Regioisomerism in Symmetric Dimethyl Dialdehydes Dictates their Photochemical Reactivity

We herein report the first light-driven
selective monoderivatization
(desymmetrization) of two chemically equivalent carbonyl groups in
a single chromophore. By comparing of four symmetric regioisomers,
featuring two equivalent ortho-methylbenzaldehyde
units, we identify dimethyl­therephtalaldehydes (DMTAs) which
can be activated in a dual wavelength-selective fashion. Under visible
light and UV-light irradiation, DMTAs undergo two consecutive Diels–Alder
reactions exhibiting near-quantitative endo-selectivity
(>99%) and provide excellent yields (96–98%). The influence
of the regioisomerism of the dialdehydes on their photochemical behavior
is profound, evidenced by an in-depth investigation of their photochemical
performance. We exemplify the capability of the photosystems via the
synthesis of complex Diels–Alder adducts with various dienophiles,
including alkynes.