STEREOCHEMICAL STUDIES ON HOMOTERPENE BIOSYNTHESIS IN HIGHER-PLANTS - MECHANISTIC, PHYLOGENETIC, AND ECOLOGICAL ASPECTS

In higher plants, the two homoterpenes 4,8-dimethylnona-1,3,7-triene (1) and 4,8,12-trimethyltrideca-1,3,7,11-tetraene (2) originate from nerolidol (3) or geranyllinalool (4) by an oxidative cleavage of their C-skeletons. The reaction proceeds with exclusive loss of H(s)-C(5) of 3 and formal production of a C4 fragment. The site specificity of the enzyme(s) is identical for all of the hitherto examined plant families (Agavaceae, Asclepiadaceae, Asteraceae, Leguminosae, Magnoliaceae, and Saxifragaceae). The enzyme tolerates a wide range of structural modifications at the polar head of 3. Instead of 3, also geranylacetone 12 and the secondary alcohol 13 can be cleaved to the homoterpene 1 and as yet unidentified carbonyl fragments. The C = C bonds within the aliphatic chain of 3 seem to be essential for the oxidative bond cleavage as well as for recognition and embedding of the substrate into the active center of the enzyme(s). The feed-induced biosynthesis of 1 and 2 in leaves of the Lima bean Phaseolus lunatus infested with the spider mite Tetranychus urticae probably requires a preceding release of nerolidol (3) or geranyllinalool (4) from phytogenic glycosides prior to the fragmentation reaction. The microbial reduction of the trienoic acids 6 and 6a is the key step for the synthesis of deuterium labelled nerolidol (3RS,5R)- and (3RS,5S)-9.