Reinvestigation of the structure of a tetracyclic terpenoid from geological samples: A novel marker for fungi involved in the degradation of buried wood?

A tetracyclic compound tentatively assigned as A-nor-friedel-8-en-10-one was recently reported by Bakar et al. (2011) [Bakar, N.A., Tay, K.S., Omar, N.Y.M.J., Abas, M.R.B., Simoneit, B.R.T., 2011. Applied Geochemistry 26, 1433-1445] from a Malaysian lake sediment. It was postulated to derive from friedelin via a sequence of diagenetic transformations involving the unprecedented removal of ring A and several hypothetical intermediates. We considered that this structural assignment needed to be reassessed, notably because we could detect the compound in wood samples devoid of friedelin-related triterpenes. Firm identification was also needed since the compound is becoming used as a proxy in recent publications and its occurrence interpreted in terms of palaeoenvironmental reconstruction. In the course of archaeometric studies of ancient buried wood, the presence of the compound as a predominant constituent of the lipid extract of a buried alder wood sample gave us the opportunity to firmly establish its structure. We report here its unambiguous identification using 1D and 2D nuclear magnetic resonance (NMR) spectroscopy after its isolation. Based on its structure, geochemical distribution and stable carbon isotopic composition, it appears that the compound likely does not derive from triterpenoids biosynthesized by higher plants, but might derive from a precursor terpenoid synthesized by microorganisms (bacteria, fungi) living within the buried wood, and possibly involved in the degradation of plant organic matter (wood, notably).