Diastereoselective Photocycloaddition Reaction of Vinyl Ether Tethered to 1,4-Naphthoquinone

The intramolecular asymmetric photocycloaddition between 1,4-naphthoquinone and vinyl ether with a small (R)-point-chiral group on the tether was studied. Under photoirradiation, [2+2] cyclobutane products were exclusively obtained for this intramolecular system. The effect of solvent polarity on the stereoselectivity was significant, with predominant formation of the (2R,S³) over the (2R,R³) isomer in non-polar solvents being reversed in slightly polar dichloromethane. The detailed temperature-dependent study revealed the dominant diastereo-differentiating processes, which were switched between the ground-state equilibrium, the relative rate of bond formation in the triplet manifold, as well as deactivation processes between the pro-(S³) and pro-(R³) precursors, depending on the temperature domain examined. The enthalpic contribution (ΔΔH^≠) was always compensated by the entropic factor (ΔΔS^≠), implying the importance of solvation on the diastereo-differentiation steps. The mechanism of photocycloaddition, especially for the face-selective processes, is thoroughly discussed, which is supported by quantum chemical calculations on the ground-state circular dichroism (CD) spectral behavior as well as on the diastereomeric transition states in the triplet excited state.