Solid-Phase Synthesis of Isoxazoles and Isoxazolines:  En Route to a New Class of Ionophores

Solid-phase organic synthesis (SPOS) was employed in the construction of isoxazole and/or isoxazoline polyheterocyclic systems. The [3 + 2] cycloaddition reactions of nitrile oxides, generated from the corresponding primary nitro compounds, with resin-bound alkynes or alkenes were highly efficient both chemically and with regard to purification. A parallel solution-phase synthesis was carried out which compared the synthetic efficiency in both media. SPOS of isoxazole/isoxazoline 11, employing a functionalized polystyrene resin, was found to be more efficient than the solution-phase approach. Three other oligoisoxazoles were prepared with average yields per step being 82, 84, and 89%.