Viaticene A – An Unusual Tetraterpene Cuticular Lipid Isolated from the Springtail Hypogastrura viatica

Article abstract of DOI:10.1002/ejoc.201900224

The cuticles of springtails are extremely wear- and friction-resistant, super-hydrophobic, non-fouling, and self-cleaning. As such, the chemistry of the lipids covering these cuticles is of great interest as a model for biomimetic super-hydrophobic surfaces, although only few of these lipids have been structurally elucidated. Hypogastrura viatica, a surface-dwelling springtail, produces highly branched tetraterpene hydrocarbons with an unprecedented [6+2]-terpene connectivity as components of the epicuticular lipid layer. The structure of the major lipid component, viaticene A, was elucidated through isolation, spectroscopic analysis, chemical derivatization, synthesis, as well as stereochemical analysis of the core unit obtained from ozonolysis of the isolated lipid. Viaticenes A and B represent a new class of irregular tetraterpenoid natural products.

Full text of DOI:10.1002/ejoc.201900224

A Journal of
Accepted Article
Title: Viaticene A, an Unusual Tetraterpene Cuticular Lipid Isolated
from the Springtail Hypogastrura viatica
Authors: Jan Bello, Patrick Stamm, Hans Petter Leinaas, and Stefan
Schulz
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Viaticene A, an Unusual Tetraterpene Cuticular Lipid Isolated
from the Springtail Hypogastrura viatica
Jan E. Bello,[a] Patrick Stamm,[a] Hans Petter Leinaas,[b] and Stefan Schulz*[a]
Abstract: The cuticles of springtails are extremely wear- and friction-
resistant, super-hydrophobic, non-fouling, and self-cleaning. As such,
the chemistry of the lipids covering these cuticles is of great interest
as model for biomimetic super-hydrophobic surfaces, although only
few of these lipids have been structurally elucidated. Hypogastrura
viatica, a surface-dwelling springtail, produces highly branched
tetraterpene hydrocarbons with an unprecedented [6+2]-terpene
connectivity as components of the epicuticular lipid layer. The
structure of the major lipid component, viaticene A, was elucidated
through isolation, spectroscopic analysis, chemical derivatization,
synthesis, as well as stereochemical analysis of the core unit obtained
from ozonolysis of the isolated lipid. Viaticenes A and B represent a
new class of irregular tetraterpenoid natural products.
reproductive behavior, as well as development,[8] but it is not yet
known if springtails utilize cuticular compounds as
semiochemicals. For insects, which are closely related to
Collembola, cuticular lipids are well known to act as contact sex
pheromones inducing sexual behavior in many species at close
range.[9–11]
Hypogastrura viatica is a medium sized species (2 mm)
commonly found on the soil surface in arctic and temperate
coastal areas, feeding on terrestrial cyanobacteria, algae, and
decaying seaweed.[12] H. viatica may form groups of many
thousands animals, a behavior possibly mediated by a yet
unknown chemical cue.[13] In this study, the cuticular lipids from a
cyanobacteria feeding population of H. viatica found in the high
arctic Svalbard Islands were analyzed.
Introduction
Springtails (Collembola) are small, wingless, soil dwelling
hexapods that consist of approximately 8,700 species found
throughout the world.[1] The cuticles of springtails are robust, non-
fouling, self-cleaning, and super-hydrophobic surfaces that
protect these arthropods from harsh soil conditions.[2] Recent
work has shown that the cuticular surfaces of springtails are not
only water-resistant but are also able to withstand wetting from
several low surface tension liquids.[3] Due to their unique
properties, such cuticles are of great interest due to possible
applications in biomimetic engineered surfaces. The springtail
surfaces contain small nanoscopic comb-like structures covered
by a thick lipid layer.[2,3] Although the morphology of Collembola
cuticles has been well studied, very little is known about the
chemical composition of their epicuticular lipid layers. Cuticular
compounds from only two Collembola species, Podura aquatica
producing
the
irregular
tetraterpene
poduran[4]
and
Tetrodontophora bielanensis producing lycopene derivatives,[5–7]
have been structurally elucidated. Aside from protection and
prevention of dehydration, the cuticular lipids of springtails may
also serve a secondary purpose in the chemical communication
of these soil dwelling hexapods, releasing information on the
species and sex of an individual. Previous studies have shown
that Collembola produce pheromones mediating aggregation and
Figure 1. A) Total ion chromatogram of a pentane extract of Hypogastrura
viatica. Alkaloids (AK), fatty acid amide (FA), squalene (SQ), cholesterol (CH),
desmosterol (DE), lipids A and B, and phythyl palmitate and oleate (PS, PO)
were present in the extract. B) EI mass spectrum of unknown compound A,
identified as viaticene A.
[a]
Dr. J. E. Bello, P. Stamm, Prof. Dr. S. Schulz
Institute of Organic Chemistry,
Technische Universität Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
E-mail: Stefan.Schulz@tu-bs.de
Homepage: http://www.oc.tu-bs.de/schulz/index_en.html
Prof. Dr. H. P. Leinaas
Department of Biosciences
University of Oslo
Postboks 1066, Blindern, 0316 Oslo (Norway)
Results and discussion
[b]
Short term extraction of the springtails with pentane furnished
extracts that were analyzed by GC/MS. Major components were
cholesterol and an unknown compound A (Figure 1). They were
accompanied by minor amounts of some unknown alkaloids and
a fatty acid amide, squalene, desmosterol and fatty acid phythyl
Supporting information for this article is given via a link at the end of
the document.
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esters. GC/HR-MS of the cuticular compound A revealed the
molecular formula to be C40H76 (m/z obs. 556.5947, calc.
556.5944), with three double-bond equivalents (Figure 1).
Although the mass spectrum of A gave little information about the
structure of the compound, the gas chromatographic retention
index (I) of 3325 suggested a highly branched backbone, hinting
that compound A was possibly a tetraterpene lipid.
which made integration difficult (see SI Figure S3). The 13C NMR
resonances were assigned to three sp2 methine carbons, two sp2
quaternary carbons and one sp2 methylene carbon confirming the
presence of three double bonds in the structure. The remaining
13C NMR resonances were assigned to sp3 methylene carbons, 7
sp3 methine and 10 CH3 carbons.
Column chromatography with a gradient solvent system
was utilized to isolate 2.3 mg of compound A (>90 % purity) from
20 g of H. viatica (see SI Figure S2). The IR spectrum of A,
obtained by GC/IR, showed significant absorptions between 2955
cm-1 and 2850 cm-1, which are typical of a hydrocarbon.
Absorptions at 3075 cm-1, 1643 cm-1, 995 cm-1, and 909 cm-1
indicated the presence of a terminal vinyl group in the structure
(see SI Figure S14). Several microscale derivatizations were
performed to obtain more structural information for compound A.
Hydrogenation resulted in a single compound with a molecular
mass of 562, indicating the loss of three double-bonds. Microscale
ozonolysis of A produced three major compounds 1-3 (Figure 2),
which were structurally elucidated based on mass spectral
fragmentation patterns, GC/HR-MS, GC/IR, and calculation of
retention indices (see SI section 7). Ozonolysis product 1 was
proposed to be 4,8-dimethylnonanal and product 2 to be 2,6-
dimethylnonanedial. These suggestions were verified by
synthesis of both compounds as shown in the SI (section 7).
Product 3 was identified as 3-(3,7-dimethyloctyl)heptane-2,6-
dione, also verified by synthesis (see below, Scheme 1). For
convenience, compound 3 obtained by ozonolysis from isolated
A will referenced as natural 3 in this article.
Scheme 1: Synthesis of (3R,3’R)-(3,7-dimethyloctyl)heptane-2,6-dione
((3R,3’R)-3) respective cyclohexenone 12.
Two-dimensional NMR experiments (1H,1H-COSY, 1H,1H-
TOCSY, 1H,13C-HMBC, and 1H,13C-HSQC) were performed to
gain information about the connectivity of the compound.
H2C=CH-CH(CH3)-CH2, -H2C-HC=C(CH3)-CH2, and -H2C-
HC=C(CH3)-CH-(CH2)2 spin systems were deduced, signifying
that one terminal vinyl group, and two trisubstituted double-bonds
were present in compound A. The last spin system also revealed
the presence of a connection point unusual in terpenes in the
molecule (see Figures S4-S9 in the SI).
Combination of the results of the microscale derivatization
and NMR experiments allowed us to propose a tetraterpene
hydrocarbon structure 4 for compound A containing five chiral
centers, three double-bonds, and unusual connectivity (Scheme
2). The mass spectrum of compound A is in full agreement with
the proposed structure. The fragmentation can be explained by
ions at m/z 415, 336, and 222 corresponding to preferred
cleavage at the allylic positions and at the main branching point
of the molecule (see SI Figure S13). Unlike most tetraterpenoid
compounds that are biosynthesized via the reaction of two
geranylgeranyl pyrophosphate subunits resulting in a [4+4]-
terpene connectivity (see SI section 10 for the introduction of a
Figure 2. A) Total ion chromatogram of the ozonolysis products of compound
A. The three major products are numbered. EI mass spectra of ozonolysis
products 1, 4,8-dimethylnonanal (B), 2, 2,6-dimethylnonanedial (C), and 3, 3-
(3,7-dimethyloctyl)heptane-2,6-dione (D).
new terpene terminology),[14] compound
A has a [6+2]
The 13C and 1H NMR spectra of A revealed a total number
of 40 carbon atoms in agreement with the molecular formula.
However, the number of hydrogen atoms could not be clearly
determined due to overlapping CH2 signals in the 1H NMR spectra,
connectivity, involving the head to tail linking of six isoprene units,
coupled to a branch consisting of two isoprene units forming a ‘Y’-
shaped structure.
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and natural diketone core molecules proved the naturally
occurring 3-(3,7-dimethyloctyl)heptane-2,6-dione to be identical
to the (3R,3’R)/(3S,3’S)-diastereomer of 3. However, attempts to
separate all four stereoisomers on various chiral stationary
phases failed. Therefore, we returned again to microscale
derivatization, this time of the synthetic and viaticene derived
diketones 3 to achieve separation of the resulting derivatives.
Scheme 2. Structure of viaticene A (4) and viaticene B (5). The tetraterpenes
show an unusual [6+2]-terpene connectivity. The origin of the diketone 3 core
obtained by ozonolysis is highlighted in red.
To confirm the structure of compound A and gain insight into
the stereochemistry of the natural compound, we synthesized the
four
stereoisomers
of
the
diketone
core
(3-(3,7-
dimethyloctyl)heptane-2,6-dione) of compound
A
that was
obtained as the ozonolysis product 3 of the lipid (Scheme 1).
Comparison of the synthetic stereoisomers and the degradation
product 3 via chiral-GC would allow for the determination of the
absolute configuration of the core structure of compound A.
(R)-Citronellyl bromide (6) served as starting material.
Hydrogenation of 6[15] followed by a Finkelstein reaction afforded
(R)-1-iodo-3,7-dimethyloctane (7).[16] In the next step 5-hexenoic
acid was converted into 5-hexenoyl chloride and immediately
treated with (1R,2R)-pseudoephedrine (8) to form the tertiary
carboxamide 9.[17,18] Pseudoephedrine amides are easily
transformed into highly enantiomerically enriched ketones, which
dictated our use of pseudoephedrine as the chiral auxiliary in this
synthesis.[17,18] Treatment of 9 with LiHMDS and 7 afforded the
alkylated amide 10 in 95:5 diastereomeric ratio. Reaction of 11
with methyl lithium by acidic work-up produced the respective
Figure 3. Determination of the absolute configuration of the naturally occurring
diketone core (3R,3’R)-3 from viaticene A by chiral GC on a βdex-225 column.
A) Mixture of all enantiomers of 3-(3,7-dimethyloctyl)heptane-2,6-dione (3), B)
(3R,3’R)-3, C) (3R,3’S)-3, D) (3S,3’S)-3, E) (3S,3’R)-3, F) 3 derived from
naturally occurring 4, G) coinjection of natural 3 and (3R,3’R)-3, indicating that
the (RS,RS)-3-diastereomer occurs naturally, H) mixture of synthetic
cyclohexanone derivatives (3R,3’R)-12 and (3S,3’S)-12, I) derivative 12
obtained from natural 3, J) (3R,3’R)-12, K) (3S,3’S)-12, L) coinjection of
(3S,3’S)-12 and the viaticene derived 12, M) coinjection of (3R,3’R)-12 and the
viaticene derived 12.
ketone
(3S,6R)-3-(but-3-en-1-yl)-6,10-dimethylundecan-2-one
(11) with no detectable epimerization of the α-stereogenic center.
Ketone 11 was then treated with Pd(OAc)2 and Dess-Martin
periodinane (DMP) in a Wacker-type oxidation to afford (3R,3’R)-
3-(3,7-dimethyloctyl)heptane-2,6-dione (3R,3’R)-3 in 38% overall
yield.[19] Synthesis of the three other enantiomers of 3 was
performed by respective combinations of enantiomers of 6 and/or
8 in the corresponding steps of the described synthesis to afford
the diketones (3R,3’S)-3, (3S,3’S)-3, and (3S,3’R)-3. The
synthetic diketone diastereomer (3R,3’R)-3/(3S,3’S)-3 had
identical mass spectra and gas chromatographic retention indices
to the ozonolysis product 3, giving further confirmation that the
proposed structure of A was correct. We propose the name
viaticene A for natural compound A.
Formation of a pyridine ring system derived from the 1,5-
diketones was attempted by microscale treatment of a 1:1 mixture
of the synthetic diketones with hydroxylamine and 3Å molecular
sieves in dry Et2O.[20] However, the resulting product was not the
desired trisubstituted pyridine, but 6-(3,7-dimethyloctyl)-3-
methylcyclohex-2-enone (12) formed by
a
regioselective
intramolecular base-catalyzed aldol condensation of 3 with 3Å
mol sieves serving as the base (see SI section 7). Molecular
sieves have been reported to function as a base in aldol and
Mannich type reactions.[21] Independent synthesis confirmed the
structure of 12 (see SI section 7). Fortunately, the
cyclohexenones 12 formed from (3R,3’R)- and (3S,3’S)-3 were
separable on the β-dex225 column (Figure 3). The derivatization
reaction proceeds without or with only low epimerization (0-20 %).
Chiral gas chromatography was then performed to clarify
the absolute configuration of the core structure of the natural
product. Separation of the diastereomers of 3 was possible on a
chiral β-dex-225 column (Figure 3). Coinjection of the synthetic
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Co-injection of (3R,3’R)-12 and 12 obtained from natural 3 proved
GC analysis revealed that the stereochemistry of the core
segment of the molecule has (3’R,14R)-configuration. Analysis of
the different populations of H. viatica showed that the viaticene
compositions from different populations can vary in composition
and is not dependent on the food source. Currently, the total
synthesis of viaticene A is in progress to determine the physical
properties and ecological significance of this unique lipid.
that
both
are
derived
from
(3R,3’R)-3-(-3,7-
dimethyloctyl)heptane-2,6-dione (Figure 3).
The stereochemistry of the stereogenic centers at C7’ and
C19 could not be assigned because the instability of dial 2 and
lack of any successful separation of a 4-methylalkanal such as 1
or their derivatives by chiral GC as of yet.[22] The stereochemistry
of the trisubstituted double-bonds of A was determined by 1H,1H-
NOESY experiments (see SI section 5). Viaticene A can therefore
be assigned as (3’R,10E,14R,15Z)-14-(3,7-dimethyloctyl)-
3,7,11,15,19,23-hexamethyltetracosa-1,10,15-triene.
Acknowledgements
Compound B showed a molecular mass two units lower
than 4, m/z 554, indicating an additional double-bond. The mass
spectra of A and B were similar. The ion m/z 413 located the
additional double-bond outside the geranyl side-chain, while m/z
336 placed it between C-1 and C-12. Minor signals in the NMR
spectra of A that originated from a small impurity of B in the
sample showed the presence of a terminal 1,3-butadiene-system
(see SI Figure S5). Therefore, we propose (3’R,10E,14R,15Z)-14-
(3,7-dimethyloctyl)-7,11,15,19,23-pentamethyl-3-
We thank the Alexander von Humboldt Foundation for financial
support to JB. We would also like to thank Christian Schlawis,
Markus Menke, and Serdar Dilek for their help with GC/IR and
chiral GC experiments.
Keywords: super-hydrophobic lipid • Collembola • cuticular
lipids • pheromones • tetraterpenes
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Entry for the Table of Contents
Key topic: Irregular terpenes
COMMUNICATION
Jan E. Bello,[a] Patrick Stamm,[a] Hans Petter Leinaas,[b]
and Stefan Schulz*[a]
Page No. – Page No.
Viaticene A, an Unusual Tetraterpene Cuticular Lipid
Isolated from the Springtail Hypogastrura viatica
A unique highly branched tetraterpene constitutes the
epicuticular layer of this Collembola. The structure was
elucidated by combination of NMR techniques and chemical
synthesis of degradation products. A Y-shaped structure is
formed by combination of two terpene units made up six and
two prenyl units, respectively. Viaticene is the first member of
a new terpene compound class.
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