Of germacrone

Some defenses are so effective that herbivores do not consume any amount of a plant. Snowshoe hares are deterred from feeding on Labrador tea. Ledum groenlandicum, by germacrone, a sesquiterpene, the major compound of its essential oil. Leaves and fresh growth internodes contain large amounts of germacrone (Reichardt etal, 1990a). Yet beaver, C. canadensis, in Saskatchewan,... 

Hikino et al. [153] have investigated by enzymatic means the stereospecific epoxidation reactions of olefinic double bonds in the plant Curcuma zedoaria Roscoe. They studied the bioconversion of germacrone (207), a constituent of C. zedoaria, by microorganisms in the hope of obtaining stereoselective epoxidation as in the case of the plant. Cuminghamella blakesleeana yielded three major products (208 - 210) from germacrone, Fig. (42). 

More recently, Asakawa et al. [154] described the biotransformation of germacrone by Aspergillus niger. A very unstable allylic alcohol (211) was obtained from the metabolite of germacrone along with (213), Fig. (42). 

Fig. (42). Biotransformation of germacrone by Cunninghamella blakesleeana (after [153]) and by Aspergillus niger (after [154])...

Scheme 53. Molecular models do not make this mechanism seem very likely and further work on the structures of caulolcatone A and B may be required. A further study of the photochemistry of germacrone (340) has shown that direct irradiation causes isomerization only of the A °-double bond whereas acetophenone-sensitized irradiation induces isomerization of both the and...

The microbial transformation of humulene, a substrate showing a structure similar to that of germacrone, was studied by Abraham and Stumpf using a screen of about 300 strains1175 . This led the authors to select the fungi Diplodia gossypina and Chaetonium cochlioides for preparative scale experiments. It was thus observed that the main reaction path starts with the epoxidation of the 1,2-double bond, as shown by direct biotransformation of this monoepoxide obtained by chemical synthesis. This is then further oxidized to yield a multitude of products including diepoxides and hydroxy-epoxides (Fig. 16.1-28). 

About 50 elemanes known to date comprise p-elemenone from the oil of myrrh, representing the Cope rearrangement product of germacrone, (-)-bicycloelemene from peppermint oils of various provenance (e.g. Mentha piperita or Mentha ar-vensis), and p-elemol which is not only a minor component of Javanese oil of citro-nella but is also found in the elemi oil with an odor like pepper and lemon, expressed from the Manila elemi resin of the tree Canarum luzonicum (Burseraceae). 

FIGURE 20.41 Biotransformation of germacrone (118) by Curcuma zedoaria and Curcuma aromatica cells. 

Sakamoto, S., N. Tsuchiya, M. Kuroyanagi, and A. Ueno. 1994. Biotransformation of germacrone by suspen sion cultured cells. 35 1215-1219. 

Hikino, H., T. Konno, T. Nagashima, T. Kohama, and T. Takemoto, 1971. Stereoselective epoxidation of germacrone by Cunninghamella blakesleeana. Tetrahedron Lett., 12 337-340. 

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