Rearrangement of Isoxazoline-5-spiro Derivatives. 1. Synthesis of 4,5-Dihydroisoxazole-5-spirocyclopropanes and Their Rearrangement to 5,6-Dihydro-4-pyridones

Article abstract of DOI:10.1021/jo00246a007

The title spiroheterocycles 3 are prepared by cycloaddition of nitrile oxides to methylenecyclopropane or its ring-substituted derivatives.Thermolysis of the cycloadducts 3 affords, presumably via N-O bond homolysis, the title pyridones 7 besides minor amounts of the isomeric enaminones 6: the amounts of 6 relative to 7 are reduced when thermolysis is carried out by FVT rather than in solution.Similar results are obtained by photolysis of 3b in solution.Phenyl substitution in the cyclopropane ring selectively causes the C(spiro)-C(benzylic) bond fission with production of a single regioisomer, 7d.The rearrangement of the cycloadduct 3e from methylenenorcarane is selective, too: the cis-fused quinolinone 7e predominates over the trans-fused isomer, as a result of "torsional strain" in the latter transition state.