Asymmetric total synthesis of the epoxykinamycin FL-120 B′

Article abstract of DOI:10.1002/anie.201104504

Turn up the heat: The synthesis of the title compound was achieved and a route to epoxide-containing diazobenzofluorenes, which could potentially serve as monomers to the dimeric lomaiviticins, was established. Key steps to construct the FL-120B′ core structure include Sharpless asymmetric epoxidation, Stille coupling, and intramolecular Friedel-Crafts acylation of atropisomeric carboxylic acids at elevated temperatures. Copyright

Full text of DOI:10.1002/anie.201104504

Communications
DOI: 10.1002/anie.201104504
Natural Product Synthesis
Asymmetric Total Synthesis of the Epoxykinamycin FL-120B’**
Stephen S. Scully and John A. Porco Jr.*
Diazobenzofluorene natural products are a family of struc-
turally complex molecules with a tetracyclic (ABCD) frame-
work bearing a diazo moiety, a functionality rarely found in
nature (Scheme 1).[1] Members of this family differ in the
damage to DNA mediated by bioreductive pathways, thus
leading to loss of the diazo functional group.[6b,7] This unique
family of natural products gained significant attention upon
isolation of the dimeric diazobenzofluorenes lomaiviticins A
(5) and B (6) by He et al. in 2001.[8] Demonstrated to be DNA-
damaging agents, the lomaiviticins were found to display
antibiotic acitivity against Gram-positive bacteria and potent
cytotoxicity in several cancer cell lines. The C2-symmetric
ꢀ
lomaiviticins may originate from the C2 C2’ linkage of
precursors that closely resemble the monomeric kinamycins.
Since 2006, numerous research groups have reported total
syntheses of monomeric diazobenzofluorene natural prod-
ucts[9] as well as studies towards the dimeric lomaiviticins.[10]
Recently, Herzon et al. reported a remarkable 11-step
synthesis of the lomaiviticin aglycon.[11] Their dimerization
approach utilizes an oxidative homocoupling of a silyl enol
ether derived from a protected monomer, which resembles 7
(Scheme 2a). Given the abundance of diazobenzofluorene
natural products bearing an epoxide or oxygenated function-
Scheme 1. Representative diazobenzofluorene natural products.
levels of functionalization of the cyclohexene moiety (D ring).
Kinamycin C (1),[2] which is biosynthetically[3] derived from
the epoxide-containing ketoanhydrokinamycin (2),[4] contains
a D ring with four contiguous stereocenters. Other epoxy-
kinamycins include FL-120B (3) and the closely related FL-
120B’ (4).[5] Monomeric diazobenzofluorenes have been
shown to exhibit antitumor properties.[2,6] Numerous studies
have suggested that these biological activities may result from
[*] S. S. Scully, Prof. Dr. J. A. Porco Jr.
Department of Chemistry and Center for Chemical Methodology
and Library Development (CMLD-BU), Boston University
590 Commonwealth Avenue, Boston, MA 02215 (USA)
E-mail: porco@bu.edu
Scheme 2. Biomimetic approaches to the lomaiviticins.
[**] This work was presented in part at the 237th American Chemical
Society National Meeting, Boston, MA, August 19-23, 2007; ORGN
abstract 667. Financial support from the National Institutes of
Health (RO1 CA137270) is gratefully acknowledged. We thank
Arthur Su (Boston University) for helpful discussions, Dr. Paul
Ralifo (Boston University) for NMR assistance, Dr. Norman Lee
(Boston University) for HPLC assistance, and Dr. Jenn-jong Young
(Institute of Preventive Medicine (Taiwan, ROC)) for generously
providing a sample of FL-120B.
ality at the C2-position, we believe that a biosynthetic
precursor to the lomaiviticins may also be depicted by the
proposed monomer 8 (Scheme 2b). This dimerization process
may result from a reductive epoxide-opening[12] event leading
to a pivotal carbon–carbon bond formation.[13] In this regard,
we report the total synthesis of FL-120B’ (4) and development
of methods to prepare diazobenzofluorenes with intact
epoxides; these methods may also allow future access to the
lomaiviticins and related compounds.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104504.
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Angew. Chem. Int. Ed. 2011, 50, 9722 –9726

In our retrosynthetic analysis, we believed that the
installation of the diazo group of FL-120B’ (4) could be
achieved by a two-step process starting from a benzofluor-
enone intermediate such as 9 (Scheme 3). This strategy would
first involve the formation of a sulfonylhydrazone from a
Scheme 4. Enantioselective synthesis of epoxide 13. a) tBuOOH
(2.0 equiv), Ti(OiPr)4 (0.10 equiv), l-DIPT (0.13 equiv), 4 ꢀ M.S.,
CH2Cl2, 08C, 60 h. DIPT=diisopropyl tartrate, M.S.=molecular sieves.
[23]
Oxidation with NaClO2
afforded carboxylic acids 20a–c
for evaluation in the pivotal intramolecular Friedel–Crafts
acylation to form the tetracyclic framework of FL-120B’.
Treatment of carboxylic acids 20a–c with trifluoroacetic
anhydride (TFAA) in a variety of solvents and at different
temperatures gave varying ratios of the desired ketone
products 21 and lactone by-products 22 (Table 1). By adding
TFAA to a preheated (808C) solution of 20a (R = Ac) in
nitromethane, as the optimal solvent, a 10:1 (determined by
1H NMR analysis of the reaction mixture) mixture of C-
acylation to O-acylation products (21a/22a) was observed.
Scheme 3. Retrosynthesis for FL-120B’ (4). PG=protecting group,
MOM=methoxymethyl, TBS=tert-butyldimethylsilyl.
ketone precursor and subsequent oxidation with spontaneous
desulfination of the hydrazone to form the requisite diazo
functionality.[9c–e] The synthesis of 9 may be achieved from the
naphthalene fragment 10 and epoxide 11 utilizing Stille
coupling and intramolecular Friedel–Crafts acylation using
reaction conditions previously described in the synthesis of
kinamycin C by our group.[9b]
Table 1: Studies on the intramolecular Friedel–Crafts acylation.
In our prior synthesis,[9b] asymmetric nucleophilic epox-
idation (d-DIPT, Ph3COOH, and NaHMDS)[14] was utilized
to access chiral, nonracemic 11. Unfortunately, high levels of
enantioselectivity were not achieved on a larger scale.
However, Sharpless asymmetric epoxidation of quinone
monoketal 12 was performed on a 4.4 g scale to provide 13
in 98% yield with moderate enantioselectivity (68% ee;
Scheme 4).[15] A single recrystallization provided 13 in high
enantiomeric excess (99% ee). Epoxide 13 was further
elaborated to our desired epoxyketone fragment 11 as
previously reported.[9b]
Entry Substrate
T
Selectivity Yield [%][c] Yield [%][c]
[8C][a] (21/22)[b] of 21
of 22
For the synthesis of the AB ring subunit, quinone 14[9b]
was reduced and subsequently methylated to provide bromo-
naphthalene derivative 15 (Scheme 5). Stannylation[16] of 15
afforded aryl stannane 10. Stille coupling[17] of stannane 10
and bromide 11 afforded epoxyketone 16 in excellent yield
(90%) as a 1.5:1 mixture of atropisomers,[18] as indicated by
1H NMR analysis.[19] Acetylation of 16 followed by reduction
with Super-Hydride (LiHBEt3) provided allylic alcohol 17 as
a single diastereomer.[19] At this stage in the synthesis, three
protecting groups for the secondary alcohol of substrate 17
were explored. Alcohol 17 was masked as acetate (Ac), 4-
azidobutyrate (C(O)(CH2)3N3),[20] and tert-butoxycarbonyl
(Boc) groups to provide protected intermediates 18a, 18b,
and 18c, respectively. Desilylation[21] of 18a–c and oxidation
with Dess–Martin periodinane[22] gave aldehydes 19a–c.
1
2
20a
80
23
10:1
4:1
89
58
8
(R=Ac)[d]
20b
15
(R=C(O)(CH2)3N3)[d]
3
4
5
20b[d]
20b
50
80
80
>20:1
>20:1
>20:1
81
–
–
–
84
20c
72[f]
(R=Boc)[e]
[a] TFAA was added to the reaction mixture at the indicated temperature.
[b] Selectivity was measured by 1H NMR analysis of the reaction mixture.
[c] Yields are for isolated products after column chromatography on
silica gel. [d] The crude reaction mixture was further treated with TFA for
complete MOM deprotection. [e] Treatment with pyridine[24] in EtOH was
required to deprotect trifluoroacetylated intermediates. [f] The Boc group
was cleaved during reactions: R=H. TFAA=trifluoroacetic anhydride,
TFA=trifluoroacetic acid.
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1
able temperature (VT) H NMR experiments indicated that
the atropisomers equilibrate upon warming (H8 proton shows
coalescence at 808C; Figure 1), and the barrier to rotation
(DG°) for acid 20b was calculated to be 18 kcalmmolꢀ1.[25]
1
Figure 1. Variable temperature (VT) H NMR (500 MHz, CD3NO2)
stackplot of acid 20b.
This significant barrier to rotation may be directly correlated
to the atropisomeric acylium intermediates A1 and A2
(Scheme 6). At lower temperatures, A1 and A2 may not
freely equilibrate and independently may give mixtures of
intermediates B1/C1 and B2/C2, respectively (Scheme 6; par-
tial structures shown for clarity).[19,26] At higher temperatures,
A1 and A2 may rapidly equilibrate, thus leading to a
thermodynamically favored ketone intermediate. We believe
that ketone intermediate B2 should be energetically preferred
owing to an unfavorable steric interaction between the C16
aryl methoxy and C1 protected alcohol in intermediate B1. In
Scheme 5. Forward synthesis to carboxylic acids 20a–c. a) Na2S2O4
(6.0 equiv), Et2O, H2O, RT, 10 min; b) MeI (6.0 equiv), K2CO3
(6.5 equiv), DMF, 08C!RT, 18 h, 66% yield (2 steps); c) (SnBu3)2
(1.3 equiv), [Pd(PPh3)4] (0.1 equiv), toluene, 1108C, 48 h, 74%; d) 11
(1.0 equiv), 10 (1.1 equiv), [Pd2(dba)3]·CHCl3 (0.1 equiv), AsPh3
(0.3 equiv), CuCl (0.6 equiv), iPr2NEt (1.1 equiv), MeCN, 728C, 3 h,
90% yield; e) Ac2O (30 equiv), pyr, RT, 6 h; f) Super-Hydride (LiHBEt3;
2.1 equiv), THF; 74% (2 steps) g) 18a: Ac2O (30 equiv), pyr, RT, 3 h,
96%; 18b: ClC(O)(CH2)3N3 (1.1 equiv), CH2Cl2/pyr, 08C, 1 h, 73%;
18c: Boc2O (5.0 equiv), DMAP (5.5 equiv), CH2Cl2, RT, 4 h, 93%;
h) HF·pyr (excess), THF/pyr, 08C, 15–20 h; i) DMP (2.0 equiv), pyr
(2.1 equiv), CH2Cl2, RT, 3–5 h, 73–88% (2 steps); j) NaClO2 (9 equiv),
NaH2PO4 (8 equiv), 2-methyl-2-butene (excess), tBuOH/H2O, 08C!
RT, 2–3 h, 78–90%. Boc=t-butoxycarbonyl, dba=dibenzylideneace-
tone, DMAP=N,N-dimethyl-4-aminopyridine, DMF=N,N-dimethyl-
formamide, DMP=Dess–Martin periodinane, pyr=pyridine.
Higher ratios (21/22) were observed with use of the bulkier
azidobutyrate (C(O)(CH2)3N3) and Boc groups (> 20:1;
Table 1, entries 3–5).[19] Interestingly, we found that higher
reaction temperatures were required to achieve higher
selectivities and yields for the desired ketone products. For
example, treatment of 20b with TFAA at RT gave a 4:1 (21b/
22b) selectivity and a 58% yield for ketone 21b (Table 1,
entry 2). The ratio was significantly increased to > 20:1 when
the reaction was performed at 508C and 808C affording 21b
in 81% and 84% yields, respectively (Table 1, entries 3 and
4).[19]
These results may be accounted for by the observation
that acid substrates 20a–c were found to exist as a mixture of
atropisomers (1.2–1.6:1 mixture by H NMR analysis). Vari-
Scheme 6. Proposed cyclization manifolds for intramolecular Friedel–
Crafts acylation and lactone formation.
1
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intermediate B2, the interaction between the C16 aryl
methoxy and H1 is minimized and upon rearomatization
affords the observed ketone products.
To complete the synthesis of FL-120B’, ketone 21c,
derived from Boc-protected acid 20c, was protected using
TBDPSCl to afford the bis(silylated) ketone 23 (Scheme 7).
Initial attempts using various Lewis and Bronsted acids to
form mesylhydrazone 24 failed to retain the sensitive epoxide
moiety. Ultimately, trifluoroacetic acid (TFA) proved to be a
cross-coupling, and intramolecular Friedel–Crafts reactions
as key steps. Notably, the high reaction temperatures utilized
for Friedel–Crafts acylation allowed selective formation of an
intermediate that led to the desired ketone products in a
process likely involving atropisomers. The synthesis of FL-
120B’ represents the first total synthesis of an epoxide-
containing, diazobenzofluorene natural product. Studies
involving evaluation of reductive coupling of epoxykinamy-
cins to access the lomaiviticins and related compounds will be
reported in due course.
Received: June 29, 2011
Published online: August 30, 2011
Keywords: antitumor agents · atropisomerism · Friedel–
.
Crafts reaction · natural products · total synthesis
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1
FL-120B’ gave matching H NMR and IR spectra as well as
TLC and HPLC retention values. Furthermore, similar
23
optical rotations for synthetic (½aꢁD ¼ꢀ1328) and semisyn-
23
thetic (½aꢁD ¼ꢀ1288) FL-120B’ allowed assignment of the
absolute configuration for both FL-120B (3) and FL-120B’ (4)
as depicted in Scheme 1.
In summary, an asymmetric synthesis of FL-120B’ has
been achieved using Sharpless asymmetric epoxidation, Stille
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