A new approach to difficult Fischer synthesis: The use of zinc chloride catalyst in triethylene glycol under controlled microwave irradiation

Application of triethylene glycol with catalytic quantity of zinc chloride (ZnCl2/TEG) is described as a new and efficient reaction medium for a difficult Fischer synthesis, leading to sensitive indoles. Transformation of the 3-acetyl-1-methylthiocycloalka[c]pyridine phenylhydrazones and p-methoxyphenylhydrazones into the 2-(2-pyridyl)indoles and 5-methoxy-2-(2-pyridyl)indoles, which are the synthons in our total synthesis of the sempervirine-type alkaloids, is carried out under controlled microwave irradiation in dry zinc chloride solution (0.16 M) in TEG. This protocol produces indoles from acetophenone or cyclohexanone via their phenylhydrazones in excellent yields.