The search for the MECP has been performed with SA-CASSCF method with (8e,6o) active space at 6-31+g(d) basis set. The orbitals included in the (8e,6o) active space are given in section S3 of SI. Intermediate geometries between FC region and MECP have been generated using linear interpolation and will be denoted as linearly interpolated internal coordinates (LIIC). LIIC potential energy surfaces (LIIC-PES) have been constructed with SA-CASSCF/6-31+g(d) level of theory followed by MS-CASPT2 level of theory with (10e,12o) active space. Minimum energy pathway (MEP) of Ds has been constructed by constraint optimizations at different C 12 -C 2 -C 3 -C 8 dihedral angles. The dihedral angle constrained optimization has been performed with TDDFT level of theory using CAM-B3LYP exchange functional at 6-311++g(d,p) basis set. The MEP is subsequently constructed with singlepoint calculations at TD-DFT optimized geometries with SA-CASSCF/6-31+g(d) followed by SS-CASPT2/6-31+g(d) to include dynamic correlation.