Thermal Rearrangement of Nitrone and Nitrile Oxide Cycloadducts to Bicyclopropylidene.¹ Synthesis of 3-Spirocyclopropane-4-pyridone and Furo[2,3-<i>c</i>]pyridine Derivatives

Abstract

Bicyclopropylidene smoothly undergoes 1,3-dipolar cycloadditions to nitrones or nitrile oxides under different reaction conditions. The strained bisspirocyclopropanated isoxazolidines obtained from nitrones easily rearrange upon heating with selective opening of one of the two spirofused cyclopropane rings. This process produces 4-pyridone, 7-indolizinone, and 2-quinolizinone derivatives containing a spirocyclopropane moiety in the alpha-position to the carbonyl group in good yields. The same sequence of cycloaddition and rearrangement can be achieved in a "one-pot" operation with considerable benefit for the reaction yield. Bisspirocyclopropaneisoxazolines obtained from nitrile oxides are 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-substituted dihydrofuro[2,3-c]pyridine derivatives.