An unusual aromatization process of higher plant triterpenes in sediments

The NMR structural elucidation of D:B-friedo-3-methyl-24-noroleana-1,3,5(10)-triene (3-methyl-24-nor-friedela-1,3,5(10)-triene) isolated from the Messel shale (Germany) highlights an unusual ring-A aromatization process occurring during diagenesis of higher plant 3-oxygenated triterpenes. This process, which involves the shift of one of the gem-dimethyl groups at C(4) to C(3), instead of its commonly observed elimination, is probably induced by structural features of the precursors (most likely the Δ5 or Δ5(10)-gluten-3-ones). An acid(clay)-catalysed enone-benzene transposition can indeed account for a direct abiotic ring-A aromatization of such precursors.