On the Synthesis of Isomeric Dithiophene Analogues of Phenathridine- N-oxides

Six of nine o,o\u27-formylnitrobithienyls have been synthesized by the tetrakis(triphenylphosphine)palladium(O)-catalyzed coupling of the three o-bromonitrothiophenes with two of the three o-formylthiopheneboronic acids with sodium carbonate or sodium bicarbonate as base and an ethylene glycol dimethyl ether-water mixture as solvent. In the reaction with 3-formyl-2-thiopheneboronic acid, the coupling was carried out by using triethylamine as base and N,N-dimethylformamide as solvent in an attempt to avoid the facile deboronation of 3-formyl-2-thiopheneboronic acid, but without success. Reduction of the o,o\u27-formylnitrobithienyls gave high yields of the N-oxides of the six isomeric dithienopyridines, which are analogues of phenanthridine-N-oxide. A direct synthesis of one of the dithienopyridines, dithieno[2,3-c:2\u27,3\u27-c]-pyridine, was achieved by the palladium(O)-catalyzed coupling of 2,3-dibromothiophene with 2-formyl-3-thiopheneboranic acid to 3-bromo-2\u27-formyl-2,3\u27- -bithienyl, which was transformed to the 3-azido-2\u27-formyl-2,3\u27- -bithienyl, which upon reduction with hydrogen sulfide underwent ring closure to the phenanthridine analogue