Stereochemical Studies on Homoterpene Biosynthesis in Higher Plants; Mechanistic, Phylogenetic, and Ecological Aspects

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Abstract
In higher plants, the two homotcrpenes 4,8‐dimethylnona‐l,3,7‐triene (1) and 4,8,12‐trimethyltrideca‐1,3,7,11‐teiracnc (2) originate from nerolidol (3) or geranyllinalool (4) by anoxidative cleavage of their C‐skele‐lons. The reaction proceeds with exclusive loss of H8–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 gcranylacclone 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 lunatm infested with the spider mite Tetranychus urticae probably requires a preceding release of nerolidol (3) or geranyllinalool (4) from phylogenic glycosides prior to the fragmentation reaction. The microbial reduction of the trienoic acids 6 and 6ais the key stop for the synthesis of deuterium labelled nerolidol (3RS,5R)‐and (3RS,5S)‐9.