Biosynthesis of 1-alkenes in higher plants: stereochemical implications. A model study with Carthamus tinctorius (Asteraceae).

Odd numbered 1-alkenes, such as 1-pentadecene or 1,8,11,14-heptadecatetraene are formed from palmitic or linolenic acid by fragmentative decarboxylation. Incubation studies with germinating safflower (Carthamus tinctorius) and (2R,3R)-12-phenyl[2,3-2H2]dodecanoic acid, (2S,3S)-12-phenyl[2,3-2H2]dodecanoic acid, (2R)-12-phenyl[2-2H]dodecanoic acid and (2S)-12-phenyl[2-2H]dodecanoic acid instead of the natural alpha-linolenic acid precursor revealed the fragmentation to be an overall anti elimination of the 3-pro(S) hydrogen and the carboxyl group (anti-periplanar transition state geometry). Externally offered 3-hydroxy acids are not fragmented to 1-alkenes. The most probable mechanistic alternatives are in agreement with abstraction of the 3-pro(S) hydrogen as a radical followed by electron transfer and fragmentation, or transient insertion of oxygen into the 3-pro(S) C-H bond and subsequent fragmentation into an 1-alkene and CO2 after appropriate activation. The mechanism seems to be of general importance for the biosynthesis of vinylic substructures of natural products from oxygenated precursors.