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
