Mono- and dicyclic unsaturated triterpenoid hydrocarbons in ...

888 R. de Mesmay et al. / Organic Geochemistry 39 (2008) 879–893
the present case of b, the molecular ion at m/z 470 is consistent
with a tetrahydrobotryococcene (C34H62). Fragment
ions at m/z 123, 177 and 231 suggest the presence of a
trimethylated cyclohexenyl ring in the left half part of the
molecule. In addition, a diagnostic ion at m/z 259 resulting
from the cleavage of the C-12/C-13 bond indicated that
two additional unsaturations were present at C-11 and
C-26. The ozonides O11 and more especially O1, although
not specific for the degradation of b (Table 1), also support
structure b as an unprecedented tetrahydrogenated
derivative of k (David et al., 1988), with the C-17/C-29
and C-21/C-22 double bonds being hydrogenated.
3.1.5. C36 and C37 monocyclic botryococcenes and partially
reduced counterparts
GC-MS analysis of the hydrogenated hydrocarbon fraction
revealed, in addition to the aforementioned botryococcanes,
the presence of two C 36 (H4) and two C 37 (H6)
monocyclic botryococcanes (Fig. 3A). Each of these two couples
exhibited strictly identical mass spectra, with diagnostic
ions indicative of the presence of a ring in the left part of
the molecule (ions at m/z 236/237, Fig. 3D and F). The mass
spectra of H4 and H6 show similarities to that of H2. Two
(respectively three) additional carbons are situated on the
right hand moiety of the molecules (ions at m/z 322/323
for H4, Fig. 3D and at m/z 336/337 for H6, Fig. 3F).
C36 botryococcanes H4 probably result from the catalytic
hydrogenation of two partially coeluting C 36 botryococcenes
(M +. at m/z 494, f1 and f2) with identical mass spectra
(Figs. 2A and 7C) and two dihydrogenated counterparts (g1
and g2), also partially coeluting (Fig. 2A) with identical
spectra (M +. at m/z 496; Fig. 7C). Fragments at m/z 123,
177, 231 and 285 in f1, f2, g1 and g2 confirmed the occurrence
of a trimethyl cyclohexenyl ring in the left hand moeity
and of two unsaturations at C-11 and C-26. The
structures of the right hand parts of the compounds were
deduced from the identification in the ozonolysis products
of the di- and tri-oxygenated compounds O4 and O6, respectively
(Table 1). The McLafferty fragment at m/z 86 in their
spectra (e.g. Fig. 6B for O4) suggests the presence in the
structure of an ethyl ketone exhibiting a methyl group in
the a position. Consequently, an ethyl group would be present
at ‘‘C-21” (numbered according to botryococcene structures)
in f1, f2, g1 and g2. Moreover, the molecular formulae
of O4 and O6 (C16H30O2 and C16H26O3, respectively) established
that a C2 moiety was lost from the right hand part
during ozonolysis, suggesting the presence of a CH3CH=C
pattern in the C36 botryococcenes f1 and f2, and the dihydro
derivatives g1 and g2. Finally, the McLafferty fragment at m/
z 170 in the spectrum of O6 indicates the presence of a ketone
at ‘‘C-17” (data not shown), and the two discrete ions
at m/z 113 and 141 in the spectrum of O4 show the presence
of a methyl at ‘‘C-17” (Fig. 6B).
These results allow us to tentatively assign f1 and f2 as
monocylic C 36 botryococcenes and g1 and g2 as their dihydro
derivatives. The occurrence of two pairs of isomers with
identical mass spectra is probably due to the existence of
stereoisomers with respect to the C-21/C-22 double bond
stereochemistry. Based on semi-empirical topological
methods for the prediction of the chromatographic retention
of alkene isomers (Heinzen et al., 1999; Junkes et al.,
2002), it can be assumed that the stereochemistry of the
C-21 double bond is E in f1 and g1 and Z in f2 and g2.
Similarly, botryococcanes H6 were probably produced by
the catalytic hydrogenation of C37 botryococcenes (C37H64,
M +. at m/z 508) and dihydro counterparts (C 37H 66, M +. at
m/z 510) (Table 1 and Fig. 3). Among the ozonolysis products,
only the di- and tri-oxygenated O5 and O7, respectively
(Table 1) could be related to C37 hydrocarbon
precursors. MS comparisons showed that O5 (C 17H 32O 2)
and O7 (C17H28O3) were higher homologues of O4 and O6,
respectively. Moreover, a base peak at m/z 43 in the spectrum
of O7 (Fig. 6D) suggested that the carbon skeleton
probably has a terminal isopropyl group. The occurrence
of an isopropyl group in O5, justasinh and i, was also supported
by the presence in the spectra of significant ions at
m/z [(M-43) + ; Figs. 6C and 7D, E]. The presence of a
CH3CH=C pattern at C-21 in h and i is supported by the presence
of a ketone a to the isopropyl group both in O5 and O7
and by biogenetic considerations (see below). The combination
of O7 with O9,allowsustoproposeh as a higher homologue
of f1 and f2 with one more carbon. The spectra of O5
and O7 exhibit some identical fragments (m/z 100, 113, 155,
225, 240), but an ion at m/z 184 in that of O7, due to McLafferty
rearrangement, and two discrete ions at m/z 113 and
155 in that of O5, indicate that O5 and O7 differ by way
of the presence of a methyl group and a ketone, respectively.
These data suggest that i and h only differ in the presence
of an exomethylene and a methyl group at C-17,
respectively. In contrast to the C36 homologues, h (respectively
i) does not appear as a mixture of two compounds
in the GC chromatogram. Nevertheless, h (respectively i)
may occur as a mixture of stereoisomers that are not separated
under the GC conditions used.
3.2. Occurrence of masokocenes in ancient ecosystems
Dicyclobotryococcenes have been recently reported to
occur in sediments of a Norwegian fjord (Smittenberg
et al., 2005) and in freshwater wetlands of the Florida Everglades
(Gao et al., 2007), but their structures were not
unambiguously determined. On the other hand, a hypothetical
dicyclobotryococcene exhibiting a planar structure
similar to that of c had already been assumed to be the precursor
of a major dicyclobotryococcane (tentatively assigned
as H3) detected after Raney nickel desulfurisation
of a polar lipid fraction from Miocene/Pliocene immature
hypersaline sediments (Sdom Formation near the Dead
Sea; Grice et al., 1998). The authors supposed that c was
incorporated into a macromolecular matrix via sulfurisation.
The lack of c among the extractable biomarkers of
the Sdom formation was explained by its ease of sulfurisation
due to the presence of four double bonds. The strong
dominance of intact masokocene c in a ca. 32,000 year
old sediment indicates excellent conditions of preservation
in Lake Masoko sediments, even though the compound
might be sensitive to sulfurisation as well as to oxidative
or reductive attack. Although B. braunii race B is known
to produce large amounts of hydrocarbons, the lack of a
hydrocarbon biomarker from other sources in Lake Masoko
ca. 32000 years ago is a clue for an ecosystem dominated at
the time by blooms of B. braunii. This situation strongly