics exhibited, in addition to
antitumor properties, impressive inhibitory activities against a
variety of bacteria.1,2 Antibiotic BE-43472B (1) demonstrated
potent inhibitory activities against an expanded panel of clinical
bacterial isolates of Gram-positive pathogens, including methi-
cillin-susceptible Staphylococcus aureus [MSSA, MIC50 ) 0.11
µM (range 0.054-0.22, 25 isolates)], methicillin-resistant
Staphylococcus aureus [MRSA, MIC50 ) 0.23 µM (range
0.11-0.90, 25 isolates)], vancomycin-resistant Enterococcus
faecalis [VRE, MIC50 ) 0.90 µM (range 0.22-3.6, 25 isolates)],
and tetracycline-resistant Staphylococcus aureus [TRSA, MIC50
) 0.11 µM (range 0.11-0.23, 11 isolates)]. Most significantly,
in a time-kill study, BE-43472B (1) exhibited strong bactericidal
activity (>99.9% kill) against MSSA, MRSA, and VRE.2 This
(1) Socha, A. M.; Garcia, D.; Sheffer, R.; Rowley, D. C. J. Nat. Prod.
2006, 69, 1070.
(2) Socha, A. M.; LaPlante, K. L.; Rowley, D. C. Bioorg. Med. Chem.
2006, 14, 8446.
(3) Kushida, H.; Nakajima, S.; Koyama, T.; Suzuki, H.; Ojiri, K.; Suda,
H. Antitumoric BE-43472 Manufacture with Streptomyces. Japan
Patent JP 08143569, 1996.
9
14812 J. AM. CHEM. SOC. 2009, 131, 14812–14826
10.1021/ja9073694 CCC: $40.75  2009 American Chemical Society

Total Synthesis of Bisanthraquinone Antibiotic BE-43472B
A R T I C L E S
Scheme 1. Initial Retrosynthetic Analysis of Bisanthraquinone
(-)-1
Figure 2. Enantiomers of bisanthraquinone antibiotic BE-43472B [(-)-1
and (+)-1] (top) and their computer-generated molecular models (bottom,
fully optimized at the B3LYB/6-31G* level, Spartan ’06 suite of programs).
Carbon, gray; hydrogen, white; oxygen, red; H-bonds, dotted gray.
rings, each containing an oxygen atom and featuring a ketal
functionality. The two anthraquinone structural motifs are held
together by a crowded carbon-carbon bond and a carbon-
oxygen-carbon bridge. Its five stereogenic centers reside in a
cluster resulting in considerable distortion of the molecule’s
otherwise flat regions. Figure 2 presents the two enantiomeric
forms of BE-43472B (1) and their computer energy-minimized
frame models. Since the absolute stereochemistry of the target
molecule was unknown at the outset of our work, our synthetic
plan had to accommodate both enantiomers, one at a time, from
readily available starting materials and with subsequent
stereocontrol.
In our initial retrosynthetic analysis, shown in Scheme 1 (with
the originally depicted1,2 absolute configuration, which eventu-
ally turned out to be antipodal to that of the natural product),
we first removed the rather labile C-3 hydroxyl group1 and
protected the two phenolic groups of the BE-43472B (1)
molecule to reveal structure 5 as a potential precursor to our
target. By removing the C-3 hydroxyl group, we ensured
stability for the intermediates on the way toward the final stages
where we would seek an opportunity to install it. The ketal
functionality within the advanced precursor 5 was then dis-
mantled, unraveling intermediate 6, protected at the secondary
alcohol site. Holding the two, now conspicuous, anthraquinone
moieties together in 6 was only a single bond (C-4a-C-7′). This
key carbon-carbon bond was envisioned to be formed through
a Michael-type addition of the enolate derived from bromoke-
tone 8 (or its debromo analogue) to enetrione 7, followed by
HBr elimination and aromatization (or oxidation/aromatization)
to generate the requisite phenolic structural motif of 6 (R ) H
or protecting group in 5-7).
compound was found to possess no activity against the Gram-
negative pathogens Klebsiella pneumoniae (ATCC 700603) and
Escherichia coli (ATCC 35218), while it demonstrated signifi-
cant potency (IC50 ) 2.0 µM) against the human colon cancer
cell line HCT-116.
In view of the continuing search for new antibacterial agents
to combat infections due to drug-resistant bacteria,4 and because
of the unprecedented structures of these new antibiotics, their
chemical synthesis was deemed important.5 In a recent com-
munication, we reported the first total synthesis and absolute
configuration of bisanthraquinone antibiotic BE-43472B (1) and
its enantiomer (Figure 2).6,7 In this article, we provide the full
account of our work in this area that includes the evolution of
the synthetic strategy toward both enantiomers of 1 and related
compounds and their biological evaluation.
Results and Discussion
General Retrosynthetic Analysis: The Michael Reaction
Approach. The T-shaped molecular architecture of the bisan-
thraquinone antibiotics as exemplified by BE-43472B (1) is both
unprecedented and challenging from the synthetic point of view.
Its C32H24O9 formula reveals its highly conjugated/unsaturated
nature with two nonidentical anthraquinone moieties serving
as its two dominant regions. These domains are fused together
through a bicyclic system comprised of two five-membered
(4) For selected references, see: (a) Nicolaou, K. C.; Chen, J. S.; Edmonds,
D. J.; Estrada, A. A. Angew. Chem., Int. Ed. 2009, 48, 660. (b) Roberts,
L.; Simpson, S. Science 2008, 321, 355, and references therein. (c)
Walsh, C. T.; Wright, G. Chem. ReV. 2005, 105, 391. (d) Nicolaou,
K. C.; Boddy, C. N. C.; Bra¨se, S.; Winssinger, N. Angew. Chem., Int.
Ed. 1999, 38, 2096.
Initial Model Studies: Formation of Key Carbon-Carbon
Bond and Aromatization. In order to test the feasibility of the
key carbon-carbon (C-4a-C-7′) bond-forming step of our
designed strategy toward antibiotic BE-43472B (1), a model
study was undertaken using simplified aryl ketone 9 as the donor
and quinone 10 as the acceptor in the projected Michael reaction
(5) For previous synthetic studies inspired by BE-43472B, see: (a)
Takikawa, H.; Hikita, K.; Suzuki, K. Angew. Chem., Int. Ed. 2008,
47, 9887. (b) Suzuki, K.; Takikawa, H.; Hachisu, Y.; Bode, J. W.
Angew. Chem., Int. Ed. 2007, 46, 3252.
(6) For a preliminary communication on this work, see: Nicolaou, K. C.;
Lim, Y. H.; Becker, J. Angew. Chem., Int. Ed. 2009, 48, 3444.
(7) Highlight: Rowley, D. C. Nature Chem. 2009, 1, 110.
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J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 14813

A R T I C L E S
Nicolaou et al.
Scheme 2. Initial Model Studies: Investigation of the Key Michael Reaction Stepa
a Reagents and conditions: (a) 9 (1.0 equiv), LiHMDS (1.0 equiv), THF, -78 °C, 2 h, then 10 (1.05 equiv), 15 min, quant. (11:epi-11 ca. 1:1); (b) 12 (1.0
equiv), LiHMDS (1.0 equiv), THF, -78 °C, 15 min, then 10 (1.1 equiv), 30 min, -78 f 25 °C, 1.5 h, 63%; (c) CAN (2.2 equiv), MeCN/H2O (2:1), 0 °C,
30 min, then CAN (1.5 equiv), 30 min, 93%.
regiochemical outcome of the addition of the enolate of 9 to
the enetrione system (10).
Having achieved the requisite carbon-carbon bond formation,
we then turned our attention to the oxidation/aromatization of
the so-obtained product 11. However, the seemingly simple task
of aromatizing ring F of this product proved much more
challenging than expected. Thus, and much to our dismay, all
attempts (e.g., exposure to acids, bases, heat, microwave
conditions) at aromatizing and/or oxidizing ring F of adduct 11
proved in vain, leading only to decomposition (mainly retro-
Michael products) or nonselective oxidations to an array of
unidentified products. In the end, a way forward was found by
abandoning ketone 9 and resorting to bromoketone 12 as a “pre-
oxidized” substrate for the Michael reaction with enetrione 10
(Scheme 2). Thus, treatment of bromoketone 12 (prepared from
9 by exposure to NBS) with LiHMDS (1.0 equiv, THF, -78
°C), followed by reaction of the generated enolate with enetrione
10 (1.1 equiv), resulted in the formation of Michael adduct 13,
albeit in lower yield than before (63%). It was interesting,
Figure 3. X-ray-derived ORTEP drawing of compound 11.
however, to observe in this case that only a single diastereomer
(stereochemistry unassigned) of 13 was observed (by 1H NMR
(Scheme 2). Thus, exposure of the known aryl ketone 98 to
spectroscopy). Exposure of this adduct to CAN in MeCN:H2O
LiHMDS (1.0 equiv, THF, -78 °C), followed by quenching of
(2:1) at 0 °C led to concurrent oxidation of the protected
hydroquinone ring G, HBr elimination, and aromatization of
ring F to furnish the desired phenolic quinone 14 in a pleasing
93% yield. It was with these encouraging results that we
embarked on our first attempt to synthesize BE-43472B (1) using
the Michael reaction as the key step to unite the two bisan-
thraquinone fragments of the molecule.
Initial Approach toward BE-43472B: Synthesis of Building
Blocks. Having completed successfully the model study for the
projected Michael addition/aromatization approach to antibiotic
BE-43472B (1), we then proceeded to construct the required
building blocks for the natural product, quinone 7 and bro-
moketone 8 (Scheme 1). For building block 8 and related
fragments, we developed an improved synthesis through modi-
the resulting enolate with enetrione 10,8,9 furnished in quantita-
tive yield the desired racemic Michael adducts 11 and epi-11,
which upon flash column chromatography (silica) equilibrated
to a ca. 1:1 mixture of inseparable epimers. Fortunately,
crystallization from acetone led to preferential formation of
crystals of 11 (mp ) 188-190 °C, acetone), whose structure
was proven by X-ray crystallographic analysis (see ORTEP
drawing, Figure 3). This crystallographic analysis not only
proved the identity of the two epimers, but also confirmed the
(8) Nicolaou, K. C.; Lim, Y. H.; Piper, J. L.; Papageorgiou, C. D. J. Am.
Chem. Soc. 2007, 129, 4001.
(9) Nicolaou, K. C.; Lim, Y. H.; Papageorgiou, C. D.; Piper, J. L. Angew.
Chem., Int. Ed. 2005, 44, 7917.
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Total Synthesis of Bisanthraquinone Antibiotic BE-43472B
A R T I C L E S
Scheme 4. Synthesis of Aryl Fragments 8, 21, 22, 23, and 26a
Scheme 3. Synthesis of Nitriles 19a and 19ba
a Reagents and conditions: (a) (COCl)2 (1.2 equiv), DMF (catalytic),
CH2Cl2, 0 °C, 30 min, 25 °C, 6 h, then Et2NH·HCl or Me2NH·HCl (1.3
equiv), Et3N (3.0 equiv), CH2Cl2 or THF, 0 f 25 °C, 12 h, 74% (16a),
88% (16b); (b) PMBCl (1.08 equiv), K2CO3 (1.5 equiv), TBAI (0.025
equiv), acetone, reflux, 48 h, 99%; (c) MOMCl (1.1 equiv), i-Pr2NEt (1.2
equiv), DMF, 60 °C, 48 h, 97%; (d) TMEDA (2.5 equiv), t-BuLi (2.5 equiv),
THF, -78 °C, then 17a (1.0 equiv) or 17b (1.0 equiv), 1 h, then DMF (3.0
equiv), 30 min, 25 °C, 2 h, 67% (18a), 95% (18b); (e) KCN (0.2 equiv),
18-crown-6 (0.2 equiv), TMSCN (1.4 equiv), 0 °C, 3 h, then AcOH, 25
°C, 12 h, 76% (19a), 85% (19b).
a Reagents and conditions: (a) 19a or 19b (1.0 equiv), LiHMDS (1.1
equiv), -78 °C, 2 h, then 20 (1.1 equiv), -78 f 25 °C, 18 h; (b) Me2SO4
(3.0 equiv), Cs2CO3 (3.3 equiv), acetone, reflux, 12 h, 37% (21a) from
18a, 59% (21b) over the two steps; (c) LiHMDS (1.0 equiv), THF, -78
°C, 1 h, then NBS (1.0 equiv) or (PhSO2)2NF (1.05 equiv), 1-1.5 h, 51%
(8), 98% (22); (d) NBS (1.05 equiv), AIBN (0.1 equiv), benzene, 70 °C,
15 min, then AIBN (0.1 equiv), 1 h, 52%; (e) LiHMDS (2.4 equiv), THF,
-78 °C, 1 h, then NBS (2.0 equiv), 1.5 h; (f) DBU (1.2 equiv), -78 f 25
°C, 10 h; (g) MOMCl (2.5 equiv), Cs2CO3 (2.5 equiv), DMF, 0 f 25 °C,
10 h, 28% over the three steps.
fication of our previous route8 based on the Hauser annulation
reaction. The required nitrile components 19a and 19b were
prepared as shown in Scheme 3. Thus, exposure of 4-methyl-
salicylic acid (15) to oxalyl chloride (1.2 equiv) in the presence
of catalytic amounts of DMF in CH2Cl2, followed by reaction
of the resulting acid chloride with either Et2NH·HCl (1.3 equiv)
or Me2NH·HCl (1.3 equiv), afforded phenolic amides 16a (74%
yield) or 16b (88% yield), respectively. These intermediates (16a
or 16b) served as precursors to the corresponding protected
derivatives (17a or 17b) which were prepared by protection of
the free phenolic hydroxy group with either a PMB (PMBCl,
K2CO3, n-Bu4NI cat., 99% yield of 17a) or a MOM (MOMCl,
i-Pr2NEt, 97% yield of 17b) group. o-Lithiation10 of aryl amides
17a or 17b (t-BuLi, THF, -78 °C), followed by quenching with
freshly distilled DMF, furnished aldehydes 18a or 18b in 67
and 95% yield, respectively. Treatment of aldehydes 18a or 18b
with TMSCN in the presence of catalytic amounts of KCN and
18-crown-6, followed by quenching with AcOH, then led to
nitriles 19a or 19b in 76 and 85% yield, respectively, as shown
in Scheme 3.
cyclohex-2-enone (20), gave the corresponding dihydroquinones,
which upon methylation (Cs2CO3, Me2SO4) led to methylated
ketones 21a (37% yield from aldehyde 18a over the three steps)
or 21b (59% yield from nitrile 19a over the two steps),
respectively. Treatment of ketone 21a with 1.0 equiv of
LiHMDS in THF at -78 °C, followed by quenching with 1.0
equiv of NBS, furnished the desired bromoketone 8 in 51%
yield. On the other hand, the use of 2.4 equiv of LiHMDS and
2.0 equiv of NBS in the above reaction led to dibromoketone
24, which upon exposure to DBU furnished bromoanthracene
25, demonstrating that the aromatization of such systems is
possible and providing access to aryl bromide precursors for
coupling reactions with suitable Michael acceptors. Finally,
protection of the phenolic group within 25 with MOMCl
(Cs2CO3) led to MOM derivative 26 in 28% overall yield for
the three steps.
The conversions of nitriles 19a and 19b to the desired
bromoketone 8, hydroquinone derivatives 21-23, and bromoan-
thracene 26 (which were also called upon to act as Michael
donors later on in the campaign, Vide infra) through a Hauser
annulation11 are shown in Scheme 4. Thus, treatment of 19a or
19b with LiHMDS in THF at -78 °C, followed by addition of
The required quinone fragment 7a and related compounds
7b and 7c were envisaged to be constructed through a route
featuring a diastereoselective Diels-Alder reaction between
chiral diene 33a and juglone (35) as the dienophile.12 The
requisite diene 33a was synthesized starting from (S)-ethyl
(10) Reviews: (a) Snieckus, V. Chem. ReV. 1990, 90, 879. (b) Whisler,
M. C.; MacNeil, S.; Snieckus, V.; Beak, P. Angew. Chem., Int. Ed.
2004, 43, 2206.
(12) For Diels-Alder reactions with juglone as dienophile, see: (a) Trost,
B. M.; Ippen, J.; Vladuchick, W. C. J. Am. Chem. Soc. 1977, 99, 8116.
(b) Stork, G.; Hagedorn, A. A., III. J. Am. Chem. Soc. 1978, 100,
3609. (c) Boeckman, R. K., Jr.; Dolak, T. M.; Culos, K. O. J. Am.
Chem. Soc. 1978, 100, 7098.
(11) (a) Hauser, F. M.; Rhee, R. P. J. Org. Chem. 1978, 43, 78. (b) Hauser,
F. M.; Chakrapani, S.; Ellenberger, W. P. J. Org. Chem. 1991, 56,
5248. (c) Mal, D.; Ray, S.; Sharma, I. J. Org. Chem. 2007, 72, 4981.
Review: (d) Mal, D.; Pahari, P. Chem. ReV. 2007, 107, 1892.
9
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A R T I C L E S
Nicolaou et al.
Scheme 5. Synthesis of Dienes 33 and 34a
Scheme 6. Diels-Alder Reaction of Diene 33a with Juglone (35)
and Subsequent Hydrogenationa
a Reagents and conditions: (a) Red-Al (1.3 equiv), Et2O, -78 f 25 °C,
24 h, then I2 (1.5 equiv), -78 f 25 °C, 4 h; (b) MEMCl (1.8 equiv),
i-Pr2NEt (2.2 equiv), CH2Cl2, -20 f 25 °C, 18 h, 88% over the two steps;
(c) CrCl2 (3.0 equiv), NiCl2 (0.04 equiv), then 29 (1.0 equiv), (S)-(-)-30a
(1.05 equiv), DMF, 0 f 25 °C, 12 h, 90% (31a:31b ca. 4:1); (d) MOMCl
(1.7 equiv), i-Pr2NEt (1.9 equiv), CH2Cl2, 18 h, 91% (32a), 90% (32b); (e)
s-BuLi (1.3 equiv), THF, -78 °C, 2 h, 76% (from 32a:32b ca. 4:1); (f)
TBAF (1.5 equiv), THF, 25 °C, 16 h, 94%.
lactate and 2-butyn-1-ol (27) as outlined in Scheme 5.13 Thus,
addition of Red-Al to alkynol 27 in ether at -78 f 25 °C,14
followed by quenching the resulting aluminate with iodine,
furnished, upon protection with MEMCl, vinyl iodide MEM
ether 29 in 88% overall yield. A Nozaki-Hiyama-Kishi
coupling15 of 29 with known lactaldehyde (S)-(-)-30a16 medi-
ated by CrCl2-NiCl2 afforded a mixture of syn and anti alcohols
31a and 31b (ca. 4:1 ratio) in 90% yield. Reaction of this
mixture (or each of the chromatographically separated
diastereomers) with MOMCl and i-Pr2NEt led to the corre-
sponding MOM derivatives (32a and 32b, 91 and 90% yield,
respectively), which underwent 1,4-elimination on exposure
to s-BuLi in THF at -78 °C to afford diene 33a, in 76%
yield.17 Finally, removal of the TBDPS group from the latter
compound with TBAF gave hydroxy diene 34 in 94% yield,
as shown in Scheme 5.
a Reagents and conditions: (a) 33a (1.0 equiv), 35 (1.3 equiv), CH2Cl2,
25 °C, 48 h, 81% (38:39 ca. 5:1); (b) H2, 10% Pd/C (20 mol %), MeOH,
59% (40) and 15% (41) starting from 33a.
As depicted in Scheme 6, reaction of diene 33a with
juglone (35, 1.3 equiv) proceeded smoothly in CH2Cl2
(13) The synthesis of dienes 33a and 33b was inspired by the work of
McDougal et al.: (a) McDougal, P. G.; Jump, J. M.; Rojas, C.; Rico,
J. G. Tetrahedron Lett. 1989, 30, 3897. (b) McDougal, P. G.; Rico,
J. G. J. Org. Chem. 1987, 52, 4817.
(ambient temperature, 48 h) to afford Diels-Alder adduct
38 as the major product, together with its regioisomer 39
(81% combined yield, ca. 5:1 ratio). These compounds proved
to be rather labile due to their susceptibility toward oxidation
by atmospheric oxygen and partial decomposition on silica
gel through elimination of the OMEM group. Stereoselective
hydrogenation (10% Pd/C, MeOH) of the crude Diels-Alder
reaction mixture gave the more stable adducts 40 and 41 in
59 and 15% yield, respectively, starting from 33a. While the
regioselectivity of this Diels-Alder reaction was expected
on the basis of electronic complementarity [dominating FMO
interactions between the dienophile (C-2, δ+) and the diene
(C-4, δ-); see structures 35 and 33a, Scheme 6], its facial
(14) Denmark, S. E.; Jones, T. K. J. Org. Chem. 1982, 47, 4595.
(15) Review: (a) Fu¨rstner, A. Chem. ReV. 1999, 99, 991. For an example
where the (Z)-iodoalkene retains its stereochemistry after coupling,
see: (b) Nicolaou, K. C.; Baran, P. S.; Zhong, Y.-L.; Barluenga, S.;
Hunt, K. W.; Kranich, R.; Vega, J. A. J. Am. Chem. Soc. 2002, 124,
2233.
(16) Overman, L. E.; Rishton, G. M. Org. Synth., Coll. Vol. IX. 1998, 139.
Overman, L. E.; Rishton, G. M. Org. Synth. 1993, 71, 56.
(17) Isomers 32a and 32b exhibited similar reactivities in this elimination
reaction on exposure to s-BuLi as noted in their reactions either as
individual compounds or as a mixture. Partial elimination of MeOH
from the terminal MEM group of diene 33a to form the corresponding
triene was observed on prolonged reaction times or when excessive
amounts of base were used.
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Total Synthesis of Bisanthraquinone Antibiotic BE-43472B
A R T I C L E S
Scheme 7. Synthesis of Quinones 7a-ca
Figure 4. X-ray-derived ORTEP drawing of quinone 7c.
Scheme 8. Michael Reaction/Aromatization Attempts To Construct
the Crowded Quaternary C-C Bonda
a Reagents and conditions: (a) MnO2 (10 wt equiv), CH2Cl2, 25 °C, 24 h,
88%; (b) ZnBr2 (6.0 equiv), CH2Cl2, 25 °C, 17 h, quant.; (c) IBX (1.2 equiv),
MeCN, 50 °C, 16 h, 94%; (d) RBr (3.0-4.2 equiv), Ag2O (4.2-5.6 equiv),
DMF, 25 °C, 2-4 h, 86% (42b), quant. (42c); (e) 1% HCl, MeOH, 50 °C,
4-6 h; (f) IBX (1.4 equiv), MeCN, 50 °C, 4-8 h, 89% (7b), 78% (7c),
over the two steps.
selectivity required consideration of steric factors in order
to trace its origins. Thus, minimization of 1,3-allylic strain
within the conformation of diene 33a and minimization of
steric repulsion in the transition states TS-36 and TS-37 may
explain the exquisite diastereoselectivity in the formation of
both regioisomers 38 and 39.
a Reagents and conditions: (a) for 44a, 21a (1.0 equiv), LiHMDS (1.1
equiv), THF, -78 °C, 1 h, then 7b (1.0 equiv), 30 min, 25 °C, 1.5 h, 31%;
for 44b or 44c, 22 (1.0 equiv), LiHMDS (1.0 equiv), THF, -78 °C, then
7b or 7c (1.0 equiv), B(n-Bu)2OTf (1.0 equiv), -78 f 25 °C, 2 h, 11%
(44b, dr ca. 1:1), 9% (44c, dr ca. 1:1); (b) 23 (1.0 equiv), LiHMDS (1.06
equiv), THF, -78 °C, 15 min, then 7b (1.0 equiv), -78 f 0 °C, 1 h, 49%
(dr ca. 1:1).
Proceeding with the synthesis, exposure of the major regioi-
somer 40 to MnO2 in CH2Cl2 at ambient temperature furnished
the desired quinone 42a in 88% yield (Scheme 7). Treatment
of the latter compound with ZnBr2 (CH2Cl2, 25 °C) furnished,
in quantitative yield, hydroxy compound 43a, whose oxidation
to the targeted quinone 7a was accomplished with IBX (MeCN,
50 °C, 94% yield). Alternatively, phenolic quinone 42a was
first protected as either benzyl ether (BnBr, Ag2O, DMF, in
the dark) to afford 42b (86% yield) or p-nitrobenzyl ether
(p-NO2BnBr, Ag2O, DMF, in the dark) to furnish 42c
(quantitative yield). Exposure of the latter compounds to
aqueous HCl in MeOH led to the corresponding alcohols (43b
or 43c), which were subsequently oxidized with IBX (MeCN,
50 °C) to afford quinone 7b (89% yield) or 7c (78% yield),
respectively. p-Nitrobenzyl quinone derivative 7c crystallized
in beautiful yellow-orange crystals (mp ) 159-161 °C, H2O/
MeCN) whose X-ray crystallographic analysis (see ORTEP
drawing, Figure 4) proved its absolute configuration and
confirmed the initial structural assignment of the Diels-Alder
adduct 38 (Scheme 6).
Coupling of the Two Fragments through Michael-Type
Reaction and Further Exploration toward the Target Molecule.
With the two key requisite building blocks for the construction
of the target molecule now available, we set out to explore their
coupling through the intended Michael addition/aromatization
sequence along the pathway to antibiotic BE-43472B (1).
Scheme 8 summarizes our advances and setbacks as we
attempted to break through certain barriers toward the target
molecule. Initial reaction of the enolate of 21a with Michael
acceptor anthraquinone 7a (R ) H, see Scheme 7) had shown
to result in low conversion, presumably due to protonation of
the enolate by the free phenol moiety of 7a. Thus, treatment of
aryl ketone 21a with LiHMDS (1.0 equiv) in THF at -78 °C,
followed by addition of benzyl-protected quinone 7b (-78 f
9
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A R T I C L E S
Nicolaou et al.
Scheme 9. Second-Generation Retrosynthetic Analysis of
(-)-BE-43472B [(-)-1]: Diels-Alder Approach
25 °C), furnished adduct 44a in 31% yield as a mixture of
diastereomers (ca. 1:1), isomerizing over time to a single
diastereomer (stereochemistry unassigned).
Unfortunately, exposure of bromoketone 8 (see Scheme 4)
to the same conditions as those shown in Scheme 8 failed to
produce any coupling product, presumably due to the steric bulk
of the bromine residue. This hypothesis prompted us to
synthesize the corresponding fluoroketone 22 (X ) F, Scheme
8) by quenching the enolate of ketone 21a (LiHMDS) with
(PhSO2)NF (98% yield, see Scheme 4) in order to explore its
coupling with Michael acceptor 7b. In the event, the enolate of
22 reacted with enetrione 7b to afford the desired product (44b),
albeit in only 11% yield, as a mixture of diastereomers (ca. 1:1
ratio) as shown in Scheme 8. A number of other partners,
including p-nitrobenzyl quinone 7c and regioisomeric bromoke-
tone 23 (prepared from ketone 21b by heating at 70 °C with
NBS in the presence of AIBN in benzene, 52% yield, see
Scheme 4), were also explored for their reactivity toward the
desired goal. Thus, partnering fluoroketone 22 with quinone 7c
under the developed coupling conditions led to a very low yield
(9%) of adduct 44c (ca. 1:1 mixture of diastereomers), while
the union of the regioisomeric bromoketone 23 with quinone
7b under the standard conditions led to 45 in 49% yield (ca.
1:1 mixture of diastereoisomers), as shown in Scheme 8.
However, aromatization attempts to convert these adducts to
the desired anthraquinone systems 6 (Scheme 1) proved in vain,
as only decomposition and undesired products, primarily due
to retro-Michael reactions, were observed. In order to avoid the
problem of the competing retro-Michael reactions, the coupling
of the aromatized bromoanthracene 26 (see Scheme 4) and
enetrione 7b via metalation and various cross-coupling-type
reactions was investigated. Disappointingly, all attempts to form
the challenging C-7′-C-4a bond failed. Faced with low yields
in the Michael coupling reactions and the inability to form the
desired anthraquinone 6 from the resulting products, we decided
to abort this approach and seek, instead, an entirely new strategy
for the set goal of synthesizing BE-43472B (1).
desired regioisomeric Diels-Alder product. This expectation
was in part based on wishful thinking, since at this stage we
were cognizant of the uncertainty surrounding the regio- and
stereochemical outcome of the designed Diels-Alder reaction
because we were unsure about the overall electronic effect of
the anthraquinone moiety attached onto the juglone structural
motif within dienophile 48. The answer would come through
experimentation, as we shall describe below.
The construction of dienophile 48 commenced from hydroan-
thracene 21b (see Scheme 4 for preparation) and proceeded as
summarized in Scheme 10. Thus, treatment of 21b with
LiHMDS in the presence of NBS produced dibromoketone 49
in 98% yield, whose aromatization (DBU) and methylation
(Cs2CO3, Me2SO4) furnished aryl bromide 51 (74% overall yield
over two steps) through intermediate phenol 50. Oxidation of
51 with CAN afforded anthraquinone 52 (77% yield), which
reacted with Me3SnSnMe3 in the presence of Pd(PPh3)4 catalyst
to afford trimethyl stannane 53 in 94% yield. Stille coupling18,19
of 53 with bromoquinone 5420 proceeded smoothly in the
presence of catalytic amounts of Pd(PPh3)4 and CuI, furnishing
the desired dienophile 48 in 65% yield. Later on, the MOM
group was removed from the latter compound (1% HCl in
MeOH, 85% yield) to generate the diphenolic compound 55,
which was subsequently also called upon to act as a dienophile
in these studies.
Second-Generation Diels-Alder Approach: The All-Carbon
Skeleton Encompassing Strategy. Despite our failure to reach
the desired framework of the BE-43472B molecule (1) through
the Michael reaction/aromatization approach, during these
studies we came to recognize the usefulness of the Diels-Alder
reaction in forming the C-ring of our target. In a speculative
but daring move, we considered generating the entire carbon
skeleton of the antibiotic 1 through a Diels-Alder reaction-
based strategy between the appropriate components. This
second-generation Diels-Alder approach is shown retrosyn-
thetically in Scheme 9 using the structure depicted by (-)-1
(although at this stage we did not know the absolute stereo-
chemistry of the natural product, as mentioned above). Thus,
dehydrating (-)-1 retrosynthetically toward the enol moiety led
to conjugated system 46, whose forward manipulation would
require regio- and stereoselective epoxidation, followed by
regioselective opening of the resulting oxirane, to afford the
natural product. Dismantling the two five-membered ether rings
within the latter intermediate through sequential C-O bond
disconnections, inserting a MeO group at C-8′, and reducing
the enol moiety revealed structure 47 as a possible precursor to
46. Applying a retro-Diels-Alder reaction on 47 unraveled
diene 34 and dienophile 48 as the required building blocks for
this strategy. The adoption of diene 34 was based on our findings
regarding its regiochemical reactivity toward juglone (35), as
discussed above (Scheme 6), which was expected to favor the
Scheme 11 presents the results of the Diels-Alder reaction
between diene 34 and dienophile 48. Thus, heating a mixture
(18) Reviews: (a) Farina, V.; Krishnamurthy, V.; Scott, W. K. Organic
Reactions, Vol. 50; Wiley-VCH: New York, 1997; p 652. (b) Fugami,
K.; Kosugin, M. Top. Curr. Chem. 2002, 219, 87. (c) Mitchell, T. N.
In Metal-Catalyzed Cross-Coupling Reactions; de Meijere, A., Dieder-
ich, F., Eds.; 2nd ed.; Wiley-VCH: Weinheim, 2004; p 125.
(19) Echavarren, A. M.; Tamayo, N.; Ca´rdenas, D. J. J. Org. Chem. 1994,
59, 6075.
(20) Grunwell, J. R.; Karipides, A.; Wigal, C. T.; Heinzman, S. W.; Parlow,
J.; Surso, J. A.; Clayton, L.; Fleitz, F. J.; Daffner, M.; Stevens, J. E.
J. Org. Chem. 1991, 56, 91.
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Total Synthesis of Bisanthraquinone Antibiotic BE-43472B
A R T I C L E S
Scheme 10. Construction of Quinone Dienophiles 48 and 55a
transition state TS-56′ that was expected to form the desired
adduct 47 as shown in Scheme 11. The stereochemistry of
compounds 57 and 58a/58b could not be completely discerned
through NMR spectroscopy. It was presumed to be as shown
on the basis of the most favorable facial orientation of diene
34 and dienophile 48 as they merge through transition state TS-
56 (Scheme 11) to form the Diels-Alder product 57. However,
from these failed experiments we drew the conclusion that the
electronic (-I) effect of the electron-withdrawing anthraquinone
substituent attached onto the juglone dienophile overrides the
polarizing effect of the intramolecular H-bond within this
system,23 enlarging the C-3 orbital coefficient of the LUMO of
the juglone system. The overall polarized characters of diene
34 and dienophile 48 (as shown in 34′ and 48′, Scheme 11)
dictate the regiochemical outcome of their union to afford
cycloadduct 57. This path-pointing study led us to the next phase
of our campaign toward the total synthesis of antibiotic BE-
43472B (1).
Final Diels-Alder Approach: Total Synthesis of Antibiotic
BE-43472B. Based on the latest intelligence gathering, we
redesigned our second-generation Diels-Alder strategy toward
the target molecule in order to accommodate the realities of
the electronics within the dienophile component of the cycload-
dition reaction. Scheme 12 outlines, in retrosynthetic format,
the newly devised synthetic strategy. Note that in this scheme
we again use the antipodal structure [(-)-1] of the natural
product (we will switch to the structure of the enantiomer (+)-1
in our final drive toward BE-43472B). The main provision in
this plan was the adoption of diene (S)-61 (with a 2-oxa as
opposed to a 1-oxa substituent), which was expected to possess
the reverse polarity (enlargement of the C-1 orbital coefficient
of the HOMO) from that of the originally used diene (34) and,
therefore, match the demonstrated polarity of dienophile 55 (see
Scheme 12). The adoption of (S)-61, in turn, required placement
of an oxygen atom at C-2 within the Diels-Alder product (ent-
60) that would have to be removed subsequently. Additionally,
two new oxygen atoms will have to be introduced into the
emerging structures (ent-60 and ent-59) at C-3 and C-1 before
reaching (-)-1. Another interesting feature of the new synthetic
design was the choice of the naked dienophile 55, whose
anthraquinone intramolecular H-bonding was expected to fa-
cilitate the intended intramolecular ipso substitution in order to
cast the last bond of the ring framework of the target molecule.
These new design modifications highlighted our expectations
for success with the possibility of a cascade sequence, beginning
with the Diels-Alder reaction and ending with an octacyclic
structure whose molecular complexity would be impressively
close to that of the targeted natural product.
a Reagents and conditions: (a) LiHMDS (2.2 equiv), NBS (2.2 equiv),
THF, 0 f 25 °C, 1 h, 98%; (b) DBU (1.0 equiv), CH2Cl2, 25 °C, 12 h; (c)
Me2SO4 (1.2 equiv), Cs2CO3 (1.2 equiv), acetone, reflux, 5 h, 74% over
the two steps; (d) CAN (2.0 equiv), CH2Cl2, MeCN, H2O, 0 °C, 30 min,
77%; (e) Pd(PPh3)4 (0.1 equiv), Me3SnSnMe3 (1.5 equiv), toluene, 110 °C,
2 h, 94%; (f) 53 (1.0 equiv), 54 (1.5 equiv), Pd(PPh3)4 (0.1 equiv), CuI
(0.2 equiv), THF, 70 °C, 22 h, 65%; (g) 1% HCl in MeOH, CH2Cl2, 25 °C,
16 h, 85%.
of 34 (4.0 equiv) with 48 (1.0 equiv) in benzene at 80 °C for
72 h furnished a single Diels-Alder adduct whose structure
was determined, through subsequent chemistry and NMR
spectroscopy, to be that of the undesired regioisomer 57.21,22
Interestingly, the silylated dienes 33a and 33b (prepared through
a route similar to that shown in Scheme 5 for 33a) failed to
react with dienophile quinone 48 under various conditions,
presumably due to steric hindrance, underscoring the relevance
of the free secondary alcohol in the diene system to the success
of the Diels-Alder reaction. Exposure of adduct 57 to the action
of ZnBr2 resulted in the simultaneous removal of the MOM
and MEM protecting groups and the formation of the isomeric
heptacyclic lactols 58a and 58b (84% yield, ca. 1:1 ratio, not
separable by chromatography). Their structures were determined
through 1H NMR ROESY studies that revealed the NOEs
indicated on their structures in Scheme 11. Manual molecular
models are supportive of the rotational barrier between these
two atropisomeric structures (58a and 58b). These studies also
provided the crucial evidence for the regioisomeric nature of
the Diels-Alder adduct 57, apparently formed through endo
transition state TS-56 which is favored over its regioisomeric
The required diene (S)-61 became readily available through
the concise synthetic route summarized in Scheme 13. Reaction
of commercially available phosphorane 62 with known aldehyde
(S)-(-)-30b24 gave R,ꢀ-unsaturated ester (S)-63 in 92% yield
(E:Z > 98:2), which was converted first to Weinreb amide (S)-
64 through the action of HNMe(OMe) and Me2AlCl and then
to methyl ketone (S)-65 by reaction with MeLi (77% yield for
the two steps).25 Desilylation of the latter compound with TBAF
(21) (a) Tietze, L. F.; Gericke, K. M.; Singidi, R. R.; Schuberth, I. Org.
Biomol. Chem. 2007, 5, 1191. (b) Boeckman, R. K., Jr.; Dolak, T. M.;
Culos, K. O. J. Am. Chem. Soc. 1978, 100, 7098. (c) Kelly, T. R.;
Montury, M. Tetrahedron Lett. 1978, 45, 4311. (d) Trost, B. M.; Ippen,
J.; Vladuchick, W. C. J. Am. Chem. Soc. 1977, 99, 8116.
(23) Rozeboom, M. D.; Tegmo-Larsson, I.-M.; Houk, K. N. J. Org. Chem.
1981, 46, 2338.
(24) Marshall, J. A.; Yanik, M. M.; Adams, N. D.; Ellis, K. C.; Chobanian,
H. R. Org. Synth. 2005, 81, 157.
(22) For Diels-Alder reactions with similar chiral 1,3-dienes, see: (a)
Barriault, L.; Thomas, J. D. O.; Cle´ment, R. J. Org. Chem. 2003, 68,
2317. (b) Carren˜o, M. C.; Garcia´-Cerrada, S.; Urbano, A.; Di Vitta,
C. J. Org. Chem. 2000, 65, 4355. (c) Reference 12a.
(25) (a) Nahm, S.; Weinreb, S. M. Tetrahedron Lett. 1981, 22, 3815. (b)
Sibi, M. P. Org. Prep. Proc. Int. 1993, 25, 15. (c) Mentzel, M.;
Hoffmann, H. M. R. J. Prakt. Chem. 1997, 339, 517. (d) Singh, J.;
Satyamurthi, N.; Aidhen, I. S. J. Prakt. Chem. 2000, 342, 340.
9
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A R T I C L E S
Nicolaou et al.
Scheme 11. Diels-Alder Reaction of Diene 34 with Dienophile 48 Leading to Undesired Regioisomeric Adduct 57 and Atropisomers 58a
and 58ba
a Reagents and conditions: (a) 48 (1.0 equiv), 34 (4.0 equiv), benzene, 80 °C, 72 h, 77%; (b) ZnBr2 (5.0 equiv), CH2Cl2, 25 °C, 24 h, 84% (58a:58b ca.
1:1).
led to hydroxy ketone (S)-66 (93% yield), which was treated
with 2.3 equiv of LDA to afford, after quenching the resulting
dianion with 1.0 equiv of TBSOTf, diene (S)-61 in 33% yield
and 95% ee, as determined by HPLC analysis using a chiral
column (plus 47% recovered starting material).
Indeed, the Diels-Alder reaction between diene (S)-61 and
dienophile 55 this time proceeded as planned and was followed
by a pleasing sequence of transformations that eventually led
to a successful total synthesis (Vide infra). However, the obtained
synthetic BE-43472B exhibited optical rotation opposite to that
exhibited by the natural BE-43472B, an observation that led to
the true configurational identity of the natural product. Thus,
by utilizing diene (S)-61, we reached the enantiomer of BE-
43472B [(-)-1, Figure 2]. We then synthesized diene (R)-61
starting with aldehyde (R)-(+)-30b and entered it into the
developed synthetic sequence, arriving at BE-43472B in its
naturally occurring enantiomeric form [(+)-1, Figure 2]. It is
this sequence that we now describe below.
At this stage, and in order to obtain insight into the behavior
of (S)-61 toward relevant quinone-type dienophiles, this diene
was allowed to react with juglone (35). As shown in Scheme
14, the reaction proceeded in CH2Cl2 at room temperature,
leading, as expected, to the opposite regioisomer (i.e., 68, >9:1
regioselectivity) to that obtained previously with the terminally
(1-oxa) substituted diene (33a, see Scheme 6). However, in this
case, the resulting Diels-Alder product (68) was found to be
rather labile, undergoing facile oxidation by air in the presence
of silica gel, forming quinone 69 (62% yield overall from 35).
The regiochemical identity of 69 was based on HMBC NMR
correlations as designated in Scheme 14, while the stereochem-
istry of its precursor (68) was based on transition state TS-67
as the sterically most favorable (1,3-allylic strain). This regio-
and stereochemical outcome pointed to the high likelihood that
diene (S)-61 will react with dienophile 55 with the desired regio-
and diastereoselectivity.
Scheme 15 depicts the first phase of the reaction sequence
that was initiated upon mixing (R)-61 and dienophile 55. Thus,
heating (R)-61 (2.0 equiv) with 55 (1.0 equiv) in CH2Cl2 solution
in a sealed tube (85 °C external temperature) for 48 h, followed
by addition of toluene and refluxing with a Dean-Stark
(26) Reviews: (a) Nicolaou, K. C.; Edmonds, D. J.; Bulger, P. G. Angew.
Chem., Int. Ed. 2006, 45, 7134. (b) Tietze, L. F.; Brasche, G.; Gericke,
K. Domino Reactions in Organic Synthesis; Wiley-VCH: Weinheim,
2006; p 672. (c) Nicolaou, K. C.; Montagnon, T.; Snyder, S. A. Chem.
Commun. 2003, 551.
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Total Synthesis of Bisanthraquinone Antibiotic BE-43472B
A R T I C L E S
Scheme 12. Final Retrosynthetic Analysis of (-)-BE-43472B
[(-)-1]
Scheme 14. Diels-Alder Model Study with Diene (S)-61 and
Juglone (35)a
a Reagents and conditions: (a) 35 (1.0 equiv), (S)-61 (1.1 equiv), 25 °C,
5 h; (b), air, SiO2, 25 °C, 30 min, 62% over the two steps.
through internal H-bonding, which activates the vinylogous-
type ester for the initiating attack by the hemiketal hydroxyl
group as shown in structures 59 and 71. Under the toluene
azeotropic refluxing conditions that removed the released MeOH
from the reaction mixture, product 72 suffers epimerization at
C-9a to afford 9a-epi-72, a phenomenon also observed on silica
gel. In refluxing benzene, we also observed (by 1H NMR
spectroscopy) epimerization of 59 to 9a-epi-59 as shown in
Scheme 15. With ample amounts of 72 and 9a-epi-72 in hand,
the stage was now set for an attempt to reach the final target,
antibiotic BE-43472B [(+)-1].
Scheme 13. Synthesis of Diene (S)-61a
The obligatory employment of diene (R)-61 in our synthesis
necessitated excision of the superfluous oxygen atom from the
C-2 position of the growing molecule; furthermore, two new
oxygen atoms, one at C-1 and one at C-3, had to be introduced
in their proper positions before the final target could be reached.
These objectives were achieved as shown in Scheme 16. Thus,
treatment of the mixture of 9a-epi-72 and 72 (ca. 2:1 ratio)27
with m-CPBA at -20 °C led to a mixture of four oxirane
epimers (ca. 4:1 ratio of ꢀ:R oxirane pairs, R-epimers not
shown). Desilylation of these oxiranes as a mixture (HF ·py,
TMSOMe) was accompanied by ring-opening to afford a
mixture of four hydroxy ketones (74, ca. 4:1 ꢀ:R ratio, R-epimers
not shown), which were subsequently, and without isolation,
subjected to SeO2 oxidation in AcOH (120 °C) to afford enone
75 (49% overall for the three steps) together with its C-3 epimer
(3-epi-75, 39%, chromatographically separated), indicating that
epimerization of the C-3 stereogenic center took place under
the oxidation conditions. Crystallization of 75 from CH2Cl2 led
to single crystals (mp >250 °C, CH2Cl2/hexanes), whose X-ray
crystallographic analysis (see ORTEP drawing, Figure 5)
revealed its relative configuration and confirmed the stereo-
chemical outcomes of both the Diels-Alder and the epoxidation
reactions.
a Reagents and conditions: (a) (S)-(-)-30b (1.0 equiv), 62 (1.1 equiv),
CH2Cl2, 25 °C, 6 h, 92%; (b) HN(OMe) ·HCl (5.0 equiv), Me2AlCl (5.0
equiv), CH2Cl2, 0 f 25 °C, 20 h; (c) MeLi, (2.0 equiv), Et2O, -78 °C,
1 h, 77% over the two steps; (d) TBAF (1.1 equiv), THF, 25 °C, 3 h, 93%;
(e) LDA (2.3 equiv), THF, -78 °C; then TBSOTf (1.0 equiv), -78 °C,
1.5 h, -78 f 25 °C, 30 min, 33% (95% ee), plus 47% recovered starting
material.
apparatus for another 24 h, led, through a remarkable cascade
sequence,26 to an inconsequential mixture of the two C-9a
epimers 72 and 9a-epi-72 (9a-epi-72:72 ca. 2:1) in a 98%
combined yield. This aesthetically pleasing cascade became
possible only because the initial Diels-Alder cycloaddition
reaction proceeded, through transition state TS-70, regio- and
stereoselectively, as desired. The resulting cycloadduct 60 was
observed by NMR spectroscopic analysis to exist in equilibrium
with its hemiketal form 59 (59:60 ca. 4:1, benzene-d6). Inspec-
tion of manual molecular models indicates that hemiketal 59
finds itself poised and in a favorable conformation to undergo
an intramolecular aromatic ipso substitution [SN(Ar)-type in-
tramolecular transesterification], expelling a molecule of MeOH
to form octacycle 72 through tetrahedral intermediate 71 as
shown in Scheme 15. The presence of the adjacent ꢀ-phenolic
quinone moiety apparently facilitates this novel cyclization
Having accomplished the installation of the C-3 hydroxyl
group in its desired configuration, we then proceeded to address
the challenging task of selectively removing the superfluous
(27) Since epimers 72 and 9a-epi-72 are hardly separable and because the
subsequent intermediates 73 and 74 were also shown to epimerize
under the reaction conditions employed for their generation, it was
more expedient and efficient to carry the mixture of 72 and 9a-epi-72
through the three-step sequence and separate the obtained enones (75
and its C-3 epimer 3-epi-75) by chromatography.
9
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A R T I C L E S
Nicolaou et al.
Scheme 15. Diels-Alder Reaction/Double THF Cyclization Cascadea
a Reagents and conditions: 55 (1.0 equiv), (R)-61 (2.0 equiv), CH2Cl2, 85 °C, sealed tube, 48 h; then toluene, 135 °C, Dean-Stark, 24 h, 9a-epi-72:72
(ca. 2:1 mixture of epimers), 98% overall yield based on 55.
Scheme 16. Late-Stage Functional Group Transformations and Synthesis of Enone 79a
a Reagents and conditions: (a) m-CPBA (1.2 equiv), CH2Cl2, -20 °C, 2 h; (b) HF·py (10.0 equiv), THF, 25 °C, 20 min; then TMSOMe; (c) SeO2 (5.0
equiv), AcOH, 120 °C, 15 h, 49% (75) and 39% (3-epi-75) over the three steps, chromatographically separated; (d) HSCH2CH2SH (4.0 equiv), BF3 ·OEt2
(6.0 equiv), CH2Cl2, 25 °C, 28 h, 55% (ent-76); (e) HSCH2CH2SH as solvent, BF3 ·OEt2 (2.6 equiv), 25 °C, 20 min, 78% (ent-76); (f) HSCH2CH2SH as
solvent, BF3 ·OEt2 (15 equiv), 25 °C, 30 min; then 0.1 M HCl, 63% (78); (g) Raney-Ni 2400, MeOH, air, 25 °C, 1.5 h, then aq. HCl, 45% (plus 19%
recovered starting material, 56% yield of 79 brsm).
oxygen atom from C-2. To this end, and as shown in Scheme
under various conditions. After considerable experimentation
16, we subjected enone 75 to the action of ethane-1,2-dithiol
we found that reaction of the enantiomeric ketone ent-75 with
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Total Synthesis of Bisanthraquinone Antibiotic BE-43472B
A R T I C L E S
Scheme 17. Final Steps of the Total Synthesis of BE-43472B
[(+)-1]a
Figure 5. ORTEP drawing of enone 75 derived from X-ray crystallographic
analysis.
4.0 equiv of ethane-1,2-dithiol and 6.0 equiv of BF3 ·OEt2 in
CH2Cl2 led to dithioketal ent-76 (55% yield), in which the C-9
carbonyl group (activated through intramolecular H-bonding)
was preferentially protected out of the four carbonyl moieties
present in the substrate (i.e., ent-75). Use of ethane-1,2-dithiol
as solvent and 2.6 equiv of BF3 ·OEt2 improved the efficiency
of this reaction, furnishing ent-76 in 78% yield. However, when
the reaction was carried out with enone 75 under more forceful
conditions (ethane-1,2-dithiol as solvent, 15 equiv of BF3 ·OEt2),
followed by quenching with 0.1 M HCl, the desired C-2
dithioketal 78 was isolated in 63% yield, presumably through
intermediate bis-dithioketal 77, which apparently collapses upon
exposure to H2O by hydrolysis of the C-9 dithiolane. The latter
process is rendered facile and selective, presumably through
intramolecular H-bonding of the phenolic OH with the neigh-
boring sulfur atom, now made possible by the increased electron
density on the latter as a result of the suppression of the electron-
withdrawing effect of the C-2 carbonyl moiety (upon its
engagement as a dithiolane system). This remarkably selective
reaction offered the sought-after window of opportunity for the
desired C-2 deoxygenation, which was accomplished through
reductive desulfurization (Raney-Ni 2400) to afford compound
79 in 45% yield (plus 19% recovered starting material; 56%
yield brsm). With this operation behind us, all that remained to
reach the target molecule was the insertion of an oxygen atom
in the C-H bond of the enone moiety within 79.
Having failed to achieve a direct Wacker-type oxidation28
of enone 79, and due to the ease of elimination of the axial C-3
hydroxyl group of the natural product [(+)-1],1 we resorted to
the mild two-step procedure involving epoxidation/oxirane
rearrangement, as shown in Scheme 17. Thus, epoxidation of
79 with t-BuOOH and DBU furnished the ꢀ-oxirane 80 as the
major product (84% yield), together with small amounts of the
epimeric R-oxirane (R-80, 7% yield, chromatographically
separated). While a number of metal- and Lewis acid-catalyzed
protocols failed to induce the desired transformation of epoxy
ketone 80 to the natural product, simple irradiation of this
precursor in benzene with UV light at ambient temperature (and
in the absence of any sensitizer)29 smoothly converted it to
a Reagents and conditions: (a) t-BuOOH (5.0 equiv), DBU (3.0 equiv),
CH2Cl2, -25 °C, 84% (80) and 7% (R-80); (b) hν (mercury lamp), benzene,
25 °C, 7 h, 77% (83% yield based on 92% conversion).
bisanthraquinone antibiotic BE-43472B [(+)-1] in 77% yield
(83% yield based on 92% conversion). Synthetic (+)-1 exhibited
physical properties identical to those reported for the natural
product and essentially the same optical rotation ([R]2D0 (c )
0.14 in CHCl3) ) +411.4) as that of natural (+)-1 ([R]2D0 (c )
0.14 in CHCl3) ) +403.1), and so did (-)-1, except for its
optical rotation, which was of the opposite sign ([R]2D0 (c ) 0.14
in CHCl3) ) -417.1).1 Because our data for synthetic (+)-1
(29) (a) Bodforss, S. Ber. Deutsch. Chem. Ges. 1918, 51, 214. (b) Jeger,
O.; Schaffner, K. Pure Appl. Chem. 1970, 21, 247.
(28) Tsuji, J.; Nagashima, H.; Hori, K. Chem. Lett. 1980, 257.
9
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A R T I C L E S
Nicolaou et al.
Scheme 18. Synthesis of the C-3 Epimeric Oxirane 3-epi-80 and
gave dithiolane 3-epi-78 in 66% yield. Reductive desulfurization
of this compound (Raney-Ni 2400, MeOH, then aquous HCl)
furnished enone 3-epi-79 (27% yield, unoptimized), which was
subjected to epoxidation (t-BuOOH, DBU, -30 °C) to afford
ꢀ-oxirane 3-epi-80 as a single isomer and in 97% yield.
Unfortunately, irradiation of the latter oxirane under the same
conditions (benzene, ambient temperature) used for the suc-
cessful rearrangement of oxirane 80 (see Scheme 17) failed to
induce any change, leading only to recovered starting material.
This outcome did not change, neither by irradiating in benzene
at 60 °C nor by changing the solvent to MeOH (irradiation at
ambient temperature). Interestingly, we observed the same
reluctance of R-oxirane R-80 (Scheme 17) to undergo the
photolytically-induced rearrangement to the desired enol struc-
ture. These results may be explained by considering the frontier
molecular orbitals of the epoxy ketone structural motif in each
of the three substrates subjected to the photolytic conditions
(80, R-80, and 3-epi-80), as illustrated in Figure 6. As seen,
the structure of the oxiranyl ketone 80 allows almost perfect
alignment of the C-9asO bond and its associated σ* orbital
with the π* orbital of the adjacent CdO bond, as required for
the oxirane rupture (see drawing 80-o, Figure 6). The indicated
H-bonding between the C-3 hydroxyl group and the oxirane
oxygen [downfield shift of the OH proton to δ ) 3.37 ppm in
Attempted Photolytic Rearrangementa
1
the H NMR spectrum, CDCl3, 500 MHz; higher Rf value of
oxiranyl ketone 80 (TLC, silica) as compared to that of enone
79] is presumed to play a subtle role in bringing about this
alignment by pulling the oxirane oxygen atom closer to the C-3
hydroxyl group. In contrast to the situation with the ꢀ-oxirane
80, in the C-3 epimeric ꢀ-oxirane3-epi-80 the steric repulsion
between the C-3 methyl group and the pseudoaxial oxirane
moiety results in the bending of the latter out of the required
orthogonal geometry to the adjacent carbonyl moiety, resulting
in insufficient overlap of the C-9a-O σ* orbital with the π*
orbital of the carbonyl group for fragmentation to occur (see
drawing 3-epi-80, Figure 6). A similar situation exists for the
R-oxirane, R-80 (which could have provided BE-43472B had
the epoxide rearrangement occurred), in which the stereoelec-
tronics are even worse (see drawing R-80-o, Figure 6). Indeed,
the preferred conformation of the cis-decalin system within
which the epoxy ketone structural motif resides forces the
oxirane into an equatorial position, resulting in inadequate orbital
overlap between the relevant σ* and π* orbitals. Manual
molecular modeling is sufficient to recognize these crucial
stereoelectronic effects within the oxiranes discussed above.
a Reagents and conditions: (a) HSCH2CH2SH as solvent, BF3 ·OEt2 (15
equiv), 25 °C, 30 min, 66%; (b) Raney-Ni 2400, MeOH, air, 25 °C, 2 h,
then aq. HCl, 3-epi-79, 27%; (c) t-BuOOH (5.0 equiv), DBU (3.0 equiv),
CH2Cl2, -30 °C, 97%.
matched the data reported both by Rowley1 and in the Japanese
patent,3 we concluded that all three compounds are identical,
that is to say one and the same.
For the photolytically induced rearrangement of epoxy ketone
80 to BE-43472B [(+)-1], we propose the radical-based
mechanism shown in Scheme 17. Thus, it is presumed that initial
nfπ* photoexcitation of 80 forms transient excited-state
diradical 81, which suffers homolytic epoxide rupture as shown
to generate diradical 82. The latter possesses an ideal geometry
for a 1,4-hydrogen shift (see σ and n orbital arrangement,
structure 82-o in box) as shown in Scheme 17 to give enol 83,
whose facile and spontaneous tautomerization leads to the
observed product [(+)-1]. The photolytically-induced rearrange-
ment of R,ꢀ-oxiranyl ketones to conjugated enol ketones was
observed as early as 1918 by Bodfoss,29 and recently revisited
by Jang, Park, and co-workers30 in a flash laser spectroscopic
study that revealed a 1,4-hydrogen shift as a key element of its
mechanism.
Having completed the total synthesis of both (+)-1 and (-)-
1, we then attempted to construct the C-3 epimer of the natural
product [(+)-1] starting from the C-3 epimeric enone 3-epi-75,
which we obtained from the SeO2-mediated oxidation of 9a-
epi-72/72 (see Scheme 16). As shown in Scheme 18, selective
thioketalization of 3-epi-75 under the previously developed
conditions (ethane-1,2-dithiol as solvent, 15 equiv of BF3 ·OEt2)
Biological Evaluation of Both Enantiomers of Antibiotic
BE-43472B and Related Compounds. The set of the synthesized
compounds enabled us to biologically evaluate a selected
number of them in order to obtain insights on structure-activity
relationships (SARs) within the class. The results are listed in
Table 1 (none of the compounds tested showed activity against
E. coli at 100 µM).
The antimicrobial activities of both enantiomers of the
bisanthraquinone BE-43472B [(+)-1 and (-)-1] and related
compounds thereof were determined against Gram-positive
methicillin-resistant Staphylococus aureus (MRSA), vancomy-
cin-resistant Enterococcus faecalis (VRE), and Gram-negative
Escherichia coli. In accord with the values reported by Rowley
and co-workers,1,2 (+)-1 (natural enantiomer) exhibited strong
bactericidal activity against Gram-positive pathogens (MRSA
and VRE). Given the architectural complexity of 1, it is notable
that the unnatural enantiomer [(-)-1] displayed similar activity.
The fact that all of the derivatives screened in these assay (55,
(30) Kim, H.; Kim, T. G.; Hahn, J.; Jang, D.-J.; Chang, D. J.; Park, B. S.
J. Phys. Chem. A. 2001, 105, 3555.
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Total Synthesis of Bisanthraquinone Antibiotic BE-43472B
A R T I C L E S
Figure 6. Conformations of the diastereomeric oxiranes 80, R-80, and 3-epi-80 (oxirane orbitals are shown on a straight line for convenience).
72, 75, 3-epi-75, ent-75, ent-3-epi-75, 78, ent-3-epi-78, 79, ent-
79, 80, R-80, 3-epi-79, 3-epi-80) lack activity implies that the
enol functionality (C-1 hydroxyl) is critical for antibacterial
activity.
Table 1. Biological Activities of (+)- and (-)-BE-43472B and
Related Compounds
MIC (µM)a
MRSAb VREc
IC50 (µM)d
The initial cytotoxicity evaluation of 1 and derivatives thereof
was performed across a panel of three cell lines of different
histological origin (HCT-116, colon; MCF-7, breast; and HeLa,
cervix). It is worth noting that both (+)-1 and (-)-1 displayed
higher potency against HeLa and HCT-116 cells than against
MCF-7 cells. While for HeLa and HCT-116 cells (-)-1 is
slightly more active than its enantiomer (+)-1, in the case of
MCF-7 cells the (+)-1 enantiomer exhibited an IC50 value
almost 2 times higher than that of the (-)-1 enantiomer.
Additionally, the simple dienophile 55 (for structure, see Scheme
15) displayed activity in all three tested cell lines with IC50
values in the same order of magnitude as those of (+)-1 and
(-)-1 (entry 3, 7 µM against HCT-116, 5 µM against HeLa,
and 15 µM against MCF-7). While none of the tested com-
pounds exhibited significant antibacterial properties against the
bacterial strains employed, a number of them showed notable
activities against some or all of the tumor cell lines used. Those
included enones 75 (entry 5, 16 µM against HCT-116 and 18
µM against HeLa), 3-epi-75 (entry 6, 10 µM against HCT-116
and 7.7 µM against HeLa), ent-3-epi-75 (entry 8, 16 µM against
HCT-116 and 14 µM against HeLa), and ent-3-epi-78 (entry
10, 31 µM against HCT-116 and 30 µM against HeLa), as well
entry
compound
HCT-116e
HeLaf
MCF-7g
1
2
3
4
5
6
7
8
(+)-1
0.15-0.3 0.6-1.5
0.10-0.2 0.3-2.0
4.1
3.8
7
NA
16
10
NA
16
NA
31
32
NA
NA
NA
8
2.6
2.2
5
NA
18
7.7
NA
14
NA
30
NA
30
NA
NA
9
26
40
15
(-)-1
55
72
75
3-epi-75
ent-75
ent-3-epi-75 NA
78 NA
ent-3-epi-78 NA
79
ent-79
80
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
40
9
10
11
12
13
14
15
16
NA
NA
NA
NA
NA
NA
R-80
3-epi-79
3-epi-80
NA
NA
NA
a Minimum inhibitory concentration. b Methicillin-resistant Staphylo-
coccus aureus. c Vancomycin-resistant Enterococcus faecalis. d Concen-
tration that causes 50% of cell growth inhibition. Experiments were done in
triplicate as described in the Supporting Information. e Human colon cancer
cell line. f Human cervical cancer cell line. g Human breast cancer cell line.
NA, not active at the highest concentration tested (100 µM for the
antibacterial assay and 40 µM for the cytotoxicity assay). Further
information is available in the Supporting Information.
9
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A R T I C L E S
Nicolaou et al.
as alkenes 79 (entry 11, 32 µM against HCT-116), ent-79 (entry
12, 30 µM against HeLa), and 3-epi-79 (entry 15, 8 µM against
HCT-116, 9 µM against HeLa, and 40 µM against MCF-7).
The latter compound showed IC50 values against all three cell
lines that are reminiscent of the natural product itself, despite
lacking the C-1 hydroxyl group, which appears to be necessary
for the aforementioned antibacterial activity. In addition, it is
interesting to note that epoxidation of the C-1/C-9a olefin leads
to a complete loss of growth inhibition activity (72, 80, R-80,
and 3-epi-80).
are less active as antitumor agents. Although the various
derivatives and intermediates en route to 1 were not as potent
antibacterial agents, important information has been gathered
from the biological evaluation of these compounds regarding
useful structure-activity relationships to aid in the design of
future analogues. With a synthetic pathway to the BE-43472B
structure now available and initial biological data in hand, the
design and synthesis of analogues of antibiotic BE-43472B as
part of an effort to discover and develop new antibacterial agents
is now feasible.
These initial investigations into the biology of 1 and related
compounds led to the following conclusions: (1) The potencies
of the natural and unnatural enantiomers of 1 against bacteria
and tumor cells are similar despite their rigid polycyclic but
antipodal structures. (2) Antibacterial activity is abolished with
even slight modifications of the C-ring of the molecule. (3)
Whereas the enol form of the 1,3-dicarbonyl function (C-1
hydroxyl) is critical for antibacterial activity, an R,ꢀ-unsaturated
carbonyl (C-1 hydrogen) is, in some cases (i.e., 75, 3-epi-75,
ent-3-epi-75, 79, ent-79, 3-epi-79), sufficient to retain cytotoxic
activity. On the other hand, epoxidation of the enone functional-
ity leads to complete loss of activity.
Acknowledgment. This article is dedicated to Professor Jean-
Marie Lehn on the occasion of his 70th birthday. Insightful
discussions with Prof. A. Eschenmoser regarding the mechanistic
aspects of this work are gratefully acknowledged. We thank Prof.
D. C. Rowley and A. Socha for a sample of BE-43472B [(+)-1].
We also thank Dr. D. H. Huang and Dr. L. Pasterneck, Dr. G.
Siuzdak, and Dr. R. Chadha for NMR spectroscopic, mass
spectrometric, and X-ray crystallographic assistance, respectively.
Financial support for this work was provided by Evonik Industries
(Germany), the National Institutes of Health (USA), the National
Science Foundation (CHE-0603217), the Skaggs Institute for
Chemical Biology, the Alexander von Humboldt Foundation
(Germany, postdoctoral fellowship to J.B.), A*STAR (Singapore,
predoctoral fellowship to Y.H.L.), the Skaggs-Oxford scholarship
program (predoctoral fellowship to Y.H.L.), the NSERC (Canada,
postdoctoral fellowship to A.L.), and the Max Weber-Programm
(Germany, student internship stipend to T.N.).
Conclusion
Based on a cascade sequence, initiated by a thermally induced
Diels-Alder reaction, the described synthetic strategy led to
an efficient total synthesis of (+)- and (-)-bisanthraquinone
antibiotic BE-43472B [(+)-1 and (-)-1] and the assignment of
the absolute configuration of this natural product as (+)-1. In
addition to demonstrating the power of cascade reactions in total
synthesis,26 the developed chemistry highlights the use of heat
and light as tools for “green” and efficient chemistry. Biological
evaluation of (+)-1 and (-)-1 revealed that both compounds
are endowed with almost equipotent antibacterial properties and
Supporting Information Available: Experimental procedures
and full compound characterization, including X-ray crystal-
lographic files in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA9073694
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14826 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009