Germacranes, Eudesmanes, Guaianes, Cadinanes, and Cubebanes

The biogenesis of the germacrane, eudesmane, and guaiane sesquiterpenes is initiated by a cyclization between the terminal double bond 

The electrophilic cyclizations of trans, rra .y-l,5-cyclodecadienes has been extensively investigated, principally with germacrane-type sesquiterpenes (121), but also with model substrates (122). Electrophiles effect regiospecific addition to the 1,2-double bond and stereospecific cyclization to /ran -decalins, transformations closely parallel to the biogenesis of the eudesmanes (725). Similar cyclizations have been observed with numerous oxygenated germacrane derivatives (727). 

Another stereochemical characteristic of these cyclizations is the equatorial disposition of both the entering electrophile and nucleophile, as expected for overall anti addition to both double bonds (727—123). The anti mode of addition appears to be followed in the initial electrophilic attack in all cases. However, the stereoselectivity in the approach of the nucleophile is evidently less since minor amounts of axial acetates are formed in the additions to the above model cyclodecadienes (73) and (74) (722). 

It is possible to effect an anti-Markovnikov cyclization to the per-hydroazulene skeleton of the guaianes by the action of acid catalysts upon the 5,6-epoxides of various germacrene sesquiterpenes (727, 124). Markovnikov opening of the epoxode with participation of the transannular double bond (cyclobutane-like strain in transition state) does not compete. For contrast, the cyclization of the isomeric epoxide to eudesmanediol is also shown. 

In contrast to the trans, /ra 5-l,5-cyclodecadienes and germacradienes, the cis, rr z/15-isomers undergo electrophilic cyclization to cis-decalins 121, 131—134). The parent diene itself reacts with the electrophile at the cis double bond initially. As in the case of the trans, trans-compounds, the stereochemistry in both addition steps is anti. 

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