compounds.
It
is
prepared
by
the
Diels–Alder
reaction
of
hexachlorocyclopentadiene and cyclopentadiene to chlordene, an unsaturated
hexachlorinated, tricyclic compound. Subsequent chlorination leads to products
containing two to three additional chlorines in the cyclopentane ring, that is, octaand nona-chlordanes. The main constituents in technical chlordane, cis- and transchlordane and trans-nonachlor, belong to these groups. Furthermore, there is a
number of more complex structures present in the technical mixture, resulting
from rearrangement reactions, in particular, Wagner-Meerwein rearrangements
(Dearth and Hites 1991). In Fig. 8.14, the molecular structures of cis- and transchlordane, oxychlordane, heptachlor, heptachlor exoepoxide, U82, MC4, MC5,
MC6, MC7 and trans- and cis-nona-chlor are shown. All derivatives possess an
endo configuration, which can be easily explained on the basis of the frontier orbital
theory valid for pericyclic reactions like the Diels–Alder reaction (Fleming 2009).
Furthermore, all compounds shown in Fig. 8.14 are chiral, apart from cis- and transnona-chlor. The chlordane components detectable in all four biota samples investigated by Buser et al. (1992) were the chiral congeners cis- and trans-chlordane,
MC4, MC5, MC6, MC7 and U82, the prochiral cis- and trans-nona-chlor, as well as
various minor components. Component K was not found to be present in these
extracts. For some congeners, a complete enantiomer separation was achieved by
Buser et al., and for other components enantiomer separation remained incomplete
(see Tables 8.11 and 8.12). The largest enantiomeric excess (ER value of 0.24) was
observed for MC5 in Baltic seal.
The major octa-chlordane component in both fish samples was cis-chlordane,
wherein herring the earlier and in salmon the later eluting enantiomer predominated.,
that is, the enantiomeric ratios of cis-chlordane between the two fish species are thus
reversed. For trans-chlordane, the authors also observed a reversal in the enantiomeric ratios between the two species. In this case, the later eluting enantiomer
predominated in herring and the earlier eluting in salmon. In seal and penguin, the
concentrations of cis- and trans-chlordane were much lower and, therefore, enantiomeric ratios were much more difficult to determine. In these two warm-blooded
species, the components U82 and MC5 were dominating. U82 is a 5+3 type chiral
octa-chlordane of, at that time, unknown configuration (Buser et al. 1992). At first,
Buser and co-workers were not able to resolve the enantiomers of U82, but their
endeavour was successful in a subsequent investigation, when they applied a
different chiral selector, heptakis(6-O-tertbutyldimethylsilyl-2,3-di-O-methyl)-βcyclodextrin (TBDMS-CD, to a technical chlordane mixture) (Buser et al. 1992).
Furthermore, Karlsson et al. meanwhile successfully elucidated the structure of U82
(see Fig. 8.14) (Karlsson et al. 1998, 1999). By contrast, MC5 was clearly separated
into enantiomers (Table 8.12).
The later eluting enantiomer predominated in seal tissue, and some preference for
this enantiomer was still observed in tissues of the other aquatic species included in
the study. Some differences in the enantiomeric composition of components MC4
and MC7 were also inferred from the data set. For component MC4, the chromatograms showed a clear predominance of the earlier eluting enantiomer in the warmblooded species, whereas this can hardly be observed in the two fish. For component
8.2 Transformation/Accumulation of Chiral Xenobiotics in Higher Organisms
151
It
is
prepared
by
the
Diels–Alder
reaction
of
hexachlorocyclopentadiene and cyclopentadiene to chlordene, an unsaturated
hexachlorinated, tricyclic compound. Subsequent chlorination leads to products
containing two to three additional chlorines in the cyclopentane ring, that is, octaand nona-chlordanes. The main constituents in technical chlordane, cis- and transchlordane and trans-nonachlor, belong to these groups. Furthermore, there is a
number of more complex structures present in the technical mixture, resulting
from rearrangement reactions, in particular, Wagner-Meerwein rearrangements
(Dearth and Hites 1991). In Fig. 8.14, the molecular structures of cis- and transchlordane, oxychlordane, heptachlor, heptachlor exoepoxide, U82, MC4, MC5,
MC6, MC7 and trans- and cis-nona-chlor are shown. All derivatives possess an
endo configuration, which can be easily explained on the basis of the frontier orbital
theory valid for pericyclic reactions like the Diels–Alder reaction (Fleming 2009).
Furthermore, all compounds shown in Fig. 8.14 are chiral, apart from cis- and transnona-chlor. The chlordane components detectable in all four biota samples investigated by Buser et al. (1992) were the chiral congeners cis- and trans-chlordane,
MC4, MC5, MC6, MC7 and U82, the prochiral cis- and trans-nona-chlor, as well as
various minor components. Component K was not found to be present in these
extracts. For some congeners, a complete enantiomer separation was achieved by
Buser et al., and for other components enantiomer separation remained incomplete
(see Tables 8.11 and 8.12). The largest enantiomeric excess (ER value of 0.24) was
observed for MC5 in Baltic seal.
The major octa-chlordane component in both fish samples was cis-chlordane,
wherein herring the earlier and in salmon the later eluting enantiomer predominated.,
that is, the enantiomeric ratios of cis-chlordane between the two fish species are thus
reversed. For trans-chlordane, the authors also observed a reversal in the enantiomeric ratios between the two species. In this case, the later eluting enantiomer
predominated in herring and the earlier eluting in salmon. In seal and penguin, the
concentrations of cis- and trans-chlordane were much lower and, therefore, enantiomeric ratios were much more difficult to determine. In these two warm-blooded
species, the components U82 and MC5 were dominating. U82 is a 5+3 type chiral
octa-chlordane of, at that time, unknown configuration (Buser et al. 1992). At first,
Buser and co-workers were not able to resolve the enantiomers of U82, but their
endeavour was successful in a subsequent investigation, when they applied a
different chiral selector, heptakis(6-O-tertbutyldimethylsilyl-2,3-di-O-methyl)-βcyclodextrin (TBDMS-CD, to a technical chlordane mixture) (Buser et al. 1992).
Furthermore, Karlsson et al. meanwhile successfully elucidated the structure of U82
(see Fig. 8.14) (Karlsson et al. 1998, 1999). By contrast, MC5 was clearly separated
into enantiomers (Table 8.12).
The later eluting enantiomer predominated in seal tissue, and some preference for
this enantiomer was still observed in tissues of the other aquatic species included in
the study. Some differences in the enantiomeric composition of components MC4
and MC7 were also inferred from the data set. For component MC4, the chromatograms showed a clear predominance of the earlier eluting enantiomer in the warmblooded species, whereas this can hardly be observed in the two fish. For component
8.2 Transformation/Accumulation of Chiral Xenobiotics in Higher Organisms
151