Synthesis of the 2-methylene analogue of the HRV 3C protease inhibitor thysanone (2-carbathysanone)

Article abstract of DOI:10.1039/c3ob41951g

The Human Rhinovirus (HRV) is the major aetiological agent for the common cold, for which only symptomatic treatment is available. HRV maturation and replication is entirely dependent on the activity of a virally encoded 3C protease that represents an attractive target for the development of therapeutics to treat the common cold. Herein we report the synthesis and biological evaluation of the 2-methylene analogue of the HRV 3C protease inhibitor (-)-thysanone (1) namely 2-carbathysanone (2), in an attempt to decipher the structural features in the natural product that are responsible for the 3C protease activity. 2-Carbathysanone (2) (and related analogues (±)-cis-23, (±)-cis-30, (±)-31) did not inhibit HRV 3C protease, indicating that the lactol functionality present in (-)-thysanone (1) is a critical structural feature required for inhibition.

Full text of DOI:10.1039/c3ob41951g

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Synthesis of the 2-methylene analogue of the
HRV 3C protease inhibitor thysanone
(2-carbathysanone)†
Cite this: Org. Biomol. Chem., 2014,
12, 905
Katrin Schünemann,a,b Daniel P. Furkert,a Eun Cho Choi,a Stephen Connelly,c
John D. Fraser,b Jonathan Sperrya and Margaret A. Brimble*a
The Human Rhinovirus (HRV) is the major aetiological agent for the common cold, for which only symp-
tomatic treatment is available. HRV maturation and replication is entirely dependent on the activity of a
virally encoded 3C protease that represents an attractive target for the development of therapeutics to
treat the common cold. Herein we report the synthesis and biological evaluation of the 2-methylene ana-
logue of the HRV 3C protease inhibitor (−)-thysanone (1) namely 2-carbathysanone (2), in an attempt to
decipher the structural features in the natural product that are responsible for the 3C protease activity.
2-Carbathysanone (2) (and related analogues ( )-cis-23, ( )-cis-30, ( )-31) did not inhibit HRV 3C pro-
tease, indicating that the lactol functionality present in (−)-thysanone (1) is a critical structural feature
required for inhibition.
Received 26th September 2013,
Accepted 9th December 2013
DOI: 10.1039/c3ob41951g
www.rsc.org/obc
Introduction
The Human Rhinovirus (HRV) is the major cause of the
common cold, which typically infects nasal epithelia cells in
the upper respiratory tract.1 The virus contains a positive
strand RNA genome, which is directly translated into a viral
polyprotein precursor of 200 kDa. In order to produce active
viral proteins and enzymes the polyprotein undergoes a series
of proteolytic cleavages. These cleavages are governed by two
virally encoded proteases designated as 2A and 3C. These pro-
teases are therefore interesting targets for drug development
against the common cold. (−)-Thysanone (1) was isolated
from the fungus Thysanophora penicilloides by the process of
screening microbial extracts for bioactive lead structures.2
(−)-Thysanone (1) was found to be an effective inhibitor of
HRV 3C protease with an IC50 of 13 µg mL−1 (47 µM).2
Fig. 1 Structure of (−)-thysanone (1), ( )-2-carbathysanone (2) and
analogues 3–8 previously synthesised by our group.6–12,15
To date, two total syntheses and one formal synthesis of the
natural product (−)-thysanone (1) have been published as well
as syntheses of several different analogues 3–8.3–17 Given our
continued interest in synthesizing (−)-thysanone (1) and its
analogues (Fig. 1),7–15 we herein report the synthesis and bio-
logical evaluation of 2-carbathysanone (2) prepared from sub-
stituted naphthalene ( )-9 that is assembled via Hauser–Kraus
annulation of cyanophthalide 10 with cyclohexenone ( )-11
(Scheme 1).
aSchool of Chemical Sciences, The University of Auckland, 23 Symonds St., Auckland,
New Zealand. E-mail: m.brimble@auckland.ac.nz
bDepartment of Molecular Medicine and Pathology, The University of Auckland,
85 Park Road, Auckland, New Zealand
cDepartment of Molecular Biology, The Scripps Research Institute, BCC 265,
Results and discussion
10550 Nr. Torrey Pines Rd., La Jolla, CA 92037, USA
†Electronic supplementary information (ESI) available: 1H and 13C NMR data for
all new compounds and experimental data for compounds 10, 13, 14, 17–21, 23
and 24. See DOI: 10.1039/c3ob41951g
Despite the fact that our earlier work suggested that
Hauser–Kraus annulation using a 4,6-dimethoxy substituted
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Scheme 3 Reagents and conditions: (i) KOtBu, tBuOH, 0 °C → 90 °C,
20 h, 27%.20
Table 1 Investigation of the Hauser–Kraus annulation. Reagents and
conditions: (i) (a) base, solvent, r.t., 1 h; (b) Me2SO4, KOH, 60 °C, 16 h
Scheme 1 Retrosynthesis of 2-carbathysanone (2) via Hauser–Kraus
annulation.
Entry
Base
Solvent
Yield
1
2
3
4
KOtBu
KOtBu
KHMDS
DBU
THF
DMSO
THF
42%
19%
35%
Decomposition
THF
Scheme 2 Reagents and conditions: (i) H2SO4, MeOH, reflux, 5 d,
quant.; (ii) Cs2CO3, nBu4NI (TBAI), 2-bromopropane, acetone–DMF
(10 : 1), reflux, 17 h, quant.; (iii) AlMe3, Et2NH, PhMe, reflux, quant.; (iv)
tBuLi, tetramethylethylenediamine (TMEDA), DMF, −78 °C → r.t., 17 h,
quant.; (v) (a) TMSCN, KCN, 18-crown-6, CH2Cl2, 3 h, 0 °C → r.t.; (b)
AcOH, 17 h, r.t., 53% over two steps.
a one-pot Hauser–Kraus annulation, methylation strategy.
Selected examples of this reaction are shown in Table 1. The
optimum conditions involving reaction of cyanophthalide 10
with potassium tert-butoxide in THF at 0 °C followed by the
addition of enone ( )-11 (entry 1) afforded naphthol ( )-17 in a
moderate 42% yield after methylation using dimethyl sulfate
cyanophthalide did not result in successful annulation with a and potassium hydroxide.
variety of electrophiles,15 we nevertheless decided to start our
The nature of the leaving group on the phthalide can
synthesis of 2-carbathysanone (2) by reinvestigation of this substantially influence the yield obtained in Hauser–Kraus
approach. We therefore synthesized cyanophthalide 10 from annulations.21–26 We therefore decided to prepare phospho-
2,4-dihydroxybenzoic acid (12) (Scheme 2). 2,4-Dihydroxy- nylphthalide 18, phenylthiophthalide 19 and phenylsulfo-
benzoic acid (12) was readily converted to the diisopropyl- nylphthalide 20 from the common intermediate benzaldehyde
protected methyl benzoate 13 in two steps using sulfuric acid 14 (Scheme 3). Phosphonylphthalide 18 was obtained follow-
in methanol to effect the methyl esterification followed by pro- ing a procedure published by Van der Veken and co-workers.27
tection of both hydroxyl groups as isopropyl ethers.18 Methyl Treatment of benzaldehyde 14 with trimethylphosphite in the
benzoate 13 was then converted to benzaldehyde 14 by treat- presence of oxalic acid gave phosphonylphthalide 18 in 81%
ment with trimethylaluminium and diethylamine followed by yield. Phenylthiophthalide 19 was prepared in a two-step, one-
formylation using tBuLi, TMEDA and DMF. Cyanophthalide 10 pot procedure.28 Potassium hydroxide was used first to gene-
was synthesized following a general literature procedure by rate an unstable hydroxyphthalide from benzaldehyde 14,
reaction of benzaldehyde 14 with trimethylsilyl cyanide which was then treated in situ with thiophenol to form
(TMSCN) in presence of catalytic amounts of KCN and thiophenylphthalide 19 in moderate 36% yield over two
18-crown-6, followed by treatment of the α-trimethylsilyl- steps. The phenylthiophthalide 19 was then oxidized with
oxybenzyl cyanide intermediate with glacial acetic acid.19 m-CPBA to afford phenylsulphonylphthalide 20 in 97% yield
Cyanophthalide 10 was obtained in a moderate 53% yield over (Scheme 4).
two steps.
Disappointingly, use of all three phthalides 18–20 in the
Hauser–Kraus annulation partner cyclohexenone ( )-11 was key-Hauser–Kraus annulation with cyclohexenone ( )-11 using
synthesized following a one-pot procedure described by Yudin potassium tert-butoxide in different solvents (e.g. DMSO and
and co-workers (Scheme 3).20
THF) led to decomposition of both starting materials
Hauser–Kraus annulation between cyanophthalide 10 and (Scheme 5).
enone ( )-11 requires investigation of a variety of different
It was therefore decided to continue the synthesis with
bases and solvents. Due to the instability of the initial cyanophthalide 10 using the previously established Hauser–
naphthalenediol annulation product it was decided to perform Kraus annulation conditions. Reduction of the ketone in the
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Fig. 2 Observed nOe signals and proposed configuration.
Scheme 4 Reagents and conditions: (i) oxalic acid, P(OEt)3, reflux, 16 h,
81%; (ii) (a) KOH, H2O, THF, r.t., 18 h; (b) PhSH, PhMe, p-TsOH, 18 h, 36%
over two steps; (iii) m-CPBA, CH2Cl2, 0 °C → r.t., 97%.
Scheme 6 Reagents and conditions: (i) CAN, MeCN–H2O, 74%.
( )-cis-naphthoquinone 23. ( )-cis-Naphthol 21 was oxidized
using ceric ammonium nitrate (CAN) in aqueous acetonitrile
affording ( )-cis-naphthoquinone 23 in moderate 74% yield
(Scheme 6). At this stage completion of the synthesis of 2-car-
bathysanone (2) only required one step, namely cleavage of the
isopropyl protecting groups. Disappointingly, treatment of
( )-cis-23 with AlCl3, BCl3 or Bi(OTf)3 only resulted in
decomposition of the starting material.29–31
Scheme 5 Unsuccessful Hauser–Kraus annulation using phthalides 18,
19 and 20.
annulation product ( )-17 using borane-dimethyl sulfide
complex gave an inseparable mixture of ( )-cis-alcohol 21 and
( )-trans-alcohol 21 in excellent yield (cis–trans, 5 : 1, Table 2),
with some of the over-reduced product ( )-22 also isolated.
In an attempt to improve the diastereoselectivity of the
reduction some sterically hindered boranes possessing two
hydrides were investigated. Pleasingly, use of both cyclohexyl-
borane and isopropoxyborane did result in the diastereoselec-
tive formation of the ( )-cis-alcohol 21, however the amount of
over-reduced product 22 also increased.
Due to the inability to deprotect the isopropyl ethers in 23
we next decided to change the isopropyl protecting groups to
methoxymethyl (MOM)-groups. The synthesis of MOM-
protected cyanophthalide 27 is shown in Scheme 7. Methyl
benzoate 24 was readily converted to TBS-protected benz-
aldehyde 25 in three steps using sodium hydride and tert-
butyldimethylsilyl chloride (TBSCl) to afford TBS-protected
methyl benzoate, which was then converted to the corres-
ponding amide using trimethylaluminium and diethyl-
The observed nOe between H-1 and H-3 established the
cis-stereochemistry in ( )-cis-alcohol 21, consistent with the
C-1 hydroxyl and the C-3 methyl groups occupying pseudo-
equatorial positions (Fig. 2).
t
amine.32 Formylation of the amide using BuLi, TMEDA and
Despite not being able to isolate a diastereomerically pure
sample of the trans-product 21 possessing the cyclohexyl sub-
stituents in the same relationship as observed in thysanone,
we aimed to complete the synthesis using the cis-product 21
and the next step required its oxidation to the corresponding
DMF afforded the desired benzaldehyde 25 in 82% yield over
three steps. The synthesis of MOM-protected phthalide 27
from benzaldehyde 25 was first attempted using TMSCN in the
presence of a catalytic amount of KCN and 18-crown-6, fol-
lowed by treatment of the α-trimethylsilyloxybenzyl cyanide
Table 2 Reduction of ketone ( )-17 using different boranes. Reagents and conditions: (i) borane, THF, 0 °C → r.t., 1 h
Entry
Borane
( )-cis-21 : ( )-trans-21
( )-22
1
2
3
BH3·SMe2
(C6H11)BH2
iPrOBH2
93% (5 : 1)
72% (1 : 0)
56% (1 : 0)
7%
25%
32%
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alcohol 29 in excellent yield. Subsequent oxidative demethyla-
tion of ( )-cis-29 using CAN afforded ( )-cis-naphthoquinone
30 in 33% yield. Removal of the MOM protecting groups was
first attempted using 1 M aq. hydrochloric acid; however, only
decomposition was observed and milder conditions were
therefore investigated. Bismuth triflate has been shown to
cleave MOM-acetals in a THF–water mixture at room tempera-
ture.31 However, under these conditions, only starting material
was observed after two hours. However, heating the reaction
mixture to 80 °C did effect deprotection, affording the desired
2-carbathysanone (2) in 87% yield.
The synthesis of 2-carbathysanone (2) described above also
resulted in two analogues ( )-23 and ( )-cis-30, good candi-
dates to be screened for HRV 3C protease activity. ( )-Naphthol
22 was also oxidised to ( )-naphthoquinone 31 using CAN in
81% yield in order to provide an additional analogue to evalu-
Scheme 7 Reagents and conditions: (i) NaH, TBSCl, 0 °C → r.t., 2 h,
quant.; (ii) AlMe3, Et2NH, PhMe, reflux, quant.; (iii) tBuLi, TMEDA, DMF,
THF, −78 °C → r.t., 18 h, 82%; (iv) (a) TMSCN, KCN, 18-crown-6, CH2Cl2,
0 °C → r.t., 3 h; (b) AcOH, 18 h; (c) P4O10, CH2(OMe)2, CH2Cl2, 0 °C →
r.t., 3 h, 37%.
intermediate with glacial acetic acid, which afforded
cyanophthalide 26. Disappointingly, the isolation of 26 was ate for inhibition of HRV 3C protease (Scheme 9). The cytotoxi-
difficult due its high solubility in water. Furthermore, the sub- city of 2-carbathysanone (2) and the three analogues ( )-cis-23,
sequent protection of 26 using MOMCl and Hünig’s base
( )-cis-30 and ( )-31 was also investigated.
resulted in alkylation at C-1 of the resulting cyanophthalide.
The HRV 3C protease screening was carried out using our
To circumvent these issues, we developed a one-pot procedure previously reported solid phase based fluorescent substrate
assay.33 The cytotoxicity (LD50) of the compounds was evalu-
(Scheme 7). Benzaldehyde 25 was treated with TMSCN in the
presence of a catalytic amount of KCN and 18-crown-6 followed ated using a cell-proliferation assay in which the human lung
by glacial acetic acid, delivering cyanophthalide 26 that adenocarcinoma epithelia cell-line NCI-H441 was grown in the
had undergone concomitant desilylation under the reaction
presence of the synthesized inhibitors 2-carbathysanone (2),
conditions. Subsequent treatment of 26 with phosphorus ( )-cis-23, ( )-cis-30 and ( )-31 and treated with 3H-thymidine
pentoxide and dimethoxymethane afforded cyanophthalide 27 to detect cell-proliferation. The results are summarized in
in 37% yield over three steps.
Table 3. All analogues did not inhibit HRV 3C protease, infer-
Disappointingly, the previously established conditions used ring the 2-oxa-lactol functionality to be a key structural feature
to effect Hauser–Kraus annulation of diisopropyl-protected in the mechanism of inhibition of HRV 3C protease by thysa-
cyanophthalide 10 with enone ( )-11 using potassium tert-
none (1). All four analogues also exhibited high cytotoxicity
butoxide followed by methylation using dimethyl sulfate and against NCI-H441.
potassium hydroxide, were unsuccessful when applied to
MOM-protected cyanophthalide 27. Fortunately, changing the
base to sodium hydride followed by methylation using
dimethyl sulfate and sodium hydride afforded dimethoxy-
naphthalene ( )-28 in 79% yield over two steps (Scheme 8). In
this case, methylation of the 10-naphthol group occurred as
the sodium hydride was able to deprotonate the hydrogen
bonded C-10-hydroxyl-group. Although attempted reduction of
( )-28 using cyclohexylborane only resulted in decomposition,
employing iBu2AlH exclusively afforded the desired ( )-cis- 81%.
Scheme 9 Reagents and conditions: (i) CAN, MeCN–H2O, r.t., 10 min,
i
Scheme 8 Reagents and conditions: (i) (a) NaH, THF, 0 °C → r.t., 1 h; (b) Me2SO4, NaH, 0 °C → r.t., 3 h, 79%; (ii) Bu2AlH, CH2Cl2, 0 °C → r.t., 87%,
(iii) CAN, MeCN–H2O, 10 min, 33%; (iv) Bi(OTf)3, THF–H2O, 80 °C, 87%.
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29.84 0.01 for 13C NMR). Coupling constants (J) are reported
in Hertz (Hz). 1H-NMR data is reported as chemical shift in
ppm, followed by the relative integral, the multiplicity (“s”
singlet, “d” doublet, “dd” doublet of doublets, “ddd”, doublet
of doublets of doublets, “dt” doublet of triplets, “t” triplet, “q”
quartet, “quint.” quintuplet, “sept.” septet, “m” multiplet, “b”
broad) and coupling constant where applicable and relative
integral. 13C NMR spectra are reported as chemical shift in
ppm. High resolution mass spectra were recorded using a
Bruker microTOF-QII mass spectrometer.
Table 3 IC50 and LD50 values of thysanone analogues prepared in this
study
Compound
R1
R2
IC50 [µM]
LD50 [µM]
93.4 36
2-Carbathysanone (2)
( )-cis-23
( )-cis-30
OH
OH
OH
OH
H
n.i.d.
n.i.d.
n.i.d.
n.i.d.
OiPr
52.0
28.4 11
37.4
0
OMOM
( )-31
OiPr
0
2,4-Bis((tert-butyldimethylsilyl)oxy)-N,N-diethyl-6-
formylbenzamide 25
n.i.d. = no inhibition detected.
To a solution of methyl 2,4-dihydroxybenzoate 24 (1.5 g,
8.9 mmol) in THF (45 mL) was added sodium hydride (0.9 g,
22.3 mmol) at 0 °C. The reaction mixture was stirred for
30 min and TBSCl (2.8 g, 22.3 mmol) was added. The reaction
mixture was stirred for 2 h then satd. aq. NH4Cl solution
(20 mL) was added. The aq. layer was extracted with EtOAc (3 ×
30 mL) and the combined organic layers were washed with
brine (30 mL) and dried over MgSO4. The solvent was removed
in vacuo and the residue purified by flash chromatography
using hexanes–EtOAc (9 : 1) as eluent to afford methyl 2,4-bis-
((tert-butyldimethylsilyl)oxy)-benzoate (3.5 g, 8.8 mmol, quant.)
Conclusion
In conclusion, we have developed a synthesis of 2-carbathysa-
none (2) and closely related analogues using a Hauser–Kraus
annulation. 2-Carbathysanone (2), and analogues ( )-cis-23,
( )-cis-30, ( )-31 did not inhibit HRV 3C protease thus indicat-
ing that the lactol functionality present in thysanone (1) is a
critical structural feature required for inhibition of HRV 3C
protease.
1
as a yellow oil. Rf (hexanes–ethyl acetate = 9 : 1): 0.81. H NMR
(400 MHz, CDCl3): δ = 7.72 (1H, d, J = 8.8 Hz), 6.40 (1H, dd, J =
8.8 Hz, 2.4 Hz), 4.46 (1H, d, J = 2.4 Hz, H-3), 3.83 (3H, s), 1.01
(9H, s), 0.97 (9H, s), 0.21 (6H, s), 0.21 (6H, s). 13C NMR
(100 MHz, CDCl3): δ = 167.0, 160.3, 157.1, 133.4, 116.0, 113.6,
112.8, 51.7, 25.8, 25.8, 18.5, 18.4. The spectroscopic data were
Experimental
General details
Unless otherwise stated, all non-aqueous reactions and distilla- in agreement with that reported in the literature.34 To a solu-
tions were performed under a nitrogen or argon atmosphere in tion of trimethylaluminum (2 M in toluene, 7 mL, 14 mmol)
oven- or flame-dried glassware. Tetrahydrofuran was freshly in toluene (10 mL) was added diethylamine (1.5 mL, 14 mmol)
distilled from sodium/benzophenone. Dichloromethane at −6 °C under an atmosphere of nitrogen. After 10 min the
(CH2Cl2) and toluene were freshly distilled from calcium reaction mixture was warmed to r.t. and methyl 2,4-bis((tert-
hydride. TMEDA was distilled from calcium hydride and butyldimethylsilyl)oxy)-benzoate (1.4 g, 4.2 mmol) was added.
stored under a nitrogen atmosphere over MgSO4. Reactions The resulting mixture was heated at reflux overnight. The
performed at low temperature were either cooled in an mixture was cooled to 0 °C then 10% aq. HCl (10 mL) was
acetone-dry ice bath for temperatures below 0 °C or using a added. The layers were separated and the organic layer was
water-ice bath for 0 °C. Flash chromatography was carried out washed with 10% aq. HCl (20 mL). The combined aq. layers
using 0.063–0.1 mm silica gel (Davisil R LC60A 40-63 Micron) were extracted with EtOAc (3 × 20 mL). The combined organic
with the indicated solvent. Preparatory TLC was carried out on layers were washed with brine (20 mL) and dried over MgSO4.
500 μm Uniplate™ (Analtech) silica gel (20 × 20 cm) thin layer The solvent was removed in vacuo and the residue purified by
chromatography plates. Infrared spectra were recorded using a flash chromatography using hexane–EtOAc (4 : 1) as eluent
Perkin Elmer R Spectrum 1000 Fourier Transform Infrared to afford 2,4-Bis((tert-butyldimethylsilyl)oxy)-N,N-diethylbenza-
spectrometer. Values are expressed in wavenumbers (cm−1
)
mide (1.54 g, 4.2 mmol, quant.) as a yellow oil. Rf (hexane–
and recorded in a range of 4000 to 450 cm−1. NMR spectra ethyl acetate = 4 : 1): 0.42. H NMR (400 MHz, CDCl3): δ = 7.04
were recorded at 21 °C in CDCl3 or C6D6 on either a Bruker® (1H, d, J = 8.3 Hz), 6.47 (1H, dd, J = 8.3 Hz), 6.31 (1H, d, J = 2.2
Avance 300 spectrometer operating at 300 MHz for 1H nuclei Hz), 3.69–3.02 (4H, bm), 1.22 (3H, t, J = 7.1 Hz), 0.99 (3H, t, J =
and 75 MHz for 13C nuclei or on a Bruker® DRX400 or 7.1 Hz), 0.98 (9H, s), 0.98 (9H, s), 0.21 (6H, s), 0.19 (6H, s). 13C
1
1
Bruker® 400 spectrometer operating at 400 MHz for H nuclei NMR (100 MHz, CDCl3): δ = 169.3, 156.9, 152.3, 128.6, 123.4,
and 100 MHz for 13C nuclei. All chemical shifts are reported in 113.6, 111.4, 43.0, 39.4, 25.8, 25.8, 18.4, 18.3, 14.2, 13.4. The
parts per million (ppm) from tetramethylsilane (δ = 0) and spectroscopic data were in agreement with that reported in the
were measured relative to the solvent in which the sample was literature.34 A solution of 2,4-bis((tert-butyldimethylsilyl)oxy)-
analysed (CDCl3: TMS δ = 0.00 for 1H NMR and CDCl3 δ = N,N-diethylbenzamide (2.5 g, 6.8 mmol) and TMEDA (1.1 mL,
1
77.16 for 13C NMR; acetone-d6: δ = 2.05 for H NMR and δ = 7.5 mmol) in THF (10 mL) was cooled to −78 °C under an
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atmosphere of nitrogen and t-BuLi (6.4 mL, 10.2 mmol) was 9,10-Dimethoxy-6,8-bis(methoxymethoxy)-3-methyl-3,4-
added slowly. The solution turned yellow and was stirred for a dihydroanthracen-1(2H)-one ( )-28
further 10 min. DMF (1.1 mL, 13.6 mmol) was added and the
mixture warmed to r.t. overnight. Satd. aq. NH4Cl solution
(20 mL) was added and the mixture stirred for 30 min. The aq.
To a solution of 4,6-bis(methoxymethoxy)-3-oxo-1,3-dihydroiso-
benzofuran-1-carbonitrile 27 (120 mg, 0.43 mmol) and cyclo-
hexenone ( )-1119 (95 mg, 0.86 mmol) in THF (1 mL) was
layer was extracted with EtOAc (3 × 20 mL). The combined
added NaH (19 mg, 60% in mineral oil, 0.47 mmol) in THF
organic layers were washed with brine (20 mL) and dried over
(0.5 mL) under an atmosphere of nitrogen. The reaction
MgSO4. The solvent was removed in vacuo and the residue puri-
mixture was stirred for 1 h then dimethyl sulfate (0.41 mL,
4.3 mmol) and NaH (38 mg, 0.95 mmol) were added. The
mixture was stirred at r.t. for 3 h then satd. aq. NH4Cl
solution (5 mL) was added. The aq. layer was extracted with
fied by flash chromatography using hexanes–EtOAc (4 : 1) as
eluent to afford the title compound (2.2 g, 5.5 mmol, 82%) as a
yellow oil. Rf (hexanes–EtOAc = 4 : 1): 0.42. νmax (film/cm−1):
2956, 2934, 2894, 2861, 1699, 1633, 1594, 1570, 1332, 1162,
EtOAc (3 × 10 mL). The combined organic extracts were
1
783, 733. H NMR (400 MHz, CDCl3): δ = 9.86 (1H, s, H-CHO),
washed with brine (10 mL) and dried over MgSO4. The solvent
6.97 (1H, d, J = 2.3 Hz), 6.53 (1H, d, J = 2.3 Hz), 3.70 (1H, dq.
was removed in vacuo and the residue purified by flash chrom-
J = 7.0 Hz, 14.2 Hz), 3.93 (1H, dq, J = 7.0 Hz, 14.2 Hz),
atography using hexanes–EtOAc (7 : 3) as eluent to afford the
3.14 (1H, dq, J = 7.3 Hz, 14.4 Hz), 3.07 (1H, dq, J = 7.3 Hz,
title compound (132 mg, 0.34 mmol, 79%) as a yellow oil. Rf
14.4 Hz), 1.24 (3H, t, J = 7.3 Hz), 0.98 (3H, t, J = 7.0 Hz),
(hexanes–EtOAc = 7 : 3): 0.54. νmax (film/cm−1): 3071, 2954,
0.95 (9H, s), 0.94 (9H, s), 0.22–0.19 (12H, m). 13C NMR
2933, 2859, 1715, 1613, 1529, 1335, 1150, 930, 765. 1H NMR
(100 MHz, CDCl3): δ = 190.4, 166.2, 156.7, 153.1, 134.8, 125.8,
(400 MHz, CDCl3): δ = 7.27 (1H, d, J = 2.4 Hz), 6.87 (1H, d, J =
116.9, 113.0, 43.2, 39.6, 25.7, 25.6, 18.3, 18.2, 14.0, 13.0. HRMS
2.4 Hz), 5.31 (2H, d, J = 0.9 Hz), 5.30 (2H, s), 3.90 (3H, s), 3.84
(3H, s), 3.58 (3H, s), 3.52 (3H, s), 3.32 (1H, dq, J = 16.2 Hz,
2.0 Hz), 2.75 (1H, dq, J = 16.2 Hz, 2.0 Hz), 2.52 (1H, dd, J =
(ESI): Calcd for C24H44NO4Si2 [MH]+: 466.2803, found:
466.2817; calcd for C24H43NNaO4Si2 [MNa]+: 488.2623, found:
488.2625.
16.2 Hz, 10.6 Hz), 2.34 (1H, dd, J = 16.2 Hz, 11.5 Hz), 2.28–2.19
(1H, m), 1.16 (3H, d, J = 6.4 Hz). 13C NMR (100 MHz, CDCl3):
δ = 197.8, 158.0, 147.7, 134.9, 131.4, 121.7, 117.1, 105.2, 98.3,
96.3, 94.5, 63.1, 60.8, 56.7, 56.5, 49.4, 32.4, 29.3, 21.5. HRMS
4,6-Bis(methoxymethoxy)-3-oxo-1,3-dihydroisobenzofuran-
1-carbonitrile 27
(ESI): Calcd for C21H27O7 [MH]+: 391.1751, found: 391.1738;
calcd for C21H26NaO7 [MNa]+: 413.1571, found: 413.1557; calcd
To
a
solution of 2,4-bis((tert-butyldimethylsilyl)oxy)-N,N-
for C21H26KO7 [MK]+: 429.1310, found: 429.1295.
diethyl-6-formylbenzamide 25 (4.0 g, 10 mmol) in CH2Cl2
(20 mL) was added KCN (13 mg, 0.2 mmol) and 18-crown-6
(53 mg, 0.2 mmol) under an atmosphere of nitrogen. The reac-
tion mixture was cooled to 0 °C and TMSCN (1.9 mL,
15 mmol) was added dropwise. The solvent was removed
under reduced pressure after 15 min and the residue taken up To
cis-9,10-Dimethoxy-6,8-bis(methoxymethoxy)-3-methyl-1,2,3,4-
tetrahydroanthracen-1-ol ( )-cis-29
a
solution of 9,10-dimethoxy-6,8-bis(methoxymethoxy)-
in acetic acid (20 mL) and the mixture stirred overnight at 3-methyl-3,4-dihydroanthracen-1(2H)-one ( )-28 (60 mg,
80 °C. The solvent was removed under reduced pressure and 0.15 mmol) in CH2Cl2 (1 mL) was added at −78 °C DIBAL-H
the residue taken up in CH2Cl2–dimethoxymethane (1 : 1, (170 µL, 1 M in cyclohexane, 0.17 mmol) under an atmosphere
80 mL) and phosphorus pentoxide (10 g, 70 mmol) was added of nitrogen and the mixture stirred at −78 °C for 30 min then
at 0 °C. The mixture was warmed to r.t., stirred for 3 h then allowed to warm to r.t. Methanol (1 mL) and half-satd.
poured into ice cold satd. aq. Na2CO3 solution (100 mL). The Rochelle’s salt (3 mL) were added and the reaction was stirred
aq. layer was extracted with EtOAc (3 × 50 mL). The combined for 20 min. The aq. layer was extracted with Et2O (3 × 5 mL)
organic layers were washed with brine (50 mL) and dried over and the combined organic extracts were dried over MgSO4. The
MgSO4. The solvent was removed in vacuo and the residue solvent was removed in vacuo and the residue purified by flash
purified by flash chromatography using hexanes–EtOAc (9 : 1) chromatography using hexanes–EtOAc (7 : 3) as eluent to
as eluent to afford the title compound (1.03 g, 3.7 mmol, 37%) afford the title compound (40 mg, 0.10 mmol, 87%) as a yellow
as a colourless oil. Rf (hexanes–EtOAc = 1 : 1): 0.70. νmax (film/ oil. Rf (hexane–EtOAc = 7 : 3): 0.39. νmax (film/cm−1): 3505,
cm−1): 2980, 2938, 1785, 1697, 1614, 1598, 1319, 1150, 972, 2934, 2833, 1751, 1619, 1595, 1452, 1335, 1227, 1147, 1024,
1
920, 686. 1H NMR (400 MHz, CDCl3): δ = 6.95 (1H, d, J = 928. H NMR (400 MHz, CDCl3): δ = 7.27 (1H, d, J = 2.3 Hz),
1.6 Hz), 6.92 (1H, d, J = 1.6 Hz), 5.92 (1H, s), 5.36 (2H, s), 6.88 (1H, d, J = 2.3 Hz), 5.32 (2H, s), 5.29 (2H, s), 5.33–5.24
5.30 (1H, d, J = 7.2 Hz), 5.23 (1H, d, J = 7.2 Hz), 3.54 (3H, s), (1H, m), 4.68 (1H, bs), 3.90 (3H, s), 3.81 (3H, s), 3.59 (3H, s),
3.51 (3H, s). 13C NMR (100 MHz, CDCl3): δ = 165.3, 157.9, 3.53 (3H, s), 3.13 (1H, dq, J = 16.6 Hz, 2.2 Hz), 2.36–2.29 (2H,
146.0, 114.2, 103.7, 105.8, 102.5, 95.1, 94.8, 64.8, 57.0, 56.8. m), 1.86–1.76 (1H, m), 1.52–1.44 (1H, m), 1.17 (3H, d, J =
HRMS (ESI): Calcd for C13H14NO6 [MH]+: 280.0816, found: 6.5 Hz). 13C NMR (100 MHz, CDCl3): δ = 155.6, 154.5, 151.4,
280.0823; calcd for C13H13NNaO6 [MNa]+: 302.0635, found: 148.5, 131.2, 129.2, 128.9, 115.9, 104.7, 98.6, 96.3, 94.7, 67.5,
302.0631.
61.9, 60.6, 56.7, 56.4, 40.0, 33.2, 27.7, 22.3. HRMS (ESI): Calcd
910 | Org. Biomol. Chem., 2014, 12, 905–912
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for C21H28NaO7 [MNa]+: 415.1727, found: 415.1729; calcd for 5,7-Diisopropoxy-2-methyl-1,2,3,4-tetrahydroanthracene-9,10-
C21H28KO7 [MK]+: 431.1467, found: 431.1474.
dione ( )-31
To a solution of 6,8-diisopropoxy-10-methoxy-3-methyl-1,2,3,4-
tetrahydroanthracen-9-ol ( )-22 (13 mg, 36 μmol) in aceto-
nitrile (0.5 mL) was slowly added a solution of CAN (40 mg,
72 μmol) in water (0.5 mL) at 0 °C. The mixture was stirred at
0 °C for 5 min. then warmed to r.t., stirred for 5 min. then
poured into ice cold water (10 mL). The aq. layer was extracted
with EtOAc (3 × 10 mL) and the combined organic extracts
were dried over MgSO4. The solvent was removed in vacuo and
the residue purified by preparative TLC using hexanes–EtOAc
(7 : 1) as eluent to afford title compound (10 mg, 29 μmol, 81%)
as a yellow oil. Rf (hexanes–EtOAc = 7 : 3): 0.83. νmax (film/
cm−1): 2980, 2932, 2859, 1727, 1652, 1591, 1457, 1310, 1266,
1156, 1109. 1H-NMR (300 MHz, acetone-d6): δ = 7.15 (1H, d, J =
2.5 Hz), 6.83 (1H, d, J = 2.5 Hz), 4.85 (1H, sept., J = 6.0 Hz),
4.74 (1H, sept., J = 6.0 Hz), 2.73–2.67 (2H, m), 2.45–2.29 (1H,
m), 2.01–1.91 (1H, m), 1.91–1.81 (1H, m), 1.78–1.64 (1H, m),
1.64–1.55 (1H, m), 1.37 (6H, d, J = 6.0 Hz), 1.36 (6H, d, J =
6.0 Hz), 1.07 (3H, d, J = 6.6 Hz). 13C-NMR (75 MHz, acetone-
d6): δ = 184.4, 181.7, 162.4, 160.3, 146.0, 140.6, 136.2, 107.5,
105.0, 71.6, 70.3, 30.7, 28.8, 27.4, 23.4, 21.4, 21.2, 20.8. HRMS
(ESI): Calcd for C21H27O4 [MH]+: 343.1904, found: 343.1878;
calcd for C21H26NaO4 [MNa]+: 365.1723, found: 365.1712; calcd
for C21H26KO4 [MK]+: 381.1463, found: 381.1466.
cis-1-Hydroxy-6,8-bis(methoxymethoxy)-3-methyl-1,2,3,4-
tetrahydroanthracene-9,10-dione ( )-cis-30
To a solution of cis-9,10-dimethoxy-6,8-bis(methoxymethoxy)-
3-methyl-1,2,3,4-tetrahydro-anthracen-1-ol ( )-cis-29 (10 mg,
25 μmol) in acetonitrile (0.5 mL) was slowly added a solution
of CAN (28 mg, 51 μmol) in water (0.5 mL) at 0 °C. The mixture
was stirred at 0 °C for 5 min then warmed to r.t., stirred for
5 min then poured into ice cold water (10 mL). The aq. layer
was extracted with EtOAc (3 × 10 mL) and the combined
organic extracts were dried over MgSO4. The solvent was
removed in vacuo and the residue purified by preparative TLC
using hexanes–EtOAc (7 : 1) as eluent to afford the title com-
pound (3 mg, 8 μmol, 33%) as a yellow oil. Rf (hexanes–EtOAc =
7 : 3): 0.27. νmax (film/cm−1): 3521, 2954, 2922, 2877, 2855,
1740, 1711, 1648, 1595, 1569, 1458, 1295, 1149, 1026, 927.
1H NMR (300 MHz, acetone-d6): δ = 7.37 (1H, d, J = 2.4 Hz),
7.12 (1H, d, J = 2.4 Hz), 5.37 (2H, s), 5.34 (2H, d, J = 0.9 Hz),
4.97–4.88 (1H, m), 3.51 (3H, s), 3.48 (3H, s), 2.76–2.71 (1H, m),
2.18–2.08 (1H, m), 2.02–1.19 (1H, m), 1.85–1.71 (1H, m),
1.41–1.31 (1H, m), 1.10 (3H, d, J = 6.6 Hz). 13C NMR (75 MHz,
acetone-d6): δ = 185.4, 183.1, 162.9, 160.1, 146.8, 143.1, 136.6,
110.3, 107.7, 96.2, 95.2, 66.9, 56.8, 56.7, 39.6, 32.2, 27.3, 21.9.
HRMS (ESI): Calcd for C19H22NaO7 [MNa]+: 385.1258, found:
385.1253.
Acknowledgements
The authors are grateful to the Maurice Wilkins Centre for
Molecular Biodiscovery for financial support.
cis-1,6,8-Trihydroxy-3-methyl-1,2,3,4-tetrahydroanthracene-
9,10-dione (2) (2-carbathysanone)
To
a
solution of cis-1-hydroxy-6,8-bis(methoxymethoxy)-
Notes and references
3-methyl-1,2,3,4-tetrahydroanthracene-9,10-dione
( )-cis-30
(4.5 mg, 12.3 µmol) in THF–H2O (1 : 1, 1 mL) was added
Bi(OTf)3 (1.6 mg, 2.5 µmol). The reaction mixture was heated at
80 °C for 3 h. Satd. aq. NH4Cl solution (5 mL) was added and
the aq. layer was extracted with EtOAc (3 × 10 mL). The com-
bined organic extracts were dried over MgSO4 and the solvent
was removed in vacuo. Purification of the residue by prepara-
tive TLC using hexanes–EtOAc (19 : 1) as eluent then afforded
the title compound (2.9 mg, 10.7 μmol, 87%) as an unstable
yellow oil; Rf (hexanes–EtOAc = 1 : 1): 0.51. νmax (film/cm−1):
3352, 2976, 1924, 1854, 1650, 1623, 1596, 1579, 1447, 1393,
1359, 1311, 1276, 1231, 1208, 1179, 1167, 1142, 1081, 1071,
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1051, 1021, 1008, 984, 875, 857, 788, 744. H NMR (400 MHz,
CDCl3): δ = 12.08 (1H, s, H-OH), 7.09 (1H, d, J = 2.4 Hz), 6.60
(1H, d, J = 2.4 Hz), 6.43 (1H, bs), 5.03–4.96 (1H, m), 4.06 (1H,
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297.0743.
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