A mixture of 3.0 g of 4-cyano-3-fluorobenzaldehyde obtainable from 2-fluoro-4-nitrotoluene by oxidation to the corresponding benzoic acid, conversion into the corresponding nitrile, reduction to 2-fluoro-4-aminobenzonitrile and conversion of the corresponding diazonium salt with formaldoxime into the aldehyde in accordance with the method of Beech (J. Chem. Soc. 1297 (1954)); the target product is also obtainable from 2-fluoro-4-aminotoluene by conversion into 3-fluoro-4-methylbenzonitrile by the Sandmeyer method, hydrolysis of the nitrile, bromination of the side chain of the ethyl 3-fluoro-4-methylbenzoate obtainable, conversion into the aldehyde by the Sommelet method, conversion of the resulting oxime into ethyl 3-fluoro-4-cyanobenzoate, reduction to 3-fluoro-4-cyanobenzyl alcohol and subsequent oxidation to the target product) and 3.5 g of 2-n-heptylpropane-1,3-diol is heated together with 1 g of strongly acid cation exchanger in 100 ml of toluene for 8 hours using a water separator. The catalyst is then filtered off and the toluene is removed. After customary working-up, 2-(4-cyano-3-fluorophenyl)-5-n-heptyl-1,3-dioxane is obtained.
