Sex pheromone of the longtailed mealybug: A new class of monoterpene structure

Article abstract of DOI:10.1021/ol802164v

The sex pheromone of the longtailed mealybug, identified as 2-(1,5,5-trimethylcyclopent-2-en-1-yl)ethyl acetate, represents the first example of a new monoterpenoid skeleton. A [2,3]-sigmatropic rearrangement was used in a key step during construction of the sterically congested tetraalkylcylopentene framework.

Full text of DOI:10.1021/ol802164v

ORGANIC
LETTERS
2009
Vol. 11, No. 12
2683-2685
Sex Pheromone of the Longtailed
Mealybug: A New Class of Monoterpene
Structure
Jocelyn G. Millar,*,† Jardel A. Moreira,† J. Steven McElfresh,† Kent M. Daane,‡
and Amy S. Freund§
Department of Entomology, UniVersity of California, RiVerside, California 92521,
Department of EnVironmental Science, Policy, and Management, UniVersity of
California, Berkeley, California 94720, and Bruker Biospin Corp., 15 Fortune DriVe,
Billerica, Massachusetts 01821
Jocelyn.millar@ucr.edu
Received April 13, 2009
ABSTRACT
The sex pheromone of the longtailed mealybug, identified as 2-(1,5,5-trimethylcyclopent-2-en-1-yl)ethyl acetate, represents the first example of
a new monoterpenoid skeleton. A [2,3]-sigmatropic rearrangement was used in a key step during construction of the sterically congested
tetraalkylcylopentene framework.
The longtailed mealybug, Pseudococcus longispinus, is a
widely distributed polyphagous insect species that infests
crops such as grapes, citrus, apples, pears, and a wide variety
of ornamental plants.1 The sessile females produce a
pheromone that is highly attractive to the ephemeral, winged
males. As part of an ongoing study of the unusual chemistry
of the terpenoid pheromones of mealybugs in the Pseudo-
coccidae family,2 we report here the identification and
synthesis of the sex pheromone of this species.
To collect sufficient pheromone to identify, large numbers
of unmated female mealybugs were required as sources of
pheromone. Thus, colonies of immature mealybugs reared
on squash fruit were treated with a discriminating dose of
pyriproxifen, an insect growth regulator, to selectively kill
the males,3 which must undergo a complete metamorphosis
to the winged adult form. The females, which do not undergo
full metamorphosis, are much less susceptible to pyriprox-
ifen. The resulting cohorts containing only unmated females
were placed in glass chambers swept with a continuous flow
of clean air, with activated charcoal traps at the air outlets
being used to collect the headspace odors. Comparisons of
the gas chromatography profiles of the resulting extracts from
cohorts of sexually mature and immature female mealybugs
on squash, and from squash fruit controls, showed one
compound (Kovat’s index [KI] 1327 on an HP-5MS GC
column)4 that was consistently present only in the headspace
volatiles from sexually mature females. The 70 eV EI mass
spectrum of the compound showed a base peak at m/z 109
amu, with m/z 136 amu being the largest mass ion detected,
suggestive of a monoterpenoid structure. The compound was
hydrolyzed by treatment with NaOH in EtOH, indicative of
an ester, and the resulting alcohol showed an EI-MS base
peak at m/z 109 amu and a small molecular ion at m/z 154
amu, suggestive of a monoterpene alcohol with molecular
formula C10H18O, with two rings or unsaturations. The KI
† University of California, Riverside.
‡ University of California, Berkeley.
§ Berkeley Bruker Biospin Corp.
(1) McKenzie, H. L. Mealybugs of California; University of California
Press: Berkeley, 1967 pp 303-305.
(2) Millar, J. G. In Semiochemicals in Pest and Weed Control; ACS
Symposium Series 906; Petroski, R. J., Tellez, M. R., Behle, R. W., Eds.;
American Chemical Society: Washington, D.C., 2005; pp 11-27.
(3) Moreno, D. S.; Fargerlund, J.; Shaw, J. G. J. Econ. Entomol. 1976,
69, 292–297.
(4) Kovats, E. In AdVances in Chromatography, Vol. 1; Giddings, J. C.,
Keller, R. A., Eds.; Edward Arnold, Ltd.: London, 1965; pp 229-247.
10.1021/ol802164v CCC: $40.75
Published on Web 05/18/2009
 2009 American Chemical Society

value of the alcohol (1197), different by only 130 KI units
from the parent compound, indicated that the ester had to
be an acetate. Catalytic reduction of the parent compound
or the alcohol (5% Pd on carbon, H2) resulted in the uptake
of two hydrogens, indicating one carbon-carbon double
bond present; thus, the final site of unsaturation had to be a
ring. Furthermore, the facts that reduction of either the parent
compound or its alcohol produced only a single product in
each case suggested that the double bond was endocyclic
and 1,2-disubstituted, because reduction of a tri- or tetra-
substituted endocyclic double bond or a double bond exo to
the ring would likely have produced more than one stere-
oisomeric product in each case.
Scheme 2. Synthesis of Natural Pheromone (2)
To complete the identification, headspace extracts collected
from multiple cohorts of virgin female mealybugs over more
than one year were pooled, concentrated, and fractionated
by normal phase HPLC (2% ether in pentane isocratic). The
fraction enriched in the pheromone compound was further
purified by preparative gas chromatography (SP-1000 packed
column, 135 °C isothermal), yielding a few micrograms of
the pure compound. The sample in deuterobenzene was
analyzed by microprobe NMR spectroscopy. The resulting
proton spectrum showed four methyl singlets, with the first
at 1.72 ppm corresponding to the acetate methyl, and the
other three at 0.86, 0.84, and 0.77 attributed to methyls on
quarternary carbons. A broadened singlet integrating for two
protons at 1.98 ppm suggested an isolated allylic CH2 group.
Two geminally coupled, one-proton ddds at 1.66 and 1.48
ppm, which were further coupled to two, one-proton ddds
at 4.19 and 4.09 ppm suggested a -CH2CH2OAc subunit
attached to a chiral quaternary carbon, which rendered the
protons in each of the CH2 groups diastereotopic. The final
two, one-proton multiplets at 5.51 and 5.46 were assigned
to alkene protons on a 1,2-disubstituted alkene in the ring.
In total, these data suggested two possible structures, 1 and
2 (Schemes 1 and 2). The available NMR data did not allow
plausible late-stage intermediate in a synthesis of 1, was
readily available from camphoric acid 3,5 and so this
synthesis was carried out first (Scheme 1). Thus, alcohol 4
was obtained from camphoric acid 3 in 45% yield over four
steps, according to the published procedure.5 Oxidation of
4 to aldehyde 5, followed by Wittig reaction with (meth-
oxymethylene)triphenylphosphorane gave methylvinyl ether
6, simultaneously adding the final carbon and an oxygen
functional group to the skeleton. The straightforward se-
quence of hydrolysis to aldehyde 7, reduction to alcohol 8,
and acetylation provided acetate ester 1. The spectra did not
match those of the insect-produced compound but this isomer
still provided valuable information, because catalytic reduc-
tion of acetate 1 and alcohol 8 produced the same two
compounds that had been obtained by reduction of the insect-
produced analogs, proving that the carbon skeleton was
correct, and that isomer 2 must therefore be the natural
product.
Because the structure of acetate 1 was so close to that of
the target compound, we first attempted to simply transpose
the double bond to the desired position, by the sequence of
allylic oxidation to an R,ꢀ-unsaturated ketone, reduction of
the alkene, conversion of the ketone to the corresponding
enol triflate, and reduction. However, the final product of
this sequence was not the desired compound 2, but the
starting material 1, indicating that the initial allylic oxidation
occurred at the sp2 carbon adjacent to the geminal dimethyl
group with transposition of the double bond, rather than at
the desired allylic carbon. Throughout this sequence, the
exact position of the double bond and functional group could
not be determined with any degree of certainty, and so it
was not obvious that the oxidation had occurred with
transposition until the end of the sequence.
Scheme 1
.
Synthesis of First Pheromone Candidate (1) from
Camphoric Acid
The target compound 2 was synthesized successfully by
the route shown in Scheme 2, using a [2,3] sigmatropic
rearrangement to place the double bond in the desired
definitive determination as to which was the correct structure,
so both were synthesized.
Examination of the literature revealed that either enanti-
omer of (1,2,2-trimethylcyclopent-3-enyl)-methanol 4, a
(5) Fuganti, C.; Serra, S. J. Org. Chem. 1999, 64, 8728–8730.
Org. Lett., Vol. 11, No. 12, 2009
2684

position.6 Thus, the known cyclopentenone 10, prepared in
one step from isobutyl 2-butenoate 9 by treatment with hot
polyphosphoric acid,7 was converted into the key allyl
stannane 12 by reduction of 10 with LiAlH4 to give alcohol
11 (89%), followed by deprotonation with KH in THF at
0 °C, and treatment with (n-Bu)3SnCH2I (91%).6 Upon
treatment with n-BuLi at -100 °C, stannane 12 underwent
[2,3] sigmatropic rearrangement in low yield (25%), furnish-
ing cyclopentenol 13a with the double bond and alkyl
substituents correctly placed.
tion of alcohol 17 with O-acetyl lactic acid chloride or
Mosher’s reagent also were not separable on polar DB-Wax
or nonpolar DB-5 columns. Fortunately, preliminary results
from field trials have shown that the racemic pheromone is
highly attractive to male longtailed mealybugs.
To our knowledge, this pheromone structure constitutes
the first example of the 1,2,2-trimethylcyclopentane skeleton
in naturally occurring monoterpenoids, although this struc-
tural motif has been found in sesquiterpenes in the cuparene8
and herbertene9 families, and in higher terpenoids such as
the carotenoids capsanthin and capsorubin.10 The structure
of the pheromone suggests that it might be assembled by
the standard 4′-1 connection of two isoprene units to form
a linear geranyl-type structure, followed by a 3′-3 connec-
tion to form the 1,1,2,2-tetraalkylcyclopentane core, and then
functional group introduction and/or modification. Formation
of the 5-membered ring might occur via cyclization of a
linalyl cation, as occurs in biosynthesis of borneol, or via
protonation of the distal olefin followed by cyclization,
analogous to reactions mediated by abietadiene synthase. In
either case, initial cyclization would proceed by an unusual
anti-Markovnikov formation of the 5-membered ring. Thus,
this structure continues the trend of unusual terpenoid
pheromones within the mealybugs and scale insects. Field
trials testing the biological activity of the racemic pheromone
are in progress in California, South America, New Zealand,
Australia, and South Africa. The results of these trials will
be reported in due course.
The low yield in this step was a result of competing
reactions, specifically, [1,2] rearrangement to give 13b (4%),
elimination to give 13c (61%), and reduction to give methyl
ether 13d (10%) (Figure 1).
Figure 1. Side products from [2,3] sigmatropic rearrangement.
The synthesis was completed by straightforward one-
carbon homologation as described for 1 above. Thus, alcohol
13a was oxidized to aldehyde 14, which was immediately
reacted with (Ph)3PdCHOMe to give methyl vinyl ether 15
in 53% yield over two steps. Acid hydrolysis of 15, reduction
of the resulting aldehyde 16 with NaBH4 (71% over two
steps), and acetylation of alcohol 17 with acetyl chloride and
pyridine gave (1,5,5-trimethylcyclopent-2-en-1-yl)-ethyl ac-
etate 2 in 92% yield, the spectra of which matched those of
the insect-produced compound.
It has not yet been possible to determine the absolute
configuration of the naturally occurring pheromone. In
particular, racemic 2, the corresponding alcohol 17, and the
saturated analogs of 2 and 17 were not resolved on a
Cyclodex B chiral stationary phase GC column. Diastere-
oisomeric derivatives of the alcohol produced by esterifica-
Acknowledgment. This work was supported by the US
Department of Agriculture, Western Regional Grants Pro-
gram, and the Viticulture Consortium.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for compounds 1-17. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL802164V
(8) Melching, S.; Blume, A.; Ko¨nig, W. A.; Muhle, H. Phytochem. 1998,
48, 661–664.
(9) Connolly, J. D.; Hill, R. A. In Dictionary of the Terpenoids, 1st ed.;
Chapman and Hall: London, 1991; Vol. 1, pp 299-300.
(10) (a) Barber, M. S.; Jackman, M. L.; Warren, C. K.; Weedon, B. C. L.
Proc. Chem. Soc. 1960, 19–20. (b) Barber, M. S.; Jackman, L. M.; Warren,
C. K.; Weedon, B. C. L. J. Chem. Soc. 1961, 4019–4024.
(6) Still, W. C.; Mitra, A. J. Am. Chem. Soc. 1978, 100, 1927–1928.
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