Figure 6: Overview of all reactions steps for the proposed mechanism.
(a) Calculated reaction path for photo-excited 8. Shown are the ΔG values (in eV) for the isolated molecules. Excitation of 8 with UV-A light and sub-ps ISC to the degenerate T4 state and IC to the T1 state allows the formation of the intermediate diradical 14(T1). Next, a hydrogen atom is transferred to afford the triplet diradical 6(T1). This intermediate either undergoes a stepwise 1,3 dipolar cycloaddition, including an ISC, to afford 4 (pathway ), or relaxes to the ground state oxidopyrylium 6(S0) (via pathway ). (b) Oxidopyrylium 6(S0) can undergo further photochemical reactions. After excitation with blue light to the S2 state, a S1/S0 conical intersection seam can lead to epoxy cyclopentenone 16 in a reaction that could be interpreted as a conrotatory 4π-electrocyclization. (c) Graphical representation of the reaction path. Vinylogous nucleophilic substitution with water affords 11 from 16.
