We then investigated the cycloaddition pathway from each of these conformers. For conformer I, we were unable to locate a concerted (4+2) transition state. Instead, the reaction follows a 'stepwise' mechanism, where only the bond between C3 of the indole and C5 of the TDO ring (𝑟 ! "" ) is formed at TS I (ΔG ‡ = 26.5 kcal mol -1 ); the second C-C bond (𝑟 # "" ) cannot be formed due to steric clash between the camphorsultam and the indole ring, leading to the high energy adduct III (ΔG = 24.4 kcal mol -1 ). However, for conformer II, a concerted, asynchronous (4+2) mechanism was found, which proceeds via TS II (DG ‡ = 23.1 kcal mol -1 ); this is followed by the unexpected, spontaneous extrusion of SO2 to form the dienamine product IV. Attempts to locate the sulfur-bridged (4+2) cycloadduct where unsuccessful. This pathway, which leads to the experimentally observed product, is 3.1 kcal mol -1 lower in energy than the stepwise mechanism.