Context <p>Lewis acid catalysis plays a central role in controlling the reactivity and selectivity of pericyclic reactions by polarizing the reacting fragments and stabilizing the corresponding transition states. In this work, the endo reaction pathway is examined in the absence of catalysis and in the presence of the Lewis acids BF₃ and AlCl₃ to clarify how differences in Lewis acidity translate into changes in transition-state structure and electronic organization. Emphasis is placed on identifying the origin of catalytic enhancement and stereochemical effects through a combined topological and spectroscopic analysis of the transition states, providing a consistent mechanistic picture based on electron density redistribution and noncovalent interactions.</p> Methods <p>All stationary points were located using density functional theory, and transition states were confirmed by frequency and intrinsic reaction coordinate calculations. Infrared spectra were obtained directly from harmonic frequency analyses and used to assess changes in carbonyl activation upon Lewis acid coordination. The electronic structure of the transition states was further analyzed using the quantum theory of atoms in molecules (QTAIM) to characterize bond critical points associated with forming bonds and Lewis acid–substrate interactions. Noncovalent interaction (NCI) analysis, combining reduced density gradient scatter plots and real‑space isosurfaces, was employed to visualize and compare weak stabilizing interactions across uncatalyzed and catalyzed systems. Together, these complementary methods provide a unified description of transition‑state polarization and catalytic effects.</p> Graphical abstract <p></p>