Occurrence of dammar-13(17)-enes in sediments: Indications for a yet unrecognized microbial constituent?
A pair of isomeric dammarenes has been identified in numerous Pleistocene to Jurassic sediment samples, most of them from continental margins. Their structures have been determined to be (20 R)- and (20 S)-dammar-13(17)-ene based on comparison of mass spectra and Chromatographic data with those of synthetic standards. Their saturated counterparts have also been found in a few samples. Based on rigorous nuclear magnetic resonance (NMR) studies with one synthetic standard and molecular mechanics calculations, it is suggested that geologically occurring dammaranes possess 13β,17α(H) stereochemistry. Compounds with a dammarane skeleton are well known natural products of several families of land plants, but the widespread occurrence of dammarenes in marine sediments suggests an alternative origin, supported by their carbon isotopic signature. Dammarenes can be envisaged as direct proton-induced cyclisation products from squalene and may represent a contribution from microorganisms. Although we are in favour of this hypothesis, an origin from geologically induced rearrangements of other compounds cannot, at present, be excluded.