Synthetic Studies on the Natural Product Myrsinoic Acid F Reveal Biologically Active Analogues

Article abstract of DOI:10.1021/acs.orglett.8b01558

The synthesis of the structure, 1, assigned to the anti-inflammatory natural product myrsinoic acid F is reported together with a means for preparing its Z-isomer 21. While neither of these compounds corresponds to the natural product, both of them are anti-inflammatory agents (as determined using a mouse ear edema assay) with congener 1 being notably more potent than the widely prescribed NSAID indometacin.

Full text of DOI:10.1021/acs.orglett.8b01558

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthetic Studies on the Natural Product Myrsinoic Acid F Reveal
Biologically Active Analogues
Jiri Mikusek,† Jeremy Nugent,† Jas S. Ward,† Brett D. Schwartz,† Alison D. Findlay,‡ Jonathan S. Foot,‡
and Martin G. Banwell*,†
†Research School of Chemistry, Institute of Advanced Studies, The Australian National University, Canberra, ACT 2601, Australia
‡Pharmaxis Ltd., 20 Rodborough Road, Frenchs Forest, NSW 2086, Australia
S
* Supporting Information
ABSTRACT: The synthesis of the structure, 1, assigned to the anti-inflammatory natural product myrsinoic acid F is reported
together with a means for preparing its Z-isomer 21. While neither of these compounds corresponds to the natural product, both
of them are anti-inflammatory agents (as determined using a mouse ear edema assay) with congener 1 being notably more potent
than the widely prescribed NSAID indometacin.
n 2002, Hirota and co-workers reported1 the isolation of a
Iterpeno-p-hydroxybenzoic acid and certain cyclic analogues
from the methanolic extracts of the leaves and twigs of Myrsine
seguinii, a shrub found in a range of countries, including China,
Japan, Cambodia, Vietnam, and Myanmar. These compounds
were evaluated as anti-inflammatory agents, and the most active
proved to be the one named myrsinoic acid F (MA-F) and for
which the cyclic structure 1 (Figure 1) was assigned on the basis
of various NMR and other spectroscopic analyses. MA-F and its
congeners may exert their anti-inflammatory effects by inhibiting
DNA polymerase λ.2
of any cyclic members of the myrsinoic acid class,8 we sought to
prepare compound 1. Herein, we report the outcomes of such
studies.
The opening stages of the route we established to access
compound 1 are shown in Scheme 1. This involved the C-
prenylation of p-bromophenol (2) under conditions defined by
Scheme 1. Synthesis of Aldehyde 6 from p-Bromophenol (2)
Figure 1. Structure assigned to the anti-inflammatory natural product
myrsinoic acid F.
In the intervening period, MA-F and its various cometabolites,
which can also be obtained from certain other plant sources,3
have been identified as HB-EGF inhibitors4 (and hence represent
potential anticancer agents), as key elements of formulations for
treating halitosis,5 as antimicrobial agents,6 and as compounds
with notable antileishmanial properties.7 Given these pro-
nounced activities and the absence of studies on the preparation
Received: May 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01558
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Bates9 and thus affording compound 3 in 64% yield. Formylation
of phenol 3 using paraformaldehyde in the presence of
triethylamine/magnesium chloride under the conditions re-
ported by Skattebøl10 then gave the salicylaldehyde 4 (68%) that
was subjected to treatment with in situ generated methoxy-
methylenetriphenylphosphorane and pivaloyl chloride (Piv-Cl)/
potassium t-butoxide to give the styrenyl ether 5 (82%), acid
catalyzed hydrolysis of which afforded the arylacetaldehyde 6
(81%).
Scheme 3. Final Steps in the Synthesis of Target 1
The completion of the installation of the geranyl-like
substructure associated with target 1 is shown in Scheme 2 and
Scheme 2. Completing the Installation of the Geranyl-Type
Substructure Associated with Target 1
On the basis of the foregoing, we considered the possibility
that the Z-isomeric form of compound 1 might represent the true
structure of MA-F. Accordingly, a route to this compound was
devised. This started (Scheme 4) with the monocrotoylation of
p-bromophenol 2 in the presence of sodium hydride, thus
affording compound 11 in 86% yield. Reaction of the latter
compound with VO(acac)2 in the presence of tert-butylhy-
droperoxide (TBHP)14 resulted in the coformation of the
chromatographically inseparable pyran 12 and the isomeric
dihydrobenzofuran 13 (67% combined yield), each of which was
obtained as a mixture of diastereoisomers and, presumably, as a
consequence of the cyclization of the initially formed epoxides.
Oxidation of this mixture of four compounds with the Dess−
Martin periodinane (DMP)15 afforded the corresponding
ketones, the major one, viz. compound 14 (78%), being obtained
in pure form after flash column chromatography. Reduction of
compound 14 using sodium borohydride afforded a ca. 1:1
mixture of the diastereoisomeric forms of compound 13 (99%)
that could be regioselectively iodinated using molecular iodine in
the presence of Ag2SO4 to afford the dihalide 15 (80%) that,
upon oxidation with DMP, then gave the ketone 16 (78%), the
structure of which was confirmed by single-crystal X-ray analysis
(see SI for details). Attempts to effect the iodination of ketone 14
and thus form compound 16 directly only led to complex
mixtures of products.
The three-step conversion of ketone 16 into the target Z-
configured olefin is shown in Scheme 5 and involved first
subjecting the former compound to a regioselective Suzuki−
Miyaura cross-coupling reaction with boronate ester 17,16 thus
affording the expected prenylated product 18 (82%). Reaction of
compound 18 with the readily obtained ylide 19 in diethyl ether
at −40 to 0 °C then gave the Z-configured olefin 20 in 72% yield.
Finally, sequential treatment of compound 20 with t-BuLi then
carbon dioxide gave, after acid workup, the target olefin 21 in
88% yield. Compounds 19, 20, and 21 were each contaminated
with ca. 15% of the isomer incorporating a terminal double bond
(see SI for details).
involved reacting an ethereal solution of the readily prepared
alkenyl iodide 711 with t-butyllithium (t-BuLi) and then adding
the resulting alkenyl lithium to aldehyde 6 and so affording, after
aqueous workup, the 2°-alcohol 8 (48%) as a clear, light-yellow
oil. Reduction of this last compound with di-iso-butylaluminum
hydride (DIBAL-H) resulted in cleavage of the associated ester
moiety, thus affording phenol 9 in 75% yield.
The conversion of phenol 9 into target 1 was readily achieved
by the pathway shown in Scheme 3. Thus, the former compound
was engaged in an intramolecular Mitsunobu reaction using
N,N,N,N-tetramethylazodicarboxamide (TMAD) in the pres-
ence of triphenylphosphine,12 thereby affording the anticipated
dihydrobenzofuran 10 in 90% yield.13 Finally, compound 10 was
treated with t-BuLi and the resulting aryllithium was then
trapped with carbon dioxide. By such means, and after acidic
work up, the target benzoic acid 1 was obtained in 48% yield. All
of the spectral data acquired on compound 1 were in complete
accord with the assigned structure. Of particular note was the
observation of a NOE between the resonances due to the
oxymethine and proximate olefinic protons, although the
quantitation of this proved difficult because of the proximity of
the signals in question. Disappointingly, a comparison of the 1H
and 13C NMR data acquired on compound 1 with those
reported1 for MA-F revealed significant differences [see the
Supporting Information (SI) for tabulated comparisons of the
1
13C and H NMR spectral data sets]. These discrepancies
All the spectral data acquired on acid 21 were in complete
accord with the assigned structure. Notable differences between
the NMR spectra of compound 21 and its E-isomer 1 include a
persisted even with variations in the pH of the medium in which
the NMR spectra of carboxylic acid 1 were recorded.
B
DOI: 10.1021/acs.orglett.8b01558
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Organic Letters
Letter
Scheme 4. Preparation of the Dihydrobenzofuran 16 from p-
Scheme 5. Completion of the Synthesis of Target 21
Bromophenol (2)
Table 1. Outcomes of the Biological Evaluation of
Compounds 1, 15, 16, and 21 in a TPA-Induced Mouse Ear
a
Edema Assay
compound
concentration (μM)
inhibition rate (%)
blank
0
indometacin
0.56
1.4
9.9
43.1
42.1
49.0
2.9
1
0.56
1.4
15
16
21
0.56
1.4
4.5
0.56
1.4
2.0
0.3
0.56
1.4
6.5
14.6
a
A sample (0.56 or 1.4 μM) of the test compound was applied to one
mouse ear, and after 0.5 h, TPA (0.5 μg) was applied to both ears of
the mouse. The edema was evaluated after 7 h, the cited inhibition rate
being determined as detailed in the SI. Five mice were used for each
experiment.
much higher field resonance due to the oxymethine carbon in the
former compound (81.5 vs 88.8 ppm) and a significantly lower
field shift for the signal due to the methyl group associated with
the proximate double bond (17.1 vs 11.0). While all these data
clearly support the acquisition of compound 21 by the means just
described, they do not match those reported1 for MA-F (see the
SI for a tabulated comparison of the relevant NMR data sets).
Accordingly, the true structure of MA-F remains unclear at the
present time.
The likely structural similarity of compounds 1 and 21
reported here to MA-F prompted their biological evaluation. In
particular, these compounds were subjected to a TPA-induced
mouse ear edema assay17 using the NSAID indometacin18 as
positive control. These tests reveal, as shown in Table 1, that the
synthetically derived materials 1 and 21 are also anti-
inflammatory agents with the former showing an inhibition
rate (IR) significantly greater than the control at the two
micromolar concentrations employed.19,20
The studies outlined above suggest that cyclic analogues of
terpeno-p-hydroxybenzoic acids exhibit “tunable” anti-inflam-
matory effects and are thus worthy of further investigation as
sources of potential therapeutic agents.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b01558.
C
DOI: 10.1021/acs.orglett.8b01558
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Experimental procedures, spectroscopic data, copies of the
Letter
(13) For examples of phenols participating, as nucleophiles, in
intramolecular Mitsunobu reactions leading to dihydrobenzofurans, see:
Lan, P.; Banwell, M. G.; Willis, A. C. J. Org. Chem. 2014, 79, 2829.
(14) Lattanzi, A.; Scettri, A. Synlett 2002, 2002, 942.
NMR spectra of compounds 1, 3−6, 7, precursor to 8−11,
13−16, 18, and precursor to 19, 20, 21 (PDF)
(15) Boeckman, R. K.; Shao, P.; Mullins, J. J. Org. Synth. 2000, 77, 141.
(16) Mao, L.; Bertermann, R.; Emmert, K.; Szabo, K. J.; Marder, T. B.
Org. Lett. 2017, 19, 6586.
(17) Gschwendt, M.; Kittstein, W.; Furstenberger, G.; Marks, F. Cancer
Lett. 1984, 25, 177.
Accession Codes
CCDC 1842970 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
(18) Shen, T. V. Semin. Arthritis Rheum. 1982, 12, 89.
(19) Compounds 1 and 21 are both slowly oxidized on exposure to air
(ca. 50% decomposition at −5 °C over 2 months) and were, therefore,
stored under argon, including while being shipped for biological
evaluation. As such, the reported biological activities need to be taken as
indicative of anti-inflammatory activity rather than defining such
properties in a truly quantitative manner. Mass spectrometric analyses
suggest the decomposition pathways involve the incorporation of
oxygen (rather than dehydrogenation to form benzofurans).
(20) Compounds 1 and 21 were tested as racemates. Since the
observed anti-inflammatory activities could arise from a single
enantiomer, the IR values for the relevant homochiral forms may be
higher than cited in Table 1.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: martin.banwell@anu.edu.au.
ORCID
Martin G. Banwell: 0000-0002-0582-475X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Australian Research Council and the Institute of
Advanced Studies for financial support. We thank the Australian
Government’s Department of Industry Innovation and Science
for the award of a Research Connections Grant (to Pharmaxis
Ltd). Professor Dean Tantillo (University of California, Davis) is
thanked for useful discussions regarding the possible true
structure of MA-F, while the assistance of Dr. Ping Lan (Jinan
University, Guangzhou, China) in acquiring the cited anti-
inflammatory data on compounds 1, 15, 16, and 21 is gratefully
acknowledged.
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DOI: 10.1021/acs.orglett.8b01558
Org. Lett. XXXX, XXX, XXX−XXX