25th International Meeting on Organic Geochemistry IMOG 2011

P-224
Diagenetic transformation of 2,3-dioxygenated triterpenoids
from higher plants in buried wood and sediments
Philippe Schaeffer 1 , Claude Le Milbeau 2 , Pierre Adam 1
1 Laboratoire de Biogéochimie Moléculaire - UMR7177 / Université de Strasbourg - CNRS, Strasbourg,
France, 2 Institut des Sciences de la Terre d'Orléans / Université d'Orléans, Orléans, France (corresponding
author:padam@unistra.fr)
Following the recent identification of novel aromatic
oleanane-related triterpenoids bearing an oxygenated
functionality at C-2 in buried oak wood (1, 2, 3a, 4a;
Fig. 1), a novel diagenetic transformation pathway of
2,3-dioxygenated triterpenoids widely distributed in
higher plants has been proposed [1] and involves
aromatization from ring D to ring A (as opposed to the
―classical― pathway starting from ring A; e.g. [2]).
These aromatic triterpenoids were indeed postulated
to derive from functionalized precursors such as 5a
and 6a occurring in oak as glycosides (e.g. [3]) We
report here the identification by mass spectrometry
(GC-(HR)MS) and NMR of some of the postulated
diagenetic intermediates (7a/b-14) in a series of
buried wood samples collected from riverine and
freshwater lake sediments.
GC-MS investigation of the solvent extracts of wood
samples shows that straight-chain lipids have almost
completely disappeared upon diagenetic alteration,
whereas C-2- and C-2,C-3- oxygenated triterpenoids
appear to be predominant along with steroids. In
particular, the structural assignment of the alcohols 7a
and 8a relies on NMR investigations and chemical
correlation with the ketones 3a and 4a. The
monoaromatic diols 9a and 10a, key intermediates in
the pathway leading from the biological triterpenes
(e.g. 5a and 6a) to C-2 oxygenated triterpenoids
presented in Fig. 1, could be recognized based on
mass spectrometry studies. In addition to these
triterpenoids belonging to the oleanane series, oak
wood samples proved to contain also ursane
analogues in varying amounts. Furthermore, the
identification of ring E demethylated analogues (3b,
4b, 7b-10b) in one wood sample suggests the
occurrence of functionalized molecule precursors
bearing, for instance, a carboxyl group at C-20 (e.g.
5b and 6b) from which the demethylated analogues
might derive by decarboxylation.
The variety of structural modifications (e.g. loss of
methyl groups, ursane vs. oleanane skeletons,
presence of aromatic rings) observed on the reported
C-2- and C-2,C-3- oxygenated triterpenes reflects the
structural variety of their parent molecules, and might
thus be interpreted in terms of biological source(s).
These triterpenoids might therefore be used as
specific chemotaxonomic markers for higher plant
species synthesizing C-2,C-3-oxygenated
triterpenoids.
Fig. 1: Diagenetic pathway leading from C-2,C-3- dioxygenated triterpenoid
precusors to C-2 oxygenated sedimentary triterpenoids. All the compounds
presented (except 5a/b and 6a/b) were detected in buried wood samples.
References
[1] C. Le Milbeau, P. Schaeffer, J. Connan, P. Albrecht, P. Adam
(2010) Org. Lett. 12, 1504-1507.
[2] G.A. Wolf, J.M. Trendel, P. Albrecht (1989) Tetrahedron 45, 6721-
6728.
[3] G. Arramon, C. Saucier, D. Colombani, Y. Glories (2002)
Phytochem. Anal. 13, 305-310.
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