Novel tri- and tetra-unsaturated highly branched isoprenoid (HBI) alkenes from the marine diatom Pleurosigma intermedium

Two previously uncharacterised C25 highly branched isoprenoid (HBI) alkenes have been identified in a laboratory culture of the diatom Pleurosigma intermedium and their structures determined using a combination of NMR spectroscopy and gas chromatography-mass spectrometry (GC-MS). These new tri- and tetra-unsaturated HBI alkenes are stereoisomers of previously characterised HBIs, with a Z stereochemistry for the C5-C6 double bond in each case. HBIs with an isomeric (E/Z) C5-C6 double bond and with 1-4 double bonds, in total, have extremely similar mass spectra and a consistent difference in retention index (ΔRI 28), with a Z before E elution order in all cases.