Chemoenzymatic Synthesis of the Antifungal Compound (–)-Pestynol by a Convergent, Sonogashira Construction of the Central Yne-Diene

Article abstract of DOI:10.1002/ejoc.201801470

A total synthesis of the fungal-derived natural product pestynol is reported via a convergent chemoenzymatic approach from the readily available precursors geranyl bromide, ethyl acetoacetate, trimethylsilylacetylene, and bromobenzene. Synthetic (–)-pestynol proved to be identical in all respects to the natural material, allowing confirmation of the structure including absolute stereochemistry.

Full text of DOI:10.1002/ejoc.201801470

A Journal of
Accepted Article
Title: Chemoenzymatic synthesis of the Antifungal Compound (-)-
Pestynol via a Convergent, Sonogashira Construction of the
Central Yne-diene
Authors: Suresh Borra, Manoj Kumar, James McNulty, Daler Baidilov,
and Tomas Hudlicky
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201801470
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10.1002/ejoc.201801470
European Journal of Organic Chemistry
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DOI: 10.1002/ejoc.2018xxx((will be filled in by the editorial staff))
Chemoenzymatic synthesis of the Antifungal Compound (-)-Pestynol via a
Convergent, Sonogashira Construction of the Central Yne-diene**
Suresh Borra,a Manoj Kumar,a James McNulty,a*
Daler Baidilov,b and Tomas Hudlickyb*
Abstract: A total synthesis of the fungal-derived natural product pestynol is reported via a convergent chemoenzymatic
approach from the readily available precursors geranyl bromide, ethyl acetoacetate, trimethylsilylacetylene and bromobenzene.
Synthetic (-)-pestynol proved identical in all respects to the natural material, allowing confirmation of its absolute
stereochemistry.
Introduction
The natural product pestynol (1), recently isolated from the
fungus Pestalotiopsis humus employing a yeast (S. cerevisiae)
multiple efflux-pump deletion mutant bio-assay-guided
protocol, was shown to possess antifungal activity.1 The
interesting structure assigned to pestynol on the basis of NMR
and Mosher ester analysis is intriguing, consisting of what
appears to be a C15 modified farnesyl unit fused to an unusual
conduritol-E type fragment. Although unique, pestynol
appears structurally related to a small class of arene or
cyclohexenyl conjugates of geraniol, including foenicuiloxin
(2),2 phomentrioloxins A (3),3a B and C (not shown),3b and
tricholomenyn A (4), Figure 1.4 The phomentrioloxins were
isolated from the mycoherbicidal fungi Diaporthegulyae
(Phomopsis sp.),5 a genus of endophytic, saprobic and plant
Figure 1. Proposed structure of the recently isolated unusual
antifungal natural product pestynol (1) and related natural products
containing internal conjugated yne-dienes.
pathogenic fungi6 under investigation as a biocontrol agent for
invasive weeds. Indeed, phomentrioloxin and its semi-
synthetic derivatives were recently shown to possess
herbicidal activities.7,8 While pestynol has not been
synthesized to date, Banwell and co-workers have prepared
phomentrioloxin A (3) from an iodo-analog of 5a (Scheme 1),
resulting in the reassignment of its absolute configuration as
shown (Fig. 1).8 The presence of the sensitive conjugated yne-
diene core in these molecules presents an attractive synthetic
target.
We decided to investigate a total synthesis of pestynol for
several reasons. In addition to opening a synthetic route to
pestynol and analogs of this type for biological studies, the
cycloisomerization chemistry of conjugated 1,3-enynes has
received recent interest, but has not been investigated in
detail.9 Surprisingly, the chemistry of yne-dienes appears to be
largely unexplored, in contrast to that of the ene-diynes, that
engage in a rich, well established chemistry and biology.10
A
total synthesis of pestynol (1) was thus initiated to confirm the
structure and absolute stereochemistry of the natural product,
and to explore the general entry to conjugated enynes and yne-
dienes in order to investigate their chemistry in more detail. In
this communication we report a highly convergent total
synthesis of pestynol, confirming its structure and absolute
stereochemistry as reported.1
a) Dr. S. Borra, Dr. M. Kumar, Dr. J. McNulty,*
Department of Chemistry and Chemical-Biology
McMaster University
1280 Main Street West
Hamilton, Ontario, L8S 4M1, Canada
E-mail: jmcnult@mcmaster.ca
http://www.chemistry.mcmaster.ca/mcnulty/index.html
b) Mr. D. Baidilov, Dr. T. Hudlicky,*
Department of Chemistry and Centre for Biotechnology,
Brock University,1812 Sir Isaac Brock Way,
St. Catharines, Ontario, L2S 3A1, Canada
E-mail: thudlicky@brocku.ca
https://brocku.ca/tomas-hudlicky/
[] Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc***.
1
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10.1002/ejoc.201801470
European Journal of Organic Chemistry
overexpresses toluene dioxygenase.15 Diol 7 is available in
reproducible yields of ~20g/L on medium scale (10-15L
fermentor) as previously reported.16 Extraction of the
centrifuged broth with based-washed EtOAc and
crystallization from pentane/EtOAc reliably provides 90-120
grams of diol 7. The diol was protected as the acetonide
derivative 6, which was then subjected to OsO4-catalyzed
dihydroxylation
O
O
OH
OH
O
O
OH
OH
i
9
+
Scheme 1. Retrosynthetic analysis for pestynol from bromobenzene
(8), geranyl bromide (14) and ethyl acetoacetate (13) proceeding
through the 1,3-enyne intermediate 9 and vinyl bromide 5b.
Br
5b
15
OH
HO
OH
OH
ii
Results and Discussion
A retrosynthetic analysis of pestynol (1) is outlined in Scheme
1. We envisioned a Sonogashira11 coupling of the vinyl
bromide 5a or 5b, available from bromobenzene via enzymatic
dihydroxylation,12 with the 1,3-enyne 9. The enyne was
expected to be available from coupling of TMS-acetylene (11)
to the vinyl triflate 10, available from geranylacetone (12),
prepared through alkylation of ethyl acetoacetate (13) with
geranyl bromide (14).
1
Scheme 3. Sonogashira cross-coupling and completion of the
synthesis of pestynol (1). Reagents: (i) PdCl2(CH3CN)2, CuI, piperidine:
benzene (1:1), 55 ºC, 4 h (72%); (ii) AcOH:H2O (5:1), 60 ºC, 6 h (81%).
to provide the vinyl bromide 5b (63% from 7), as first reported
in connection with the enantiodivergent synthesis of (+)- and
(-)-pinitol.17 Hydrolysis of the acetonide in 5b furnished tetrol
5a (91%), making both available for investigation as coupling
partners.
Reaction of the enolate of ethyl acetoacetate with geranyl
bromide followed by saponification and decarboxylation
proceeded without incident,13 and geranylacetone (12) was
isolated as an oil in 65% isolated yield, Scheme 2. Reaction of
the kinetic enolate of 12 with the Comins triflimide,14 followed
by the Miller-Johnson protocol11b proved problematic. The
vinyl triflate 10 was formed, but proved to be very unstable,
and could not be isolated by chromatography. On standing,
decomposition rapidly led to formation of several new
compounds. A solvent extraction protocol was developed that
allowed partition of the triflate 10 into pentane, the salt of the
pyridyl-monotriflate and other side products being removed
into the aqueous phase. Compound 10 was then coupled
directly with TMS-acetylene to give the trimethylsilyl
derivative of 9 in 40% overall yield from 12. Desilylation
proceeded without incident to give 9, which was prepared
routinely in 50 – 100 mg quantities.
The initial attempts at Sonogashira coupling of 9 with vinyl
bromides 5a and 5b, following close precedents
(CuI/(PPh3)2PdCl2, iPr2NH),8,11b were not successful, Scheme
3. Starting materials were not consumed and only trace of the
products were observed. We note that such cross-couplings,
described previously, have employed vinyl iodide reactants. In
the present case, the terminal alkyne 9 is highly lipophilic and,
tetraol 5a, in particular, is highly polar, leading us to suspect a
miscibility issue. When the reaction was repeated with the less
polar acetonide 5b in benzene containing triethylamine, the
product 15 was isolated in about 5% yield. A brief screen of
other palladium precursors and bases identified the acetonitrile
complex PdCl2(CH3CN)2 and CuI as well as the use of
piperidine as base as optimal conditions. With this catalyst
o
system in piperidine:benzene (1:1) as the solvent at 55 C,
complete consumption of diol compound 5b was observed and
the yne-diene 15 was obtained in 72% isolated yield. The
insoluble tetraol 5a failed to couple with 9 under these
conditions. Removal of the acetonide from 15 was investigated
under mild conditions, given the sensitivity of the yne-diene,
and was achieved in water-acetic acid mixture at 55 °C,
yielding synthetic pestynol in 81% yield. Inspection of the 1H,
13C and two-dimensional NMR spectra showed synthetic
pestynol to be identical in all respects to the natural material
reported by Sakai and co-workers.1 The absolute
stereochemistry of natural pestynol was assigned through
application of the Mosher method and optical data showed the
compound to be levorotatory, exhibiting a specific rotation
[α]D 27 -49.4 (c 0.1, methanol). Synthetic pestynol as prepared
here demonstrated a specific rotation of -51.7 in methanol, thus
confirming the absolute stereochemistry as originally
assigned.1
Scheme 2. Synthesis of the conjugated enyne 9. Reagents: (i)
MeCOCH2CO2Et, K2CO3, acetone, 70 ºC, 4 h; (ii) KOH, MeOH, reflux,
3 h, then 1 M HCl (65% over 2 steps); (iii) (iPr)2NH, nBuLi (1.6 M in
hexane), N-(2-pyridyl)triflimide, HMPA, THF, -78 ºC , 30 min, then -20
ºC, 30 min; (iv) PdCl2(PPh3)2, CuI, (iPr)2NH, TMS-acetylene, THF, 20
ºC, 1 h, (40% over 2 steps); (v) K2CO3, MeOH, r.t., 4 h (68%).
Vinyl bromides 5a and 5b were prepared from bromobenzene
in three and four steps respectively. The first step involved the
whole-cell fermentation of bromobenzene with E. coli JM109
(pDTG601A), a recombinant strain developed by Gibson that
2
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10.1002/ejoc.201801470
European Journal of Organic Chemistry
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Conclusion
In conclusion, we report the first enantioselective synthesis of
the natural product pestynol confirming the overall structure
and absolute stereochemistry. Conditions were developed to
allow preparation and direct coupling of the sensitive vinyl
triflate 10. The use of a benzene/piperidine solvent system was
developed to allow Sonogashira coupling of the lipophilic
alkyne 9. These developments, in conjunction with protocols
developed by Miller-Johnson11b and Banwell and co-
workers,8,11c should allow access to terminal enynes such as 9
and ynedienes of wide structural scope. Exploration of the
metal-mediated cycloisomerization9 chemistry of these
compounds and full screening of the biological activity of
pestynol are under active investigation in our laboratories.
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Acknowledgments
The authors are grateful to Natural Sciences and Engineering
Research Council of Canada (NSERC) [TH and JM], Canada
Foundation for Innovation (CFI) [TH and JM], Canada
Research Chair Program, TDC Research Foundation and The
Advanced Biomanufacturing Centre at Brock University
[TH].
______________________________________________________________
Keywords: Alkenes· alkynes·1,3-enyne ·biocatalysis·Sonogashira
reaction· antifungal
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3
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