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. Author manuscript; available in PMC: 2019 Nov 1.
Published in final edited form as: Nat Ecol Evol. 2018 Oct 15;2(11):1709–1714. doi: 10.1038/s41559-018-0676-2

Fig. 2|. Chemical structures of stelliferasterol, isostelliferasterol and strongylosterol.

Fig. 2|

These are the three known natural sterol precursors of the 26-mes sterane biomarker20,21,23,39,40 They are found only in certain demosponges, but not detected in other groups of eukaryotes. Note:(1) the methyl-substituent at the terminal position of the sterol side chain, which remains preserved at C-26 in 26-mes; and (2) the unusual double bond positions in the side chains of stelliferasterol and strongylosterol. The biological configuration is 20R for all three sterols, 24R for strongylosterol and stelliferasterol, and 25S for isostelliferasterol. Three stereogenic carbon atoms exist in the side chain of 26-mes (chirality at C-20, C-24 and C-25), but only C-20 stereoisomers give separate compound peaks, producing up to four regular stereoisomers of 26-mes (αααS, αββR, αββS and αααR) in ancient rocks and oils, as has also been found for other sterane compounds.