3,6-Disubstituted Cyclohexenones in the Synthesis of New Three Ring Liquid Crystalline Compounds with the Negative Dielectric Anisotropy

New improved approach for preparation of the three ring liquid crystalline compounds with 2,3-difluorobenzene moiety is proposed. The key stage is the synthesis of 3,6-disubstituted cyclohex-2-en-1-ones via the condensation of the corresponding Mannich salts with 2-substituted acetoacetic esters (or methyl benzyl ketones) in the presence of base. The chlorination with phosphrous pentachloride of these cyclohexenones or methylation with methylmagnesium iodide followed oxidative aromatization give new three ring compounds, which could be used in the LC compositions with negative dielectric anisotropy.