Cineole, biosynthesis

In contrast to the above-ground organs of plants, roots represent an unexplored area of terpene biosynthesis and function. To date, just a small number of terpene synthases have been identified in plant roots. In Arabidopsis, the terpene synthase genes that encode 1,8-cineole synthase and (Z)-y-bisabolene synthase are expressed differentially in the stele of younger root growth zones and in the cortex and epidermis of older roots (reviewed... 

Croteau, R. and F. Karp, Biosynthesis of monoterpenes Partial purification and characterization of 1,8-cineole syntetase from Salvia officinalis. Arch. Biochem. Biophys., 179, 257-265 (1977). 

The substrate specificity of 1,8-cineole, a-terpineol, limonene, and terpin-olene synthetases from sage has not yet been determined however, it is quite clear that the sage preparations, and similar preparations from Citrus (George-Nascimento and Cori, 1971 Chayet et al., 1977), can utilize both GPP and NPP for the biosynthesis of monocyclic monoterpenes. Only when such cell-free preparations are free of all competing enzyme activities can the true specificities of the monoterpene synthetases be accurately determined. 

Figure 3.11 shows the biosynthesis of (+)-bornyl pyrophosphate (the precursor of (-l-)-borneol (28) and (+)-camphor (29)) and of (+)-sabinene (30) (the precursor of the thujones) from 3R)-20 cyclized in an anti,endo conformation (Wise et al. 1998). Other related products include camphene (31) and 1,8-cineole (32). The chemistry involved in the formation of the final products includes Wagner-Meerwein rearrangements of hydride and (in the case of camphene) a skeletal carbon-carbon bond as well as simple cyclizations. The biosynthesis of (-)-a-and P-pinene (33 and 34) proceeds along similar lines from (3S)-20 and is shown in Figure 3.12. 

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