Variable stereochemistry in highly branched isoprenoids from diatoms

C-25 highly branched isoprenoid (HBI) alkenes are ubiquitous lipids found i n geochemical samples around the globe. The origins of these widespread geo chemicals are believed to be restricted to a limited number of diatoms, inc luding Haslea ostrearia (and related species), Rhizosolenia setigera, and P leurosigma intermedium. The unsaturation of the HBI alkenes ranges from 2-6 in different species and cultures. The number of stereogenic centres is us ually limited to two in the HBI alkenes due to double bond positions. The r elative and/or absolute configurations for these have been determined for a range of HBI alkenes produced from different diatoms cultured under a numb er of growth conditions. These determinations have involved a combined spec troscopic and chromatographic analysis using NMR spectroscopy and chiral ga s chromatography, respectively. HBIs isolated from Haslea spp. belong to a specific structural type which exhibit configurational diastereoisomerism, while those isolated from P. intermedium and R. setigera represent a differ ent structural type and usually exist as mixtures of geometric isomers only . HBIs are reported from a new species of diatom whose stereochemical prope rties he between those found for Haslea spp. and P. intermedium. (C) 2001 W iley-Liss, Inc.