Targeted Synthesis of 4-Chloro-3-formylthieno[2,3-b]pyridine and/or 4-Chlorothieno[2,3-b]pyridine by Reaction between N-Protected 3-Acetyl-2-aminothiophenes and Vilsmeier–Haack Reagent

Formylated chlorothieno[2,3-b]pyridine derivatives were synthesized by reaction between N-protected 3-acetyl-2-aminothiophenes and Vilsmeier–Haack reagent under classical conditions. These products were not accessible without N-protection of the starting materials or by reaction between the reagent and 4-chlorothieno[2,3-b]pyridine under any conditions. The conditions of the reaction could be altered to produce unformylated derivatives in better yields than reaction with unprotected aminothiophene.