Fig. 2. (A) Plausible pathway for the cyclization and partial racemization of 5′. (B) Detailed energy profile of racemization of (M,R_a)-INT1. Calculations were conducted at B3LYP-D3(BJ)/6-311+G(2df,p)//B3LYP-D3(BJ)/6-31+G(d) for C, H, O, S and SDD for Cs with SMD (DMF). ΔG^‡ values represent the activation free energies (kcal mol⁻¹) of the forward reaction. The numbers in parentheses are the relative free energies (kcal mol⁻¹).