Hydroxylated germacrene

Chan and coworkers [23] and Izac et al. [24] isolated germacrenes derivatives, 13-19, as the major secondary metabolites from Pseudopterogorgia americana. Izac et al. [24] and Chan [23] isolated the new elemanolide 18 and elemanolide 19, respectively along with C2 and C5 hydroxylated germacrenes. 

Franssen et al. [24] pointed out an alternative method of production of nootkatone from valencene catalysed by (-i-)-germacrene A hydroxylase, an enzyme of the cytochrome P450 monooxygenase type that was isolated from chicory roots. In general, this enzyme appeared to accept a broad range of sesquiterpenes and hydroxylates exclusively at the side-chain s isopropenyl group. Valencene is an exception it was not hydroxylated at the side chain, but -nootkatol was formed in the first step (Scheme 22.5) it is not yet clear if the second step is enzyme-catalysed. 

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... 

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