The Geography of Phytochemical Races

218 5 Wide Disjunctions
consisting of 78 genera and 1225 species, according to Mabberley (1997, p. 297),
and one that he defi nes as cosmopolitan in distribution. Xanthones are a well-known
class of secondary metabolite, many of which have been isolated from members
of Gentianaceae. They are biosynthesized through the condensation of a benzoyl
CoA derivative with three “acetate” units (actually, three malonyl CoA units), followed
by ring closure via phenol coupling, as illustrated in Fig. 5.1. Two routes
of ring closure are possible so that the fundamental, or base structures, that result
are 1,3,5,6-tetrahydroxyxanthone [389] and 1,3,6,7-tetrahydroxyxanthone [390].
Adaptations to this system can involve elaboration of either ring, or in several cases,
both. Xanthones are known in members of Gentianaceae that have all positions
on the A-ring oxygenated, others are known that have all positions on the B-ring
oxygenated. Still others are known that have lost one or more, or in some cases even
most, of the hydroxyl groups from the base structures.
With this preamble, we now look at the application of these structural alterations in
a search for trends within Gentianaceae, as outlined by Gottlieb and Kubitzki (1983)
in their paper on “Ecogeographical phytochemistry.” First, these workers determined
relative specialization values depending on the oxygenation patterns of the two rings,
that is, the level of oxygenation of each ring. (Note that O-methylation does not affect
the oxidation level of the ring and is thus not a consideration.) Points are accumulated
for each ring by counting the number of steps necessary to achieve the oxidation
level in any given compound. The total number of points accumulated for each
compound is then divided by the number of compounds that possess that pattern to
provide a measure of “evolutionary advancement” for each species, EA A for A-ring
patterns, EA B for the B ring. A plot of these parameters against each other resulted
Fig. 5.1 Compounds 389 and 390; xanthones from Gentianaceae showing their formation; and a
graph of oxidation indices