Stereoselective synthesis of some novel heterocyclic estrone derivatives by intramolecular 1,3-dipolar cycloaddition

16,17-seco-3-Methoxyestra-1,3,5(10),16-tetraen-17-al undergoes intramolecular nitrone 1,3-dipolar cycloaddition with both hydroxylamine and N-methylhydroxylamine to produce a single isoxazolidine isomer in each case. The ring-closures of the hydrazones and the aldazine derived from the secoaldehyde lead to fused N-containing heterocycles via Lewis acid-induced cyclization of the intermediate azomethine imines.