Syntheses of Dimeric Tetrahydroxanthones with Varied Linkages: Investigation of "shapeshifting" Properties

Article abstract of DOI:10.1021/jacs.5b09825

The 2,4′- and 4,4′-linked variants of the cytotoxic agent secalonic acid A and their analogues have been synthesized. Kinetic resolution of an unprotected tetrahydroxanthone scaffold followed by copper-mediated biaryl coupling allowed for efficient access to these compounds. Evaluation of the "shapeshifting" properties of 2,2′-, 2,4′-, and 4,4′-linked variants of the secalonic acids A in a polar solvent in conjunction with assays of the compounds against select cancer cell lines was conducted to study possible correlations between linkage variation and cytotoxicity.

Full text of DOI:10.1021/jacs.5b09825

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Article
Syntheses of Dimeric Tetrahydroxanthones with Varied
Linkages: Investigation of “Shapeshifting” Properties
Tian Qin, Takayuki Iwata, Tanya T. Ransom, John A. Beutler, and John A. Porco
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b09825 • Publication Date (Web): 06 Nov 2015
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Syntheses of Dimeric Tetrahydroxanthones with Varied
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Linkages: Investigation of “Shapeshifting” Properties
Tian Qin,†,┴ Takayuki Iwata,† Tanya T. Ransom,§ John A. Beutler,§ and John A. Porco, Jr.*,†
†Department of Chemistry and Center for Molecular Discovery (BU-CMD), Boston University, 590 Commonwealth Avenue,
Boston, Massachusetts 02215, United States
§Molecular Targets Laboratory, Frederick National Laboratory for Cancer Research, National Cancer Institute, Frederick,
Maryland 21702, United States
Supporting Information Placeholder
ABSTRACT: 2,4’- and 4,4’-Linked variants of the cytotoxic agent secalonic acid A and their analogues have been synthesized. Kinetic
resolution of an unprotected tetrahydroxanthone scaffold followed by copper-mediated biaryl coupling allowed for efficient access to these
compounds. Evaluation of the “shapeshifting” properties of 2,2’-, 2,4’- and 4,4’-linked secalonic acids A in a polar solvent in conjunction
with assays of the compounds against select cancer cell lines was conducted in order to study possible correlations between linkage
variation and cytotoxicity.
properties of the tetrahydroxanthone scaffold can be compared
INTRODUCTION
Dimeric tetrahydroxanthone natural products belong to a family
of secondary metabolite mycotoxins.
to other natural products such as coleophomones 12 and the
“shapeshifting” bullvallene system reported by Bode and
1
Their interesting
coworkers.13
biological properties and varied structures have attracted
significant attention from both the biological and chemical
communities. Among these natural products, the secalonic acids,
2,2’-linked dimeric tetrahydroxanthones,2 were found to exhibit
interesting bioactivities. For instance, secalonic acid A (1)3 has
antitumor activity and also reduces colchicine toxicity in rat
cortical neurons.4 In addition to the 2,2’-linked dimeric natural
products, 2,4’- and 4,4’-linked secalonic acids are also found in
nature. For example, the 2,4’-linked isomer penicillixanthone A,
(2)5 and the 4,4’-linked secalonic acid E (talaroxanthone, 4)6
have recently been isolated and characterized (Figure 1). In
addition to the secalonic acids, biaryl linkage variation has been
observed in related natural products. Recently, a related subclass
of tetrahydroxanthones, phomoxanthones (5), have been isolated
and characterized. 7 The presence of a 4,4’ linkage was
Due to their interesting chemical and biological properties,
tetrahydroxanthones have drawn attention from synthetic
organic chemists. Recently, our laboratory 14 as well as the
laboratories of Bräse,15 Nicolaou,16 Tietze 17, and other groups18
have accomplished syntheses of monomeric chromone lactones,
tetrahydroxanthones,
and
2,2’-linked
dimeric
tetrahydroxanthone natural products. We considered that
chemical syntheses of 2,4’-linked and 4,4’-linked secalonic
acids would be not only be synthetically challenging but may
provide access to dimeric linkage variations to study their
“shapeshifting”13 properties of this class of compounds.
Moreover, along with the previously synthesized 2,2’-linked
compounds,14 these compounds may be used to ultimately
construct
a
tetrahydroxanthone library with potentially
interesting biological activities. In this paper, we describe
syntheses of 2,4’- and 4,4’-linked variants of the cytotoxic agent
established by X-ray crystallography. The corresponding 2,2’-
8
linked dimer dicerandrol
C
(6) and 2,4’-linked dimer
secalonic acid
A and its analogues. Evaluation of the
phomoxanthone B (7) have also been isolated from natural
“shapeshifting” properties of 2,2’-, 2,4’- and 4,4’-linked
secalonic acids A in conjunction with assays of the compounds
against select cancer cell lines was conducted in order to study
preliminary correlations between linkage variation and
cytotoxicity.
sources.
Due to the presence of varied biaryl linkages, the shape of
molecules may change dramatically which may have
a
substantial influence on their biological properties. For example,
the 4,4’-linked compound phomoxanthone A exhibits a strong
cytotoxicity to L5178Y cancer cells (IC50 = 0.3 µM),7c whereas
the 2,2’-linked congener dicerandrol C was found to be
somewhat less potent (IC50 = 2.8 µM).8c For the secalonic acids,
previous studies indicated that the linkage may undergo facile
isomerization in polar solvents.9 A similar transformation was
observed for the monomeric tetrahydroxanthone parnafungins to
generate a mixture of isomers A1 (8)/A2 (9)/B1 (10)/B2 (11)
from each pure isomer at room temperature (Figure 2).10 The
authors propose that retro-oxa-Michael reaction of the
tetrahydroxanthone to acetophenone intermediates 12/13 was
responsible for linkage isomerization. Interestingly, it was
determined by affinity-selection/mass spectrometry (AS-MS)
that parnafungin A1 (8) was the active inhibitor of the fungal
11
enzyme polyadenosine polymerase (PAP).
The fluxional
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NBS led to a mixture of brominated products 15, 16, and 17
(Scheme 1). Interestingly, use of In(OTf)3 as catalyst21 for the
bromination led to the production of ortho-bromide 16 in 62 %
yield along with para isomer 15 (37%). After further evaluation
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of additives, we found that a catalytic amount of AuCl3
significantly changed the outcome of the bromination (Scheme
1, b)). Treatment of 14 with NBS in the presence of 5 mol% of
AuCl3 favored production of the para-brominated chromone
lactone 15. This phenomenon was also observed in other related
chromone lactone substrates.23 To understand possible operative
mechanisms, chromone lactone 14 was treated with AuCl3 (72 h)
(Scheme 2, a)) in which case several chlorinated products 19-21
were obtained.24 We considered that the Au(III) catalyst may
chelate with the lactone and/or methyl ester moieties of
substrate 14 to direct the bromination, potentially via a direct
auration process 25 to generate the chelated gold (III) aryl
intermediate 22 (Scheme 2, b)). This hypothesis is also
supported by the fact that AuCl3 failed to enhance the yield of
the para product on tetrahydroxanthone substrates. However, an
auration pathway cannot completely explain the unselective
chlorination observed using AuCl3. As an alternative
mechanism, AuCl3 may also chelate to the phenol26 of substrate
14 to afford a putative aurate complex 23 which may effectively
block the ortho-position to bromination with NBS (Scheme 2,
c)). The latter mechanism is also supported by 1H NMR
experiments involving complexation of substrate 14 with AuCl3
in which case loss of the phenol resonance of 14 was
observed.23 Our current results highlight the importance of the
chromone lactone ring system in the gold (III)-catalyzed
bromination process.
Scheme 1. Bromination of a Chromone Lactone Substrate
Figure 1. 2,2’-Linked and Related 2,4’- and 4,4’-Linked
Secalonic Acids
Scheme 2. Possible Mechanisms for para-Bromination
Figure 2. Shapeshifting, Dynamic Equilibration of the
Parnafungins
RESULTS AND DISCUSSION
Synthesis of 4,4’-Linked Chromone Lactone Dimers.
In addition to the dimeric tetrahydroxanthones, there have
been a number of related 4,4’-linked chromone lactone dimers
isolated from nature, including gonytolide A19 and paecilin A.20
Based on our strategy for the synthesis of 2,2’-linked chromone
lactones,14b we anticipated that the corresponding 4,4’-linked
dimers may be accessed using our previously developed copper-
mediated stannane coupling approach if prefunctionalization of
the para-position of the chromone lactone monomer could be
accomplished. However, after investigating various iodination
and bromination conditions on monomer 14, we found that the
para-halogenated chromone lactone was generally the minor
product. For example, treatment of substrate 14 with 1 equiv. of
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With the para-bromide 15 in hand, it was smoothly converted
From these results, we hypothesized that production of the
desired para-bromo-tetrahydroxanthone (cf. 30) was not favored
under Dieckmann cyclization conditions. Therefore, we changed
our strategy to prefunctionalize the tetrahydroxanthone moiety.
However, when tetrahydroxanthone 33 was treated with
chlorinating reagents such as N-chlorophthalimide, the chloro-
diketone 34 was isolated in near quantitative yield as a single
diastereomer (Scheme 5). The anti-configuration between the
chlorine and the methyl ester moieties was confirmed by X-ray
crystal structure analysis (Figure 3).23 The high
diastereoselectivity observed is likely due to steric repulsion
between the electrophile and the methyl ester.
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to the corresponding para-stannane 18 in 79 % yield (Scheme
1). We next investigated stannane dimerization with different
metals (e.g. CuCl) and oxidants (e.g. air, O2, CuCl2).14b
Surprisingly, phenolic stannane 18 did not generate the desired
4,4’-linked dimers under these conditions; the main side
reaction generally observed was protodestannylation likely due
to the low pKa of the proximal phenol. Accordingly, we altered
our strategy and compound 14 was O-methylated with Me2SO4
(Scheme 3). Treatment of 24 with nBu4NBr3 led to the
27
production of bromide 25 in 93% yield.
Subsequent
stannylation provided compound 26 in 70 % yield. When we
applied the previously developed CuCl/air mediated coupling
conditions,14b stannane 26 was only converted to dimeric
products in trace amounts with a large amount of the starting
material remained intact. We thought that use of air as oxidant
was difficult to accurately control and that this could lead to the
variable conversions observed in the reaction. Fortunately, use
of CuCl2 as oxidant afforded the C2 and Cs dimers 27 and 28 in
reproducible yields (23 %).
As the vinylogous acid should be the most nucleophilic
moiety in the tetrahydroxanthone substrate, we thought that use
of the enol-protected compound 35 could avoid this problem.
After evaluating different iodination and bromination conditions,
the ortho-halogenated product (cf. 36) was found to be dominant.
We reasoned that the vinylogous acid moiety in the
tetrahydroxanthone could be deactivated under acidic conditions.
After forming the presumed protonated tetrahydroxanthone 37,
the phenol may become the more nucleophilic site. As an
alternative, the added TFA could protonate NIS which may
generate a more active iodination reagent with different
selectivity. Indeed,
Scheme 3. Synthesis of
a Chiral, Racemic 4,4’-Linked
Chromone Lactone
Scheme 4. Dieckmann Cyclization of Brominated
Tetrahydroxanthones
Synthesis of 4,4’-Linked Tetrahydroxanthone Dimers.
The successful synthesis of 4,4’-linked chromone lactones
encouraged us to further investigate syntheses of the
corresponding 4,4’-linked dimeric tetrahydroxanthones. We
envisioned
that
an
enantiopure,
para-functionalized
Scheme 5. Synthesis of a para-Iodo Tetrahydroxanthone
tetrahydroxanthone could serve as a key intermediate. Our
initial thought was that a para-brominated tetrahydroxanthone
could be obtained from a brominated chromone lactone through
Dieckmann cyclization (Scheme 4). However, potential linkage
isomerization of tetrahydroxanthones could create difficulties in
this transformation. Accordingly, we treated the ortho-
brominated chromone lactone 16 with 10 equivalents of NaH in
THF for 3 h. In this case, the corresponding tetrahydroxanthone
29 was isolated in 43 % yield as a single diastereomer.
Surprisingly, when we treated the bromide 15 with 10
equivalents of NaH, we obtained three different
tetrahydroxanthone products. The desired para-brominated
tetrahydroxanthone 30 was obtained in only 10 % yield. The
main byproduct observed was the ortho-bromide 31 with an
anti-configuration between the hydroxyl and ester groups. Even
when we shortened the reaction time or lowered the reaction
temperature, the undesired tetrahydroxanthones 29/31 were still
observed. Thus, we considered that under Dieckmann
cyclization conditions, para-bromide 15 may generate
intermediate 32 in situ. Subsequent oxa-Michael reaction could
convert 32 to the corresponding tetrahydroxanthone products.
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Scheme 6.
Tetrahydroxanthone Substrate
Kinetic Resolution of an Unprotected
Figure 3. X-ray Crystal Structure of Chlorinated
Tetrahydroxanthone 34
when substrate 33 was treated with NIS in TFA/CH2Cl2
(Scheme
5),
both
para-iodo
and
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tetrahydroxanthones were generated in a 1:1 ratio by crude H
NMR analysis. The mixture was further methylated using
trimethylsilyldiazomethane to afford para-iodide 38 and ortho-
iodide 36 in 31 % and 32 % isolated yields, respectively (two
steps).
In order to access chiral, non-racemic 4,4’-linked secalonic
acid A, synthesis of an enantiopure, monomeric
tetrahydroxanthone was required. Relying on our recently
developed kinetic resolution of the tetrahydroxanthone moiety
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using homobenzotetramisole (HBTM) catalyst 39,
we
irreproducible. Fortunately, stannylation could be cleanly
performed on the unprotected substrate (-)-44. Remarkably, the
para-iodide (-)-38 was not transformed to a dimeric product
under our previous one-pot Suzuki dimerization conditions14c
(Pd-SPhos-II, BPin2, K3PO4).
anticipated that an enantiopure para-iodide could be obtained in
a similar fashion. However, contrary to the excellent kinetic
resolution observed for ortho-iodide 36, HBTM catalyst 39 did
not successfully catalyze acetylation of para-iodide 38 or only
yielded moderate levels of enantioselectivity (Scheme 6). This
unexpected result indicated that substitution on the para-
position of the tetrahydroxanthone jeopardized its reactivity
with the HBTM catalyst system.
As there are no literature reports of stannane dimerizations
with substrates containing a free phenol, we were concerned that
a
free phenol could jeopardize the dimerization step.
Nevertheless, the resulting stannane (-)-44 was subjected to
dimerization conditions. Applying our previously developed
CuCl/air conditions to stannane (-)-44 cleanly produced a
protodestannylated product. After further evaluation of oxidants
(e.g. Cu(ethylhexanoate)2, Cu(OAc)2, FeCl3, Mn(acac)3), we
found that CuCl2 and ferrocenium tetrafluoroborate could
provide the corresponding 4,4’-linked dimer (-)-45 in 40-50 %
conversion (Scheme 7). However, the yield/conversion
remained inconsistent over several experimental trials. After
further screening of additives (e.g. Na2SO4, CaCO3, NaHCO3),
we found that trace amounts of water decreased the yield
dramatically. Use of freshly-dried stannane (-)-44 afforded the
corresponding C2 symmetric dimer (-)-45 in 39 % yield.
Treatment of (-)-45 with 3M HCl led to the formation of the
deprotected 4,4’-linked tetrahydroxanthone dimer (+)-46 in
91 % yield.
After the unsuccessful kinetic resolution attempts with the
para-iodo tetrahydroxanthone, we considered that the kinetic
resolution
could
be
performed
on
unprotected
tetrahydroxanthone scaffolds. Indeed, HBTM catalyst 3914b
could convert 33 to the acylated product. However, even with a
substoichiometric
amount
of
the
base
(N,N-
diisopropylethylamine, DIEA), the tetrahydroxanthone still
underwent syn/anti and retro-Dieckmann rearrangement (not
shown). As tetrahydroxanthone substrates were found to be
stable under acidic conditions, we wondered whether base was
necessary for acylation. Gratifyingly, without any base, kinetic
resolution proceeded smoothly on tetrahydroxanthone 33 to
generate (-)-33 and (+)-41 in excellent yield and in excellent
enantiomeric excess (Scheme 6). The s factor for the latter
reaction was above 200. For blennolide B (42) as substrate, we
were concerned that the additional methyl group on C ring could
dramatically decrease the acylation reactivity of the secondary
alcohol. For this case, enantioselectivities using propionic
anhydride never exceeded 40 % conversion after screening
various conditions. Fortunately, after switching to the less
hindered acylating reagent acetic anhydride, (-)-42 and the
acylated tetrahydroxanthone (+)-43 were obtained in high yield
and ee. Notably, the s factor observed for the unprotected
blennolide B (s=159) was even higher than for the enol-
protected blennolide B shown previously (s=93).14b
Scheme 7. Asymmetric Synthesis of a Model 4,4’-Linked
Tetrahydroxanthone Dimer
Using the enantioenriched tetrahydroxanthones, the para-
iodo tetrahydroxanthones were prepared in a similar manner.
With compound (-)-38 in hand (Scheme 7), we followed our
previous sequence to protect the free phenol. However, in this
case MOM protection was found to be low yielding and
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Scheme 8. Asymmetric Synthesis of a 4,4’-Linked Secalonic
Acid A
Scheme 10. Asymmetric Synthesis of a 2,4’-Linked Secalonic
Acid A (Penicillixanthone A)
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Synthesis of 2,4’-Linked Tetrahydroxanthone Dimers.
With our success in synthesizing of 2,2’- and 4,4’- secalonic
acids, we anticipated that we could also obtain the 2,4’-linked
tetrahydroxanthones from heterodimerization of 2- and 4-
stannyl tetrahydroxanthone monomers. Equimolar amounts of (-
)-54 and (-)-44 were treated with CuCl/CuCl2 in DMA. The
desired 2,4’-linked dimer (-)-56 was obtained in 32 % yield
(Scheme 9), along with 15 % of the 2,2’ dimer (-)-55 and less
than 5 % of the 4,4’ dimer (-)-45. After acidic deprotection, the
2,4’-linked model tetrahydroxanthone dimer (-)-57 was obtained
in 85 % yield. In a similar manner, protected 2,4’-linked
secalonic acid A (-)-59 was constructed from (-)-58 and (-)-49
(Scheme 10). The corresponding 2,2’-linked dimer (-)-62 and
4,4’-linked dimer (-)-50 (not shown) were also produced in this
reaction in around 20 and <5% yields, respectively. The 2,4’-
linked dimer secalonic acid A (penicillixanthone A) (-)-2 was
obtained smoothly after deprotection with 3M HCl. NMR
spectra and αD values for (-)-2 were found to be identical to
those reported for the natural sample.5a, d
After successfully establishing access to a model 4,4’-linked
dimeric tetrahydroxanthone, we considered that 4,4’-secalonic
acid A could be synthesized in a similar manner (Scheme 8).
Blennolide B (-)-42 was iodinated with NIS in TFA/CH2Cl2.
The resulting para- and ortho-iodides were O-methylated to
afford (-)-47 and (-)-48 (32% yield for both compounds).
Stannylation proceeded smoothly following the previously
described protocol to generate (-)-49. The C2 dimer (-)-50 was
successfully synthesized by treating freshly-dried stannane (-)-
49 with CuCl/CuCl2. The presence of a 4,4’-linkage was further
verified through a key HMBC correlation between C-2 and 1-
OH.23 Finally, 4,4’-linked secalonic acid A (+)-51 was obtained
after acidic deprotection. In a similar manner, the Cs symmetric
tetrahydroxanthone dimer 53 was synthesized from dimer 52,
the latter obtained from the chiral, racemic monomer 42.23
Shapeshifting Properties of 2,2’-, 2,4’- and 4,4’-Secalonic
Acids A.
Burobane and coworkers9 reported that 2,2’- secalonic acids
could isomerize to the corresponding 2,4’- and 4,4’-linked
congeners in polar solvents (e.g. pyridine and CH3CN).
Intrigued by this phenomenon, and with the synthetic 2,2’-, 2,4’-
and 4,4’- linked secalonic acids A in hand, we were interested to
monitor this process using UPLC analysis in order to determine
the ratio of products obtained based on thermodynamic
equilibration. We first considered use of DMSO as solvent for
our isomerization study based on use of this solvent in
biological assays. 29 We first prepared a solution of the 4,4’-
linked secalonic acid (+)-51 in DMSO (0.5 mg/mL) at room
temperature and the solution was monitored by UPLC analysis.
As shown in Figure 4, we observed that the 4,4’-linked
secalonic acid (+)-51 was converted to 2,2’- (-)-1 and 2,4’-
linked secalonic acids (-)-2 slowly at room temperature. After
13 h, the secalonic acids reached thermodynamic equilibrium. A
similar equilibrium process was observed starting with the pure
2,2’- or 2,4’-linked secalonic acids A.23 The thermodynamic
ratio of 2,2’-, 2,4’- and 4,4’- secalonic acids was determined to
be 3.2:2:1 as determined by crude 1H NMR analysis.23 The
isomerization process was also found to be faster with 10%
pyridine in DMSO23 which indicates that base can promote
isomerization (cf. Figure 2). Forming the less sterically
hindered biaryl bond may be a driving force for this equilibrium
Scheme 9. Asymmetric Synthesis of Model 2,4’-Linked
Tetrahydroxanthone Dimer
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(-)-45
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a Conflicting values observed in two experiments.
and the basis for favoring the 2,2’-linkage isomer in the
equilibration process.
Biological Studies.
We explored the biological activity of the synthetic xanthones
using a 48 h cancer cell growth inhibition assay with two renal
cancer cell lines (A498, UO-31) and two colon cancer cell lines
(Colo205 and KM12). The results are tabulated in Table 1 and
dose response curves are provided in the Supporting
Information.23 The known compounds secalonic acids A (-)-1, D
(+)-1, and penicillixanthone (-)-2 all showed potent activity
similar to that reported previously in L1210 leukemia cells30 and
MDA-MB-435 melanoma and SW-620 colon cancer cells.5d The
response for (+)-1 was also similar in comparison to the NCI 60
screen wherein GI50 values of 550, 150, 66 and 55 nM were
recorded for the above mentioned cell lines.30 A major
difference between the present biological assay and the NCI 60
screen is the use of a formazan XTT endpoint in contrast to the
protein stain SRB in the NCI 60, as well as the use of higher cell
densities and a 384 well format in the present assays. It is
interesting to observe that both (-)-1 and (+)-1 have similar
activities, which was also reported for both enantiomers of the
anticancer agent simaomicin .31
Figure 4. Equilibration of 4,4’-Linked Secalonic Acid A in
DMSO
Table 1.
Cytotoxicities of secalonic acids and related
compounds with varied linkages against four human tumor cell
lines, IC50 values in µM.
compound
A498
UO31
0.12
>40
4.9
Colo205
0.044
>40
KM12
0.072
>40
linkage
2,2
(-)-1
6.0
(-)-6014c
(-)-6114b
(-)-6214b
(+)-1
>40
5.5
2,2
Chromone lactones 27 and 28 had no activity against any of
the four cell lines at 40 µM, showing that the
tetrahydroxanthone skeleton is necessary for cell growth
inhibition. Comparable patterns of cell growth inhibition were
seen for all three dimer linkage types, e.g. (-)-1, (-)-2, and (+)-
51 via likely interconversion and equilibration of linkage
isomers to a mixture of 2,2’-, 2,4’-, and 4,4’-linked compounds
as shown in Figure 4. While no specific attempt was made to
control this process during bioassays, dry compounds were
dissolved in DMSO and immediately frozen at -20°C and then
thawed the day of assay. While we are not able to judge the
extent of equilibration that has occurred during the 48 h cell
growth assays, the similar patterns observed for the three
compounds support interconversion of the secalonic acid A
linkage isomers in the assay. Interestingly, in our studies we
found that the O-methyl enol ether appears to prevent dynamic
(shapeshifting) behavior. However, methyl enol ethers such as (-
)-56 (2,4’-linked), (-)-50 (4,4’-linked), were found to have
reduced biological activities, while the protected compounds (-)-
62 and (+)-63 (2,2’-linked) maintained modest bioactivities.
Nevertheless, all methyl-protected substrates have reduced
bioactivities in comparison to the unprotected dimeric
tetrahydroxanthones, which leads us to believe that the
tetrahydroxanthone moiety is crucial for their cytotoxicity. It is
also noteworthy that compounds such as (-)-61 and (-)-62
(Figure 5) appear to have different patterns of cell growth
inhibition from the others, perhaps indicating a different mode
of action. Of the tetrahydroxanthones, only the rugulotrosin
derivative (-)-6014c was found to be inactive against all cell lines
tested. Moreover, compound (+)-46 has a reduced cytotoxicity
in comparison to (+)-51, which contains an additional methyl
group on the tetrahydroxanthone C ring. A similar pattern also
was observed for compounds (-)-57 and (-)-2. These results
indicate that the methyl group on the C ring may provide a
2.8
2.8
2,2
3.9
3.6
2.9
n.d.a
0.19
n.d.a
8.2
2,2
3.9
0.40
6.8
0.052
4.0
2,2
(+)-6314b
(-)-56
4.8
2,2
6.3
9.9
8.6
2,4
(-)-57
14
35
>40
31
2,4
(-)-2
n.d.a
11
0.10
33
0.067
>40
0.099
>40
2,4
(+)-46
(+)-51
(-)-50
4,4
1.1
0.060
18
0.010
>40
0.012
39
4,4
>40
4,4
Figure
5.
Structure
of
Additional
Dimeric
Tetrahydroxanthones Investigated
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hydrophobic site which is crucial for cytotoxicity. These
conclusions about selectivity and potency are preliminary and a
more meaningful analysis will require testing in the full NCI 60
screen.
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1 (a) Bräse S.; Encinas A.; Keck J.; Nising, C. F. Chem. Rev. 2009, 109,
3903. (b) Masters, K.-S.; Bräse, S. Chem. Rev. 2012, 112, 3717.
2 Wezeman, T.; Bräse, S.; Masters, K.-S. Nat. Prod. Rep. 2015, 32, 6.
CONCLUSION
We have developed a copper-mediated aryl stannane coupling
protocol to access chromone lactone dimers as well as 4,4’- and
2,4’-linked dimeric tetrahydroxanthones. A highly selective
kinetic resolution was performed on an unprotected
tetrahydroxanthone scaffold to enable access to chiral, non-
racemic monomers. Asymmetric syntheses of 2,4’-linked
(penicillixanthone A) and 4,4’-linked secalonic acids A and
their analogues have been achieved. Our investigation also led
to the discovery of a gold (III)-catalyzed para-bromination of a
chromone lactone substrate. With a varied linkage series of
secalonic acids A, the “shapeshifting” properties were evaluated
and monitored by UPLC analysis. Initial biological studies of
3 (a) Franck, B.; Gottschalk, E. M.; Ohnsorge, U.; Baumann, G. Angew.
Chem. Int. Ed. Engl. 1964, 3, 441. (b) Franck, B.; Baumann, G.;
Ohnsorge. U. Tetrahedron Lett. 1965, 6, 2031. (c) Franck, B.;
Gottschalk, E. M.; Ohnsorge, U.; Hüper, F. Chem. Ber. 1966, 99, 3842.
4 Zhai, A.; Zhang, Y.; Zhu, X.; Liang, J.; Wang, W.; Lin, Y.; Chen, R.
Neurochem. Int. 2011, 58, 85.
5 (a) Wen, L.; Guo, Z.; Liu, F.; Wan Q.; Yu, Z.; Lin, Y.; Fu, L. Huaxue
Yanjiu Yu Yingyong, 2009, 21, 198. (b) Jiang, T.; Tian, L.; Guo, A.; Fu,
H.; Pei, Y.; Lin W. Yaoxue Xuebao, 2002, 37, 271. (c) Bao, J.; Sun, Y.-
L.; Zhang, X.-Y.; Han, Z.; Gao, H.-Z.; He, F.; Qian, P.-Y.; Qi, S.-H. J.
Antibiot. 2013, 66, 219. (d) El-Elimat, T.; Figueroa, M.; Raja, H. A.;
Graf, T. N.; Swanson, S. M.; Falkinham, J. O. III; Wani, M. C.; Pearce,
C. J.; Oberlies, N. H. Eur. J. Org. Chem. 2015, 109.
secalonic acid
A analogues revealed that the dimeric
tetrahydroxanthone moiety was crucial for cytotoxicity against
select cancer cells and that isomerization of the biaryl linkage
likely occurs during the cell-based assays. Further biological
studies of secalonic acid derivatives, as well as the chemistry of
the secalonic acid core structure, are ongoing and will be
reported in due course.
6
Koolen, H. H. F.; Menezes, L. S.; Souza, M. P.; Silva, F. M. A.;
Almeida, F. G. O.; de Souza, A. Q. L.; Nepel, A.; Barison, A.; de Silva,
F. H.; Evangelista, D. E.; de Souza, D. L. J. Braz. Chem. Soc. 2013, 24,
880.
7
(a) Isaka, M.; Jaturapat, A.; Rukseree, K.; Danwisetkanjana, D.;
Tanticharoen, M; Thebtaranonth, Y. J. Nat. Prod. 2001, 64, 1015. (b)
Elasasser, B.; Krohn, K.; Florke, U.; Root, N.; Aust, H.-J.; Draeger, S.;
Schulz, B.; Antus, S.; Kurtan, T. Eur. J. Org. Chem. 2005, 4563. (c)
Ronsberg, D.; Debbab, A.; Mandi, A.; Vasylyeva, V.; Bohler, P.; Stork,
B.; Engelke, L.; Hamacher, A.; Sawadogo, R.; Diederich, M.; Wary, V.;
Lin, W.; Kassack, M. U.; Janiak, C.; Scheu, S.; Wesselborg, S.; Kurtan,
T.; Aly, H.; Proksch, P. J. Org. Chem. 2013, 78, 12409.
ASSOCIATED CONTENT
Supporting Information Available Experimental procedures
and characterization data for all new compounds, including X-
ray structure analysis of compound 34, X-ray crystallographic
data (CIF), and detailed biological methods and results (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
8 (a) Wagenaar, M. M.; Clardy, J. J. Nat. Prod. 2001, 64, 1006. (b)
Erbert, C.; Lopes, A. A.; Yokoya, N. S.; Furtado, N. A. J. C.; Conti, R.;
Pupo, M. T.; Lopes, J. L.C.; Debonsi, H. M. Bot. Mar. 2012, 55, 435. (c)
Rukachaisirikul, V.; Sommart, U.; Phongpaichit, S.; Sakayaroj, J.;
Kirtikara, K. Phytochemistry, 2008, 69, 783. (d) Choi, J. N.; Kim, J.;
Ponnusamy, K.; Lim, C.; Kim, J. G.; Muthaiya, M. J.; Lee, C. H. J.
Antibiot. 2013, 66, 231. (e) Ding, B.; Yuan, J.; Huang, X.; Wen, W.;
Zhu, X.; Liu, Y.; Li, H.; Lu, Y.; He, L.; Tan, H.; She, Z. Mar. Drugs
2013, 11, 4961.
AUTHOR INFORMATION
Corresponding Author
*porco@bu.edu
Present Address
┴10550 N Torrey Pines Road, La Jolla, CA 92037 United States
9
Burobane, I.; Vining, L. C.; McInnes, A. G. Ger. Offen. DE 80-
3002761, 1980.
ACKNOWLEDGMENTS
Financial support from the NIH (GM-073855, GM-099920,
J.A.P., Jr.), and the NIGMS CMLD Initiative (P50 GM067041,
J.A.P., Jr.) is gratefully acknowledged. Work at the BU-CMD is
supported by NIH R24 grant GM-111625. We thank Vertex
Pharmaceuticals, Inc. for a graduate fellowship to T.Q. and the
Uehara Memorial Foundation for a postdoctoral fellowship to
T.I. This research was also supported in part by the Intramural
Research Program of NIH, National Cancer Institute, Center for
Cancer Research and by the Developmental Therapeutics
Program, Division of Cancer Diagnosis and Treatment. We
thank Dr Jeffrey Bacon (Boston University) for X-ray crystal
structure analysis and Dr. Lauren Brown (BU-CMD) for
experimental assistance.
10 Parish, C. A.; Smith, S. K.; Calati, K.; Zink, D.; Wilson, K.; Roemer,
T.; Jiang, B.; Xu, D.; Bill, G.; Platas, G.; Peláez, F.; Díez, M. T.; Tsou,
N.; McKeown, A. E.; Ball, R. G.; Powles, M. A.; Yeung, L.; Liberator,
P.; Harris, G. J. Am. Chem. Soc. 2008, 130, 7060.
11 Adam, G. C.; Parish, C. A.; Wisniewski, D.; Meng, J.; Liu, M.; Calati,
K.; Stein, B. D.; Athanasopoulos, J.; Liberator, P.; Roemer, T.; Harris,
G.; Chapman, K. T. J. Am. Chem. Soc. 2008, 130, 16704.
12 Nicolaou, K. C.; Montagnon, T.; Vassilikogiannakis, G.; Mathison, C.
J. N. J. Am. Chem. Soc. 2005, 127, 8872.
13 (a) Lippert, A. R.; Naganawa, A.; Keleshian, V. L.; Bode, J. W. J. Am.
Chem. Soc. 2010, 132, 15790. (b) He, M.; Bode, J. W Proc. Nat. Acad.
Sci. U.S.A. 2011, 36, 14752. (c) Larson, K. K.; He, M.; Teichert, J. F.;
Naganawa, A.; Bode, J. W. Chem. Sci. 2012, 3, 1825. (d) He, M.; Bode,
J. W. Org. Biomol. Chem. 2013, 11, 1305. (e) Teichert, J. F.; Mazunin,
D.; Bode, J. W. J. Am. Chem. Soc. 2013, 135, 11314.
14 (a) Qin, T.; Johnson, R. P.; Porco, J. A., Jr. J. Am. Chem. Soc. 2011,
131, 1714. (b) Qin, T.; Porco, J. A., Jr. Angew. Chem. Int. Ed. 2014, 53,
3107. (c) Qin, T.; Skraba-Joiner, S. L.; Khalil, Z. G.; Johnson, R. P.;
Capon, R. J.; Porco, J. A., Jr. Nat. Chem. 2015, 7, 234.
REFERENCES
7
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Page 8 of 9
1
2
3
4
5
6
7
8
9
10
1
12
13
14
15
16
17
18
19
20
21
2
23
24
25
26
27
28
29
30
31
32
3
34
35
36
37
38
39
40
41
42
43
4
45
46
47
48
49
50
51
52
53
54
5
56
57
58
59
60
15 (a) Nising, C. F.; Ohnemüller, U. K.; Bräse, S. Angew. Chem. Int. Ed.
30
Kurobane, I.; Iwahashi, S.; Fukuda, A. Drugs Exp. Clin. Res. 1987,
2006, 45, 307. (b) Gérard, E. M. C.; Bräse, S. Chem. Eur. J. 2008, 14,
8086. Additional synthetic studies: (c) Lesch, B.; Bräse, S. Angew.
Chem. Int. Ed. 2004, 43, 115. (d) Ohnemüller, U. K.; Nising, C. F.;
Nieger, M.; Bräse, S. Eur. J. Org. Chem. 2006, 1535. (e) Nising, C. F.;
Friedrich, A.; Bräse, S. Synlett 2007, 2987. (f) Ohnemüller, U. K.;
Nising, C. F.; Encinas, A.; Bräse, S. Synthesis 2007, 2175. (g) Volz, N.;
Bröhmer, M. C.; Nieger, M.; Bräse, S. Synlett 2009, 550. (h) Meister, A.
C.; Encinas, A.; Sahin, H.; Singer, E. M. C.; Nising, C. F.; Nieger, M.;
Bräse, S. Eur. J. Org. Chem. 2014, 4861.
13, 339.
31
Wang, Y.; Wang, C.; Butler, J. R.; Ready, J. M. Angew. Chem. Int.
Ed. 2013, 52, 10796.
16 Nicolaou, K. C.; Li, A. Angew. Chem. Int. Ed. 2008, 47, 6579.
17 (a) Tietze, L. F.; Spiegl, D. A.; Stecker, F.; Major, J.; Raith, C.; Große,
C. Chem. Eur. J. 2008, 14, 8956. (b) Tietze, L. F.; Jackenkroll, S.; Raith,
C.; Spiegl, D. A.; Reiner, J. R.; Claudia, M.; Campos, O. Chem. Eur. J.
2013, 19, 4876. (c) Tietze, L. F.; Ma, L.; Reiner, J. R.; Jackenroll, S.;
Heidemann, S. Chem. Eur. J. 2013, 19, 8610. (d) Tietze, L. F.;
Jackenkroll, S.; Hierold, J.; Ma, L.; Waldecker B. Chem. Eur. J. 2014,
20, 8628.
18
(a) Sudhakar, G.; Bayya, S.; Kadam, V. D.; Nanubolu, J. B. Org.
Biomol. Chem. 2014, 12, 5601. (b) Liu, J.; Li, Z.; Tong, P.; Xie, Z.;
Zhang, Y.; Li, Y. J. Org. Chem. 2015, 80, 1632.
19 (a) Kikuchi, H.; Isobe, M.; Sekiya, M.; Abe, Y.; Hoshikawa, T.; Ueda,
K.; Kurata, S.; Katou, Y.; Oshima, Y. Org. Lett. 2011, 13, 4624. (b)
Kikuchi, H.; Isobe, M.; Kurata, S.; Katou, Y.; Oshima, Y. Tetrahedron
2012, 68, 6218.
20 Guo, Z.; She, Z.; Shao, C.; Wen, L.; Liu, F.; Zheng, Z.; Lin, Y. Magn.
Reson. Chem. 2007, 45, 777.
21
Zhou, C.-Y.; Li, J.; Peddibhotla, S.; Romo, D. Org. Lett., 2010, 12,
2104.
22 Mo, F.; Yan, J. M.; Qiu, D.; Li, F.; Zhang, Y.; Wang, J. Angew. Chem.
Int. Ed. 2010, 49, 2028.
23 See the Supporting Information for complete experimental details.
24
For arene chlorination using AuCl3, see: (a) Kharasch, M. S.; Isbell,
H. S. J. Am. Chem. Soc. 1931, 53, 3053. (b) Shi, Z.; He, C. J. Am. Chem.
Soc. 2004, 126, 13596. (c) Lavy, S.; Miller, J. J.; Pažický, M.;
Rodrigues, A.-S.; Rominger, F.; Jäkel, C.; Serra, D.; Vinokurov, N.;
Limbach, M. Adv. Synth. Catal. 2010, 352, 2993. For copper-mediated
iodination, see: (d) Butler, J. R.; Wang, C.; Bian, J.; Ready, J. M. J. Am.
Chem. Soc. 2011, 133, 9956.
25
For auration of aromatic compounds with AuCl3, see: (a) de Graaf, P.
W. J.; Boersma, J.; van der Kerk, G. J. M. J. Organomet. Chem. 1976,
105, 399. (b) Fuchita, Y.; Utsunomiya, Y.; Yasutake, M. J. Chem. Soc.,
Dalton Trans. 2001, 2330. (c) Shi, Z.; He, C. J. Org. Chem. 2004, 69,
3669.
26
For examples of aurate complexes of phenols, see: (a) Barnholtz, S.
L.; Lydon, J. D.; Huang, G.; Venkatesh, M.; Barnes, C. L.; Ketring, A.
R.; Jurisson, S. S. Inorg. Chem. 2001, 40, 972. (b) Debono, N.; Iglesias,
M.; Sánchez, F. Adv. Synth. Catal. 2007, 349, 2470.
27
Tietze, L. F.; Ma, L.; Jackenkroll, S.; Reiner, J. R.; Hierold, J.;
Gnanaprakasam, B.; Heidemann, S. Heterocycles 2014, 88, 1101.
28
(a) Birman, V. B.; Li, X. Org. Lett. 2006, 8, 1351. (b) Birman, V. B.;
Li, X. Org. Lett. 2008, 10, 1115.
29
For a recent report describing interconversion of biaryl linkages for
tetrahydroxanthone-chromanone dimers in DMSO, see: Wu,G.; Yu, G.;
Kurta, T.; ndi,A.; Peng, J.; Mo, X.; Liu,M.; Li,H.; Sun, X.; Li, J.;
Zhu, T.; Gu, Q.; Li, D. J. Nat. Prod. 2015, DOI:
10.1021/acs.jnatprod.5b00636
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