Highly branched isoprenoid (HBI) hydrocarbons have been widely reported in Recent sediments. It is known that two species of diatomaceous algae, the diatoms Haslea ostrearia and Rhizosolenia setigera are able to biosynthesise C25 HBI alkenes. Large scale cultures of H. ostrearia studied previously, allowed isolation and characterisation of five HBI polyenes. The stereochemistry of the chiral centres was not reported however. The concentrations of HBIs often decrease with depth in sediment cores but the reasons for this are unknown.
In the present study, two previously unidentified C25 HBIs have been isolated from H. ostrearia and the structures unambiguously assigned by nuclear magnetic resonance spectroscopy (NMR) and mass spectral analysis. They have been identified as 2,10,14-trimethyl-6-methylene-7-(3-methylpent-4-enyl)pentadecane and the triene 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)pentadeca-5,9-diene.
A different C25 triene from the diatom R. setigera has been isolated and tentatively identified by 1H NMR spectroscopy as 2,6,10,14-tetramethyl-7-(3-methylpent-1,4-dienyl)pentadec-9-ene. This compound appears to be common in sediments, judging from published retention indices and mass spectra.
The partial configurations of C25 HBI alkenes have been established, for the first time. A combination of NMR spectroscopy studies of the alkenes with chiral shift reagents in conjunction with soluble silver B-diketonate complexes and enantioselective gas chromatography of oxidation products of the HBIs was used. Differences in HBI isomer configurations were observed between different laboratory cultures of H. ostrearia.
A C25:2 diene identified in Antarctic sea-ice diatoms and surface layer sediments from Antarctica was identified as 2,10,14-trimethyl-6-methylene-7-(3-methylpent-4-enyl)pentadecane. The identification was made by comparison of mass spectra and GC retention indices on both polar and apolar GC stationary phases with those of the diene isolated from H. ostrearia. Stereochemical studies identified the C25:2 diene from sea-ice diatoms as 2,10S,14-trimethyl-6-methylene-7-(3S-methylpent-4-enyl)pentadecane whilst the structurally similar diene C25:2 from the sediments was identified as 2,10S,14-trimethyl-6-methylene-7-(3RS-methylpent-4-enyl)pentadecane.
Individual HBI alkenes reacted rapidly under mild acid-catalysed conditions (i.e. K-10 montmorillonite clay or TSOH-HOAc). For the dienes, double bond migration and geometric isomerisation was observed as the only reactions. In contrast, the trienes underwent both isomerisation and rapid cyclisation reactions to yield substituted cyclopentenes and cyclohexenes. A compound with similar mass spectrum and GC retention indices was identified in sapropels from the Eastern Mediterranean Sea.
Parts of this work have been published Rullkotter et al., (1998) Proc. OPD, Sci. Results, 160, 271-283 [doi: 10.2973/odp.proc.sr.160.023.1998]; Wraige et al., (1999) Phytochemistry, 51, 69-73 [doi: 10.1016/S0031-9422(98)00694-3].
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