Umlagerung von α‐Halogen‐ in α′‐Halogen‐cyclobutanone, Schlüsselstufe einer variationsreichen Synthese von Pyrethroiden

Rearrangement of α‐Halogen‐ to α′‐Halogen‐cyclobutanones, Key Step of a Highly Versatile Synthesis of Pyrethroids α‐Halogenocyclobutanones, which are readily available by [2 + 2]‐cycloaddition of haloketenes to terminal olefins ( e. g. 5 → 6 ), undergo an efficient and stereoselective cine ‐rearrangement to α′‐halogenocyclobutanones in the presence of catalysts such as tertiary amines, HX acids or quaternary ammonium salts ( e. g. 6 → 7 , Table 1 ). Preparative as well as mechanistic aspects of the cine ‐rearrangement are discussed. The 2,4‐ cis ‐disubstituted cyclobutanones 7–32 thus formed represent valuable intermediates in a new synthesis of pyrethroids 1 . The X‐ray structure of 2‐chloro‐4‐(2,2,2‐trichloroethyl)‐3,3‐dimethylcyclobutanone ( 7 ), the most important precursor of cis ‐ 3 (X = Cl) shows the following features: a puckered cyclobutanone ring (dihedral angle 31°), 2,4‐ cis ‐di‐pseudoequatorial arrangement of the chloro and trichloroethyl substituents, and an endo ‐deviation (0.225 Å; 11°) of the carbonyl O‐atom from the plane formed by C(1), C(2) and C(4) ( Fig. 2 ).