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).