Irregular Monoterpenes

A number of natural monoterpene structures contain carbon skeletons which, although obviously derived from isoprene C5 units, do not seem to

—r regular monoterpene skeleton irregular monoterpene skeletons

Figure 5.17

fit the regular head-to-tail coupling mechanism, e.g. those in Figure 5.17. These structures are termed irregular monoterpenes and seem to be limited almost exclusively to members of the plant family the Compositae/Asteraceae. Allowing for possible rearrangements, the two isoprene units appear to have coupled in another manner and this is borne out by information available on their biosynthesis, though this is far from fully understood. Thus, although DMAPP and IPP are utilized in their biosynthesis, geranyl PP and neryl PP do not appear to be involved. Pre-eminent amongst these structures are chrysanthemic acid and pyrethric acid (Figure 5.18), found in ester form as the pyrethrins* (pyrethrins, cinerins, and jasmolins, Figure 5.18), which are valuable insecti-cidal components in pyrethrum flowers, the flower heads of Chrysanthemum cinerariaefolium (Com-positae/Asteraceae). These cyclopropane structures are readily recognizable as derived from two iso-prene units, and a mechanism for the derivation of chrysanthemic acid is given in Figure 5.19 (compare this mechanism with that involved in the formation of presqualene PP during steroid biosynthesis, page 214). This invokes two DMAPP units joining by a modification of the standard mechanism, with termination achieved by cyclopropane ring formation. Little is known about the origins of pyrethrolone, cinerolone, and jas-molone (Figure 5.18), the alcohol portions of the pyrethrins, though it is possible that these are cyclized and modified fatty acid derivatives, the cyclization resembling the biosynthetic pathway to prostaglandins (see page 53). Thus, a-linolenic acid via 12-oxophytodienoic acid could be the precursor of jasmolone, with ß-oxidation and then decarboxylation accounting for the chain shortening (Figure 5.20). Certainly, this type of

O

R1

R2

pyrethrin I

Me

CH=CH2

pyrethrin II

CO2Me

CH=CH2

cinerin I

Me

Me

cinerin II

CO2Me

Me

jasmolin I

Me

Et

jasmolin II

CO2Me

chrysanthemic acid HOx

MeO2C

vCO2H pyrethric acid

pyrethrolone phenothrin

cinerolone

Cinerolone

phenothrin

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