Asymmetric synthesis of both enantiomers of arteludovicinolide A

Article abstract of DOI:10.1021/ol401473v

The first total synthesis of either enantiomer of Arteludovicinolide A and their biological evaluation is reported, featuring a new strategy for the asymmetric construction of γ-butyrolactones with stereogenic side chains in the 4-position. Starting from the renewable resource methyl 2-furoate, the sesquiterpene lactone was synthesized in 9 steps and 4.8% overall yield via an asymmetric cyclopropanation and two diastereoselective nucleophile additions making use of a donor-acceptor-cyclopropane-lactonization cascade. At noncytotoxic concentrations (≤10 μM) (+)-1 was found to have a 15 times higher anti-inflammatory activity (4.87 ± 1.1 μM) than previously reported for concentrations of ≥45 μM.

Full text of DOI:10.1021/ol401473v

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Asymmetric Synthesis of Both
Enantiomers of Arteludovicinolide A
€
Andreas Kreuzer, Sabine Kerres, Thomas Ertl, Hannelore Rucker, Sabine Amslinger,
and Oliver Reiser*
€
€
Institut fu€r Organische Chemie, Universitat Regensburg, Universitatsstr. 31,
93053 Regensburg, Germany
oliver.reiser@chemie.uni-regensburg.de
Received May 24, 2013
ABSTRACT
The first total synthesis of either enantiomer of Arteludovicinolide A and their biological evaluation is reported, featuring a new strategy for
the asymmetric construction of γ-butyrolactones with stereogenic side chains in the 4-position. Starting from the renewable resource
methyl 2-furoate, the sesquiterpene lactone was synthesized in 9 steps and 4.8% overall yield via an asymmetric cyclopropanation and two
diastereoselective nucleophile additions making use of a donor-acceptor-cyclopropane-lactonization cascade. At noncytotoxic concentrations
(e10 μM) (þ)-1 was found to have a 15 times higher anti-inflammatory activity (4.87 ( 1.1 μM) than previously reported for concentrations
of g45 μM.
Artemisiae plants (Asteraceae) are widely distributed
throughout the world, and many species have been used
as medicinal plants especially in Chinese folk medicine.1
The sesquiterpene lactone Arteludovicinolide A ((þ)-1)2
was first discovered in 1991 in the aerial parts of Artemisia
ludoviciana and proved to possess anti-inflammatory
activity (NO inhibition via iNOS pathway).2b It belongs
to the widely occurring class of exo-methylene-γ-
butyrolactones,3 being anti-disubstituted in the 4- and
5-positions, as found for example in many paraconic
acids4 or guaianolides.5 In the latter two classes of natural
products, we have developed a methodology based on
cyclopropanecarbaldehyde 4 (Scheme 1), being readily
available in either enantiomeric form by a Cu(I)-bis-
(oxazoline) catalyzed asymmetric cyclopropanation of
methyl 2-furoate (7) with ethyl diazoacetate followed
by reductive ozonolysis.6 Upon diastereoselective addition
of a nucleophile Nu1 in agreement with the FelkinꢀAnh
paradigm7 followed by base induced hydrolysis of the
oxalic ester that triggers a retroaldolꢀlactonization cas-
cade, lactone aldehyde 8 can be obtained with high anti-
selectivity. Addition of a second nucleophile Nu2 to 8 gives
rise to lactones of type 9. This strategy, however, is limited
to the introduction of stabilized nucleophiles Nu1 such
as allyl, which precludes the synthesis of widely occurring
γ-butyrolactones with alkyl or vinyl functionnalities as
found for example in (þ)-arteludovicinolide A ((þ)-1). We
report here an alternative approach toward γ-butyrolac-
tones 3 being identical to 9 in its relative substitution
(1) Huang, Z.-S.; Pei, Y.-H.; Liu, C.-M.; Lin, S.; Tang, J.; Huang,
D.-S.; Song, T.-F.; Lu, L.-H.; Gao, Y.-P.; Zhang, W.-D. Planta Med.
2010, 76, 1710.
(2) (a) Jakupovic, J.; Tan, R. X.; Bohlmann, F.; Boldt, P. E.; Jia, Z. J.
Phytochemistry 1991, 30, 1573. (b) Lateff, A. A.; Gamal-Eldeen, A. M.;
ꢀ
Turky, F.; Hirata, T.; Pare, P. W.; Karchesy, J.; Kamel, M. S.; Ahmed,
A. A.; Hegazy, M.-E. F. Nat. Prod. Commun. 2008, 3, 851. (c) Huang,
Z.-S.; Pei, Y.-H.; Liu, V.; Lin, S.; Tang, J.; Huang, D.-S.; Song, T.-F.;
Lu, L.-H.; Gao, Y.-P.; Zhang, W.-D. Planta Med. 2010, 76, 1710.
(3) Kitson, R. R. A.; Millemaggi, A.; Taylor, R. J. K. Angew. Chem.,
Int. Ed. 2009, 48, 9426.
(4) Bandichhor, R.; Nosse, B.; Reiser, O. Top. Cur. Chem. 2005,
243, 43.
(5) Schall, A.; Reiser, O. Eur. J. Org. Chem. 2008, 2353.
(6) (a) Kalidindi, S.; Jeong, W. B.; Schall, A.; Bandichhor, R.; Nosse,
B.; Reiser, O. Angew. Chem., Int. Ed. 2007, 45, 6361. (b) Chhor, R. B.;
€
€
Nosse, B.; Sorgel, S.; Bohm, C.; Seitz, M.; Reiser, O. Chem.;Eur. J.
€
2003, 9, 260. (c) Bohm, C.; Reiser, O. Org. Lett. 2001, 3, 1315. (d) Nosse,
€
B.; Chhor, R. B.; Jeong, W. B.; Bohm, C.; Reiser, O. Org. Lett. 2003, 5,
€
941. (e) Bohm, C.; Schninnerl, M.; Bubert, C.; Zabel, M.; Labahn, T.;
Parisini, E.; Reiser, O. Eur. J. Org. Chem. 2000, 2955.
(7) Mengel, A.; Reiser, O. Chem. Rev. 1999, 99, 1191.
r
10.1021/ol401473v
XXXX American Chemical Society

pattern but allowing the introduction of alkyl, aryl, vinyl, and
alkynyl groups in the 5-position, thus greatly expanding the
scope for the synthesis of the title compounds. As an
application of this methodology, the enantioselective synth-
esis of either enantiomer of arteludovicinolide A ((þ)-1 and
(ꢀ)-1) from commercial methyl 2-furoate (7),whichisreadily
available on ton scale from hemicellulose, is demonstrated.
Saponification of the labile oxalic ester in 5a with
Ba(OH)2 8H2O in MeOH gave rise to the R/β-chiral alde-
3
hyde 6 in 82% overall yield starting from 4. The decrease
in the diastereomeric ratio in the transformation from
5a (94:6) to 6 (90:10) results from a epimerization on
the aldehyde bearing carbon center most likely caused
during saponification under the basic conditions em-
ployed. Importantly, the enantiopurity of 6 remained
unchanged, ensuring that no erosion of stereochemistry
by retroaldol/aldol processes had occurred (vide infra).
Separation of the two diastereomers of 6 turned out to
be impracticable; therefore 6 was used as the diastereo-
meric mixture.
Scheme 1. Synthetic Strategies to Chiral γ-Butyrolactones
The chemoselective addition of organolithium and or-
ganomagnesium reagents tothe aldehyde functionality in6
could be achieved in accordance with the FelkinꢀAnh
paradigm7 with concurrent lactone formation to give rise
to trans-substituted γ-butyrolactones 11 (Table 1). Addi-
tionally, chelation with the ester group is possible, which
would further contribute to the formation of the major
diastereomer 11a. The stereocenter in the 3-position could
be regarded as nonreinforcing according to the Evans
model for 1,3-inductions;8 nevertheless, the stereocenter
in the 2-position appears to be the dominating control
element in the title transformation. High diastereoselec-
tivity was observed especially when sterically demanding
Grignard reagents were employed (entry 6), while the slim
lithium(trimethylsilyl) acetylene (TMS-acetylene-Li) was
unselective (entry 5). The good result from the introduc-
tion of a vinyl nucleophile is noteworthy (Table 1, entry 3),
since it demonstrates the potential for the introduction of
vinyl moieties at the lactone scaffold, which is a prevailing
motive among the sesquiterpene lactones, particularly
in the family of the germacranolides.9 Along with those,
the arteludovicinolides (AꢀD)2 and various closely related
structures such as the iso-seco-tanapartolides10 appear to
be accessible by the synthetic approach developed in this
study.
Indeed, the synthesis of (þ)-arteludovicinolide A (þ)-1,
and in parallel the synthesis of (ꢀ)-1 starting from (ent)-6
(99% ee) obtained from (ent)-4 (vide supra), was readily
accomplished: by reacting 6 (90% ee) with vinyl-lithium
reagent 14 derived from hexane-2,5-dione,12 the key struc-
tural element 2 was obtained in 67% yield along with epi-2
in 5% yield,11 the latter structure being confirmed by X-ray
analysis (Scheme 3).
Because of the sensitive nature of R-methylene-γ-butyro-
lactones, we initially considered setting the methyl ketone
functionality in the C-3 side chain of 1 via a Wackerꢀ
Tsuji oxidation prior to the final introduction of the
Aiming ultimately at the synthesis of naturally occurring
(þ)-1, cyclopropanecarbaldehyde 4 (90% ee) was synthe-
sized in two steps from 7 in diastereomerically pure form
on gram scale following the protocol developed by us (for
detailed procedures, see Supporting Information).6 After
having worked out the underlying methodology described
below and the synthesis to (þ)-1, the sequence was repeated
starting with the analogous synthesis of (ent)-4 (99% ee) to
arrive at unnatural (ꢀ)-1to allow the biological evaluation of
both enantiomers.
Borontrifluoride mediated allylation of 4 gives rise to 5
(94:6 dr, Scheme 2); however, rather than performing the
direct base induced hydrolysis of the oxalylic ester function-
ality that would give rise to 8 (Nu1 = allyl, Scheme 1),
protection of the hydroxyl group with TIPS leading to 5a
was carried out. Hydrolysis of the oxalylic ester functionality
now leads to the acyclic aldehyde 6, which was investigated
in reactions with nucleophiles (vide infra; Table 1).
Scheme 2. Synthesis of 6 via Sakurai Allylation,
TIPS-Protection, and Saponification
(8) Evans, D. A.; Dart, M. J.; Duffy, J. L.; Yang, M. G.; Livingston,
A. B. J. Am. Chem. Soc. 1995, 117, 6619.
(9) Zidorn, C. Phytochemistry 2008, 69, 2270.
(10) (a) Huneck, S.; Zdero, C.; Bohlmann, F. Phytochemistry 1986,
25, 883. (b) Tan, R. X.; Jakupovic, J.; Bohlmann, F.; Jia, Z. J.; Huneck,
S. Phytochemistry 1991, 30, 583.
(11) 2 and 11 were formed along with minor amounts of epimers
reflecting the diastereomeric anti/syn 9:1 ratio of starting material 6.
These epimers could be removed in the following transformations; see
Supporting Information for a detailed analysis.
(12) See Supporting Information for the preparation of 13.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Formation of the Major γ-Lactone 11a from 6, Being
Rationalized via a Combined FelkinꢀAnh (FA)/Chelation
Pathway
Scheme 4
entry
nucleophile
Me-MgBr
yield [%]a
trans/cisb
1
2
3
4
5
6
68
62
52
53
57
46
84:16
79:21
89:11
71:29
52:48
96:04
Me-Li
vinyl-MgBr
Ph-MgBr
TMS-acetylene-Li
iPr-MgBr
a Isolated yields. b Based on 1H NMR of the crude.11
Scheme 3. Synthesis of the Key Structural Element of 1 via
Lithium Organyl 14 Addition to 6
WackerꢀTsuji oxidation in the presence of an exo-methy-
lene group, as it was to the best of our knowledge
unexplored. Starting from 2, the introduction of the exo-
methylene group via dimethylmethylideneammonium io-
dide (Eschenmoser’s salt)14 was determined to be the most
convenient and afforded the desired product 17 in 43%
yield over three steps along with 20% of recovered starting
material. The application of copper-free WackerꢀTsuji
oxidation conditions15 (PdCl2, O2, dimethylacetamide/
H2O 8:1) to 17 afforded 18 in 52% yield along with
approximately 7% of inseparable aldehyde contamina-
tions arising from double bond cleavage. Fortunately,
the mild Pd(quinox)Cl2 catalyzed TBHP mediated Wacker
oxidation16 gave 18 in 67% yield without byproduct for-
mation. Final deprotection of 18 with buffered TBAF in
THF17 afforded (þ)-1, with spectroscopic data in accordance
exo-methylene group (Scheme 4). Unfortunately, under
various conditions tried for the oxidation of 2, the pre-
servation of the ketal moiety was not possible, which is
necessary for the subsequently intended deprotonation
at the lactone R-position to introduce the exo-methylene
group. Several attempts to reprotect the ketone moieties
in 15 were unsuccessful: either no reaction or complete
decomposition was observed. Nevertheless, at this stage
we were able to obtain an X-ray structure of ent-15 syn-
thesized from (ent)-2, which confirmed its relative and
absolute configuration.
Also the direct conversion of the terminal olefin in 2
into a ketal with Hg(OAc)2 and PdCl2 in the presence
of ethylenglycol13 was not successful (not shown). How-
ever, 15 could be desilylated to 16 for biological evaluation
(vide infra). We therefore investigated the possibility of a
(14) Schreiber, J.; Maag, H.; Hashimoto, N.; Eschenmoser, A.
Angew. Chem., Int. Ed. 1971, 10, 330.
(15) Mitsudome, T.; Umetani, T.; Nosaka, N.; Mori, K.; Mizugaki,
T.; Ebitani, K.; Kaneda, K. Angew. Chem., Int. Ed. 2006, 45, 481.
(16) Michel, B. W.; Camelio, A. M.; Cornell, C. N.; Sigman, M. S.
J. Am. Chem. Soc. 2009, 131, 6076.
(13) Hunt, D. F.; Rodeheaver, G. T. Tetrahedron Lett. 1972, 34,
3595.
Org. Lett., Vol. XX, No. XX, XXXX
C

of 90% ee and 91% ee respectively (employed starting
material 4: 90% ee). HPLC analysis of ent-15 and ent-18
(starting from enantiopure ent-4) showed enantiopure
compounds (>99% ee), indicating that no racemization
occurs in the course of the entire synthesis.
(þ)-1, (ꢀ)-1, and 16 were tested for cytotoxicity (MTT
cell viability test, Figure 1 left charts) and anti-inflamma-
tory activity (iNOS-inhibition via Griess assay, Figure 1
right charts) in the noncytotoxic concentration range.18
In the cell viability assay, (þ)-1 and (ꢀ)-1 exhibited
similar IC50 values of 45.3 ( 2.6 and 49.3 ( 2.7 μM respec-
tively (Figure 1A,B left charts). In contrast to that (þ)-1
showed significantly higher iNOS inhibition with an IC50
value of 4.87 ( 1.1 μM19 compared to the unnatural (ꢀ)-1
(IC50 = 10.3( 5.7 μM). Sinceitis known for sesquiterpene
lactones that, besides the R-methylene group, a second
Michael acceptor like the cyclopentenone moiety can play
an important role in their biological activity,20 we also
tested 16. However, 16 had no significant cytotoxic activity
(c < 100 μM) and only weak anti-inflammatory properties
(IC50 = 50.2 ( 18.3 μM).
In conclusion we have developed the first enantio-
selective syntheses of (þ)- and (ꢀ)-arteludovicinolide A,
which were obtained in 9 steps and 4.8% overall yield
(7.0% brsm) starting from 2-furoic ester 7. Moreover, we
could show that at noncytotoxic concentrations (e10 μM)
(þ)-1 has a 15 times higher anti-inflammatory activity than
previously reported for concentrations of g45 μM.2b
Finally, the exo-methylene group in the lactone moiety
was determined to be decisive for the biological activity of
the natural product.3
Figure 1. Results of in vitro assays performed with RAW264.7
cells stimulated with 10 ng/mL of LPS. Charts on the left side
refer to MTT tests of (þ)-1, (ꢀ)-1, and 16 after an incubation
time of 24 h at different concentrations. Charts on the right
display the influence of (þ)-1, (ꢀ)-1, and 16 on NO-production
(Griess assay). Data represent at least three independent experi-
ments performed in quadruplicates. Levels of significance:
* p e 0.05, ** p e 0.01, *** p e 0.001.
Acknowledgment. We thank Sabine Stempfhuber
and Dr. Michael Bodensteiner, Department of Crystal-
lography, University of Regensburg, for X-ray structure
analysis.
with the literature and molecular mass confirmed by
HRMS analysis. Furthermore, the enantiomer (ꢀ)-1 was
prepared in an analogous way starting from enantiopure
ent-4. Chiral HPLC analysis of 1 was not met with success
due to the sensitive nature of 1, whereas the separation of
15and18 was possibleand revealedanenantiomericpurity
Supporting Information Available. Experimental pro-
cedures, NMR spectra for all compounds, and CIF-data
for epi-2 and ent-15. This material is available free of
charge via the Internet at http://pubs.acs.org.
(17) Debenham, J. S.; Rodebaugh, R.; Fraser-Reid, B. J. Org. Chem.
1997, 62, 4591.
(18) See Supporting Information for further details.
(19) Reference 2b reports an IC50 of 70.4 μM, taken however at a
much higher concentration range (>45 μM), which is according to our
measurements already in the cytotoxic region.
(20) Schmidt, T. J.; Lyss, G.; Pahl, H. L.; Merfort, I. Bioorg. Med.
Chem. 1999, 7, 2849.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX