Total synthesis of granditropone, grandirubrine, imerubrine, and isoimerubrine

Concise total syntheses of the naturally occurring tropoloisoquinolines grandirubrine (1), imerubrine (2), and isoimerubrine (3) are detailed. The regioselective total synthesis of grandirubrine (1) is based on the [4 + 2] cycloaddition reaction of the alpha-pyrone 44 with the cyclopropenone ketal 18. Subsequent retro-Diels-Alder loss of CO2, norcaradiene rearrangement to the cycloheptatrienone ketal, and ketal hydrolysis provided the tropone 7 (granditropone). Regioselective hydroxylation of granditropone (NH2NH2; KOH) provided grandirubrine (1) and O-methylation of 1 provided both imerubrine (2) and isoimerubrine (3).