Structure–Reactivity Correlations and Mechanistic Understanding of the Photorearrangement and Photosalient Effect of α‑Santonin and Its Derivatives in Solutions, Crystals, and Nanocrystalline Suspensions

The synthesis, crystal packing and
photochemical reactions of α-santonin
(1a) and its methyl, ethyl, n-propyl,
and n-butyl derivatives (1b–e) are described to explore the effect that a photochemically
benign yet structurally significant synthetic modification can have
on the solid-state photochemical reactivity. The structures of the
derivatives were determined using single crystal X-ray diffraction
and compared against the packing of α-santonin. A cage dimer
(12a) found exclusively upon irradiation of 1a in the solid state was not found when the other derivatives were
exposed to light, because the alkylation proved to perturb the crystal
packing away from an optimal dimerization alignment. Using a high-speed
camera, we monitored the photosalient effect of α-santonin and
found it to occur at an angle orthogonal to the b-axis of the unit cell, which we suspect is caused by the formation
of the cage dimer 12. The photochemistry of 1a–e in solution and crystalline suspensions was
also analyzed. The solution photochemistry was in accord with literature
precedence, and the crystalline suspensions yielded a variety of photoproducts
including a tertiary alcohol (7b–e), which is not commonly observed in neutral water. An exocyclic
alkene photoproduct (8b) was also discovered, and its
presence is hypothesized to be caused by an intermolecular deprotonation
caused by a water molecule present in the crystal.