The Mosaic of Rottlerin

Article abstract of DOI:10.1021/acs.joc.5b01827

The first total synthesis of rottlerin is described. The methodology allows the development of potential novel protein kinase C δ (PKCδ) analogues for better treatment of various diseases. Kamalachalcone A and dimeric rottlerin were synthesized in a very

Full text of DOI:10.1021/acs.joc.5b01827

Article
pubs.acs.org/joc
The Mosaic of Rottlerin
Kenneth K. C. Hong, Graham E. Ball, David StC. Black, and Naresh Kumar*
School of Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia
S
* Supporting Information
ABSTRACT: The first total synthesis of rottlerin is described.
The methodology allows the development of potential novel
protein kinase C δ (PKCδ) analogues for better treatment of
various diseases. Kamalachalcone A and dimeric rottlerin were
synthesized in a very practical and economical way using FeCl3
as a catalyst.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Malloutus philippensis, a plant commonly found throughout
Southeast Asia and Australia,1 possesses a range of biological
properties, including antioxidant, antiulcer, antitumor,2 cyto-
toxic,3,4 and antifungal activities.5 The granular hairs on the
surface of the fruits are covered with red exudates, called
kamala, that have traditionally been used as a medicine and a
dye.6 Various complex flavonoids and other molecules,
including the protein kinase C (PKC) inhibitor rottlerin, have
been isolated from this plant. Among these complex molecules,
kamalachalcones A, B, and E possess a unique pentacyclic ring
system that is probably formed from intermolecular dimeriza-
tion of a chalcone dihydropyran, also known as the “red
compound”.6 Tanaka et al.6 have hypothesized that kama-
lachalcone A could be synthesized via an acid-catalyzed
reaction. In fact, the biosynthetic origin of kamalachalcone A
and rottlerin appeared to be “red compound” related (Scheme
1).
The role of rottlerin as a specific PKCδ inhibitor has been
questioned, as researchers have provided conflicting arguments.
Despite the debate on the selectivity of PKCδ inhibition,
rottlerin is still a compound that possesses a range of potent
biological activities, including inhibition of micropinocytosism,7
anticancer (pancreatic, prostate, breast, and lung), and
antiangiogenic effects. Nevertheless, the synthesis of rottlerin
has never been reported. This is perhaps due to the sensitivity
of rottlerin toward both basic and acidic conditions,8,13 which
discourage the chemical synthesis. Therefore, it is desirable to
develop an efficient and economical synthesis of rottlerin and
its analogues and it is envisaged that the synthesis of rottlerin
will bring social impact to a certain extent. This paper describes
the first total synthesis of rottlerin and also the corresponding
kamalachalcone dimers. We propose that the final step in the
biosynthesis of kamalachalcone A involves the acid-catalyzed
dimerization of a chalcone derivative.
To evaluate the feasibility of this proposed biosynthetic
pathway, we planned a short synthesis of kamalachalcone A
employing an acid-catalyzed dimerization reaction as the final
step (Scheme 2).
The synthesis of the “red compound” was prepared in six
steps using the literature method;9 a stepwise preparation from
1 to 3 using Vilsmeier−Haack formylation10 followed by
Clemmensen reduction appeared to have a higher percentage
yield than the one-step methylation from 1 to 3 suggested by
the literature.9 Characterization data for all intermediates were
consistent with those in the literature.9,10 Although rottlerin
resembles the “red compound” 7, its synthesis is challenging
due to its methylene bridge moiety. Although many phenolic
natural products bearing a methylene bridge have been
synthesized, they are symmetrical dimers and their syntheses
employ a simple coupling using an aldehyde-containing
substrate.11,12 In fact, rottlerin-related structures such as
mallotus B13 and mallotochromene,14 which are nonsym-
metrical, have never been synthesized. Although Meikle and
Stevens’ synthesis of uliginosin A is an example of successful
synthesis of unsymmetrical phloroglucinol,15 it involves the
synthesis of a symmetrical dimer with the presence of
formaldehyde and phloroglucinol, followed by the convoluted
so-called “rottlerone change” to give the unsymmetrical dimer
via a disproportionation using NaH as a base.
Rottlerin’s biosynthesis is believed to be an oxidation of the
“red compound” 5 to form the highly reactive quinone methide
intermediate 6 and subsequent Michael addition with the
corresponding acetophenone 3 to form rottlerin (Scheme 3).16
Received: August 6, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.5b01827
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Scheme 1. Proposed Biogenesis of Kamalachalcone A and Rottlerin
Scheme 2. Preparation of the “Red Compound” 5
Scheme 3. Proposed Biosynthesis of Rottlerin
Scheme 4. Synthesis of Chromene 9
Our initial screening of the reaction between chromene 7
and acetophenone 3 in the presence of an oxidizing agent
(Ag2O, AgNO3, DDQ) failed to give rottlerin. Therefore, an
alternative approach to generate an o-quinone methide via a
nonoxidative pathway was taken.
Monoprotected chromene 8 was prepared according to the
literature procedure.17 Subsequent aldol condensation gave
chromene 8 in moderate yield (Scheme 4). Finally the
deprotection afforded the 5-hydroxyl chromene 9 in 32%
yield similar to the literature report. Spence’s synthesis of
penilactone A18 gave inspiration for the synthesis of a quinone
methide precursor using formaldehyde, acetic acid, and sodium
acetate. Its synthesis has the advantage of avoiding the use of
protecting groups and is therefore neat and economical.
However, the desired target 10 was not achieved in our case
(Scheme 5); instead, rottlerone was formed preferentially from
a homodimerization of 9.
After several screenings, we discovered that Eschenmoser’s
salt provided the desired quinone methide precursor in a mild
and high-yield fashion, similar to the synthesis of arzanol.19
When chromene 9 was heated with Eschenmoser’s salt in
CHCl3 at reflux, a full conversion of starting materials into the
B
DOI: 10.1021/acs.joc.5b01827
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and methylene protons between 1.90 and 2.65 ppm along with
a doublet proton at 4.7 ppm (d, J = 4.7 Hz), it was believed that
dimerization had occurred. A single crystal was also grown
using aqueous ethanol as solvent, and X-ray crystallography
confirmed the structure of 12 (see the Supporting Informa-
tion).
Scheme 5. Attempted Synthesis of Quinone Methide
Precursor 10
Other protic acids, including trifluoroacetic acid and glacial
acetic acid, were explored. None of these showed the formation
of 12 with prolonged reaction time. However, when a trace
amount of water was present, product 12 was observed.
However, addition of water to glacial acetic acid did not provide
any product. This is perhaps due to the glacial acetic acid being
too weak to drive the reaction. Moreover, to explore the
importance of water to the dimerization reaction, an experi-
ment used HCl gas pumped into the reaction mixture in
anhydrous methanol. Both NMR and TLC showed no product
formation in 24 h. However, when water was present,
precipitation happened after a short period of time and the
solid was determined to be dimer 12.
To confirm that concentrated HCl acts as a catalyst, a
stoichiometric amount of acid was added and no significant
change of product yield was observed. Some Lewis acids were
also screened for their effectiveness, as Huan Sun and Schaus21
demonstrated that FeCl3·6H2O can catalyze the homodimeri-
zation of flavenes to form dependensin and other dimeric
flavonoids. The results (Scheme 7) showed that, without any
water or hydronium ion, the reaction can still proceed in the
presence of anhydrous FeCl3 (40 mol %).
tertiary amine 11 was observed. Dimethylamine serves as a
good leaving group under thermal conditions to convert tertiary
amine 11 into the o-quinone methide. When acetophenone 3
was heated with chromene 11 in toluene at reflux, rottlerin was
produced effectively in 63% yield (Scheme 6).
Scheme 7. Optimized Conditions for Dimerization of
Chromene 4
Comparison of retention times of a rottlerin standard from
Sigma-Aldrich and the synthetic rottlerin using preparative
HPLC resulted in a close match (see the Supporting
Information). Both 1H and 13C NMR spectra provided a
close match with literature values.13,20
To demonstrate that “red compound” 5 could be dimerized
to give kamalachalcone A, chromene 4 was used as a model
compound to determine the optimal conditions for the
dimerization reaction. When chromene 4 was treated with
concentrated hydrochloric acid, dimer 12 was isolated as a
white solid by filtration after 1 h.
Dichloromethane was the most suitable solvent, as it
provided easy workup and gave fewer side products. Moreover,
the reaction was completed within 5 min, as indicated by both
1
The H NMR spectrum showed the disappearance of the
two doublet protons from 4. With the rise of four new aliphatic
Scheme 6. Successful Synthesis of Rottlerin
C
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Scheme 8. Mechanistic Possibilities for Conversion of 4 to 12
Scheme 9. Synthesis of Kamalachalcone A and Dimeric Rottlerin 15
NMR analysis of an aliquot and TLC analysis. These
experiments demonstrated the formation of the unique
pentacyclic ring system possibly formed via a hetero-Diels−
Alder pathway.
The mechanism of the dimerization of compound 4 was
investigated computationally using DFT at the B3LYP/6-
31G(d), B3LYP/6-311+G(2df,2p)//B3LYP/6-31G(d), and
M06-2X/6-31+G(d)//B3LYP/6-31G(d) levels of theory, the
last method being selected, as previously it has proved reliable
in 6-π-electron cyclization reactions.22 Continuum model
methanol solvent using the SMD method was employed.23
Using H+ as a model (Lewis) acid catalyst, the activation barrier
D
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Figure 1. ORTEP diagram of kamalachalcone A. Thermal ellipsoids are drawn at the 50% probability level.
for the key dimerization step was found to be lower in the case
of proceeding via the protonated intermediate 4H+ (ΔG⧧ = 110
kJ mol−1 (M06-2X; Scheme 8b) in comparison with proceeding
via an uncatalyzed path involving the neutral o-quinone
methide intermediate 4QM (ΔG⧧ = 133 kJ mol−1; Scheme
8a). The overall lower free energy barrier for the path shown in
Scheme 8b suggests that H+ interacting with the oxygen atom
of the quinone methide may indeed catalyze the dimerization
step of the reaction. We note that the errors in the calculated
solvation energies, and by extension the reaction energies, for
the sequential pathway containing changed species (Scheme
8b) are likely to be larger than for the concerted pathway
containing only neutral species (Scheme 8a).23 Since the
reaction leading to the formation of 12 is stereoselective, in that
only the isomer in which the newly formed ring with a cis
fusion is observed, this suggests that a concerted formation of
12, such as the reaction of a preformed quinone methide
intermediate, 4QM, with a second molecule of the starting
compound 4, found in Scheme 8a is more likely. However, a
sequential pathway such as that in Scheme 8b, in which an
intermediate IM1 was found using the computational method-
ology employed, may still be possible. The intermediate IM1 is
formed via formation of only the C−C bond in the
dimerization step rather than a concerted formation of C−C
and C−O bonds. We note that the barrier to the synchronous
deprotonation (using water as the base) and ring closure of
IM1 to form the C−O bond in the final product 12 is
calculated to be very low (ΔG⧧ = 24 kJ mol−1 relative to IM1),
significantly less than the barrier to rotation around the newly
formed C−C bond in IM1 (ΔG⧧ = 60 kJ mol−1 relative to
IM1).
be developed and provide a wider range of novel protein kinase
C delta inhibitors. An optimized dimerization method using
FeCl3 also provided kamalachalcone A and dimeric rottlerin 13.
The biological activities of rottlerin analogues and other
dimeric compounds will be reported in the future.
EXPERIMENTAL SECTION
■
All reactions requiring anhydrous conditions were performed in oven-
dried glassware under an argon atmosphere unless otherwise stated. All
other reagents were purchased from commercial sources and used
without further purification. Reactions were monitored using thin-layer
chromatography (TLC) using aluminum plates coated with silica gel
GF254. Compounds were detected by short- and long-wavelength
ultraviolet light. Gravity column chromatography was carried out using
40−63 μm silica gel. Flash column chromatography was carried out
using 6−35 μm silica gel. Preparative reversed phase HPLC was
performed using a PDA detector (254 nm) and C18 column (150 mm
× 19 nm) on a gradient elution of 1−100% over 60 min with a flow
rate of 5 mL/min. Formic acid (0.1%)/Milli-Q water was used as
eluent A, and formic acid (0.1%)/acetonitrile was used as eluent B.
Rottlerin standard was purchased from Sigma-Aldrich to compare the
retention time with the synthetic rottlerin using preparative HPLC.
NMR spectra were obtained in the designated solvents at 298 K on a
300, 400, or 600 MHz spectrometer as designated. Chemical shifts (δ)
are in parts per million and are internally referenced relative to the
solvent nuclei. 1H NMR spectral data are reported as follows: chemical
shift measured in parts per million (ppm) downfield from TMS (δ);
multiplicity; observed coupling constant (J) in hertz (Hz); proton
count; assignment. Multiplicities are assigned as singlet (s), doublet
(d), doublet of doublets (dd), doublet of triplets (dt), triplet, (t),
quartet (q), quintet (p), doublet of doublets of doublets (ddd),
multiplet (m), and broad singlet (bs) where appropriate. 13C NMR
spectra were recorded in the designated solvents, and chemical shifts
are reported in ppm downfield from TMS. Melting points were
measured using a Mel-Temp melting point apparatus and are
uncorrected. Infrared spectra were recorded using an attenuated
total reflection FTIR spectrometer. High-resolution mass spectra were
measured at 70 eV using a quadrupole analyzer and are reported with
ion mass/charge (m/z) ratios as values in atomic mass units.
With the success of dimerization of compound 4,
kamalachalcone A was synthesized using the optimized FeCl3
dimerization strategy and was obtained in 55% yield.
1
Characterization of H and 13C NMR showed a close match
with the literature values6 (see the Supporting Information).
Dimerization of rottlerin also gave the dimeric product 13,
similar to kamalachalcone E, in 44% yield (Scheme 9).
To give further structural confirmation of the product, single
crystals of kamalachalcone A were grown in THF and an X-ray
crystal structure was determined (Figure 1).
1-(2,4,6-Trihydroxy-3-methylphenyl)ethan-1-one (3).9
(1). Preparation of Zinc Amalgam. Mercuric chloride (1.0 g) was
dissolved in a mixture of concentrated hydrochloric acid (3 mL) and
water (50 mL). Zinc powder (15.0 g) was added, and the mixture was
stirred vigorously for 5 min. During the stirring the zinc wool was
broken into small shiny pieces. The aqueous layer was decanted. The
amalgamated zinc was used immediately for the reduction.
(2). Synthesis of Acetophenone 3. Concentrated hydrochloric acid
(30 mL) and water (20 mL) were added to a solution of acetophenone
2 (2.5 g, 12.7 mmol) in methanol (50 mL) in a 250 mL conical flask
(note: the mixture existed as a suspension). The mixture was warmed
to 50 °C, and amalgamated zinc (prepared in part 1) was added in one
CONCLUSIONS
■
In conclusion, rottlerin was synthesized for the first time using a
biomimetic approach which is practical and inexpensive. The
synthetic methodology allows potential rottlerin analogues to
E
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lot. The suspension dissolved immediately and formed a homogeneous
solution. This reaction mixture was stirred vigorously for 5 min under
open-flask conditions (note: bubbling and exothermic) and then
quickly filtered through a filter funnel with cotton employed to remove
the amalgamated zinc. The residual compound was transferred without
loss using methanol. The solution was then evaporated in vacuo to
form a yellow solid. The solid was washed with water and filtered to
give 3 as a yellow solid (1.63 g, 70%): mp 208−212 °C; 1H NMR (300
MHz, DMSO-d6) δ 13.97 (s, 1H), 10.54 (s, 1H), 10.31 (s, 1H), 6.00
(s, 1H), 2.54 (s, 3H), 1.82 (s, 3H); 13C NMR (75 MHz, DMSO-d6) δ
204.3, 165.2, 164.5, 161.9, 105.6, 103.1, 95.7, 34.4, 9.2.
using hexane/EtOAc (8/2) to give rottlerin (0.052 g, 63%) as a red-
orange solid: mp 210−214 °C; H NMR (600 MHz, CDCl3) δ 16.51
1
(s. 1H), 15.60 (bs, 1H), 9.55 (bs, 1H), 8.19 (d, J = 15.6 Hz, 1H), 7.84
(d, J = 15.6 Hz, 1H), 7.59−7.63 (m, 2H), 7.39−7.45 (m, 3H), 6.66 (d,
J = 10.1 Hz, 1H), 5.49 (d, J = 10.1 Hz, 1H), 3.81 (s, 2H), 2.71 (s, 3H),
2.08 (s, 3H), 1.53 (s, 6H); 13C NMR (150 MHz, CDCl3) δ 204.0,
193.1, 163.1, 160.9, 159.7, 159.0, 156.7, 155.6, 143.4, 135.6, 130.5,
129.1, 128.9, 128.4, 126.8 125.1, 117.2, 106.6, 105.5, 104.5, 104.0,
102.8, 102.0, 78.2, 32.6, 28.0, 16.0, 7.6; HRMS (ESI) m/z calcd for
C30H28O8 (M + H)+ 517.1857, found 517.1851.
1,1′-(5,8,10-Trihydroxy-2,2,6,9,13,13-hexamethyl-
1,7a,13a,13b-tetrahydro-2H,13H-pyrano[3,2-c:4,5,6-d′e′]-
dichromene-4,11-diyl)bis(ethan-1-one) (12). To a solution of 4
(0.020 g, 0.081 mmol) in DCM (1 mL) was added anhydrous FeCl3
(0.005 g, 0.032 mmol), and the mixture was stirred at room
temperature for 5 min. After completion of reaction as indicated by
TLC or NMR (over 90% conversion of 4), the reaction mixture was
washed with brine (30 mL) and extracted with DCM (3 × 20 mL),
dried over Na2SO4, and evaporated in vacuo. The solid was washed
with MeOH (3 × 5 mL) to give spectroscopically pure compound 12
(0.028 g, 70%) as a white solid: mp 260−264 °C; IR νmax 3589, 3385,
1598, 1422, 1367, 1159, 1126, 790 cm−1; 1H NMR (400 MHz,
CDCl3): 14.19 (s, 1H), 13.85 (s, 1H), 6.82 (s, 1H), 4.68 (d, J =5.2 Hz,
1H), 2.66 (s, 3H), 2.65 (s, 3H), 2.53−2.60 (m, 1H), 2.15−2.19 (m,
1H), 2.11 (s, 3H), 2.09 (s, 3H), 2.05−2.10 (m, 1H), 1.83−1.92 (m,
1H), 1.59 (s, 3H), 1.54 (s, 3H), 1.47 (s, 3H), 1.45 (s, 3H); 13C NMR
(100 MHz, CDCl3) δ 203.9, 203.3, 165.8, 162.5, 160.5, 158.5, 155.0,
154.3, 107.3, 105.7, 105.6, 100.7, 98.0, 78.6, 77.7, 77.4, 70.3, 47.5, 42.3,
33.7, 33.5, 30.3, 28.3, 27.1, 24.3, 20.6, 7.4, 7.3; HRMS (ESI) m/z calcd
for C28H32O8 (M + H)+ 497.2170, found 497.2166.
(E)-1-(7-Hydroxy-5-(methoxymethoxy)-2,2-dimethyl-2H-
chromen-8-yl)-3-phenylprop-2-en-1-one (8). To a solution of
chromene 7 (1.0 g, 3.59 mmol) in MeOH (15 mL) were added
benzaldehyde (0.37 mL, 3.59 mmol) and KOH pellets (0.60 g, 10.77
mmol). The mixture was stirred for 10 h, during which time the
solution gradually changed from orange to red and eventually
precipitate formed. After completion, water (100 mL) was added
and the mixture extracted with EtOAc (3 × 100 mL). The combined
organic extracts were washed with brine, dried using Na2SO4, and
evaporated in vacuo. Flash chromatography on silica gel using hexane/
EtOAc (9/1) afforded a bright orange red solid. The solid was washed
with hexane (3 × 5 mL) to give (note: hexane wash to remove excess
benzaldehyde) a fine red-orange powder of 8 (0.54 g, 41%): mp 100−
1
104 °C; IR νmax 3066, 1630, 1582, 1340, 1148, 1062, 921 cm−1; H
NMR (600 MHz, acetone-d6) δ 13.96 (s, 1H), 8.19 (d, J = 16 Hz, 1H),
7.81 (d, J = 16 Hz, 1H), 7.74−7.78 (m, 2H), 7.47−7.53 (m, 3H), 6.65
(d, J = 10 Hz, 1H), 6.26 (s, 1H), 5.65 (d, J = 10 Hz, 1H), 5.35 (s, 2H),
3.50 (s, 3H), 1.60 (s, 6H); 13C NMR (100 MHz, acetone-d6) δ 193.7,
167.7, 156.7, 143.2, 136.3, 131.3, 130.0, 129.2, 128.2, 126.2, 117.2,
107.4, 96.0, 95.3, 79.1, 56.7, 28.1; HRMS (ESI) m/z calcd for
C22H22O5 (M + Na)+ 389.1359, found 389.1358.
Kamalachalcone A.6 To a solution of 5 (0.060 g, 0.18 mmol) in
DCM (1 mL) was added anhydrous FeCl3 (0.011 g, 0.07 mmol), and
the mixture was stirred at room temperature for 5 min (note: the
solution turned from red to brown and eventually black). After
completion of reaction as indicated by TLC or NMR (over 95%
conversion of 5), the reaction mixture was washed with brine (30 mL)
and extracted with DCM (3 × 20 mL), dried over Na2SO4, and
evaporated in vacuo. The solid was washed with MeOH (3 × 5 mL) to
give spectroscopically pure kamalachalcone A (0.067 g, 55%) as a
(E)-1-(5,7-Dihydroxy-2,2-dimethyl-2H-chromen-8-yl)-3-phe-
nylprop-2-en-1-one (9). Chromene 8 (0.50 g, 1.36 mmol) was
dissolved in 20 mL of a MeOH/THF mixture (1/1) and was heated at
reflux for 4 h. The reaction was monitored by TLC. After completion,
water (100 mL) was added and the mixture extracted with EtOAc (3 ×
50 mL). The combined organic extracts were washed with brine, dried
using Na2SO4, and evaporated in vacuo. Flash chromatography on
silica gel using hexane/EtOAc (8/2) gave 9 (0.14 g, 32%) as a red oil:
IR νmax 3273, 1627, 1598, 1342, 1156, 1087, 975 cm−1; 1H NMR (400
MHz, CDCl3) δ 14.23 (s, 1H), 8.11 (d, J = 16 Hz, 1H), 7.76 (d, J = 16
Hz, 1H), 7.58−7.62 (m, 2H), 7.24−7.43 (m, 3H), 6.59 (d, J = 10 Hz,
1H), 6.01 (s, 1H), 5.48 (d, J = 10 Hz, 1H), 1.55 (s, 6H); 13C NMR
(100 MHz, CDCl3) δ 193.2, 166.5, 158.8, 156.9, 142.6, 135.7, 130.3,
129.1, 128.4, 127.5, 124.9, 116.7, 106.6, 102.7, 96.5, 78.4, 28.1; HRMS
(ESI) m/z calcd for C20H18O4 (M + H)+ 323.1278, found 323.1276.
(E)-1-(6-((Dimethylamino)methyl)-5,7-dihydroxy-2,2-di-
methyl-2H-chromen-8-yl)-3-phenylprop-2-en-1-one (11). To a
solution of chromene 9 (0.10 g, 0.31 mmol) in CHCl3 was added
Eschenomoser’s salt (0.057 g, 0.31 mmol). The mixture was heated at
reflux for 0.5 h. After completion, water (50 mL) was added and the
mixture extracted with CHCl3 (3 × 50 mL). The combined organic
extracts were washed with brine, dried using Na2SO4, and evaporated
in vacuo. A small amount of acetonitrile was added and orange
precipitate was formed, which was filtered and dried to give 11 (0.10 g,
88%) as an orange powder: mp 110−114 °C; IR νmax 2966, 2925,
1
yellow solid: mp 274−280 °C; H NMR (400 MHz, CDCl3) δ 14.40
(s, 1H), 13.95 (s, 1H), 7.99 (d, J = 15.7 Hz, 2H), 7.77 (d, J = 15.7 Hz,
2H), 7.55−7.68 (m, 4H), 7.35−7.48 (m, 6H), 6.90 (s, 1H), 4.75 (d, J
= 5.19 Hz, 1H), 2.57−2.68 (m, 1H), 2.25 (t, J = 4.66 Hz, 1H), 2.08−
2.17 (m, 1H), 2.15 (s, 3H), 2.13 (s, 3H), 1.94 (t, J = 12.89 Hz, 1H),
1.67 (s, 3H), 1.60 (s, 3H), 1.51 (s, 3H), 1.49 (s, 3H); 13C NMR (100
MHz, CDCl3) δ 193.6, 193.1, 166.5, 163.0, 160.6, 158.7, 154.5, 153.9,
142.2, 142.1, 135.7, 135.7, 130.3, 130.2, 129.1, 129.0, 128.4, 128.3,
127.9, 127.8, 107.8, 106.1, 106.0, 105.5, 101.1, 98.3, 78.7, 77.8, 77.4,
70.5, 47.5, 42.3, 30.5, 28.6, 27.1, 24.5, 20.6, 7.6, 7.4; HRMS (ESI) m/z
calcd for C42H40O8 (M + Na)+ 695.2615, found 695.2610.
(2E,2′E)-1,1′-(6,9-Bis(3-acetyl-2,4,6-trihydroxy-5-methylben-
zyl)-5,8,10-trihydroxy-2,2,13,13-tetramethyl-1,7a,13a,13b-tet-
rahydro-2H,13H-pyrano[3,2-c:4,5,6-d′e′]dichromene-4,11-
diyl)bis(3-phenylprop-2-en-1-one) (13). To a solution of rottlerin
(0.050 g, 0.097 mmol) in DCM (3 mL) was added anhydrous FeCl3
(0.006 g, 0.039 mmol), and the mixture was stirred at room
temperature for 30 min. The reaction mixture was washed with
brine (30 mL) and extracted with DCM (3 × 20 mL), dried over
Na2SO4, and evaporated in vacuo. The remaining crude product was
purified by flash column chromatography on silica gel using hexane/
EtOAc (7/3) to give dimer 13 (0.044 g, 44%) as an orange solid: mp
246−250 °C; IR νmax 3328, 2973, 2929, 1602, 1425, 1356, 1159, 1126,
1051, 969 cm−1; 1H NMR (600 MHz, CDCl3) δ 16.25 (s, 1H), 14.61
(s, 1H), 12.88 (s, 1H), 12.03 (s, 1H), 8.05 (d, J = 15.7 Hz, 1H), 7.98
(d, J = 15.7 Hz, 1H), 7.68−7.76 (m, 6H), 7.43−7.50 (m, 6H), 4.84 (d,
J = 4.37 Hz, 1H), 3.70−3.92 (m, 4H), 2.58−2.63 (m, 1H), 2.45 (s,
3H), 2.26−2.28 (m, 1H), 2.25 (s, 3H), 2.05−2.11 (m, 1H), 1.92−1.95
(m, 1H) 1.89 (s, 3H), 1.81 (s, 3H), 1.59 (s, 3H), 1.50 (s, 3H), 1.47 (s,
3H), 1.32 (s, 3H); 13C NMR (150 MHz, CDCl3) δ 203.5, 203.3,
193.1, 164.1, 160.4, 160.4, 160.3, 160.0, 160.0, 159.8, 158.4, 157.3,
1
1624, 1590, 1340, 1130, 972 cm−1; H NMR (400 MHz, CDCl3) δ
15.14 (s, 1H), 8.11 (d, J = 15.6 Hz, 1H), 7.81 (d, J = 15.6 Hz, 1H),
7.59−7.63 (m, 2H), 7.39−7.46 (m, 3H), 6.80 (d, J = 10 Hz, 1H), 5.51
(d, J = 10 Hz, 1H), 4.21 (s, 2H), 2.86 (s, 6H), 1.56 (s, 6H); 13C NMR
(150 MHz, CDCl3) δ 193.3, 166.4, 160.1, 158.2, 143.5, 135.5, 130.6,
129.2, 128.5, 127.1, 125.1, 117.5, 106.0, 105.4, 99.0, 79.1, 51.0, 42.1,
28.5; HRMS (ESI) m/z calcd for C23H25NO4 (M + H)+ 380.1856,
found 380.1858.
Rottlerin.13,19 A mixture of chromene 11 (0.060 g, 0.16 mmol)
and acetophenone 3 (0.029 g, 0.16 mmol) in toluene was heated at
reflux for 0.5 h. The reaction was monitored by TLC. After completion
of the reaction, toluene was evaporated in vacuo. The remaining crude
product was purified by flash column chromatography on silica gel
F
DOI: 10.1021/acs.joc.5b01827
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
155.5, 155.0, 142.6, 141.8, 135.0, 134.8, 130.7, 130.5, 129.3, 129.2,
128.4, 128.3, 127.2, 127.0, 108.2, 107.7, 106.6, 106.1, 105.2, 105.0,
104.9, 103.3, 102.9, 102.6, 100.5, 79.2, 79.1, 79.0, 78.9, 78.8, 70.4, 45.5,
32.3, 32.3, 29.9, 27.9, 26.7, 24.1, 20.3, 16.4, 15.9, 8.3, 8.0; HRMS (ESI)
m/z calcd for C60H56O16 (M + Na)+ 1055.3461, found 1055.3445.
(14) Arisawa, M.; Fujita, A.; Saga, M.; Hayashi, T.; Morita, N.;
Kawano, N.; Koshimura, S. J. Nat. Prod. 1986, 49, 298.
(15) Meikle, T.; Stevens, R. J. Chem. Soc., Perkin Trans. 1 1978, 1303.
(16) Crispin, M. C.; Hur, M.; Park, T.; Kim, Y. H.; Wurtele, E. S.
Physiol. Plant. 2013, 148, 354.
(17) Xia, L.; Narasimhulu, M.; Li, X.; Shim, J. J.; Lee, Y. R. Bull.
Korean Chem. Soc. 2010, 31, 664.
ASSOCIATED CONTENT
* Supporting Information
■
(18) Spence, J. T. J.; George, J. H. Org. Lett. 2013, 15, 3891.
(19) Minassi, A.; Cicione, L.; Koeberle, A.; Bauer, J.; Laufer, S.; Werz,
O.; Appendino, G. Eur. J. Org. Chem. 2012, 2012, 772.
(20) Furusawa, M.; Ido, Y.; Tanaka, T.; Ito, T.; Nakaya, K.; Ibrahim,
I.; Ohyama, M.; Iinuma, M.; Shirataka, Y.; Takahashi, Y. Helv. Chim.
Acta 2005, 88, 1048−1058.
(21) Luan, Y.; Sun, H.; Schaus, S. E. Org. Lett. 2011, 13, 6480.
(22) Tang, S.-Y.; Shi, J.; Guo, Q.-X. Org. Biomol. Chem. 2012, 10,
2673.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.5b01827.
CCDC-1406109 contains supplementary crystallographic data
for kamalachalcone A. CCDC-1406110 contains supplementary
crystallographic data for compound 12. These data can be
obtained free of charge from the Cambridge Crystallographic
Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
(23) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B
2009, 113, 6378.
1H NMR and 13C NMR spectra for 3, 8, 9, 11−13,
rottlerin, and kamalachalcone A and crystallographic and
computational data (PDF)
Crystallographic data (CIF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*N.K.: tel, +61 2 9385 4698; fax, +61 2 9385 6141; e-mail, n.
kumar@unsw.edu.au.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.K.C.H. is grateful to UNSW Australia for a Postgraduate
Scholarship. The authors are grateful to the Nuclear Magnetic
Resonance Facility and the Bioanalytical Mass Spectrometry
Facility in the Mark Wainwright Analytical Centre at UNSW
Australia for the use of the instruments. We thank Dr. Mohan
M. Bhadbhade for the single-crystal X-ray studies.
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DOI: 10.1021/acs.joc.5b01827
J. Org. Chem. XXXX, XXX, XXX−XXX