Halogenierte Cyclobutanone als variationsreiche Vorläufer von Pyrethroiden: Synthesen durch [2+2]‐Cycloadditionen von Haloketenen an 1,1‐disubstituierte Äthylene

Halogenated Cyclobutanones as Highly Versatile Precursors of Pyrethroids: Syntheses via [2+2]‐Cycloadditions of Haloketenes to 1,1‐Disubstituted Ethylenes [2+2]‐Cycloaddition reactions of the new chloro‐(2,2,2‐trichloroethyl)ketene (9) with various 1,1‐disubstituted ethylenes as well as [2+2]‐cycloadditions of highly halogenated ketenes of type 32 are described. The halogenated cyclobutanones thus obtained represent valuable intermediates in a new synthesis of insecticidal pyrethroids (Scheme 1). Ketene 9, prepared in situ from readily available precursors, parallels, if not exceeds, dichloroketene in its reactivity. With isobutene, the cyclobutanone 10 (a precursor for cis‐3‐(2,2‐dichlorovinyl)‐2,2‐dimethylcyclopropane carboxylic acid) is obtained in 70% yield. The reaction of 9 with propene is stereospecific and leads to cis‐17, exclusively. This result may be explained in terms of the energetically most favourable approach of the reacting molecules in a [π2s + π2s] process.