1,3-Dipolar cycloaddition reaction applied to synthesis of new unsymmetric liquid crystal compounds-based isoxazole

In this study, a regioselective, simple and versatile copper(I)-catalyzed procedure for preparation of a series of liquid crystals based on unsymmetrical 3,5-disubstituted isoxazole was developed. Using different substituted chloro oximes and phenyl acetylenes, 1,3-dipolar cycloaddition reaction was carried out. A second series containing the isoxazole ring and a triple bond in the rigid core was also synthesized. From the 3-(4-bromophenyl)-5-(4-(decyloxy)phenyl)isoxazole, new liquid-crystalline compounds were prepared by Sonogashira cross-coupling. All of the derivates of the isoxazole ring compounds exhibited mesomorphism. Smectic C, smectic A, and nematic phases were observed by optical microscopy and DSC analysis. The yield of these reactions varied from moderate to excellent (47–93%). The structure of the rigid core was investigated by single crystal X-ray diffraction, which confirmed the regioselectivity of the [3+2] dipolar cycloaddition reaction.