Total synthesis of the rubrolone aglycon

A total synthesis of the tubrolone 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 4 pi participation of an O-alkyl alpha,beta -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.