X=Y-ZH compounds as potential 13-dipoles. Part 29. The iminium ion route to azomethine ylides. Reaction of cyclic secondary amines with mono- and bI- functional aldehydes
give azomethine ylides. The anti-dipole is formed stereospecifically or, in the case of benzaldehyde and 2-methylbenzaldehyde, stereoselectively. The effect of the structure of the aldehyde on the stereochemistry of the derived azomethine ylide is rationalised in terms of steric and electronic effects. Inter- and intra-molecular trapping of the azomethine ylides gives cycloadducts in good yield.
decarboxylation, followed by intramolecular cycloaddtion of the resultant non-stabilised azomethine ylides. Evidence is produced for syn-anti dipole equilibration and it is found that intramolecularcycloadditions to a terminal alkyne involve only the anti-dipole and proceed via an exotransition state to give a single cycloadduct. In contrast, intramolecularcycloaddition to a terminal alkene involves