X=Y–ZH systems as potential 1,3-dipoles. Part 57: Cascade 1,3-azaprotio cyclotransfer–cycloaddition reactions between aldoximes and divinyl ketone: remarkable rate enhancement and control of cycloaddition regiochemistry by Lewis acids

The tandem 1,3-azaprotio cyclotransfer–cycloaddition reaction between aldoximes and divinyl ketone affords the exo-isomers of substituted 1-aza-7-oxabicyclo[3.2.1]octan-4-ones when a substoichiometric amount of hafnium(IV) chloride, zirconium(IV) chloride or aluminium(III) chloride is added.