Theoretical Model for N‑Heterocyclic Carbene-Catalyzed Desymmetrizing [4 + 1] and [4 + 2] Annulations of an Enal and Aryldialdehyde with 1,3-Cyclopentenedione

N-Heterocyclic carbene (NHC) catalyzed desymmetrizing reactions of olefins have rapidly developed in recent years; however, the origins of the chemo- and stereoselectivities of these reactions remain poorly understood. Herein, we propose a mechanistic map for these reactions to predict how chemo- and stereoselectivities are controlled by different active intermediates (i.e., Breslow and acylazolium intermediates). Remarkably, our findings contradict a previous proposition that product structures are determined by a transformation between a pair of isomers.