Biosynthesis of 1-alkenes in higher plants. A model study with the composite Carthamus tinctorius L

Uneven numbered 1-alkenes, such as 1-pentadecene, 1-heptadecene, 1,8-heptadecadiene, 1,8,11-heptadecatriene and 1,8,11,14-heptadecatetraene are the predominant hydrocarbons in germinating safflower (Carthamus tinctorius L.). According to their chain length and positions of double bonds they are derived from palmitic, stearic, oleic, linoleic or linolenic acid, respectively. This has been proved by administration of synthetic alpha-, beta-, or gamma-deuterated 12-phenyldodecanoic acids to germinating safflower. Mass spectrometric analysis of the resulting deuterium-labelled 11-phenyl-1-undecenes shows that only a single hydrogen from C-3 and carbon dioxide from C-1 of the precursor fatty acid is lost during vinyl group formation. A strong isotope effect (kH/kD greater than or equal to 10) during the conversion of racemic 12-phenyl[3-2h]dodecanoic acid to 11-phenyl-1-[2-2H]undecene is consistent with an initial enzymatic attack on a non-activated hydrogen at C-3 accompanied by simultaneous fragmentation into the respective 1-alkene and carbon dioxide. Mechanisms, based on ionic or radical abstraction of a hydrogen from C-3 of the fatty acid, followed by decarboxylation are discussed. The results are applicable to the formation of various vinylic substituents in natural products such as porphyrins, acetylenic hydrocarbons, pheromones from marine brown algae and terpenoids.