Total synthesis of anhydromarasmone

Article abstract of DOI:10.1002/ejoc.200200656

A synthesis of anhydromarasmone is presented using, as key step, an iridium-catalyzed double-bond isomerization from a homoallyl ether to an allyl ether. Other transition metal complexes employed for double-bond transposition resulted in the vinyl ether as the final product. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejoc.200200656

FULL PAPER
Total Synthesis of Anhydromarasmone
Johann Jauch,*[a] Carsten Wallner,[a] and Eberhardt Herdtweck[b]
Dedicated to Prof. Dr. Wolfgang Steglich on the occasion of his 70th birthday
Keywords: Iridium / Isomerization / Natural products / Total synthesis
A synthesis of anhydromarasmone is presented using, as key
step, an iridium-catalyzed double-bond isomerization from a
homoallyl ether to an allyl ether. Other transition metal com-
plexes employed for double-bond transposition resulted in
the vinyl ether as the final product.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
Since kuehneromycin A and the mniopetals are inhibitors
of the reverse transcriptase of HIV, we became interested
in the question of whether the marasmones also show this
biological activity. Therefore, we decided to synthesize
anhydromarasmone 1a as a first target molecule to provide
material for biological testing.[5]
In the course of our research towards highly func-
tionalized natural products, we recently published our
syntheses of kuehneromycin A,[1] mniopetal E,[2] and mni-
opetal F.[3] Structurally related to these compound are the
marasmones (Figure 1), which were isolated some years ago
by Ayer and co-workers[4] from Marasmius oreades. This
fungus seems to be responsible for the so-called ‘‘fairy
rings’’ in Canadian golf yards.
Here we wish to report our results towards this end.
Results and Discussion
Our syntheses of kuehneromycin A and the mniopetals
were based on a common intermediate 6, which should be
a suitable starting material for the synthesis of 1a.
In principle, we have two possibilities to convert 6 into
1a (see Scheme 1). The first strategy consists of the cleavage
of the silyl ether 6, oxidation of 5 with a concomitant
double-bond shift to the α,β-unsaturated aldehyde 4, Luche
reduction to the allyl alcohol 3, intramolecular transacetal-
ization to 2 and, finally, introduction of the 2,3-double
bond. The second strategy is shorter and makes use of silyl
ether cleavage to give 5, intramolecular transacetalization
leading to 7, double-bond isomerization to yield 2 and, fi-
nally, introduction of the 2,3-double bond. Whereas in the
first strategy the double-bond isomerization is driven by es-
tablishing an α,β-unsaturated aldehyde from a homoallyl al-
cohol, there is no such driving force in the second strategy.
In that case, it was an interesting question as to whether or
not it is possible to use transition metal methodology for
this task and, thus, we decided to explore the second strat-
egy in greater detail.
Figure 1. Anhydromarasmone 1a, marasmone 1b, isomarasmone
1c and dihydromarasmone 1d
[a]
Institut für Organische Chemie und Biochemie der Technischen
Desilylation of 6 using TBAF was straightforward as was
intramolecular transacetalization in the presence of TFA to
give 7 (92% over two steps). Acetal 7 crystallized readily
upon standing. The structure was confirmed by NMR spec-
troscopy and X-ray analysis[6] (Figure 2). The next step was
Universität München
Fax: (internat.) ϩ 49-89/289-13329
E-mail: johann.jauch@ch.tum.de
[b]
Anorganische-Chemisches
Universität München
Institut
der
Technischen
Lichtenbergstrasse 4, 85747 Garching, Germany
3060
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200200656
Eur. J. Org. Chem. 2003, 3060Ϫ3064

Total Synthesis of Anhydromarasmone
FULL PAPER
Scheme 1. Retrosynthetic strategies for the synthesis of 1a
the crucial double-bond isomerization and a survey of the
literature[7] offered, as catalysts, Pd/C, PdCl2(PhCN)2,
PtCl2(PhCN)2, and RuCl3·3H2O, which did not give the de-
sired isomerization. Using RhCl3·3H2O with or without
CaCO3,
[(PPh3)3RhCl],
[(PPh3)3RhH(CO)],
and
IrCl3·3H2O as catalysts gave the enol ether 8 as the main
product (Ͼ 80%) (Scheme 2). Compound 2 is produced in-
itially, but soon it isomerizes, so that once the disappear-
ance of 7 has occurred, almost all of 2 has been transformed
into 8.
Scheme 2. Double-bond isomerization using various transition me-
tal catalysts (see text)
ska’s complex [(PPh3)2Ir(CO)Cl][8] is the catalyst of choice
A solution to this problem would be a catalyst that me- in this reaction. After 6 h at 70 °C in toluene/ethanol (4:1,
diates the first isomerization selectively, but not the second v/v) equilibrium between 7 and 2 was reached [46% of 7,
one. After various unsuccessful attempts, we found that Va- 50% of 2, 4% of 8 (GC)]. Interestingly, in all our isomeriz-
Figure 2. X-ray structures of homoallyl ether 7 and allyl ether 2
Eur. J. Org. Chem. 2003, 3060Ϫ3064
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3061

J. Jauch, C. Wallner, E. Herdtweck
FULL PAPER
Scheme 3. a) TBAF, THF, room temp., 2 h; b) 20% TFA in CH2Cl2, room temp., 10 min, 92% over two steps; c) [(PPh3)2Ir(CO)Cl] (50
mol %), toluene/ethanol (4:1, v/v), 70 °C, 6 h, 43% (ca. 35% of 7 was recovered); d) i. LDA, THF, Ϫ78 °C, 15 min, ii. PhSeBr, Ϫ78 °C,
15 min, iii. H2O2, 0 °C to room temp., 2 h, 58%.
THF was filtered through basic alumina to remove peroxides and
then dried with K/benzophenone under N2. Methylene chloride
was dried over CaH2 and diisopropylamine was dried over NaH
prior to distillation.
ation reactions, the double bond migrates towards the
bridgehead carbon C8.
This successful isomerization would be practical only if
the two isomers could be separated readily. Indeed, this is
possible using flash chromatography. It is essential to use
silica gel with 15Ϫ40 µm particle size since it gives better
separations than the silica gel (40Ϫ63 µm particle size) that
is usually applied. The separated starting material, as well
as the catalyst, could be recycled. Isomerized acetal 2 gave
crystals suitable for X-ray analysis (Figure 2).[6]
After having solved the isomerization problem, introduc-
tion of the 2,3-double bond using standard methodology
(LDA, PhSeBr, H2O2)[9] was straightforward. The resulting
product was identical in all respects with the published
natural product. Furthermore, our synthesis confirms the
absolute configuration of anhydromarasmon suggested by
Ayer et al.[4]
Synthesis of 7: A solution of silyl ether 6 (0.81 g, 1.24 mmol) in
THF (10 mL) was cooled to 0 °C and a solution of TBAF in THF
(1.2 mL, 1.2 mmol) was added. The cooling bath was removed and
the reaction mixture was stirred at room temperature for 1 h. The
solvent was evaporated in vacuo and the residue was dissolved in
dichloromethane (ca. 10 mL) and evaporated again to remove re-
sidual water azeotropically. Crude 5, which was pure enough for the
next step, was dissolved in dichloromethane (8.5 mL) and cooled to
0 °C. TFA (2 mL) was then added dropwise with vigorous stirring
and after 10 min the cooling bath was removed. Stirring was con-
tinued for 1 h at room temperature and then the reaction mixture
was poured onto a mixture of crushed ice, saturated NaHCO3 solu-
tion and diethyl ether (ca. 50 mL). After separation of the phases,
the aqueous phase was extracted once with diethyl ether (50 mL).
The combined organic phases were washed with brine and dried
over MgSO4. Flash chromatography with pentane/diethyl ether
(3:1, v/v) gave 7 (0.3 g, 92%) as a colorless oil, which crystallized
upon standing. [α]2D0 ϭ ϩ 97.8 (c ϭ 0.99, CH2Cl2). 1H NMR
(360 MHz, CDCl3): δ ϭ 6.11 (d, J ϭ 4.6 Hz, 1 H, H-11), 5.98 (dt,
J ϭ 9.8, 3.3 Hz, 1 H, H-7), 5.74 (dt, J ϭ 9.8, 2.6 Hz, 1 H, H-6),
4.41 (t, J ϭ 8.8 Hz, 1 H, H-12), 3.76 (t, J ϭ 8.4 Hz, 1 H, H-12),
3.41 (dd, J ϭ 11.3, 4.5 Hz, 1 H, H-9), 3.01Ϫ2.88 (m, 1 H, H-8),
2.95 (td, J ϭ 14.0, 5.2 Hz, 1 H, H-3), 2.39 (ddd, J ϭ 11.7, 4.5,
3.9 Hz, 1 H, H-3), 2.37 (d, J ϭ 3.3 Hz, 1 H, H-5), 2.00 (dt, J ϭ
14.3, 4.9 Hz, 1 H, H-2), 1.69 (td, J ϭ 13.6, 3.9 Hz, 1 H, H-2), 1.48
(s, 3 H, H-14), 1.08 (s, 3 H, H-13) ppm. 13C NMR (90.6 MHz,
CDCl3): δ ϭ 205.1 (C-1), 169.7 (C-15), 129.1 (C-6), 128.3 (C-7),
106.7 (C-11), 73.3 (C-12), 64.8 (C-10), 49.0 (C-9), 48.9 (C-5), 39.8
(C-2), 36.0 (C-3), 34.6 (C-8), 32.1 (C-4), 31.8 (C-13), 21.9 (C-14)
ppm. MS (EI, 80 °C): m/z (%) ϭ 262 ([Mϩ], 5), 218 ([M Ϫ CO2]ϩ,
39), 189 (49), 162 (62), 132 (100), 119 (63), 91 (68), 55 (12). HRMS
(EI, 80 °C): calcd. 262.1205; found 262.1216.
The complete synthesis of 1a starting from 6 is shown
in Scheme 3.
Conclusion
In summary, we have synthesized 1a from 6 in four steps,
using an iridium-catalyzed double-bond isomerization as
the key transformation. Offering a valuable shortcut, this
reaction could be useful for other chemists faced with the
problem of double-bond transposition.
Experimental Section
1
General Procedures: H and 13C NMR spectra were recorded with
a Bruker AM 360 at 360 and 90 MHz, respectively. Chemical shifts
are reported in ppm and coupling constants are given in Hz. For
complete assignments of H and C atoms, HMQC, HMBC, H,H- of
COSY, HMQC-COSY and NOESY spectra were recorded on a
Bruker AV 500 at 500 MHz. Mass spectra were obtained in EI
(GC-MS) mode on a Finnigan MAT 8200.
Double-Bond Isomerization by Using Vaska’s Complex: A solution
7
(145 mg, 0.55 mmol) and (PPh3)2Ir(CO)Cl (225 mg,
0.29 mmol, 0.5 equiv.) in dry toluene (4 mL) and dry ethanol
(1 mL) and stirred for 6 h at 70 °C. After cooling to room tempera-
ture, the reaction mixture was filtered (ca. 200 mg of Vaska’s com-
3062
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Eur. J. Org. Chem. 2003, 3060Ϫ3064

Total Synthesis of Anhydromarasmone
FULL PAPER
J. Jauch, Synlett 2001, 87Ϫ89. [2b] Y. Suzuki, R. Nishimaki,
[2] [2a]
plex was recovered) and the solvents were evaporated to dryness.
Flash chromatography on silica gel (15Ϫ40 µm, Merck) as the
stationary phase and pentane/diethyl ether (5:1, v/v) as the eluent
gave 2 (62 mg, 0.24 mmol, 43%) as a colorless oil that crystallized
upon standing [α]2D0 ϭ ϩ30.8 (c ϭ 0.2, Et2O); ref.[4a] [α]2D0 ϭ ϩ28
(c ϭ 0.4, MeOH)). Additionally, 7 (50 mg, 0.19 mmol, 35%) was
recovered and 8 (6 mg, 4%) was isolated. 1H NMR (360 MHz,
CDCl3): δ ϭ 6.14 (d, J ϭ 4.2 Hz, 1 H, H-11), 5.95 (m, 1 H, H-7),
4.53 (br. d, J ϭ 10.2 Hz, 1 H, H-12), 4.45 (br. d, J ϭ 10.6 Hz, 1
H, H-12), 3.37 (m, 1 H, H-9), 2.91 (td, J ϭ 14.6, 5.1 Hz, 1 H, H-
2), 2.48Ϫ2.28 (m, 2 H, H-6), 2.38 (dt, J ϭ 14.2, 3.3 Hz, 1 H, H-2),
2.04 (dd, J ϭ 11.4, 5.9 Hz, 1 H, H-5), 1.95 (dt, J ϭ 13.5, 4.5 Hz, 1
H, H-3), 1.74 (td, J ϭ 14.6, 3.4 Hz, 1 H, H-3), 1.45 (s, 3 H, H-14),
1.06 (s, 3 H, H-13) ppm. 13C NMR (90 MHz, CDCl3): δ ϭ 205.5
(C-1), 169.3 (C-15), 131.2 (C-8), 125.9 (C-7), 105.3 (C-11), 72.2 (C-
12), 64.7 (C-10), 48.7 (C-9), 47.9 (C-5), 41.4 (C-3), 36.4 (C-2), 32.7
(C-4), 31.4 (C-13), 25.3 (C-6), 21.5 (C-14) ppm. MS (EI, 80 °C):
m/z (%) ϭ 247 ([M Ϫ CH3]ϩ, 2), 234 ([M Ϫ CO]ϩ, 2), 218 ([M Ϫ
CO2]ϩ, 100), 161 (44), 147 (25), 134 (25), 120 (36), 119 (40), 91 (40).
M. Ishikawa, T. Murata, K. I. Takao, K. I. Tadano, J. Org.
Chem. 2000, 65, 8595Ϫ8607.
[3] [3a]
[3b]
J. Jauch, Eur. J. Org. Chem. 2001, 473Ϫ476.
Y. Suziki,
A. Ohara, K. Sugaya, K. I. Takao, K. I. Tadano, Tetrahedron
2001, 57, 7291Ϫ7301.
[4a] W. A. Ayer, P. A. Craw, T. J. Stout, J. Clardy, Can. J. Chem.
[4]
[4b]
1989, 67, 773Ϫ778.
W. A. Ayer, P. A. Craw, Can. J. Chem.
Recently, Berg et al. isolated a sub-
[4c]
1989, 67, 1371Ϫ1380.
stance closely related to the marasmones from the fungus
Agrocybe sp. HKI 0259; see: A. Berg, H. Dörfelt, T. T. Kiet,
B. Schlegel, U. Gräfe, J. Antibiotics 2002, 55, 818Ϫ820.
These tests are currently under way and will be reported else-
where.
[5]
[6] [6a]
Crystal structure analysis of 7: C15H18O4, Mr ϭ 262.29,
orthorhombic, space group P212121 (No. 19), a ϭ 8.0967(1),
3
˚
˚
b ϭ 10.3170(1), c ϭ 15.5077(1) A, V ϭ 1295.41(2) A ; Z ϭ 4;
ρcalcd. ϭ 1.345 g cmϪ3, F000 ϭ 560, µ ϭ 0.097 mmϪ1. A single
crystal suitable for the X-ray diffraction study was obtained
from an Et2O/pentane solution (slow evaporation). The selec-
ted crystal was coated with perfluorinated ether, fixed in a
capillary and transferred to the diffractometer under a cold
nitrogen flow (Oxford Cryosystems). Preliminary examination
and data collection were carried out on a kappa-CCD device
(NONIUS MACH3) at the window of a rotating anode (NON-
IUS FR591) with graphite-monochromated Mo-Kα radiation
Synthesis of 1a: nBuLi in hexane (89 µL, 133.6 µmol) was added
to dry diisopropylamine (14.0 mg, 18.2 µL, 138.9 µmol) dissolved
in THF (0.5 mL) under N2 at Ϫ78 °C. The resulting solution was
stirred for 15 min at that temperature, and then a solution of 2
(28.0 mg, 106.8 µmol) in THF (0.5 mL) was added dropwise. After
15 min at Ϫ78 °C, a dark-red solution of phenylselenyl bromide
(32.8 mg, 138.9 µmol) in THF (0.7 mL) was added dropwise
(PhSeBr reacted immediately and the red color disappeared. The
color of the reaction mixture changed from yellow to red after 1
equiv. of PhSeBr had been added). The reaction mixture was
quenched with saturated NH4Cl solution (ca. 5 mL), warmed to
room temperature and then it was extracted with diethyl ether (3
ϫ ca. 10 mL). 30% H2O2 (0.5 mL) was added to the combined
organic extracts and the mixture was stirred at room temperature
for 2 h. The reaction was quenched with saturated Na2S2O3 (ca.
10 mL) and the aqueous layer was extracted with diethyl ether (2 ϫ
ca. 10 mL). The combined organic phases were dried with MgSO4,
filtered and the solvents evaporated to leave a yellow residue, which
was purified by flash chromatography with pentane/diethyl ether
(2:1, v/v) as eluent. Yield: 16 mg of pure 1 (58%). [α]2D0 ϭ Ϫ68 (c ϭ
0.3, MeOH); ref.[4a] [α]2D0 ϭ Ϫ59.8 (c ϭ 0.4, MeOH). 1H NMR
(500 MHz, CDCl3): δ ϭ 6.86 (d, J ϭ 10.5 Hz, 1 H, H-3), 6.40 (d,
J ϭ 4.5 Hz, 1 H, H-11), 6.00 (d, J ϭ 10.0 Hz, 1 H, H-2), 5.99 (m,
1 H, H-7), 4.54 (br. d, J ϭ 10.0 Hz, 1 H, H-12), 4.47 (d, J ϭ
10.5 Hz, 1 H, H-12), 3.29 (br. s, 1 H, H-9), 2.48Ϫ2.28 (m, 3 H, H-
5, H-6), 1.47 (s, 3 H, H-14), 1.22 (s, 3 H, H-13) ppm. 13C NMR
(90 MHz, CDCl3): δ ϭ 193.5 (C-1), 169.6 (C-15), 162.6 (C-3), 131.2
(C-8), 125.5 (C-7), 123.9 (C-2), 105.9 (C-11), 72.1 (C-12), 59.9 (C-
10), 49.4 (C-9), 41.9 (C-5), 35.5 (C-4), 30.8 (C-13), 24.7 (C-6), 23.9
(C-14) ppm. MS (EI, 70 eV): m/z (%) ϭ 260 (Mϩ, 1), 216 ([M Ϫ
CO2]ϩ, 54), 201 ([M Ϫ CO2 Ϫ CH3]ϩ, 38), 187 (30), 173 (20), 120
(100), 96 (34).
˚
(λ ϭ 0.71073 A). Data collection was performed at 123 K
within the Θ range of 2.37° Ͻ Θ Ͻ 25.34°. A total of 19359
reflections were integrated and corrected for Lorentz and pola-
rization effects. After merging (Rint ϭ 0.033), 2368 [2293: Io Ͼ
2σ(Io)] independent reflections remained and all were used to
refine 245 parameters. The structure was solved by a combi-
nation of direct methods and difference-Fourier syntheses. All
non-hydrogen atoms were refined anisotropically. All hydrogen
atom positions were found in the difference Fourier map calcu-
lated from the model containing all non-hydrogen atoms. The
hydrogen atoms’ positions were refined with individual iso-
tropic displacement parameters. Full-matrix least-squares re-
2
2 2
finements were carried out by minimizing w(Fo Ϫ Fc ) and
converged with R1 ϭ 0.0242 [Io Ͼ 2σ(Io)], wR2 ϭ 0.0621 [all
data], GOF ϭ 1.036 and shift/error Ͻ 0.001. The correct en-
antiomer is proved by the chemical synthesis. [6b] Crystal struc-
ture analysis of 2: C15H18O4, Mr ϭ 262.29, orthorhombic,
space group P212121 (No. 19), a ϭ 8.5170(1), b ϭ 10.4650(2),
3
˚
˚
c ϭ 14.4239(3) A, V ϭ 1285.61(4) A ; Z ϭ 4; ρcalcd. ϭ 1.355 g
cmϪ3, F000 ϭ 560, µ ϭ 0.098 mmϪ1. A single crystal suitable
for the X-ray diffraction study was obtained from an Et2O/
pentane solution (slow evaporation). The selected crystal was
coated with perfluorinated ether, fixed in a capillary and trans-
ferred to the diffractometer under a cold nitrogen flow (Oxford
Cryosystems). Preliminary examination and data collection
were carried out on a kappa-CCD device (NONIUS MACH3)
at the window of a rotating anode (NONIUS FR591) with
˚
graphite-monochromated Mo-Kα radiation (λ ϭ 0.71073 A).
Data collection was performed at 153 K within the Θ range of
2.40° Ͻ Θ Ͻ 25.42°. A total of 25579 reflections were inte-
grated, corrected for Lorentz and polarization effects. After
merging (Rint ϭ 0.042), 2372 [2152: Io Ͼ2 σ(Io)] independent
reflections remained and all were used to refine 244 parameters.
The structure was solved by a combination of direct methods
and difference-Fourier syntheses. All non-hydrogen atoms were
refined anisotropically. All hydrogen atom positions were
found in the difference Fourier map calculated from the model
containing all non-hydrogen atoms. The hydrogen atoms’ posi-
tions were refined with individual isotropic displacement par-
ameters. Full-matrix least-squares refinements were carried out
Acknowledgments
This work was generously supported by the Deutsche Forschungs-
gemeinschaft and the Leonhard-Lorentz-Stiftung. We thank Pfitzer
AG (Karlsruhe), BASF AG (Ludwigshafen), and Degussa AG
(Frankfurt) for gifts of chemicals and laboratory equipment.
2 2
by minimizing w(Fo2 Ϫ Fc ) and converged with R1 ϭ 0.0297
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 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Jauch, C. Wallner, E. Herdtweck
FULL PAPER
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