temperatures of from 20 to 160 u00b0 c and preferably of from 80 to 140 u00b0 c are used the starting materials for the isomerization reaction are compounds of the formula ii which are preferably prepared as shown in scheme 2a by route a starting from a bis oxime monoether of the formula iv and a benzyl derivative of the formula iii or by route b starting from an oxime ether of the formula v and a hydroxylamine of the formula vi the reaction a is a nucleophilic substitution which can be 25 carried out under the customary reaction conditions the benzyl compounds iii are to be understood as compounds in which x is as defined in claim 1 and l 1 is a leaving group such as halogen acyloxy alkylsulfonyloxy or arylsulfonyloxy and in particular chlorine or bromine the substituents r 1 to r 5 of the bis oxime monoethers of the formula iv are as defined in claim 1 the reaction is expediently carried out in an inert solvent such as an ether for example tetrahydrofuran or dioxane or in a polar aprotic solvent for example acetone acetonitrile dimethyl sulfoxide sulfolane dimethylformamide or dimethylacetamide the base which is employed is usually sodium carbonate or potassium carbonate sodium hydride sodium methoxide or a tertiary amine the reaction can also be carried out in a two phase system for example dichloromethane water with the aid of a suitable phase transfer catalyst work up of the reaction mixtures can be carried out for example by extraction the benzyl compounds.