Methylated diterpene resin acids

Although the resins accumulated by Eremophila species are most often composed of a mixture of diterpenes and flavones, in the case of the taxonomically related E. virgata and E. interstans the resin is a complex mixture of sesquiterpene acids A portion of the methylated... 

Cembranoids The first macrocyclic diterpene isolated from an Eremophila species was the triol (99) produced by E. clarkei (96). The resin obtained from this plant was a complex mixture of compounds from which the crystalline triol could be obtained with difficulty from the neutral fraction. Larger quantities of triol were available after methylation and lithium aluminium hydride reduction of the acidic fraction. The plane structure of 99 was established by conversion of the triol to a crystalline stereoisomer of cembrane (100) as shown in Scheme 25. The two dihydrotriols (101 and 102) obtained in this sequence also served to prove the location of the hydioxymethylene groups on the cembrane skeleton. Since both 101 and 102 are optically active, the symmetrical 4,12-cis arrangement of the primary alcohol groups can be excluded. Furthermore, the 4, 2-trans-disposition of these groups can also be excluded since elimination of the asymmetry at Cl leads to two different olefins (103 and 104). 

Spiro diterpenes E. viscida, as the name implies, is a species whose branches and leaves carry a sticky resin and this appearance gave rise to the vernacular name of varnish bush. The resin consists largely of material extractable into base. The major components are two diterpene acids (131,132) which disclosed a new diterpene skeleton (111). Although a small amount of the crystalline dihydroxy acid (131) could be obtained after tedious separation, as often found with Eremophila resins, the process was simplified by methylation of the crude acidic fraction and separation of the methyl esters. 

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