Germacrene skeleton

C15H20O2, Mr 232.32, colorless cryst., mp. 110-111 °C, [a]p +145.8° (CHCI3) soluble in chloroform, ethanol, and THE. An a,j3-unsaturated sesquiterpene lactone with the germacrene skeleton (see ger-macrenes) from Aristolochia reticulata and A. serpen-taria. The absolute configuration has been determined both by X-ray diffraction and enantiospecific synthesis. ... 

Germacrene C 4.64) has been shown to derive from mevalonate and 2- raw5-farnesol 4.60). Its formation can be seen to be the result of displacement of pyrophosphate from C-1 of farnesyl pyrophosphate 4.63) by the distal double bond. Scheme 4.13. An oxygenated relative of 4.64), ageratriol has been shown to derive similarly via oxygenated intermediates [75, 76]. Further cyclization of the germacrene skeleton [as (4.64)] would afford compounds of type (4.4). On the other hand caryophyllene 4.66), known to be derived from mevalonate, is expected to derive from 2-m-farnesol (4.65) in a dif-... 

Recently, germacrene-A (III) has been identified as the alarm pheromone of the sweet clover aphid Therioaphis trifolii (38). This alarm releaser, which has often been proposed as the progenitor of cyclic sesquiterpenes, constitutes the second compound with a germacrane-type skeleton to be identified as an insect pheromone. 

Protonation-induced cychzation of germacrene A generates the cis-decalin skeleton of the eudesmyl cation, an intermediate in the biosynthesis of the phytoalexin capsidiol (79). The eudesmane skeleton is converted to the 5-epi-aristolochene (80) skeleton by Wagner-Meerwein rearrangements of a hydride and a methyl group (Cane 1990). Hydroxylation of 80 then produces capsidiol. The entire process of formation of epi-aristolochene from FPP is mediated by a single synthase which has been purified from tobacco (Starks et al. 1997). The intermediacy of germacrene A in the biosynthesis was demonstrated by mutation of... 

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. 

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