Preparation of 3-(10′-halo-9′-anthracenyl) isoxazolecarboxylic esters

The 10-halo (Cl or Br) anthracene-9-nitrile oxides (la,b) were obtained directly from the treatment of 9-anthracenylaldoxime with N-halosuccinimide (NCS or NBS) in DMF. The 3-(10′-halo-9′-anthracenyl)-5-isoxazolecarboxylic esters (5a,b and 6a,b) were prepared via 1,3-dipolar cycloaddition between the obtained nitrile oxides 1a (or 1b) and two different dipolarophiles: ethyl β-pyrrolidinocrotonate (an enamine of ethyl acetoacetate) or dimethyl acetylenedicarboxylate (DMAD) respectively. The 10 (or 10′)-position of the anthracene in either anthracene-9-nitrile oxide or 3-(9′-anthracenyl) isoxazole molecules (3,4) is readily halogenated by N-halosuccinimide in DMF. X-ray studies showed that 5a possesses two aromatic ring systems that lie at 74.4° from coplanarity. The bond linking the two ring systems is 1.4893(18) A, indicating only partial conjugation between the two ring systems. The crystal lattice showed unique head-to-tail intermolecular stacking of anthracene rings.