Synthesis of Enantiopure Indolizinones by Cascade Ring Enlargements of 4‘-Chlorospiro[cyclopropane-1,5‘-isoxazolidines]

2-Chloro-2-cyclopropylideneacetates (1-Me and 1-Et) and their spiropentane analogues 2 cycloadd
enantiopure five-membered cyclic nitrones to give the corresponding adducts (quantitatively, four
examples), which undergo cascade ring enlargements to yield indolizinone derivatives (53−70%, four
examples). The ring enlargement process is triggered by the abstraction of a bridgehead proton induced
by a base and can be suppressed by the presence of a bulky substituent nearby, such as a (triisopropylsilyl)oxy group.