Total Synthesis of the Rubrolone Aglycon

A total synthesis of the rubrolone aglycon is detailed and is based on two key Diels−Alder reactions. The AB ring system incorporating a tetrasubstituted pyridine was assembled, enlisting the rare 4π participation of an O -alkyl α,β-unsaturated oxime in an intramolecular [4 + 2] cycloaddition reaction (70%). The C-ring oxygenated tropolone was introduced through a room-temperature, exo selective [4 + 2] cycloaddition of a cyclopropenone ketal (97%) followed by in situ generation of a norcaradiene and room-temperature electrocyclic rearrangement to a cycloheptatrienone ketal appropriately substituted for hydrolysis directly to a 2,4-dihydroxycycloheptatrienone.