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 o f the neocarzinostatin chromophore cyclonona-1,2,3,7-tetraen-5-yne to 1,5 d idehydroindene. The 298 K reaction activation enthalpy, exothermicity, and singlet-triplet splitting (H-0) of the product biradical are predicted to b e 17.8, 1.2, and -6.4 kcal/mol, respectively, at levels of theory showing n ear-quantitative agreement with experiment for the analogous cyclizations o f hex-3-ene-1,5-diyne and hepta-1,2,4-trien-6-yne. Factors controlling diff erences in the Myers-Saito cyclization compared to the Bergman cyclization are analyzed.