Trends in Chrysanthemic Acid Chemistry: A Survey of Recent Pyrethrum Syntheses

Abstract

Nearly 80 years have passed since Staudinger and Ruzicka elucidated the structure of the active constituents of the insecticidal powder obtained from pyrethrum flowers (Tactenum cinerariaefolium, formerly Chrysanthemum cinerariaefolium, Figure 1).[1–4] As soon as the structures and absolute configurations of the six insecticidal esters (1a)– (1f) (Scheme 1, Table 1) naturally present in the extract had been fully confirmed,[5–8] synthetic analogues were investigated in attempts to elucidate the principles governing their activity and to discover more potent insecticides.[9] Developments in this field have led to a new generation of pyrethroids, such as (S)-bioallethrin (2), permethrin (3), and deltamethrin (4)[10] (Scheme 2), with greater insecticidal activity or a faster knockdown effect than the natural esters, as well as enhanced photostability.[10,11] Synthetic pyrethroids have therefore emerged to replace DDT (5) in crop protection; to their advantage they combine both low mammalian toxicity[12]∗ and biodegradability with high activity against a large number of insect types.