28- Nor-spergulanes, a novel series of rearranged hopanes

Abstract

A new series of rearranged hopanes ranging from C 29 to at least C 34 has been identified as 28- nor-spergulanes (or 21-methyl-28- nor-hopanes). The C 29 member of the series was unambiguously identified as 17 β,18 α-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 C 29 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 C 29 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.