An Efficient Photo-driven Retro-Diels-Alder Reaction; Dienes on Demand

Skeletal editing enables precise structure modifications by inserting, deleting, or rearranging atoms to achieve diversification. Building on previous computational studies that highlighted the potential for cycloreversion of trans-cyclohexenes as a means of diene synthesis, and motivated by our ongoing interest in molecular transducers, we present the first experimental evidence for the retro-Diels-Alder reaction of trans-4-phenyl-3,6-dihydro-2H-pyran. Energy transfer photocatalysis leads to isomerization to the strained trans-isomer, where the strain energy is exploited to drive cycloreversion, yielding both diene and formaldehyde. Mechanistic studies support a charge-separated transition state, while computational models explain the unique reactivity of this heterocyclic ring. We employed sequential cycloreversion/cycloaddition reactions in a one-pot sequence to convert the parent aryldihydropyrans into [4+2] adducts with dienophiles, in addition to preparing 4-vinylcyclohexenes via self-dimerization, and dihydroterphenyl cores by adding 1,1-diarylethylene. This new photo-driven strategy facilitates use of dihydropyrans as diene synthons, providing routes to arylcyclohexenes via atom-pair swap skeletal editing.