Protecting-Group-Free Total Syntheses of Rubrolide R and S

Article abstract of DOI:10.1002/ejoc.201700158

The two marine natural products rubrolide R (1) and S (2) were synthesised in only three linear steps starting from commercially available tetronic acid without using protecting-group chemistry. Key steps in the syntheses were the Pd-catalysed Suzuki–Miya

Full text of DOI:10.1002/ejoc.201700158

A Journal of
Accepted Article
Title: Protecting Group Free Total Syntheses of Rubrolide R and S
Authors: Mathias Schacht, Gordon Jacob Boehlich, Jessica de Vries,
Stephanie Bertram, Gülsah Gabriel, Phyllis Zimmermann,
Peter Heisig, and Nina Schützenmeister
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700158
Link to VoR: http://dx.doi.org/10.1002/ejoc.201700158
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10.1002/ejoc.201700158
European Journal of Organic Chemistry
COMMUNICATION
Protecting Group Free Total Syntheses of Rubrolide R and S
M. Schacht,[a] G. J. Boehlich,[a] J. de Vries,[a] S. Bertram,[b] G. Gabriel,[b] P. Zimmermann,[c] P. Heisig[c]
and N. Schützenmeister*[a]
Dedicated to Professor Dr. Dr. h.c. Lutz F. Tietze in occasion to his 75th birthday
Abstract: The two marine natural products rubrolide R 1 and S 2
were synthesised in only three linear steps starting from
commercially available tetronic acid without using protecting group
chemistry. Key steps in the syntheses were the Pd-catalysed
Suzuki-Miyaura cross coupling followed by a vinylogous Aldol
condensation. Both compounds have been tested for their antibiotic
and antiviral activities. At concentration of 10 µM rubrolide R 1 and S
2 a 2-log and 1.5-log reduction in virus titre has been detected for a
seasonal influenza virus (H3N2) and the pandemic swine influenza
virus (pH1N1), respectively.
Recently, the first total syntheses of rubrolide R 1 and S 2 have
been reported showing that beside the antiviral activity both
rubrolides show significant inhibition of the NO production.[9]
Herein, we describe the more efficient syntheses of both
rubrolides R 1 and S 2 using a Suzuki-Miyaura cross coupling
and a vinylogous aldol condensation as key steps starting from
tetronic acid (Scheme 1). Due to the lack of protecting groups in
this synthetic route[10] the total syntheses were dramatically
shortened to only three linear steps starting from commercially
available tetronic acid. This route is, compared to earlier total
syntheses of rubrolides,[11] highly flexible and can be used to
easily synthesise further rubrolides and analogues to contribute
to structure activity relationship studies for the determination of
the unknown molecular target.
Introduction
Rubrolides are a marine natural product class that were first
isolated from the tunicate Ritterella rubra in 1990.[1] Since then
more than 19 different rubrolides have been isolated from
marine organisms.[2-7] Rubrolides are furanone derivatives,
which are substituted with two aromatic, mostly highly
brominated, substituents. Their biological activities range from
anti-bacterial[1,2,4,6]
,
anti-inflammatory[3] to aldose-reductase
inhibition[8] with a so far unknown mode of action. In 2014, two
new antiviral rubrolides, R 1 and S 2, have been isolated from
the marine-derived fungus Aspergillus terreus OUCMDZ-1925,
which was obtained from barracuda intestines, showing superior
activity against influenza A (H1N1) compared to ribavirin. [7]
Scheme 1. Retrosynthesis of rubrolide R 1 and S 2.
Results and Discussion
The retrosynthetic analysis of the two rubrolides 1 and 2 shows
the four required building blocks 3a, 3b, 4 and 5. The
benzaldehyde derivatives 3a[12] and 3b[13] as well as the required
triflate 4[14] can be synthesised using literature known protocols..
The commercially available tetronic acid 4[15] was transferred
into triflate 6 in 90% yield (Scheme 2).
Figure 1. Rubrolide R (1) and S (2).
[a]
M. Schacht, G. J. Boehlich, J. de Vries, N. Schützenmeister
Department of Chemistry, Institute of Pharmacy, Universität
Hamburg, Bundesstrasse 45, 20146 Hamburg (Germany)
E-mail: nina.schuetzenmeister@chemie.uni-hamburg.de
https://www.chemie.uni-hamburg.de/pha/schuetzenmeister/
S. Bertram, G. Gabriel
Heinrich-Pette-Institute, Leibniz Institute for Experimental Virology
Martinistrasse 52, 20251 Hamburg (Germany), Centre for Structural
and Cell Biology in Medicine, University of Luebeck, Ratzeburger
Allee 160, 23562 Luebeck (Germany)
[b]
[c]
Scheme 2. Synthesis of furanone 7: a) NEt3 (1.2 equiv.), Tf2O (1.2 equiv.),
CH2Cl2, 0 °C→r.t., 2 h 90%; b) 5 (1.2 equiv.), Pd(PPh3)4 (0.5 mol%), Na2CO3
(3.0 equiv.), dioxane, 70 °C, 16 h, 95%.
P. Zimmermann, P. Heisig
Department of Chemistry, Institute of Biochemistry and Molecular
Biology, University of Hamburg, Bundesstrasse 45, 20146 Hamburg
(Germany)
The introduction of the first aromatic moiety can be achieved via
a
Pd-catalysed Suzuki-Miyaura cross coupling using
Supporting information for this article is given via a link at the end of
the document.
commercially available boronic acid 5. In contrast to earlier
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10.1002/ejoc.201700158
European Journal of Organic Chemistry
COMMUNICATION
rubrolide syntheses using this strategy [16] a protecting group free
boronic acid has been chosen to avoid additional deprotection
steps. For the optimisation of the Suzuki-Miyaura cross coupling
two different catalysts, Pd(PPh3)4 and PdCl2(PPh3)2, have been
investigated. Both catalysts yielded Suzuki-product 7 at 70 °C
using Na2CO3 as base.
in 88% yield. Building block 3b was synthesised in one step from
4-hydroxybenzaldehyde 8 [13] (Scheme 3).
Table 1. Optimisation of the Pd-catalysed Suzuki-Miyaura cross coupling (6
(1.0 equiv.),
5 (1.2 equiv.), Na2CO3 (3.0 equiv.), 70 °C); catalyst A:
PdCl2(PPh3)2; catalyst B: Pd(PPh3)4.
Loading
(mol%)
entry
time
catalyst
solvent
yield
1
2
66 h
66 h
66 h
16 h
16 h
16 h
66 h
66 h
66 h
16 h
16 h
16 h
16 h
16 h
16 h
A
A
A
A
A
A
B
B
B
B
B
B
B
B
B
5.0
2.5
1.0
5.0
2.5
1.0
5.0
2.5
1.0
5.0
2.5
1.0
0.5
1.0
1.0
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
toluene
72%
54%
33%
21%
29%
29%
72%
54%
33%
45%
68%
81%
95%
81%
69%
3
Scheme 3. Syntheses of building blocks 3a and 3b: a) tert-butyl (2-methylbut-
3-en-2-yl) carbonate, Pd(PPh3)4 (1.0 mol%), THF, 4 °C, 16 h, 80% (79%[12]); b)
DMF, microwave, 180 °C, 45 min, 88%; c) isoprene, H3PO4, PE, r.t., 16 h,
52% (51.3%[13]).
4
5
6
As last step we employed a vinylogous aldol condensation
7
between our key intermediate
7 and the benzaldehyde
derivatives 3a and 3b. The condensation led to an E/Z-mixture of
the products of approximately 1:10 for rubrolide R 1 and 1:4 for
rubrolide S 2. Unfortunately, it was not possible to separate the
isomers by crystallisation or standard silica gel column
chromatography since the Z-products are prone to isomerisation
to the corresponding E-isomer after longer exposure to silica gel.
Subsequent, reversed-phase column chromatography yielded
rubrolide R 1 in 63% and rubrolide S 2 in 66% as pure Z-isomers.
8
9
10
11
12
13
14
15
2-MeTHF
In case of the use of PdCl2(PPh3)2 (A) the best yield of promising
72% (entry 1) has been achieved with relatively long stirring
times of 66 h and a catalyst loading of 5.0 mol%. Reduction of
the catalyst loading (entry 2 and 3) or stirring time (entry 4-6)
gave lower yields. Using Pd(PPh3)4 (B) as Pd-source yielded
after 66 h and 5.0 mol% of the catalyst the same yield of 72%.
Reduction of the amount of catalyst led to decreasing yields
(entry 8 and 9) as observed in case of the previously used
catalyst A. A different trend occurred while reducing the catalyst
loading at the reduced stirring time of 16 h (entry 10-13) and
reached its maximum at 0.5 mol% giving the Suzuki-Miyaura
product in 95% yield after purification by crystallisation. Change
of the solvent to toluene (entry 14) gave similar results, while
use of 2-MeTHF (entry 15) led to decreasing yields. The key
intermediate 7 can be used to synthesise further rubrolide
analogues without using protecting group chemistry.
Scheme 4. Vinylogous aldol condensation of 1 and 2: a) TBSOTf, DIPEA,
DMF, r.t., 1 h, then 3a or 3b in DMF, 2 h then DBU, 120 °C, 5 h, then H2O,
DBU, r.t. 16 h, 63% (1) 66% (2)
The benzaldehyde derivative 3a was synthesised following a
slightly optimised procedure[12] for the Claisen rearrangement
using microwave irradiation in DMF at 180 °C to give aldehyde
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The double bond configuration of 1 and 2 was analysed by
NOESY experiments confirming the Z-configuration (see
supporting information).
In addition, the structure of rubrolide S 2 was unambiguously
determined by single crystal X-ray crystallography (Figure 2).[17]
Figure 3. Antiviral effect of rubrolide R 1 and S 2 on influenza virus replication.
MDCK cells were infected with either H3N2 (A and B) or pH1N1 (C and D)
influenza viruses and co-treated with either an E/Z-mixture (rubrolide R 1: 1:9
E/Z; rubrolide S 2: 1:4 E/Z) (A and C) or the pure Z-isomer (B and D) of
rubrolide R 1 and S 2.
Figure 2. X-ray single crystal structure of rubrolide S 2. Thermal ellipsoids are
shown at 50% probability level. Hydrogen atoms are omitted for clarity.
Furthermore, the pure rubrolide R 1 and S 2 were tested for their
antibacterial activity against type strains of Escherichia coli,
Pseudomonas aeruginosa, Enterococcus faecalis and
Staphylococcus aureus. The preliminary test showed growth
inhibition for rubrolide S 2 for the gram-negative E. coli and in
weaker form for P. aeruginosa. This effect could not be
confirmed by agar diffusion nor by serial dilution tests.
Biological evaluation
Rubrolide R 1 and S 2 as pure Z-configurated and as E/Z-
mixtures, were tested for their antibacterial and antiviral activities.
We addressed whether rubrolide treatment would reduce
replication of various influenza virus subtypes. Therefore, MDCK
cells were infected with H3N2 or pH1N1 influenza viruses and
co-treated with ribavirin or rubrolids R 1 and S 2 (supporting
information).[18] The viral titer of infected and PBS (Ctr1) or
DMSO (Ctr2) treated cells served as a negative control.
Rubrolide R 1 and S 2 as E/Z-mixtures (rubrolide R 1: 1:9 E/Z;
rubrolide S 2: 1:4 E/Z) showed weaker inhibition of H3N2 and no
inhibition of pH1N1 virus replication compared to controls
(Figure 3A and C). In contrast, the pure Z-isomers of rubrolide R
1 and S 2 displayed significant inhibition of H3N2 and pH1N1
virus replication in a concentration dependent manner compared
to controls (Figure 3B and D). Furthermore, the pure Z-isomers
of rubrolide R 1 and S 2 were most effective against H3N2
(Figure 3A and B) compared to pH1N1 (Figure 3C and D) virus
replication. Treatment of virus and cells with ribavirin, which is a
guanosine analogue and inhibits viral RNA synthesis, resulted in
no significant inhibition on influenza virus replication (Figure 3).
These data demonstrate that the pure Z-isomers of rubrolids R 1
and S 2 are able to inhibit the influenza virus replication of H3
and H1 subtypes more efficiently than the E/Z-mixtures.
Conclusions
The dramatically short and protecting group free syntheses of
the two most recent isolated rubrolides R 1 and S 2 are reported.
Over three steps, rubrolide R 1 was obtained in 54% and
rubrolide S 2 in 56% total yield as single Z-isomers starting from
commercially available tetronic acid. Both rubrolides showed
inhibition of virus replication of a seasonal influenza virus
(H3N2) and the pandemic swine influenza (pH1N1) from 2009.
Acknowledgements
We thank Stefanie Thanisch for excellent technical assistance in
generating the virus inhibition experiments.
Keywords: total synthesis • cross coupling • antiviral
compounds • natural products • aldol reaction
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10.1002/ejoc.201700158
European Journal of Organic Chemistry
COMMUNICATION
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Entry for the Table of Contents
Layout 1:
COMMUNICATION
Rubrolide R and S are two new
representatives of a marine natural
product class showing activity
against the influenza viruses H1N1
and H3N2. Both natural products
have been synthesised via a
concise, protecting group free, three
step synthesis. The key steps
involve a Suzuki-Miyaura cross-
coupling and a vinylogous Aldol
condensation.
Natural Product Synthesis
M. Schacht, G. J. Boehlich, J. de
Vries, S. Bertram, G. Gabriel, P.
Zimmermann, P. Heisig and N.
Schützenmeister*
Page No. – Page No.
Protecting Group Free Total
Synthesis of Rubrolide R and S
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