2-(Methoxycarbonyl)ethyl as a Removable N-Protecting Group: Synthesis of Indoloisoquinolinones by Pd(II)-Catalyzed Intramolecular Diamination of Alkynes

Pd(II)-catalyzed double cyclization of 1,2-diarylethynes bearing an N-methyl-N-[2-(methoxycarbonyl)ethyl]amino and an aminocarbonyl group at the ortho positions of the two aromatic rings afforded the tetracyclic N-[2-(methoxycarbonyl)-ethyl]indoloisoquinolinones in good to excellent yields. The N-[2-(methoxycarbonyl)ethyl] group is readily removed under basic conditions (DBU, DMF, 120 °C) to afford the corresponding tetracycles with a free indolyl nitrogen in excellent yields. The 2-(methoxycarbonyl)ethyl as a removable N-protecting group is illustrated in other Pd(II)- and Pd(0)-catalyzed and selenium-mediated transformations.