A New Cascade Ring Enlargement of Isoxazolidines Formed from 2-Chloro-2-cyclopropylideneacetates

2-Chloro-2-cyclopropylideneacetate 1 and spiropentane analog 2 cycloadd pyrroline N-oxide 3 to give spiro[cyclopropane- 1,5A-isoxazolidine]s 4 and 5 in good yields; due to the presence of a chlorine substituent on the carbon a to the spirocyclopropane ring, which facilitates a cyclopropyl to cyclobutyl ring enlargement, these compounds undergo a cascade rearrangement to yield indolizinone derivatives 8 and 9 cleanly (70–73% yield), offering a new method for the synthesis of the indolizine skeleton.