Thermal and (Thermo-Reversible) Photochemical Cycloisomerization of 1 H -2-Benzo[ c ]oxocins: From Synthetic Applications to the Development of a New T-Type Molecular Photoswitch.

A novel T-type molecular photoswitch based on the reversible cyclization of 1 H -2-benzo[ c ]oxocins to dihydro-4 H -cyclobuta[ c ]isochromenes has been developed. The switching mechanism involves a light-triggered ring-contraction of 8-membered 1 H -2-benzo[ c ]oxocins to 4,6-fused O -heterocyclic dihydro-4 H -cyclobuta[ c ]isochromene ring systems, with reversion back to the 1 H -2-benzo[ c ]oxocin state accessible through heating. Both processes are unidirectional and proceed with good efficiency, with switching properties─including reversibility and half-life time─easily adjusted via structural functionalization. Our new molecular-switching platform exhibits independence from solvent polarity, originating from its neutral-charge switching mechanism, a property highly sought-after for biological applications. The photoinduced ring-contraction involves a [2+2] conjugated-diene cyclization that obeys the Woodward-Hoffmann rules. In contrast, the reverse process initiates via a thermal ring-opening ( T > 60 °C) to produce the original 8-membered 1 H -2-benzo[ c ]oxocins, which is thermally forbidden according to the Woodward-Hoffmann rules. The thermal ring-opening is likely to proceed via an ortho -quinodimethane ( o -QDM) intermediate, and the corresponding switching mechanisms are supported by experimental observations and density functional theory calculations. Other transformations of 1 H -2-benzo[ c ]oxocins were found upon altering reaction conditions: prolonged heating of the 1 H -2-benzo[ c ]oxocins at a significantly elevated temperature (72 h at 120 °C), with the resulting dihydronaphthalenes formed via the o -QDM intermediate. These reactions also proceed with good chemoselectivities, providing new synthetic protocols for motifs found in several bioactive molecules, but are otherwise difficult to access.