Cycloaddition of nitrones 1 with methylene-γ-butyrolactones 2, 3 and 4 afforded spiroadducts 5, 6 and 7 with high selectivity. Mixtures of diastereoisomers were usually obtained, whose structures were established by ¹H and ¹³C NMR spectroscopies or X-ray crystallography. Treatment of spiroadducts in acidic and alkaline media led to different, unexpected and novel rearrangements.