Unprecedented rearrangement of 5-benzoyl substituted bicyclic isoxazolidines to dehydro pyrrolizidin-2-ones and indolizidin-2-ones

5-Benzoyl substituted isoxazolidines 3, obtained by 1,3-dipolar cycloadditions of pyrroline N-oxide and tetrahydropyridine N-oxide to phenyl vinyl ketone, undergo a rearrangement to dehydro 3-phenyl-pyrrolizidin-2-one and 3-phenylindolizidin-2-one catalysed by Al2O3; the rearrangement products undergo a Michael addition to a second molecule of phenyl vinyl ketone.