The versatile chemistry of thiophenes: The effects of b ‐side contra c ‐side annelation on reactivity

The synthesis of the nine dithienopyridines and twenty‐four thienonaphthyridines with angular annelation is described. They are all conveniently obtained by Pd(0)‐catalysed couplings between heterocyclic o ‐formylboronic acids or o ‐formylstannyl derivatives with heterocyclic o ‐amino‐halo derivatives. The effect of the use of silver oxide and cupric oxide as co‐reagent is discussed. Differences in electrophilic substitution, metalation and cycloaddition of b ‐ and c ‐fused isomers is treated. Theoretical calculations at the RHF/3–21G* level are in agreement with the orientation and reactivity in the nitration of dithieno[3,4‐ b :3′,4′‐ d ] pyridine and dithieno[2,3‐ b :3′,2′‐ d ] pyridine observed experimentally. The preparation of N ‐oxides of the above‐mentioned ring‐systems and their reactivity will also be discussed.