Structural and morphological aspects of small 3,5-disubstituted isoxazoles

The structural and morphological features of small 3,5-disubstituted isoxazoles/isoxazolines are presented and discussed in the light of thermal analysis by differential scanning calorimetry (DSC), polarized optical microscopy (POM) technique, combined with single crystal X-ray diffraction and molecular modeling. The title compounds obtained were also characterized by H-1, C-13 and F-19 NMR. Two of the 3,5-diarylisoxazoles reported here contain totally hydrogenated or fluorinated benzene rings on both sides of the heterocycle. Partially hydrogenated and fluorinated molecules were prepared alternating the position of the benzene rings on 3- and 5-position of the isoxazoles. Other isoxazoles were synthesized to compare the influence of fluorine atoms on the transitional properties. They were synthesized in two steps, starting from [3 + 2] 1,3-dipolar cycloaddition reaction of nitrile oxides with alkenes to yield the isoxazolines and subsequent MnO2-oxidation process to reach the final isoxazoles.