Electrophile- and Lewis acid-induced nitrone formation and 1,3- dipolar cycloaddition reactions in the 13α- and 13β-estrone series

δ-Alkenyl oximes of 13α-estrone 3-methyl ether undergo intramolecular 1,3-dipolar cycloaddition reactions with BF3•OEt2 as catalyst, furnishing isoxazolidines. The reactions of the 13α- and 13β-estrone oximes with electrophiles lead to cyclic nitrones in high yields via attack of the oxime nitrogen on the intermediate halonium ions. In the intermolecular cycloadditions of 16-halomethyl nitrones to N-phenylmaleimide, single condensed aza-D-homo isoxazolidines are formed with high chemo- and stereoselectivity.