Stereoselective synthesis of trans-fused iridoid lactones and their identification in the parasitoid wasp Alloxysta victrix, Part I: Dihydronepetalactones

Article abstract of DOI:10.3762/bjoc.8.140

Starting from the enantiomers of limonene, all eight stereoisomers of trans-fused dihydronepetalactones were synthesized. Key compounds were pure stereoisomers of 1-acetoxymethyl-2-methyl-5-(2-hydroxy-1-methylethyl)-1- cyclopentene. The stereogenic center of limonene was retained at position 4a of the target compounds and used to stereoselectively control the introduction of the other chiral centers during the synthesis. Basically, this approach could also be used for the synthesis of enantiomerically pure trans-fused iridomyrmecins. Using synthetic reference samples, the combination of enantioselective gas chromatography and mass spectrometry revealed that volatiles released by the endohyperparasitoid wasp Alloxysta victrix contain the enantiomerically pure trans-fused (4R,4aR,7R,7aS)-dihydronepetalactone as a minor component, showing an unusual (R)-configured stereogenic center at position 7.

Full text of DOI:10.3762/bjoc.8.140

Stereoselective synthesis of trans-fused iridoid
lactones and their identification in the parasitoid
wasp Alloxysta victrix, Part I: Dihydronepetalactones
Nicole Zimmermann, Robert Hilgraf, Lutz Lehmann, Daniel Ibarra
and Wittko Francke*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1246–1255.
Department of Chemistry - Organic Chemistry, University of Hamburg,
Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
doi:10.3762/bjoc.8.140
Received: 16 March 2012
Accepted: 20 July 2012
Published: 07 August 2012
Email:
Wittko Francke* - francke@chemie.uni-hamburg.de
* Corresponding author
This article is part of the Thematic Series "Biosynthesis and function of
secondary metabolites". Part II [1] describes the synthesis of
enantiomerically pure trans-fused iridomyrmecins by this approach.
Keywords:
Alloxysta victrix; identification; iridoid; stereoselective synthesis;
trans-fused dihydronepetalactone
Guest Editor: J. S. Dickschat
© 2012 Zimmermann et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Starting from the enantiomers of limonene, all eight stereoisomers of trans-fused dihydronepetalactones were synthesized. Key
compounds were pure stereoisomers of 1-acetoxymethyl-2-methyl-5-(2-hydroxy-1-methylethyl)-1-cyclopentene. The stereogenic
center of limonene was retained at position 4a of the target compounds and used to stereoselectively control the introduction of the
other chiral centers during the synthesis. Basically, this approach could also be used for the synthesis of enantiomerically pure
trans-fused iridomyrmecins. Using synthetic reference samples, the combination of enantioselective gas chromatography and mass
spectrometry revealed that volatiles released by the endohyperparasitoid wasp Alloxysta victrix contain the enantiomerically pure
trans-fused (4R,4aR,7R,7aS)-dihydronepetalactone as a minor component, showing an unusual (R)-configured stereogenic center at
position 7.
Introduction
The endohyperparasitoid wasp Alloxysta victrix is part of the chemicals of the lower trophic levels such as oat plant and grain
tetratrophic system of oat plants (Avena sativa), grain aphids aphid have been identified [2-4]. However, there is nearly no
(Sitobion avenae), primary parasitoids (Aphidius uzbekistan- information about the intra- and interspecific signaling path-
icus) and hyperparasitoids (Alloxysta victrix). Chemical com- ways between primary parasitoids and hyperparasitoids. In
munication by volatile signals is considered to play a major role order to gain further information about the chemical structures
in interactions between these trophic levels, and some semio- and the biological significance of corresponding signals, we
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examined the volatile components of pentane extracts from abundant (80%). The most striking differences in the spectra
dissected heads as well as headspace volatiles of Alloxysta were pronounced signals for the molecular ion M+ = 168 and
victrix by coupled gas chromatography/mass spectrometry M+ − 15 (70% and 40%, respectively) for the cis-fused
(GC/MS). Figure 1 shows one major component and several dihydronepetalactone whilst in the spectrum of X the two
trace components which could be identified as 6-methyl-5- signals were of only low abundance (Figure 2).
hepten-2-one (1), neral (2), geranial (3), actinidine (4), and
geranylacetone (5). Bioassays revealed the main compound 1 to
be repellent to the aphid-parasitoid, Aphidius, by warning the
primary parasitoid of the presence of the hyperparasitoid [5].
The prenyl-homologue of 1, geranylacetone (5), seems to be a
component of the sex pheromone of Alloxysta victrix [6]. In
addition, GC/MS of cephalic secretions of both sexes showed a
minor component X, the mass spectrum of which suggested it to
be a trans-fused dihydronepetalactone. Since no synthetic refer-
ence compounds were available, we had to synthesize all eight
trans-fused dihydronepetalactones to unambiguously identify
the natural product X. The realization of this task is the subject
of the present paper.
Figure 2: 70 eV EI-mass spectrum of the iridoid X, a component of the
volatile secretions of the parasitoid wasp Alloxysta victrix.
Dihydronepetalactones are derivatives of nepetalactone (7)
which was first isolated by Mc Elvain in 1941 from the essen-
tial oil of catmint, Nepeta cataria (Figure 1) [9]. Relative
configurations of cis-fused nepetalactones and some related
derivatives have been investigated [10,11]. Nepetalactone and
cis- as well as trans-fused dihydronepetalactones have been
isolated from the leaves and galls of the plant Actinidia
polygama [8]. In addition, dihydronepetalactones are compo-
nents of the defensive secretions of some ant species [12], while
Figure 1: Terpenoids 1–5 present in Alloxysta victrix and cis-fused
bicyclic iridoids known from other insects (6–8).
nepetalactone and the corresponding lactol showing (1R)-con-
figuration have been identified as pheromones of aphids
[13,14]. (1R,4S,4aR,7S,7aR)-Dihydronepetalactol (8) was char-
Results and Discussion
Apart from a couple of known acyclic terpenoids (Figure 1),
acterized as a semiochemical for lacewings [15].
analysis by gas chromatography coupled with mass spectrom- The dihydronepetalactone skeleton shows four contiguous
etry (GC/MS) revealed the presence of an unknown minor stereogenic centers, giving rise to eight trans-fused stereoiso-
component X in both sexes of Alloxysta victrix. Chemical mers a–d and the corresponding enantiomers a'–d' (Figure 3).
ionization analysis (GC/CIMS) showed the molecular mass of
the compound to be M+ = 168, while high resolution mass spec- Whilst several stereoselective syntheses of the relatively wide-
trometry (GC/HRMS) proved its atomic composition to be spread and well known cis-fused nepetalactone and its dihydro
C10H16O2, suggesting an oxygenated monoterpene as the target derivatives have been carried out [16-19], only very few
structure. The fragmentation pattern, exhibited in the 70 eV approaches specifically aiming at the synthesis of trans-fused
EI-mass spectrum (Figure 2), showed some similarities to that iridoid lactones have been published. Starting from (S)-
of the known cis-fused dihydronepetalactone (6) [7], however, pulegone (9) or its enantiomer, Wolinsky [20,21] described a
differences in relative abundances of fragment ions pointed to a route to this group of iridoids that can be applied to synthesize
trans-fused dihydronepetalactone as the target structure [8]. In pure stereoisomers of dihydronepetalactones as well as the
the mass spectrum of the cis-fused compound m/z 67 and m/z 95 structurally related iridomyrmecins, another class of iridoids.
were of similarly low intensity (30%), while in that of the However, Wolinsky’s method suffers from several major disad-
unknown natural product X the two fragments were highly vantages such as high costs of (S)-pulegone and difficult separa-
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selective addition of hydrochloric acid afforded the chloride 11,
and subsequent elimination of hydrochloric acid gave a mixture
of the methyl esters 12 and 13 (methyl trans-pulegenate) [20-
22] which could be separated by chromatography on silica gel.
Hydroboration and lactonization of 12 furnished a mixture of
the C4-epimers a and b that once again needed to be separated
by chromatography on silica gel [23].
Analytical data of the first eluting component a were in accor-
dance with those reported in the literature [24]. The same
sequence starting from (R)-pulegone yielded a mixture of
diastereomers a' and b'. The relative configuration of a at C4
was assigned according to NOESY experiments. Decisive
NO-effects were found between the protons 4-H and 7a-H as
well as between 7a-H and 7-CH3 (Figure 4).
Figure 3: Structures and gas chromatographic retention times of trans-
fused dihydronepetalactones on a conventional FFAP column (FFAP)
and on an enantioselective cyclodextrin column (cyclo). For experi-
mental details see Supporting Information File 1. The racemates b/b'
and d/d', which coeluted on FFAP, could be separated from each other
on DB5 (data not shown).
tions of diastereomeric mixtures. Therefore, as an alternative,
we present a novel stereoselective route to trans-fused dihy-
dronepetalactones starting from pure, cheaply available enan-
tiomers of limonene.
Route to trans-fused dihydronepetalactones
a and b starting from (S)-pulegone
Figure 4: Configuration of the dihydronepetalactone a.
For comparison, the synthesis of a and b was carried out
following Wolinsky’s approach: (S)-Pulegone (9) was trans- Basically, the sequence developed by Wolinsky could also
formed to trans-pulegenic acid 10 via bromination, Favorskii provide access to the diastereomers c and d (and their enan-
rearrangement, and subsequent elimination (Scheme 1). Stereo- tiomers) if trans-pulegenic acid (10) would be replaced by cis-
Scheme 1: Route from (S)-pulegone to the mixture of dihydronepetalactones a and b, consequently following Wolinsky's approach [23]. Reaction
conditions: a) Br2, HOAc, 0 °C; b) KOH, H2O, reflux (15%, 2 steps); c) MeOH/HCl, rt, 96 h (82%); d) 2,6-lutidine, reflux, 72 h (75% of 12 + 13);
e) BH3·SMe2, 0 °C, THF, NaOH, H2O2 (86% mixture of diastereomers); f) p-TsOH, toluene, reflux (17% of a + b).
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pulegenic acid. A mixture of the latter and its trans-isomer sition state. We envisioned that stereoselective hydrogenation of
(60:40) can be obtained by using a different base in the the endocyclic double bond of the key intermediate 16 (and its
Favorskii-rearrangement step [22], again requiring a difficult diastereomer 16*) in either a “syn”- or “anti”-fashion could
chromatographic separation. Furthermore, this multistep route yield two pairs of diastereomeric hydroxy carboxylic acids
has several major disadvantages: The formation of mixtures of 17/17* or 18/18* after some simple functional group transfor-
epimers entails to separations at several stages which have mations. These hydroxy acids would then yield the desired
proven to be problematic. Moreover, several reaction steps trans-fused dihydronepetalactones a–d during a final lactoniza-
afford unsatisfactory yields [23]. In addition, one of the main tion step.
disadvantages is the fact that (S)-pulegone (S-9) is a highly
expensive starting material for the synthesis of four of the eight Starting from cheap and pure (S)-limonene (14'), the corres-
trans-fused dihydronepetalactones. That excludes this route for ponding trans-fused dihydronepetalactones a'–d' could be
the synthesis of larger amounts.
synthesized in the same way, showing our novel route to be a
versatile alternative to Wolinsky’s sequence [20,21]. In contrast
to the latter, which fixed the stereogenic center of pulegone at
position 7 of the final dihydronepetalactone, in our route the
Route to stereochemically pure trans-fused
dihydronepetalactones from (R)-limonene
Due to the shortcomings of the route described above, we stereogenic center of limonene is retained at position 4a of the
designed an improved strategy towards trans-fused dihy- target compound and used for the stereoselective introduction of
dronepetalactones. Starting from 1-formyl-2-methyl-5-(1- additional chiral centers.
methylethenyl)-1-cyclopentene (15) as the key intermediate, the
stereoselective synthesis of all eight stereoisomers could be Synthesis of the key intermediate 16
achieved (Figure 5). The aldehyde 15 could be readily prepared The synthesis of the key intermediate 16 – which shows two
from commercially available pure and cheap (R)-limonene (14) differentiated primary alcohol functions – started from enan-
[25-27]. Non-selective hydroboration of the double bond in the tiomerically pure (R)-limonene (14, Scheme 2). Ozonolysis fol-
side chain of 15 would yield a pair of diastereomers 16/16* lowed by reductive workup with dimethyl sulfide produced
which would have to be separated. However, we expected that (3R)-3-(1-methylethenyl-6-oxoheptanal), which yielded the
the chiral center at C5 would cause stereocontrol by forcing the formyl cyclopentene 15 upon intramolecular aldol conden-
reaction to proceed through the sterically least hindered tran- sation [25-27]. Subsequently, the aldehyde 15 was reduced to
Figure 5: Route to stereochemically pure trans-fused dihydronepetalactones a–d from (R)-limonene.
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Beilstein J. Org. Chem. 2012, 8, 1246–1255.
the allylic alcohol 19 with LiAlH4 and converted into the due to enolization of the hydrogenation product, might allow
acetate 20 [28]. Hydroboration of 20 using disiamylborane this formal “anti”-addition of hydrogen, the key intermediate 16
proceeded with high stereoselectivity affording 16 as a single was transformed to the aldehyde 23.
stereoisomer [17,28]. Similar results of highly stereoselective
hydroborations of structurally related chiral cyclopentene In the course of this short sequence, the free hydroxy group of
derivatives have been reported [20,21].
16 was protected as the TBDMS ether to yield 21 which
afforded the mono-protected diol 22 after treatment with KOH
in methanol. Subsequently, 22 was oxidized with pyridinium
dichromate to give aldehyde 23.
We expected that catalytic hydrogenation of the trisubstituted
cylopentene 23 with a heterogeneous catalyst would preferen-
tially take place from the sterically less hindered side of the
molecule. This would lead to an all-cis configured hydrogena-
tion product which would endure considerable steric strain. Due
to the CH-acidity at the α-position of the formyl group, epimer-
ization of the all-trans product 24 under acidic or basic condi-
tions could be expected. Lange et al. reported the catalytic
hydrogenation of a structurally close analogue, (5R)-1-formyl-
2-methyl-5-isopropylcyclopent-1-ene, over Pd/C (10%) to give
a 9:1 mixture of the all-cis versus the all-trans product [29]. In
our case, the application of Lange’s method to the aldehyde 23
led to the formation of a 3:1 mixture of the all-cis versus the all-
trans epimer. Subsequent treatment with sodium methoxide in
Scheme 2: Synthesis of the key compound 16. Reaction conditions:
a) O3, MeOH, −50 °C (86%); b) AcOH, piperidine, C6H6, reflux (85%);
c) LiAlH4, Et2O (82%); d) Ac2O, pyridine, rt (91%); e) B2H6, 2-methyl-
2-butene, 0 °C, THF, NaOH, H2O2 (66%).
Synthesis of trans,trans-dihydronepeta-
lactone b
To install a trans,trans-configuration between the substituents MeOH at rt for 20 h completely shifted the equilibrium to the
at C5-C1 and C1-C2 of the cyclopentane backbone – which thermodynamically more stable all-trans product 24. Unfortu-
would later reflect the trans,trans relationship between nately, these results could not be reproduced on larger reaction
substituents at C7-C7a and at C7a-C4a of the dihydronepetalac- scales (>5 mmol). After screening of a variety of other hydro-
tones a and b – a formal “anti”-addition of hydrogen to the genation conditions, we found the hydrogenation of 23 with am-
cyclopentene 16 had to be carried out (Scheme 3). Usually, both monium formate over palladium on carbon (10%) to be the
homogeneous and heterogeneous catalytic hydrogenation reac- method of choice [30]. Using this approach, the all-trans alde-
tions proceed via “syn”-addition of hydrogen to olefinic double hyde 24 was almost exclusively formed. The presence of am-
bonds. Only subsequent isomerization processes may lead to a monium formate in the reaction mixture probably leads to an
formal “anti”-addition. To obtain a suitable precursor, which, “in-situ” epimerization at C2 from the kinetically formed all-cis
Scheme 3: Synthesis of trans,trans-substituted dihydronepetalactone b. Reaction conditions: a) TBDMSCl, imidazole, DMF, 0 °C (99%); b) KOH,
MeOH, 0 °C (87%); c) PDC, MS, CH2Cl2, 0 °C to rt (61%); d) ammonium formate, Pd/C, MeOH, reflux (48%); e) KMnO4, t-BuOH, pH 4.5, 0 °C (22%);
f) TBAF, THF (100%); g) DCC, DMAP, CH2Cl2 (66%).
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to the thermodynamically more stable all-trans product. Rela- The relative configuration of the trans,trans-dihydronepeta-
tive configurations of all substituents of compound 24 were lactone b was confirmed by NOESY experiments. Decisive
confirmed by NOESY experiments (Figure 6). Strong NO-effects could be observed between 4-CH3 and 7a-H as well
NO-effects were found between the protons 5-CH3 and 1-H, as between 4-CH3 and 5-Ha, and furthermore, between 5-Ha
1-H and 1’H (the proton at C1 of the side chain), 5-H and 2-H and 7a-H as well as between 7a-H and 7-CH3 (Figure 6). The
as well as between protons of 1-CHO and 2-H which is in line enantiomer b' was synthesized via the same route starting from
with a trans,trans-configuration (using the nomenclature (S)-limonene. Analytical data of compound b' were identical to
described above).
those which were obtained of b when Wolinky’s route was fol-
lowed (see above).
Starting from 24, the trans,trans-dihydronepetalactone b was
Synthesis of cis,trans-fused dihydronepeta-
synthesized in three subsequent steps (Scheme 3). First, oxi-
dation of the aldehyde group with potassium permanganate in lactone c
the presence of a phosphate buffer (pH 4.5) afforded the For the synthesis of the cis,trans dihydronepetalactone c, a
carboxylic acid 25 without epimerization at C1 [23,31]. Subse- cis,trans-configuration between the substituents at C5-C1 and
quent deprotection of the TBDMS ether with tetrabutylammo- C1-C2 of the cyclopentane backbone needed to be established.
nium fluoride (TBAF) yielded 17, and lactonization with N,N- With acetate 16 as the key intermediate, a stereoselective “syn”-
dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyri- addition of hydrogen from the same side as the (R)-configured
dine (DMAP) in dichloromethane afforded the trans,trans- side chain at C5 would provide the desired stereochemical
dihydronepetalactone b.
outcome of the hydrogenation reaction (Scheme 4). We
Figure 6: Configurations of compound 24 and the dihydronepetalactone b.
Scheme 4: Synthesis of cis,trans-substituted dihydronepetalactone c. Reaction conditions: a) Crabtree's catalyst [Ir(cod)PCy3(py)]PF6 (11 mol %),
1 bar H2, CH2Cl2, rt (81%); b) TBDMSCl, imidazole, DMF, 0 °C (100%); c) KOH, MeOH, rt (93%); d) RuCl3·3H2O (2 mol %), NaIO4, CCl4, CH3CN,
phosphate buffer (pH 7), rt (81%); e) HF, CH3CN, rt (100%); f) DCC, DMAP, CH2Cl2 (66%).
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expected the free hydroxy group of 16 to coordinate to an (pH 7) produced the carboxylic acid 29 without epimerization at
appropriate homogenous hydrogenation catalyst, controlling the C1. After removal of the TBDMS protecting group with HF in
stereochemical course of the hydrogen transfer from the same acetonitrile, the hydroxy acid 18 was lactonized in the presence
side as the side chain at C5 through chelation. We chose Crab- of DCC and catalytic amounts of DMAP in dichloromethane at
tree’s iridium catalyst ([Ir(cod)PCy3(py)]PF6) which has been rt to afford cis,trans-dihydronepetalactone c. Its enantiomer c’
reported to furnish excellent facial selectivities during directed was synthesized from enantiomerically pure (S)-limonene,
hydrogenations of cyclic olefins [32-34].
following the same route. The relative configuration of c was
confirmed by NOESY experiments (Figure 7). Decisive
Hydrogenation of acetate 16 in the presence of 11 mol % of NO-effects could be observed between 4a-H and 7-CH3 as well
Crabtree’s catalyst under 1 bar hydrogen pressure for 1.5 h as between 4-CH3 and 7a-H, and furthermore, between 4-CH3
yielded the desired product 26 as a single diastereomer. Alter- and 5-Ha as well as between 5-Ha and 7a-H.
native hydrogenation methods using optically active catalysts
Synthesis of a mixture of trans-fused
failed. In one case we investigated the hydrogenation of the
endocylic double bond of the allylic alcohol 19 (Scheme 2) with dihydronepetalactones c and d
one of Noyori’s ruthenium BINAP catalysts ([Ru((S)- The stereogenic center at C1’ of the acetate 26 keeps (R)-con-
BINAP)](OAc)2) [35,36] but reduction occurred only at the side figuration which resulted from highly stereoselective hydrobo-
chain.
ration of the acetate 20 to yield the key intermediate 16 as
shown above (Scheme 2). For the synthesis of the cis,trans-
Relative configurations of all substituents of the acetate 26 were dihydronepetalactone d, this stereocenter needed to be isomer-
confirmed by NOESY experiments (Figure 7). Strong ized to keep the (4S)-configuration in the final product
NO-effects were observed between the protons 5-CH3 and 1’-H (Scheme 5). To achieve the required inversion, the acetate 26
(protons of the acetoxymethyl group at C1), 5-CH3 and 2-H, was oxidized to the aldehyde 30, which could be epimerized
1’-H and 2-H as well as 5-H and 1’’-CH3 (protons of the methyl using p-toluenesulfonic acid in benzene under reflux conditions
group at C1 of the side chain) which is in line with a cis,trans- to provide a 2:3 mixture of the desired aldehydes 30 and its
configuration (using the nomenclature described above).
epimer 30*. Subsequent steps were carried out with the mixture
of diastereomers. Reaction of 30/30* with sodium borohydride
Starting from 26, the synthesis of the cis,trans-dihydronepeta- at −20 °C reduced the aldehyde function to yield a mixture of
lactone c was completed in five subsequent steps (Scheme 4). the diastereomers 26/26*. The following sequence, yielding a
First, the free hydroxy group of 26 was protected as the mixture of the dihydronepetalactones c and d was essentially
TBDMS ether to yield 27. Then, the acetate group was removed the same as described above (Scheme 4).
with methanolic KOH to afford the alcohol 28. Careful oxi-
dation of the primary alcohol function [37,38] with Transformation of the free hydroxy group to the TBDMS ethers
ruthenium(III) chloride and sodium periodate in a biphasic mix- 27/27* was followed by cleavage of the acetate moiety with
ture of carbon tetrachloride, acetonitrile, and phosphate buffer methanolic KOH to give a mixture of the alcohols 28/28*. Oxi-
Figure 7: Configurations of compound 26 and the dihydronepetalactone c.
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Scheme 5: Synthesis of a 2:3 mixture of dihydronepetalactones c and d. Reaction conditions: a) (COCl)2, DMSO, CH2Cl2, −70 °C to 0 °C (71%);
b) p-TsOH, benzene, reflux (96%); c) NaBH4, MeOH, −20 °C; d) TBDMSCl, imidazole, DMF, 0 °C (78%) (over two steps); e) KOH, MeOH, rt (94%);
f) RuCl3·3H2O (2 mol %), NaIO4, CCl4, CH3CN, phosphate buffer (pH 7), rt (69%); g) HF, CH3CN, rt (98%); h) DCC, DMAP, CH2Cl2 (62%).
Formal synthesis of a mixture of trans-fused
dation of the primary alcohol function with ruthenium(III) chlo-
dihydronepetalactones a and b from
(R)-limonene
ride and sodium periodate in a biphasic mixture of carbon tetra-
chloride, acetonitrile and phosphate buffer (pH 7) afforded the
carboxylic acids 29/29* without epimerization at C1 [37,38].
After cleavage of the TBDMS ether with HF in acetonitrile, a
mixture of dihydronepetalactones c and d was formed by
lactonization of the hydroxy acids 18/18* with DCC and
DMAP in dichloromethane at rt. The C4 epimeric dihy-
dronepetalactones c and d could be separated by column chro-
matography over silica. Starting from the enantiomer of 26, a
mixture of dihydronepetalactones c' and d' was synthesized by
following the same reaction sequence.
As outlined above, six of the eight possible stereoisomers of
trans-fused dihydronepetalatones were synthesized from the
enantiomers of limonene following a new route. Compound a
and its enantiomer a' were prepared according to the procedure
described by Wolinsky [20,21]. However, our new approach
also includes a formal synthesis of a and a'. A mixture of
a and b will be easily obtained from the protected hydroxy
aldehyde 24 by the straight forward procedure outlined in
Scheme 6.
Scheme 6: Formal synthesis of a mixture of dihydronepetalactones a and b from (R)-limonene.
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Reduction of the aldehyde function of 24 and acetylation of the which was well separated from its enantiomer by an α-value of
resulting primary alcohol followed by cleavage of the silyl a':a = 1.01 (Figure 3). Consequently, the structure of X was
group will furnish the primary alcohol 31, which upon oxi- unambiguously assigned to be (4R,4aR,7R,7aS)-dihydronepeta-
dation will yield the corresponding aldehyde that can be lactone. It should be noted that Meinwald et al. identified a/a'
epimerized to the diastereomers 32/32* as shown above. Subse- (absolute configuration not assigned) as a component of secre-
quent reduction of 32/32*, silylation of the resulting primary tions of the abdominal defense glands of the rove beetle
alcohols and saponification will produce a mixture of the dia- Creophilus maxillosus [39]. Interestingly, the structure of a' is
stereoisomers 33/33*. Oxidation of the primary alcohol moiety, relatively close to that of nepetalactone 7 and lactol 8, the sex
followed by cleavage of the silyl group will yield the epimeric pheromone of the grain aphid S. avenae [13] which keeps the
hydroxy acids 34/34* which will form a mixture of the second level in the investigated tetratrophic system. Grant et al.,
dihydronepetalactones a and b after lactonization. As shown found the trans-fused (1R,4aS,7R,7aR)-1-methoxy-4,7-
above, this mixture can be separated upon column chromatog- dimethyl-1,4a,5,6,7,7a)-hexahydrocyclopenta[c]pyran, called
raphy.
(1R)-1-methoxymyodesert-3-ene, among the volatiles of the
Ellangowan poison bush, which they transformed to the corres-
ponding lactone a' [24]. Apart from this compound and very
Conclusion
In summary, we synthesized all eight trans-fused stereoiso- few others, the stereogenic center carrying the methyl group in
meric dihydronepetalactones. After having used the enan- the five-membered ring of iridoid lactones including insect
tiomers of pulegone as educts in Wolinsky’s route to semiochemicals [13-15] generally shows (S)-configuration.
(4S,4aS,7S,7aR)-dihydronepetalactone (a) and its enantiomer a' Only recently, two isomeric iridoid lactones showing (7R)-con-
[23], we developed an improved and general way for the syn- figuration have been identified from the Drosophila parasitoid
thesis of all trans-fused dihydronepetalactones, starting from Leptopilina heterotoma [40]. Compound X has been identified
pure enantiomers of limonene. Our approach is also superior to in the mandibular gland secretions of other Alloxysta species,
that starting from optically active carvone that yields the too, [41]. However, its biological significance is not yet clear
starting material for the synthesis of trans-fused iridoid lactones and will need further investigations.
only as a byproduct [15].
The differentiation of the oxygen containing functional groups
in the trisubstituted cyclopentene 16, a key-compound in our
Identification of a trans-fused
synthetic approach, provides access to a large number of
dihydronepetalactone in the parasitoid
iridoids including nepetalactones but also iridomyrmecins and
wasp Alloxysta victrix
monocyclic compounds. Consequently, having reference com-
Upon gas chromatography using FFAP as a polar achiral pounds at hand, structures of hitherto unknown iridoids [42]
stationary phase, the stereoisomers a and c could be well sep- may now be assigned. It may turn out that the chiral center
arated while b and d coeluted. However, the latter pair could be carrying the methyl group in the five-membered ring of iridoids
resolved on a less polar DB5-capillary, where b/b' eluted after may much more often show (R)-configuration than it is known
d/d' (data not shown). As a result, the relative configuration of today.
each of the trans-fused dihydronepetalactones could be unam-
biguously assigned by GC/MS.
Supporting Information
With the exception of (4S,4aS,7R,7aR)-dihydronepetalactone
Supporting Information File 1
(d) and its enantiomer d', the stereoisomers could well be
distinguished by enantioselective gas chromatography using a
1:1-mixture of OV1701 and heptakis-(6-O-tert-butyl-
dimethylsilyl-2,3-di-O-methyl)-β-cyclodextrin as an optically
active stationary phase. Figure 3 shows the corresponding reten-
tion times of all eight stereoisomers that were obtained with the
two used capillary column systems. Coupled GC/MS using
FFAP as the stationary phase revealed the target natural iridoid
lactone X to show the same mass spectrum and the same reten-
tion time as a/a', the first eluting pair of the synthetic dihy-
dronepetalactones (Figure 3). Enantioselective gas chromatog-
raphy on a cyclodextrin column showed X to coelute with a'
Experimental details and characterization data for
synthesized compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-140-S1.pdf]
Acknowledgements
The biological material was kindly provided by Prof.
Dr. Urs Wyss, University of Kiel, Germany. Financial
support by the German Science Foundation, Deutsche
Forschungsgemeinschaft (DFG – Fr 12/1) is gratefully
acknowledged.
1254

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