Figure 9. Flexible scan of the C(8)–O(14) bond.
Path 2: The H+ in solution participates in the cyclization process and attacks electron-rich O(29), leading to protonation. Intramolecular hydrogen migration occurs after intermediate med1 is converted into med2 and the H atom migrates to the carbonyl O atom. Product 3a’ is formed through the five-membered transition state, TS3. The calculation and analysis show that this path is reasonable, and the geometries of the stationary points in the reaction are shown in Figure 10.