,β-Epoxyketonitrones have been shown to be viable precursors of vinylnitrosonium cations upon treatment with trimethylsilyl trifluoromethanesulfonate. Variable-temperature NMR experiments indicate that this transformation occurs slowly at room temperature. The generated N=0 heterodiene undergoes an intramolecular [4 + 2]-cycloaddition with a pendant, unactivated olefin. A strong dependence of the outcome of the reaction on the starting nitrone geometry was observed. A number of α,β-epoxyaldonitrones have been prepared, and variable-temperature NMR experiments indicate that under identical conditions these substrates generate vinylnitrosonium cations at -30 °C and undergo analogous cycloaddition. Trapping of the intermediate N-alkyl-5,6-dihydro-4H-l,2-oxazinium cations has been accomplished with aqueous potassium cyanide. The stereochemistry of the cycloadducts has been assigned on the basis of1H NMR analysis and the geometry of the transition states deduced therefrom. A marked preference for reaction via transition states which contain an allylic (trimethylsilyl)oxy group in the plane of the heterodiene has been observed with aldonitrone precursors. The folding of the side chain containing the dienophile has been observed to be exclusively exo to the heterodiene with ketonitrone precursors and predominantly exo for aldonitrone precursors.