Electrocyclic rearrangement and MO symmetry explains unexpected reaction mechanism. We used Frontier MO (FMO) theory to rationalize the series of structural transformations in Scheme 5 by electrocyclic rearrangement around the polyene side chain within the key transition state, IX-Br (Fig. , Model 2). We show that a possible electrocyclic rearrangement at the terminal carbon (C1) and the nitrile group within IX-Br (Fig. ) favored a four-membered cyclic transition state that results in X-Br. Specifically, the two sites interacting in a conrotatory mode, resembling an intramolecular [2,3] sigmatropic rearrangement, leads to the formation of C1-C4 σ bond and a simultaneous breaking of C3-C4 σ and π bonds (Fig. ). The stereochemistry of the intramolecular [2,3] sigmatropic rearrangement followed the Woodward-Hoffman rules wherein the four-membered transition state, as well as, the π2-π, and σ1-σ MOs preserved a C2 symmetry during the conrotatory electrocyclic rearrangement (Fig. ). Mechanistically, both the Highest Occupied MO (HOMO) and the Highest Occupied MO-1 (HOMO-1) of the rearranged product were bonding orbitals that favored the four-membered transition states (X-Br and XI-Br, Scheme 5), which made the overall reaction from IX-Br to XIII-Br feasible. We performed the same calculations for the conformational isomer of III-I, represented as IX-I (Fig. ). Contrary to IX-Br, we did not observe a similar four-membered transition state with IX-I. Mechanistically, the non-bonding interactions between the HOMO (π2) of IX-I with HOMO-1 (π) of the cyclized product (Fig. ) was due to the electronic nature of the large-sized iodine atom. Finally, we observed that the sigmatropic rearrangement in IX-Br was reversible (Fig. ), which suggests that ring-closing and ring-opening occurs by using a shared transition state, such that the two molecules IX-Br and XIII-Br exist in equilibrium. This suggests that the lower experimental yield (entry 1 in Table ) of ROI may be attributed to the presence of these isomeric compounds in the reaction mixture.