Quantum Chemical Characterization of the Cyclization of the Neocarzinostatin Chromophore to the 1,5-Didehydroindene Biradical

Quantum mechanical calculations have been carried out for the cyclization of the neocarzinostatin chromophore cyclonona-1,2,3,7-tetraen-5-yne to 1,5-didehydroindene. The 298 K reaction activation enthalpy, exothermicity, and singlet−triplet splitting (H₀) of the product biradical are predicted to be 17.8, 1.2, and −6.4 kcal/mol, respectively, at levels of theory showing near-quantitative agreement with experiment for the analogous cyclizations of hex-3-ene-1,5-diyne and hepta-1,2,4-trien-6-yne. Factors controlling differences in the Myers−Saito cyclization compared to the Bergman cyclization are analyzed.