The work reveals that a chelate-type interaction in the transition state of a β-aminoalkyl radical in a copper(I)-catalyzed 5-exo-trig radical cyclization step changes the usual stereochemistry of the NH-pyrrolidine ring predicted by the Beckwith–Houk transition state model. In contrast, the rigidity in the fused β-aminoalkyl radical changes the Baldwin's predicted 5-exo to 6-endo cyclization mode, preferentially forming a piperidine ring over a pyrrolidine ring via a geometrically constrained transition state. The resultant diverse NH-pyrrolidines, pyrrolines and piperidines are sources of the bioactive natural product roseophilin and the drug Ritalin among others.