Investigations of an Unexpected [2+2] Photocycloaddition in the Synthesis of (-)-Scabrolide A from Quantum Mechanics Calculations.

We utilize ab initio quantum mechanics calculations to evaluate a range of plausible mechanistic pathways for the unexpected formation of a [6-4-4] ring system from an enone-olefin photocycloaddition in the synthesis of (-)-scabrolide A, previously reported by our group. We present a mechanistic analysis that is consistent with all current experimental observations, including the photoexcitation, the C-C bond formation, and the associated chemo- and diastereoselectivity.