Four-Component Cascade Heteroannulation of Heterocyclic Ketene Aminals: Synthesis of Functionalized Tetrahydroimidazo[1,2‑<i>a</i>]pyridine Derivatives

Abstract

An efficient and straightforward four-component synthetic protocol has been developed to synthesize imidazo­[1,2-<i>a</i>]­pyridines and imidazo­[1,2,3-<i>ij</i>]­[1,8]­naphthyridine derivatives incorporating medicinally privileged heterosystems from heterocyclic ketene aminals, aldehydes, diketene, and amines via cascade reactions, including diketene ring-opening, Knoevenagel condensation, aza–ene reaction, imine–enamine tautomerization, cyclocondensation, and intramolecular S<sub>N</sub>Ar. This strategy can provide an alternative approach for easy access to the highly substituted imidazo­[1,2-<i>a</i>]­pyridine derivatives in moderate to good yields using four simple and readily available building blocks under mild conditions. Importantly, the unusual splitting peaks in the <sup>1</sup>H NMR spectra of the products derived from heterocyclic ketene aminals with an <i>o</i>-halogen atom on the aryl ring were explained reasonably by varying the temperature in NMR analysis

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