Among commercial products containing pyrethroids, there is a predominance of
racemic formulations (equal proportions of enantiomers) and enriched isomers
[8]. Although single isomers (e.g. gamma-cyhalothrin and bioresmethrin) are more
efficient and environmentally safe due to their specific effect on target receptors,
their industrial-scale production is often limited by cost-efficient technologies
[2, 4]. On the other hand, only a few enantiomers in racemic formulations have
the desired action. For example, only one enantiomer of each pair of diastereomers
of permethrin (1R-cis and 1R-trans) and cypermethrin (1R-cis-αS and 1R-trans-αS)
has strong insecticidal activity. The remaining two enantiomers of permethrin and
six stereoisomers of cypermethrin are not as efficient or have no specific activity
[9]. Considering racemic permethrin, if only 50% of enantiomeric molecules are
efficient as insecticides, a greater environmental burden is expected due to the
expense of material resources and the need for greater volume of application. In
addition to the increased risk of contamination of urban and agricultural environments, possible impacts on nontarget organisms are expected for all permethrin
enantiomers, since toxic effects were reported for some insecticidal enantiomers,
and endocrine disruption and immunotoxicity are related to others [10–12].
Considering the widespread use of pyrethroids and their chemical complexity, it
is essential to consider their stereoisomerism to more accurately assess the persistence, risk of bioaccumulation and possible undesired impacts of pyrethroids on
nontarget organisms. In this sense, an achiral analytical approach in environmental
and toxicological studies is able to only partially assess the potential adverse effects
of pyrethroids in biological systems [3].
Therefore, this chapter presents data with the aim of discussing the stereochemical behaviour of pyrethroids in the environment. Relevant studies on the consequences of pyrethroid toxicity to nontarget organisms, the potential bioaccumulation
of pyrethroids and their fate at isomeric levels and the use of isomeric profiles as
markers of environmental origin will be discussed.
2 Pyrethroid Structure Configuration
The synthesis of pyrethroids was modelled upon esters (pyrethrins) that constitute
approximately 25–50% of pyrethrum, a natural extract of Chrysanthemum spp.
flowers used for centuries as insecticide [13]. Among the six isolated esters of
pyrethrum responsible for its insecticidal activity, there are two related groups:
three esters similar to cyclopropanecarboxylic acid, also named chrysanthemic
acid, and three esters related to pyrethric acid [14]. Both acids occur esterified
with three alcohols (cinerolone, jasmolone and pyrethrolone), known generically
as rethrolones (Fig. 1a). The esterification of the chrysanthemic acid with each
rethrolone generates pyrethrins I, while the esterification of pyrethric acid with
rethrolones forms pyrethrins II [14].
Among these main structures found in pyrethrum extract, chrysanthemic acid
served as a model for the synthesis of pyrethroids. Chrysanthemic acid has two
Stereoselectivity and Environmental Behaviour of Pyrethroids
151
racemic formulations (equal proportions of enantiomers) and enriched isomers
[8]. Although single isomers (e.g. gamma-cyhalothrin and bioresmethrin) are more
efficient and environmentally safe due to their specific effect on target receptors,
their industrial-scale production is often limited by cost-efficient technologies
[2, 4]. On the other hand, only a few enantiomers in racemic formulations have
the desired action. For example, only one enantiomer of each pair of diastereomers
of permethrin (1R-cis and 1R-trans) and cypermethrin (1R-cis-αS and 1R-trans-αS)
has strong insecticidal activity. The remaining two enantiomers of permethrin and
six stereoisomers of cypermethrin are not as efficient or have no specific activity
[9]. Considering racemic permethrin, if only 50% of enantiomeric molecules are
efficient as insecticides, a greater environmental burden is expected due to the
expense of material resources and the need for greater volume of application. In
addition to the increased risk of contamination of urban and agricultural environments, possible impacts on nontarget organisms are expected for all permethrin
enantiomers, since toxic effects were reported for some insecticidal enantiomers,
and endocrine disruption and immunotoxicity are related to others [10–12].
Considering the widespread use of pyrethroids and their chemical complexity, it
is essential to consider their stereoisomerism to more accurately assess the persistence, risk of bioaccumulation and possible undesired impacts of pyrethroids on
nontarget organisms. In this sense, an achiral analytical approach in environmental
and toxicological studies is able to only partially assess the potential adverse effects
of pyrethroids in biological systems [3].
Therefore, this chapter presents data with the aim of discussing the stereochemical behaviour of pyrethroids in the environment. Relevant studies on the consequences of pyrethroid toxicity to nontarget organisms, the potential bioaccumulation
of pyrethroids and their fate at isomeric levels and the use of isomeric profiles as
markers of environmental origin will be discussed.
2 Pyrethroid Structure Configuration
The synthesis of pyrethroids was modelled upon esters (pyrethrins) that constitute
approximately 25–50% of pyrethrum, a natural extract of Chrysanthemum spp.
flowers used for centuries as insecticide [13]. Among the six isolated esters of
pyrethrum responsible for its insecticidal activity, there are two related groups:
three esters similar to cyclopropanecarboxylic acid, also named chrysanthemic
acid, and three esters related to pyrethric acid [14]. Both acids occur esterified
with three alcohols (cinerolone, jasmolone and pyrethrolone), known generically
as rethrolones (Fig. 1a). The esterification of the chrysanthemic acid with each
rethrolone generates pyrethrins I, while the esterification of pyrethric acid with
rethrolones forms pyrethrins II [14].
Among these main structures found in pyrethrum extract, chrysanthemic acid
served as a model for the synthesis of pyrethroids. Chrysanthemic acid has two
Stereoselectivity and Environmental Behaviour of Pyrethroids
151