The solid-phase synthesis of pharmacologically interesting heterocycles is presented. The formation of a series of (5,5)-, (5,6)-, (6,5)-, and (6,6)-fused bicyclic ring systems was systematically studied by implementation of a common strategy involving N-acyliminium intermediates. These are highly reactive and transformed further in intramolecular cascade reactions with strong as well as weak C, N, S, and 0-nucleophiles. The methodology was successfully applied to the conversion of peptidomimetics into constrained small molecule core structures, such as the hexahydropyrrolo[2,1-b][1,3]oxazines, generally with full control of diastereoselectivity (>20:1) and in purities above 90%.