THERMAL REARRANGEMENT OF NITRONE AND NITRILE OXIDE CYCLOADDUCTS TO BICYCLOPROPYLIDENE - SYNTHESIS OF 3-SPIROCYCLOPROPANE-4-PYRIDONE AND FURO[2,3-C]PYRIDINE DERIVATIVES

Bicyclopropylidene smoothly undergoes 1,3-dipolar cycloadditions to ni trones or nitrile oxides under different reaction conditions. The stra ined bisspirocyclopropanated isoxazolidines obtained from nitrones eas ily rearrange upon heating with selective opening of one of the two sp irofused cyclopropane rings. This process produces Li-pyridone, 7-indo lizinone, and 8-quinolizinone derivatives containing a spirocyclopropa ne moiety in the alpha-position to the carbonyl group in good yields. The same sequence of cycloaddition and rearrangement can be achieved i n a ''one-pot'' operation with considerable benefit for the reaction y ield. Bisspirocyclopropaneisoxazolines obtained from nitrile oxides ar e more stable than their saturated counterparts and rearrange only at higher temperature less chemoselectively. Opening of both spiro-fused cyclopropyl rings followed by aromatization produces interesting 2-sub stituted dihydrofuro[2,3-c]pyridine derivatives.