Palladium-Catalyzed Double Reductive Cyclization of 2,3-Dinitro-1,4-dialkenylbenzenes. Synthesis of 1 H ,8 H -Pyrrolo[3,2- g ]indoles

A flexible route to both symmetrical and unsymmetrical 1 ,8 -pyrrolo[3,2- ]indole has been developed. The key and ultimate step is a double palladium-catalyzed, carbon monoxide mediated reductive cyclization of 1,4-dialkenyl-2,3-dinitrobenzenes. The cyclization precursors were prepared by a double Kosugi-Migita-Stille cross coupling of 1,4-dibromo-2,3-dinitrobenzene with an alkenyltin reagent to give symmetrical products. Unsymetrical cyclization precursors were prepared by two sequential cross couplings using 4-iodo-2,3-dinitrophenyl trifluoromethanesulfonate as the starting material.