Regio- and stereoselective thermal cycloaddition of α-aryl-N-phenylnitrones to 16-dehydropregnenolone acetate: π-facial selective addition of the minor rotamers of the nitrones

Thermal cycloaddition of α-aryl-N-phenylnitrones to the C16C17 π-bond in 16-dehydropregnenolone-3β-acetate (1) involves only the minor rotamer (E-form) of the nitrones and occurs regio-, stereo- and π-facial-selectively to afford steroido[16,17-d]isoxazolidines (3) in high yield.