Synthesis and structures of three isoxazole-containing Schiff bases

The synthesis and structures of three isoxazole-containing Schiff bases are reported, namely, (E)-2-{[(isoxazol-3-yl)imino]­meth­yl}phenol, C(10)H(8)N(2)O(2), (E)-2-{[(5-methyl­isoxazol-3-yl)imino]­meth­yl}phenol, C(11)H(10)N(2)O(2), and (E)-2,4-di-tert-butyl-6-{[(isoxazol-3-yl)imino]­meth­yl}phenol, C(18)H(24)N(2)O(2). All three structures contain an intra­molecular O—H⋯N hydrogen bond, alongside weaker inter­molecular C—H⋯N and C—H⋯O contacts. The C—O(H) and imine C=N bond lengths were consistent with structures existing in the enol rather than the keto form. Despite having dihedral angles <25°, none of the com­pounds were observed to be strongly thermochromic, unlike their anil counterparts; however, all three com­pounds showed a visible colour change upon irradiation with UV light.