A novel group of allenic hydrocarbons from five Australian (Melolonthine) beetles

Article abstract of DOI:10.1039/b101801i

Allenic hydrocarbons, previously unknown as a molecular class from insects, are represented by CH₃(CH₂)_n-CH=?=CH-(CH₂) ₇CH₃ (n = 11-15, 17, 19) in several Australian melolonthine scarab beetles and with demonstrated (R)-chirality when n = 11 and 13.

Full text of DOI:10.1039/b101801i

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A novel group of allenic hydrocarbons from five Australian
(Melolonthine) beetles†
Mary T. Fletcher,a Matthew J. McGrath,a Wilfried A. Ko¨nig,b Christopher J. Moore,c Bronwen W.
Cribb,d Peter G. Allsoppe and William Kitching*a
a Department of Chemistry, University of Queensland, Brisbane, Australia.
E-mail: kitching@chemistry.uq.edu.au; Fax: +61 7 3365 4299; Tel: +61 7 3365 3925
b Institut fu¨r Organische Chemie, Universita¨t Hamburg, Hamburg, Germany
c
Department of Primary Industries, Brisbane, Australia
d Department of Zoology and Entomology, University of Queensland, Brisbane, Australia
e
Bureau of Sugar Experiment Stations, Bundaberg, Australia
Received (in Cambridge, UK) 26th February 2001, Accepted 28th March 2001
First published as an Advance Article on the web 20th April 2001
Allenic hydrocarbons, previously unknown as a molecular
class from insects, are represented by CH3(CH2)n-
Similarly the minor C-27 component was concluded to be
9,10-heptacosadiene 5, based on the formation of two isomers
of 9,11-dimethoxyheptacosane, and also 9-methoxyheptacos-
10-ene and 11-methoxyheptacos-9-ene. A more varied mixture
of regioisomeric mono- and bis-methyl ethers would be
anticipated from both conjugated and non-conjugated dienic
hydrocarbons.
D
CHN•NCH-(CH2)7CH3 (n
=
11–15, 17, 19) in several
Australian melolonthine scarab beetles and with demon-
strated (R)-chirality when n = 11 and 13.
Melolonthine scarabs are pests of field crops and pastures,1 and
we have undertaken studies of the chemistry of certain
sugarcane scarabs from north-eastern Australia. Cuticular
hydrocarbons2 from female adults were of surprisingly limited
constitutional variation. In Antitrogus consanguineus, one
component represents (GC analysis) ca. 50% of the cuticular
hydrocarbons and this appeared to be a C-25 hydrocarbon with
a molecular ion m/z 348 (C25H48), confirmed by GCMS-CI
analysis. A minor component was the bis-homologue C27H52
(M+, 376). On this basis, both incorporated two degrees of
unsaturation.
A methoxymercuration–reductive demercuration (NaBH4)
protocol was conducted to locate the sites of unsaturation, with
the resulting methyl ether groups guiding the mass spectral
fragmentations.3 This reaction provided a mixture of several
mono- and bis-methyl ethers with the two most abundant bis-
methyl ethers exhibiting identical mass spectra. These were
deduced to be stereoisomers of 9,11-dimethoxypentacosane 1
on the basis of their fragmentations (Scheme 1). Two further
components were similarly deduced to be mixtures of stereo-
isomers of 11-methoxypentacos-9-ene 2 and 9-methoxypenta-
cos-10-ene 3, resulting from initial mono-methoxymercuration
and then reduction. This indicated that the C-25 hydrocarbon
was D9,10-pentacosadiene 4 (Scheme 1). GC-MS analyses of the
methyl ethers resulting from NaBD4 reduction support these
conclusions. In the case of bis-ethers 1, two deuterium atoms
were incorporated (e.g. M-15 at m/z 399 from m/z 397) but the
ions at m/z 241 and 157 were unchanged. This confirms that
both Hg atoms became attached to the central carbon of the
allene (i.e. C-10), so that on reduction 10-2H2-1 was formed.
These conclusions were supported by diagnostic ions at m/z
166 and m/z 250 (for the C-25 compound) and m/z 166 and m/z
278 (for the C-27 compound), attributable to McLafferty
rearrangement ions, known to appear in the mass spectra of
acyclic allenes4 (Scheme 2).
The major component was partially purified (HPLC:silica–
1
hexane, ca. 0.3 mg). The H NMR signal at d 5.04 (br, m)
correlated (HSQC) with the 13C NMR signal at 90.8 ppm,
(representing both sp2-carbons), both signals being consistent
with the allene structure. The signal for the central carbon of the
allene group (expected ~ 200 ppm) was not discernible from
this dilute sample.5
GCMS comparisons established that the C-25 allene 4 was
also a minor cuticular component in two other sugarcane
beetles, Lepidiota negatoria and Dermolepida albohirtum.
Extracts from D. albohirtum and L. picticollis contained as a
major component an allene with M+ 320 (C23H44) and
diagnostic ions at m/z 166 and m/z 222, again indicating 9,10-di-
unsaturation. The D9,10-tricosadiene structure 6 was proposed
for this constituent. When co-injected with a set of standard
alkanes, the allenes 4, 5 and 6 eluted just prior to the
corresponding alkane and have equivalent chain lengths of 24.9,
26.9 and 22.9, respectively.
To confirm the identity of these three components, syntheses
of 4, 5 and 6 were then undertaken and a route was developed
for delivery of both racemic and enantiomerically enriched
systems. Key steps utilised were the ‘acetylene zipper’
n
reaction,6 regioselective addition of Bu3SnH to a propargylic
alcohol, and anti elimination of nBu3SnOMs (or nBu3SnOAc)7
to install the propa-1,2-diene unit. This approach is shown in
Scheme 3 for D9,10-pentacosadiene 4.
Similar approaches furnished the racemic D9,10-tricosadiene
6 and D9,10-heptacosadiene 5. GC-MS comparisons, including
co-elution studies, established that the natural components were
Scheme 1 Reagents and conditions: i, Hg(OAc)2–MeOH then NaBH4; ii, as
for i (double addition/reduction).
† Electronic supplementary information (ESI) available: analytical data for
4, 5, 6 and 9, and proposed McLafferty rearrangements for 4. See http:
//www.rsc.org/suppdata/cc/b1/b101801i/
Scheme 2
DOI: 10.1039/b101801i
Chem. Commun., 2001, 885–886
This journal is © The Royal Society of Chemistry 2001
885

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An analogous approach with (S)-tricosa-10-yn-9-ol (80% ee)
provided predominantly (R)-D9,10-tricosadiene (77% ee, 36%
from the alcohol). Natural D9,10-tricosadiene was then shown to
be (R)-configured with 86% ee. Given the consistency of the
location of the D9,10-propadiene unit, a predominating (R)-
chirality is likely for D9,10-heptacosadiene, as well.
Examination of males of a further cane beetle species, L.
9,10
crinita has revealed the presence of the higher allene, D
-
hentriacontane, C31H60 (M+ = 432 and McLafferty ions at m/z
334 and 166). A lower level component is D9,10-nonacosadiene,
the C-29 allene (M+ = 404, m/z 306 and 166). Co-occurring
with the dominant C-25 allene in A. consanguineus, are much
lower levels of the even carbon numbered D9,10-tetracosadiene
(M+ 334, m/z 246, 166) and D9,10-hexacosadiene (M+ 362, m/z
264, 166), in the former case confirmed by synthesis.
Scheme 3 Reagents and conditions: i, nBuLi, THF 278 °C/240 °C then
nC9H19Br, HMPA (97%); ii, KH, DAP (79%); iii, nBuLi, THF 278 °C/
240 °C then nC8H17CHO (51%); iv, nBu3SnH, AIBN 90 °C; v, MsCl, Et3N
(60% from alkynol).
6, 4 and 5. In the case of the synthesised D9,10-pentacosadiene
4, its 1H and 13C NMR spectra matched precisely the spectra of
the natural component.
All allenes identified have D9,10-unsaturation, and those odd
numbered in carbon are considerably favoured. Administration
of labelled potential precursors is being conducted to reveal
possible routes to these hydrocarbons. As far as we are aware,
allenic hydrocarbons were previously unknown from insects,
although the male-produced pheromone of the dried bean beetle
(Acanthoscelides obtectus) incorporates an allene moiety.14 The
biological role of these compounds is being investigated.
The authors are grateful to the Australian Research Council
(SPIRT award) for support, to Professor R. W. Rickards for
valuable suggestions, and to David Logan (BSES, Ayr,
Australia) for provision of D. albohirtum specimens.
The chirality of the natural allenes was then addressed. The
enantiomers of (±)-D9,10-tricosadiene 6 and (±)-D9,10-pentaco-
sadiene 4 were base-line separated on a heptakis(6-O-tert-
butyldimethylsilyl-2,3-di-O-methyl) b-cyclodextrin column,
after ca. 160 min at 155 °C and ca. 200 min at 165 °C
respectively. The natural D9,10-pentacosadiene (from A. con-
sanguineus) was of high ee, and co-eluted with the first eluting
9,10
enantiomer of the racemate. This applied also to D
-
tricosadiene from D. albohirtum. The present allenes provide
remarkable examples of the separation efficacy of these
modified cyclodextrin phases.8
Nonracemic allenes of known predominating chirality were
required next. The C-25 propargylic alcohol 7 (Scheme 4) was
first oxidised to the ketone, and reduction with Me-CBS9
afforded the (S)-alcohol 8 (80% ee, Mosher’s ester analyses).10
The predominant configuration was confirmed by converting
the alcohol into decane-1,2-diol, whose rotation ([a]D 27°; c,
0.12, MeOH) may be compared with that for the authentic (S)-
isomer, ([a]D 211.9°; c, 0.43, MeOH).11 The (S)-propargylic
alcohol 8 was then hydrostannylated, acetylated and treated
with TBAF in DMSO to effect anti-elimination of nBu3-
SnOAc12 and give the (R)-enantiomer, 9 ([a]D 217° (c, 0.54,
CHCl3)13 in 72% ee (enantioselective gas chromatography).
Co-injection studies established that natural D9,10-pentacosa-
diene in A. consanguineus was predominantly the (R)-enantio-
mer (89% ee).
Notes and references
1 For a useful review see L. N. Robertson, P. G. Allsopp, K. J. Chandler
and R. T. Mullins, Aust. J. Agric. Res., 1995, 46, 1.
2 See, for example, A. G. Bagnères, J. L. Clément, M. S. Blum, R. F.
Severson, C. Joulie and C. Lange, J. Chem. Ecol., 1990, 16, 3213.
3 D. R. Nelson, C. L. Flatland and J. E. Baker, Insect Biochem., 1984, 14,
435, and references therein.
4 J. R. Wiersig, A. M. H. Yeo and C. Djerassi, J. Am. Chem. Soc., 1977,
99, 532.
5 For discussions of NMR spectra of allenes see: (a) D. H. Williams and
I. Fleming, Spectroscopic Methods in Organic Chemistry, McGraw-
Hill, London, 5th edn., 1995, pp. 157 and 163; (b) R. Steur, M. J. A. van
Dongen, W. Dreuth, J. W. de Hann and L. J. M. van de Ven,
Tetrahedron Lett., 1971, 3307.
6 (a) C. A. Brown and A. Yamashita, J. Am. Chem. Soc., 1974, 97, 891;
(b) J. C. Lindhort, G. L. van Mourik and H. J. J. Pabon, Tetrahedron
Lett., 1976, 2565.
7 Y. Araki and T. Konoike, Yuki Gosei Kagaku Kyokaishi, 2000, 58(10),
956. (Chem Abs., 133:321484).
8 J. Pietruzka, D. H. Hochmuth, B. Gehrcke, D. Icheln, T. Runge and
W. A. Koenig, Tetrahedron: Asymmetry, 1992, 661.
9 E. J. Corey and C. J. Helal, Angew. Chem., Int. Ed., 1998, 1986.
10 G. R. Sullivan, J. A. Dale and H. S. Mosher, J. Org. Chem., 1973, 38,
2143.
11 Y. Masooka, S. Masayuki and K. Mori, Agric. Biol. Chem., 1982, 46,
2319.
12 Y. Araki and T. Konoike, Tetrahedron Lett., 1992, 33, 5093.
13 Laevo-rotatory, non-cyclic, 1,3-dialkylallenes have the (R)-configura-
tion. See for example, D. J Pasto and K. D. Sugi, J. Org. Chem., 1991,
56, 4157, and references therein.
14 K. Mori, in The Total Synthesis of Natural Products, ed. J. Apsimon,
John Wiley and Sons, New York, 1992, vol. 9, pp. 212–215.
Scheme 4 Reagents and conditions: i, TPAP, NMO (82%); ii, (S)-MeCBS
(2.5 equiv.) BH3·SMe2 (5 equiv.) 230 °C 1 h (70%); iii, nBu3SnH AIBN
90 °C neat; iv, Ac2O, pyridine (14% from alkynol); v, TBAF, DMSO reflux
(77%).
886
Chem. Commun., 2001, 885–886