The [3,3] sigmatropic rearrangement of the allyl vinyl ethers to the γ,δ-unsaturated carbonyl compounds, dihedral angle, which reduced the C 1 -C 6 distance (between two terminal allyl carbon atoms) from 3.37 to 2.33 Å and resulted in a chair-like TS (Fig. ). In the SI Figs. and, the chair form of the TS was shown to be 6.0 kcal/mol more stable than the boat form, consistent with previous computational analysis. The EDC plots in Figs. and showed that the weakening of the C 4 -O 3 σ bond (shown in magenta) and the strengthening of the C 2 -O 3 bond occurred in both reactant→TS and TS→product stages. On the other hand, the formation of the C 1 -C 6 σ bond and the weakening of C 1 -C 2 and C 5 -C 6 bonds occurred mostly after the TS.