Biosynthesis of 1‐alkenes in higher plants: stereochemical implications

Abstract

Odd numbered 1‐alkenes, such as 1‐pentadecene or 1,8,11,14‐heptadecatetraene are formed from palmitic or linolenic acid by fragmenlative decarboxylation. Incubation studies with germinating safflower ( carthamus tinctorius ) and (2 R ,3 R )‐12‐phenyl[2,3‐ 2 H 2 ]dodecanoic acid, (2 S ,3 S )‐12‐phenyl[2,3‐ 2 H 2 ]dodecanoic acid, (2 R )‐12‐phenyl[2‐ 2 H]dodecanoic acid and (2 S )‐12‐phenyl[2‐ 2 H]dodecanoic acid instead of the natural α‐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 CO 2 after appropriate activation. The mechanism seems to be of general importance for the biosynthesis of vinylic substructures of natural products from oxygenated precursors.