Chirality and bioactivity
Contrary to the conventional wisdom that only pureenantiomers are bioactive, natural products with bioactivitiesare not always enantiomerically pure. For example, tribolure(4,8-dimethyldecanal), the aggregation pheromone of the redflour beetle, is a mixture of all the four stereoisomers in a ratioof (4R,8R)/.(4R,8S)/ (4S,8R)/ (4S,8S)= 4:4:1:1. Stereochemistrybioactivityrelationships among pheromones are not simplebut complicated. In the case of tribolure, a mixture with thenaturally occurring ratio of the stereoisomers is most active.Neither (R)- nor (S)-sulcatol (6-methyl-5-hepten-2-ol) isbehaviourally bioactive as the aggregation pheromoneof an ambrosia beetle, whereas their mixture is active. Inthe case of olean (1,7-dioxaspiro[5.5]undecane), the sexpheromone of the olive fruit fly, its (R)-isomer is active againstthe males, while its (S)-isomer is active against the females.