Total Synthesis of a Dimeric Thymol Derivative Isolated from Arnica sachalinensis

Article abstract of DOI:10.1002/anie.201701481

The total synthesis of a dimeric thymol derivative (thymarnicol) isolated from Arnica sachalinensis was accomplished in 6 steps. A key biomimetic Diels–Alder dimerization was found to occur at ambient temperature and the final oxidative cyclization occurs

Full text of DOI:10.1002/anie.201701481

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International Edition: DOI: 10.1002/anie.201701481
German Edition: DOI: 10.1002/ange.201701481
Natural Product Synthesis
Hot Paper
Total Synthesis of a Dimeric Thymol Derivative Isolated from Arnica
sachalinensis
Irene De Silvestro, Samuel L. Drew, Gary S. Nichol, Fernanda Duarte,* and
Andrew L. Lawrence*
Abstract: The total synthesis of a dimeric thymol derivative
(thymarnicol) isolated from Arnica sachalinensis was accom-
plished in 6 steps. A key biomimetic Diels–Alder dimerization
was found to occur at ambient temperature and the final
oxidative cyclization occurs when the substrate is exposed to
air and visible light. These results indicate that this natural
product is likely the result of spontaneous (non-enzyme-
mediated) reactivity.
I
n 1999, Passreiter and co-workers isolated a racemic thymol
derivative (1) from the flower heads of Arnica sachalinensis
(Asteraceae), a sunflower native to Sakhalin Island off the
Pacific Coast of Russia (Scheme 1).[1] For ease of discussion
within this manuscript, and for future communications, we
suggest “thymarnicol” (a portmanteau of thymol and Arnica)
as a suitable name for compound 1. Preliminary biological
testing has revealed thymarnicol (1) to have potentially useful
antifeedant,[1a] phytotoxic,[2] and anti-inflammatory[3] activity.
For its small molecular size, thymarnicol (1) possesses
significant molecular complexity and, despite its dimeric
origins, contains no element of symmetry. The dense array of
oxygen functionalities and four associated stereogenic centers
at the core of the novel spiro[benzofuran-pyranobenzofuran]
framework poses a considerable synthetic challenge. The
previous isolation of oxidized and acetylated thymol deriva-
tives from other Asteraceae plants (e.g., 2, 3 and 4)[4] led
Passreiter and co-workers to propose a biosynthetic pathway
for thymarnicol (1; Scheme 1).[1a] It begins with hetero-Diels–
Alder dimerization of enal-thymol derivative 5 to give
dihydropyran 6.[5] Hydrolysis of the phenol ester groups
then enables two cyclizations to form pentacycle 7, with
a final oxidation at the benzylic methine position giving
thymarnicol (1). We decided to embark upon efforts to
achieve a concise total synthesis of this complex and compact
Scheme 1. Structure of thymarnicol (1) and Passreiter’s proposed
biosynthetic pathway. Ac=acetyl.
natural product and to investigate the chemical feasibility of
Passreiterꢀs proposal.
The synthesis began with Wittig methylenation of com-
mercially available acetophenone 8 to give alkene 9,[6]
followed by acetylation using acetic anhydride in pyridine
(Scheme 2). Both steps proceeded in excellent yield and could
be conducted on a multigram scale. Oxidation of alkene 10 to
enal 11, using stoichiometric SeO2, was performed in several
smaller batches since the yield was found to decline with scale
(Scheme 2).[7] The chemical feasibility of Passreiterꢀs pro-
posed hetero-Diels–Alder dimerization was quickly estab-
lished, with enal 11 being found to undergo dimerization
when stored neat at ambient temperatures (ca. 50% con-
version after 5 days). Heating neat samples of enal 11 at 808C
for 42 h resulted in near quantitative dimerization. The crude
dimer was immediately subjected to basic conditions to
deprotect the two phenols. This did not give pentacycle 7 (see
Scheme 1) but instead gave lactol 12, the product of just one
cyclization (Scheme 2). Interestingly, lactol 12 exists as
a mixture of two diastereomers, and the ratio between
them, determined by analysis of the 1H NMR spectrum,
varies with the solvent (d.r. in CDCl3 72:28, CD3COCD3
78:22), presumably as a result of facile lactol epimerization.
It was observed that lactol 12 was unstable; new peaks
corresponding to thymarnicol (1) could be identified in the
1H NMR spectra of older samples. After careful experimen-
tation, we discovered that this fortuitous aerial oxidation is
[*] I. De Silvestro, Dr. G. S. Nichol, Dr. F. Duarte, Dr. A. L. Lawrence
EaStCHEM School of Chemistry, University of Edinburgh
Joseph Black Building, David Brewster Road
Edinburgh, EH9 3FJ (UK)
E-mail: fernanda.duarte@ed.ac.uk
a.lawrence@ed.ac.uk
Dr. S. L. Drew
Research School of Chemistry, Australian National University
Canberra, ACT 2601 (Australia)
and
Present address: Department of Chemistry
University of California, Irvine, CA 92697-2025 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201701481.
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investigate the factors that control the reactivity and regio-
selectivity of the hetero-Diels–Alder dimerization
(Scheme 3).[12] For the simplified model compound 11a,
which bears no ortho substituent, there are eight distinct
transition structures (TSs) possible (Scheme 3a). These TSs
are described using a notation where the regiochemical-
orientation is meta (m) or para (p), the Alder–Stein mode is
endo (N) or exo (X), and the dienophile adopts either an s-cis
(cis) or s-trans (trans) conformation. Without exception, the
para TSs were found to be significantly lower in energy than
the meta TSs owing to better orbital overlap and lower
distortion penalties (Scheme 3a; see the Supporting Informa-
tion for full distortion/interaction analysis).[13,14] The unex-
pected exo selectivity observed for the majority of the TSs is
due to favourable non-covalent (dispersion) interactions
between the two aromatic rings (as shown in Scheme 3b for
the meta-X-trans TS), which outweighs the higher distortion
penalty that normally disfavours exo TSs.[15] The lowest
energy TS, however, is not exo orientated; the para-N-cis
TS is a rare example of a C2-symmetric bis-pericyclic TS
(Scheme 3c).[16] In bis-pericyclic TSs, the [4+2] and [2+4]
cycloaddition pathways have fully merged and thus benefit
from three primary orbital interactions. Following the bis-
pericyclic TS, the pathway then bifurcates to give the
degenerate [4+2] and [2+4] cycloadducts. The lowest
energy TS for hetero-Diels–Alder dimerization of enal 11
was also found to be a para-N-cis TS, which although not C2-
symmetric still has bis-pericyclic character (Scheme 3d; see
the Supporting Information for full computational details for
enal 11).
Further insight into the origin and reactivity of thymarni-
col (1) was acquired through other unsuccessful synthetic
studies. For example, different phenol-protecting groups were
investigated (Scheme 4). The tert-butyldimethylsilyl (TBS)-
protected enal 13 underwent a less efficient Diels–Alder
dimerization, requiring higher temperatures and giving lower
yields of dihydropyran 14 (Scheme 4a). Use of the sterically
less demanding MOM (methoxymethyl) ether, however,
resulted in dimerization occurring at ambient temperature,
with a synthetically useful 77% yield of dihydropyran 16
achieved upon heating enal 15 at 808C for 22 h (Scheme 4a).
Unfortunately, dihydropyran 16 could not be successfully
advanced to give thymarnicol (1; see below). The most
interesting precursor to investigate, from a biomimetic per-
spective, was the unprotected monomer 4, a known natural
product that exists primarily as the lactol isomer (Sche-
me 4b).[4b,17] When compound 4 was stored neat at ambient
temperatures, multiple new minor peaks appeared in the
1H NMR spectrum and the Diels–Alder dimer 12 could be
identified amongst these new species. Therefore, an alter-
native biosynthetic pathway involving dimerization of the
natural product 4 is chemically feasible. Synthetically speak-
ing, however, this route was not pursued owing to difficulties
associated with the preparation and purification of lactol 4
and an apparent lack of selectivity for dimerization.
Scheme 2. Six-step synthesis of thymarnicol (1). THF=tetrahydro-
furan.
promoted by exposure to visible light.[8] Therefore, a hexane/
EtOAc solution of crude lactol 12, open to the atmosphere,
was irradiated with visible light from an 11 W compact
fluorescent lamp for 72 h. Analysis of the 1H NMR spectrum
of the resulting product, with inclusion of an internal
standard, indicated a remarkable 57% crude yield of the
two lactol epimers of thymarnicol (1) over three steps from
enal 11. Work is ongoing in our laboratory to identify other
minor products from this aerial oxidation and to interrogate
likely mechanisms.[9]
The final three-step sequence, from enal 11 to thymarnicol
(1), could be conducted on a more than 100 mg scale and
without chromatographic purification of intermediates.
Column chromatography followed by preparative HPLC
could then be used to access analytically pure samples of
thymarnicol (1; 40 mg prepared so far), but still as an
unavoidable mixture of the two lactol epimers. Crystallization
from acetonitrile resulted in crystals suitable for single-crystal
X-ray diffraction studies.[10] The crystal structure obtained
matched that reported for the natural material,[1b] which
consists of just one lactol epimer (Scheme 2). Nevertheless,
subsequent analysis of these crystals by solution-phase
1H NMR spectroscopy showed the presence of both lactol
epimers. It must be concluded that thymarnicol (1) is stereo-
dynamic; it exists as a mixture of lactol epimers in solution but
can exist as a single epimer in the solid state. Thus, a six-step
total synthesis of thymarnicol (1) has been achieved, involving
À
À
the formation of nine new bonds (three C C, six C O), three
new rings, and four new stereogenic centres.
The greatest synthetic challenge encountered during our
synthetic efforts was the propensity of the thymarnicol
nucleus to undergo acid-promoted rearrangements. For
example, attempts to deprotect dimer 16 under standard
Density functional theory (DFT) calculations at the
wB97X-D/6-31 + G(d) level of theory[11] were undertaken to
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Scheme 4. a) Hetero-Diels–Alder dimerization of differently protected
monomers 13 and 15. b) Hetero-Diels–Alder dimerization of unpro-
tected monomer 4. MOM=methoxymethyl, TBS=tert-butyldimethyl-
silyl.
the MOM ethers in epoxide 18 led to the formation of bis-
lactones 19 in high yield (Scheme 5b).[10,19] The predisposed
nature of these rearrangements, which presumably occur
through [1,2]-shift mechanisms (see Scheme 5), leads us to
Scheme 3. a) Reaction free energies (DG°80 ꢀC) at the wB97X-D/6-
311+ +G(d,p)//wB97X-D/6-31+G(d) level of theory for the eight
possible transition structures for hetero-Diels–Alder dimerization of
model compound 11a. b) The wB97X-D/6-31+G(d) optimized struc-
ture of the m-X-trans TS for model compound 11a, with a non-covalent
interaction surface (green indicates van der Waals/dispersion interac-
tions, blue indicates strong polar interactions, and red indicates steric
repulsion). The wB97X-D/6-31+G(d)-optimized structures of the p-N-
cis TS for model compound 11a (c) and for enal 11 (d).
HCl/MeOH conditions resulted in cleavage of the dihydro-
pyran ring to give dihydrobenzofuran 17 as a mixture of two
diastereomers (Scheme 5a).[18] Similarly, attempts to cleave
Scheme 5. a) Rearrangement observed during deprotection of MOM-
protected dimer 16. b) Rearrangement observed during deprotection of
MOM-protected epoxide 18. TMS=trimethylsilyl.
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In conclusion, through our synthetic efforts, we have been
able to investigate the chemical feasibility of Passreiterꢀs
suggested biosynthetic pathway for thymarnicol (1). We have
shown that the proposed hetero-Diels–Alder dimerization is
plausible, with both acetyl- and MOM-protected systems (11
and 15) undergoing spontaneous dimerization at room
temperature. We have also provided evidence in support of
an alternative biosynthetic pathway involving dimerization of
lactol 4, a known natural product previously isolated from
related Asteraceae plants.[4b] It was discovered that dihydro-
pyran 12 undergoes the final oxidative cyclization when
simply exposed to air and visible light. Therefore, our results
indicate that the complexity-generating Diels–Alder dimeri-
zation and oxidative cyclization likely proceed without the
intervention of enzymes to produce natural thymarnicol
(1).[21]
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[8] No oxidation was observed when samples of lactol 12 were
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[9] Inclusion of a photosensitizer (rose bengal) did not lead to any
appreciable rate enhancement.
[10] CCDC 1530569 (1) and 1530570 (meso-9) contain the supple-
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Acknowledgements
This work was supported by a Royal Society Research Grant
and a Marie Curie Career Integration Grant. F.D. acknowl-
edges the Royal Society for a Newton International Fellow-
ship. I.D.S. thanks the University of Edinburgh for the
provision of a studentship. Antony Crisp (Australian National
University) is thanked for conducting preliminary experi-
ments. Dr. Stephen Moggach (University of Edinburgh), Dr.
Nicholas Green (University of Edinburgh) and Prof. Mick
Sherburn (Australian National University) are thanked for
helpful discussions.
Conflict of interest
The authors declare no conflict of interest.
Keywords: biomimetic synthesis · cycloaddition · dimerization ·
natural products · terpenoids
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[1] a) C. M. Passreiter, G. Willuhn, H. Weber, K.-J. Schleifer,
Tetrahedron 1999, 55, 2997 – 3006; b) C. M. Passreiter, H.
Weber, D. Blꢁser, R. Boese, Tetrahedron 2002, 58, 279 – 282.
[2] Thymarnicol (1) has also been isolated from Hofmeisteria
schaffneri: A. Perꢂz-Vasquꢂz, A. Reyes, E. Linares, R. Bye, R.
Mata, J. Nat. Prod. 2005, 68, 959 – 962.
[18] T. W. Green, P. G. M. Wuts, Protective Groups in Organic
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[19] J. H. Rigby, J. Z. Wilson, Tetrahedron Lett. 1984, 25, 1429 – 1432.
[20] Recent examples of natural product anticipation from biomim-
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[4] a) C. M. Passreiter, U. Matthiesen, G. Willuhn, Phytochemistry
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K. Kerr, Phytochemistry 1979, 18, 489 – 490.
[5] To the best of our knowledge, there have only been two previous
reports regarding hetero-Diels–Alder dimerization of 2-aryl-
a,b-unstaurated aldehydes, and both exhibited the regioselec-
4
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[21] The question of whether thymarnicol (1) is an artefact of the
extraction/isolation/purification process or is the result of
predisposed chemical reactivity, which is useful for the plant, is
Manuscript received: February 10, 2017
Final Article published: && &&, &&&&
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Communications
Natural Product Synthesis
I. De Silvestro, S. L. Drew, G. S. Nichol,
F. Duarte,*
A. L. Lawrence*
&&&& — &&&&
Poised to (re)act: The total synthesis of
a dimeric thymol derivative (thymarnicol)
isolated from Arnica sachalinensis was
accomplished in 6 steps. A key biomim-
etic Diels–Alder dimerization was found
to occur at ambient temperature and the
final oxidative cyclization occurs when the
substrate is exposed to air and visible
light. These results indicate that this
natural product is likely the result of
spontaneous (non-enzyme-mediated)
reactivity.
Total Synthesis of a Dimeric Thymol
Derivative Isolated from Arnica
sachalinensis
6
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!