The Geography of Phytochemical Races

5.2 Nonvascular Plants 239
compounds, and showed signifi cant quantitative differences between carposporophytes
and tetrasporophytes. In the case of mertensene, there was a decrease from 26.2 to
6.4%; with violacene there was an increase from 26 to 47.7%.
Other studies of P. cartilagineum have been reported, almost all of which
revealed some level of variation among populations. An example involves plants
collected near the Isle of Wight (50°41′N, 1°5′W), which yielded fi ve cyclic monoterpenes
having both chlorine and bromine as substituents (Higgs et al., 1977), with
structures of the general nature seen above, for example, 477. Plants collected in
the vicinity of James Island on the Antarctic Peninsula (>60°S) (Stierle and Sims,
1979), and from two sites on the eastern coast of Tasmania (Mayfi eld Bay and
Schouten Beach) (Jongaramruong and Blackman, 2000) accumulated signifi cant
amounts of acyclic monoterpenes of the sort illustrated in Fig. 5.11.
It is obvious that production of polyhalogenated monoterpenes is a characteristic
feature of the red algae. Within this group of natural products, one sees an almost
bewildering degree of structural variation involving the nature of the halogen atom
present, the number of halogen atoms present, the stereochemistry of the substituents,
and whether the parent compound is cyclic or acyclic. It is diffi cult, possibly even
impossible, to assign chemotaxonomic signifi cance to the presence or absence of any
given compound, a situation that can only be remedied when detailed information
on their biosynthesis becomes available. Is there a specifi c enzyme responsible for
the placement of each halogen atom at each specifi c location? Likely not. The fundamental
terpene molecule is quite reactive by itself, but the stereochemistry of the
products does suggest highly specifi c surfaces (enzyme faces) upon which the various
reactions might occur. Despite the absence of defi nitive biosynthetic information, we
can still appreciate the remarkable chemical synthetic capacities of these organisms.
If the differences in chemical profi les do nothing other than suggest further study of
possible evolutionary relationships, then the chemicals have performed a useful function.
Examples from other genera follow.
5.2.15 Portieria hornemanii (Rhodophyllidaceae)
Widely distributed in the Pacifi c, Portieria hornemanii (Lyngbye) Silva (synonyms:
Chondrococcus hornemannii, Desmia hornemanii) has provided natural-product
chemists with another interesting assortment of secondary metabolites, including
more additions to the list of halogenated terpenes. Studies by Gunatilaka et al. (1999)
revealed structures of two compounds from plants collected from a variety of reef
sites on Guam and the Mariana Islands. Characteristic of the chemistry of this species
are the cyclic compounds, apakaochtodene-A [488], apakaochtodene-B [489] and
ochtodene [490] (see Fig. 5.12 for structures 488–492). This species also produces
large numbers of acyclic halogenated terpenes represented here by (2Z)-6-bromo-
3-chloromethyl-1,7-dichloro-7-methylocta-2-ene [491], identifi ed from collections
from Nelly Bay, Great Barrier Reef (A. D. Wright et al., 1990, 1991), and (2Z)-1,6dichloro-3-chloromethyl-7-methylocta-2,6-diene
[492], also from the Great Barrier