Intramolecular cycloadditions with isobenzofurans .12. Synthesis and reactions of furo[3,4-d]isoxazoles

The synthesis of furo[3,4-d]isoxazoles (3) and inter- and intramolecular cycloaddition reactions (to 4, 5a,b) are reported. The structures of 3k, 5b and 6 have been clarified by X-ray crystallography. Quantum chemical calculations (AM1, PM3, ab initio, density functional methods) concerning the geometry and the different reactivity of benzo[c]furans and furo[3,4-d]isoxazoles were performed.