A new series of rearranged hopanes ranging from C29 to at least C34 has been identified as 28-nor-spergulanes (or 21-methyl-28-nor-hopanes). The C29 member of the series was unambiguously identified as 17β,18α-28-nor-spergulane using NMR spectroscopy. All members of the series have typical hopane mass spectra with an m/z 191 base peak, but the m/z 369 fragment is unusually intense (70–90%). The C29 28-nor-spergulane (29Nsp) is always the dominant member of the series and 28-nor-spergulanes having more than 31 carbon atoms are insignificant in all samples. The content of 29Nsp is often positively correlated with that of other rearranged C29 hopanes (neohopanes and diahopanes). However, 29Nsp seems to be particularly abundant in some oils from lacustrine source rocks in South East Asia. Laboratory experiments suggest that the precursors of 28-nor-spergulanes are hop-17(21)-enes which are oxidized to hopadienes, rearranged to 28-nor-spergula-12,17-dienes and finally reduced, forming 28-nor-spergulanes. The two double bonds necessary for rearrangement might also be formed by migration of double bonds from the side chain after elimination of hydroxyl groups in biological hopanoids. A late eluting compound in the 398 → 191 transition used for detection of 29Nsp was tentatively identified as 24-nor-gammacerane.
- Programområde 3: Energiressourcer