. Biochemical systematics. Biochemical variation; Botany. 266 BIOCHEMICAL SYSTEMATICS A non-lactone sesquiterpene of the guajol type, partheniol, is found in Parthenium H3C OH partheniol The presence of two similar sesquiterpenes, parthenin and ambrosin in the genera Parthenium and Ambrosia, the latter sub- stance actually occurring in both genera, is suggestive of a relation- ship between the two genera not readily apparent by their taxonomic disposition (that is, they are often treated as belonging to different tribes or sub-tribes). The suggested relationship is further strengt
. Biochemical systematics. Biochemical variation; Botany. 266 BIOCHEMICAL SYSTEMATICS A non-lactone sesquiterpene of the guajol type, partheniol, is found in Parthenium H3C OH partheniol The presence of two similar sesquiterpenes, parthenin and ambrosin in the genera Parthenium and Ambrosia, the latter sub- stance actually occurring in both genera, is suggestive of a relation- ship between the two genera not readily apparent by their taxonomic disposition (that is, they are often treated as belonging to different tribes or sub-tribes). The suggested relationship is further strength- ened by the discovery of a third substance, coronopilin (1,2-dihydro- parthenin) in both genera (Herz and Hogenaur, 1961). Although the eudesmol and guajol types of sesquiterpenes may not appear to be closely similar, the principal difference between the two lies in the type of cross linkage present. In the eudesmol type a C—C linkage yields a pair of six-membered rings; in the guajol type a C—C linkage yields a seven-membered and five-membered pair. This minor difference between the two types of sesquiterpenes may indicate close biosynthetic similarity. Therefore, it is not surprising to find these compounds restricted to a few rather closely related genera or even together in a single genus. The methyl substitution at position 5 in helenalin and other guajol derivatives is considered by Herz to represent a shift from position 4 of a substance such as partheniol, illustrated above. It is important to know whether the eudesmol or the guajol type is more primitive, but evidence is insufficient at this time to allow even useful speculation. It seems that the Anthemideae are much more versatile in terpene and sesquiterpene synthesis than other tribes of the family Compositae noted. At least 75 different sesquiterpenes are reported from a num- ber of different families, including those of the gymnosperms, dicots, and monocots (Karrer, 1958). However, within any closely circu
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