Heterocyclic Interconversion. Pyridinium Salts to Indole-3-propenals

Significance

A new strategy for the synthesis of indole-3-propenals and the corresponding 7-aza series by ring opening of pyridinium salts has been developed. Pyridin-3-yl-2-anilines and the corresponding 2-aminopyridine derivatives, readily prepared by Suzuki cross-couplings, upon treatment with cyanogen bromide in warm ethanol form pyridinium ring-opened intermediates which, upon hydrolysis, afford substituted indole-3-propenals in good yields. The scope of the reaction in terms of R¹ and R² substituents was not sufficiently investigated.