Identification of a Grain Beetle Macrolide Pheromone and Its Synthesis by Ring-Closing Metathesis Using a Terminal Alkyne

Article abstract of DOI:10.1021/acs.orglett.5b02461

A major C₁₈-macrolide was found during analysis of the frass of the storage beetle Oryzaephilus surinamensis to be (9Z,12Z,15R)-octadeca-9,12-dien-15-olide (10, cucujolide XI). The synthesis used ring-closing alkyne metathesis as a key step. The highly active 2,4,6-trimethylbenzylidyne molybdenum complex [MesCMo{OC(CF₃)₂Me}₃] (12) allowed the use of a terminal alkyne and afforded the product in excellent yield. Bioassays proved the activity of the R-enantiomer 10 in the aggregation of the beetle. Cucujolide XI is the first macrolide pheromone oxidized at the ω-4 position.

Full text of DOI:10.1021/acs.orglett.5b02461

Letter
pubs.acs.org/OrgLett
Identification of a Grain Beetle Macrolide Pheromone and Its
Synthesis by Ring-Closing Metathesis Using a Terminal Alkyne
Susann Hotling,† Celine Bittner,‡ Matthias Tamm,‡ Sonja Dahn,§ Jana Collatz,§ Johannes L. M. Steidle,§
̈
̈
and Stefan Schulz*,†
†Institut fur Organische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig, Germany
̈
̈
̈
‡Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,
Germany
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§Fachgebiet Tierokologie, Institut fur Zoologie, Universitat Hohenheim, 70593 Stuttgart, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: A major C18-macrolide was found during analysis of the frass of the
storage beetle Oryzaephilus surinamensis to be (9Z,12Z,15R)-octadeca-9,12-dien-
15-olide (10, cucujolide XI). The synthesis used ring-closing alkyne metathesis as
a key step. The highly active 2,4,6-trimethylbenzylidyne molybdenum complex
[MesCMo{OC(CF3)2Me}3] (12) allowed the use of a terminal alkyne and
afforded the product in excellent yield. Bioassays proved the activity of the R-
enantiomer 10 in the aggregation of the beetle. Cucujolide XI is the first
macrolide pheromone oxidized at the ω-4 position.
acrolides are an important class of compounds used by
1
Mseveral animals for communication via pheromones.
Beetles of the genera Cryptolestes (Laemophloeidae) and
Oryzaephilus (Silvanidae), which are important grain storage
pests, use a variety of such macrolides as aggregation and sex
pheromones, often in species specific mixtures. Ground-
breaking work about 30 years ago by the group of Oehlschlager
led to the identification of cucujolides I to IX, unsaturated
macrolides from C10 to C14 containing one or two double
bonds.2 We became interested in the pheromones of the
sawtoothed grain beetle Oryzaephilus surinamensis, previously
investigated by Oehlschlager,3 because these compounds might
Figure 1. Total ion chromatogram of active fraction containing
unknown compounds A and B, as well as known cucujolides V and X.
be used by parasitoids of the beetle for host finding. These
parasitoids are potentially useful in storage protection. We
recently identified cucujolide X, (5Z,8Z,12R)-tetradeca-5,8-
dien-12-olide, as a female attracting pheromone in O.
surinamensis.4 This is the first macrolide carrying an ethyl side
chain. All others are either unbranched or carry a methyl group,
only.
treatment it would have escaped our attention. It was
accompanied by a second compound B with an identical
molecular mass of 278. Compound B was readily identified by
comparison with a synthetic sample to be coriolide known from
Heliconius butterflies.5,6
To obtain more structural information, the extract containing
compound A in the μg range was subjected to micro-
hydrogenation with Pd/C catalysis. The resulting compound
C exhibited the mass spectrum shown in Figure 2. The
molecular ion shift from m/z 278 to 282 revealed the presence
of two double bonds in A. The large ion at m/z 264 (M-18)
During the analysis of volatiles released from the frass of O.
surinamensis we obtained a fraction that was highly attractive to
the parasitoids. Besides small amounts of cucujolides V and X,
large amounts of fatty acids such as linolenic acid were present.
After removal of the acids with basic alumina, a new major peak
was found by GC/MS analysis, previously obscured by the
broad tailing of linolenic acid (Figure 1). This compound A
showed a mass spectrum closely related to that of linolenic acid
(see Supporting Information, Figure S2). Without the alumina
Received: August 26, 2015
© XXXX American Chemical Society
A
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To prove our assumption, we planned a synthesis of 10 via
ring-closing alkyne metathesis (RCAM) (Scheme 1). With the
availability of highly active alkyne metathesis catalysts,7 RCAM
has recently become an important synthetic tool for the
preparation of cyclic natural products.6,6 For recent examples
reporting the RCAM of internal alkynes, see ref 8. In previous
work, we developed an enantioselective synthesis to cucujolide
X using a enyne precursor obtained by Wittig methodology.4
Although the RCAM reaction was catalyzed in a satisfactory
manner by an imidazolin-2-iminato tungsten benzylidyne
complex,9 the more recently developed, highly active 2,4,6-
trimethylbenzylidyne molybdenum complex [MesCMo{OC-
(CF3)2Me}3] (12) might serve as an improved RCAM
catalyst.10 Moreover, this catalyst proved capable of promoting
the metathesis of terminal alkynes, which allows avoiding the
sometimes tedious installment of an internal triple bond at the
ω-1 position of a chain. For determination of the absolute
configuration of the natural product and for bioassays, both
enantiomers of the target lactone 10 were prepared.
For the synthesis of the R-enantiomer, the required
enantiomerically pure alkynol 8 was prepared, starting from
R-epichlorhydrin (6) by alkylation, epoxide formation, and its
regioselective alkynylation, as reported previously.11,12 The
enyne acid 5 was introduced by the approach reported by us
earlier.4 9-Undecen-1-ol (3) was converted into methyl 9-
oxononanoate (4) by Jones oxidation, methylation, and
K2OsO4/NaIO4 cleavage. A Z-selective Wittig reaction with
phosphonium iodide 2 furnished (Z)-tetradec-9-en-12-ynoic
(5) acid after saponification. Attempts to obtain a simpler
analog, (Z)-tridec-9-en-12-ynoic acid carrying a terminal triple
bond, failed because the basic conditions of saponification and
Wittig reaction favored formation of the conjugated allenic (Z)-
tridec-9,11,12-trienoic acid instead. The precursor 9 for the
RCAM was obtained by 1-ethyl-3-(3-(dimethylamino)propyl)-
Figure 2. EI-mass spectra of compound A (top) and its hydrogenation
product C (bottom). Diagnostic ions are marked.
together with the ions at m/z 60 and 73 indicated a macrocyclic
lactone, because many published mass spectra of macrolides
show such fragments. Two diagnostic ions can be found, m/z
239 (M-43) and m/z 210 (M-72), suggesting a 16-membered
macrocyclic ring and a propyl group next to the single bound
oxygen. We observed this mass spectrometric behavior in
various branched macrolides (unpublished results). Assuming a
potential biosynthetic relation to linolenic acid, as is the case for
cucujolide V and X, we assumed that compound A would be
(9Z,12Z)-octadeca-9,12-dien-15-olide (10).
Scheme 1. Synthesis of Cucujolide XI (10)
B
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carbodiimide (EDC) esterification of 8 and 5. The key RCAM
step with the catalyst 12 furnished macrolactone 11 in almost
quantitative yield. Final hydrogenation under Lindlar catalysis
led to the target compound R-10. In an identical sequence, S-10
was obtained starting from S-epichlorhydrin. Comparison of
GC and MS data revealed the natural compound to be indeed
(9Z,12Z)-octadeca-9,12-dien-15-olide (10), carrying a propyl
side chain. We propose the name cucujolide XI for this
compound, in congruence with previous pheromone compo-
nents of cucujid beetles.13 The absolute configuration of the
natural product is R, as determined by gas chromatography on a
chiral Hydrodex-6-TBDMS phase (see Supporting Information,
Figure S3).
Next the biological activity of 10 was tested. A bioassay was
performed to test the attractiveness for both males and females
of O. surinamensis in a four-chamber olfactometer, as described
previously.4 While males did not respond to 200 ng of 10 at all,
females showed a significantly increased walking duration in the
test field with the R-enantiomer as compared to the control
field. The S-enantiomer proved to be inactive (Figure 3). The
mandelalide A, effectively achieving RCAM with a precursor
carrying one terminal and one preterminal triple bond,17 similar
to the alkyne motif used in our synthesis. These catalysts
proved to be very useful, because the sometimes tedious
synthesis of methylated alkynes can thus be avoided.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02461.
Experimental procedures, bioassay method, and enan-
tiomer separation (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: stefan.schulz@tu-bs.de.
Notes
The authors declare no competing financial interest.
REFERENCES
■
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Figure 3. Response of female and male O. surinamensis to the R-
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