N‐alkyl unsaturated imines derived from acrolein, a toxin produced during oxidative stress, and biogenic alkyl amines occur naturally and are considered biologically relevant compounds. However, despite the recent conceptual and technological advances in organic synthesis, research on the new reactivity of these compounds is lacking. This personal account discusses research on the reactivity that has been overlooked in acrolein imines, including the discovery of new methods to synthesize biologically active compounds, the determination of new functions of relevant imines and their precursors, i. e., aldehydes and amines, and the application of these methods for clinical diagnosis. The unique reactivity of unsaturated imines has been explored for their potential in novel chemical reactions. These new reactivities of imines have led to (i) the synthesis of biologically active natural products, (ii) the discovery of new biofunctions, and (iii) medical innovation for intraoperative cancer diagnosis.