Facile Synthesis of Polysubstituted Indolizines via One-Pot Reaction of 1-Acetylaryl 2-Formylpyrroles and Enals

The Eschenmoser's Salt as a Formylation Agent for the Synthesis of Indolizinecarbaldehydes and Their Use for Colorimetric Nitrite Detection

C-H bond formylation is the most immediate way to incorporate the versatile formyl group into (hetero)aromatics. However, the type of reagents and severe conditions involved in the classical formylation methods often curtail their application, especially in the presence of other functional groups. Herein, we present the Eschenmoser's salt, a commercially available (dimethylamino)methylating chemical, as a useful reagent for the C-H formylation of indolizines and other compounds. The method is straightforward and mild, furnishing indolizinecarbaldehydes in modest-to-good yields with exclusive and remote regioselectivity. Furthermore, these compounds can be easily transformed into push-pull dyes and are highly selective in the colorimetric detection of nitrite, a substance extensively employed as preservative in the food industry, the concentration of which is crucial to control to prevent harmful effects in living organisms. The assay is simple, allowing the naked-eye detection of nitrite in solution or on a cotton swab for a wide range of concentrations.

Base-promoted [4 + 2] annulation of pyrrole-2-carbaldehyde derivatives with β,γ-unsaturated α-ketoesters: syntheses of 5,6-dihydroindolizines

A base-promoted [4 + 2] annulation of pyrrole-2-carbaldehyde derivatives with β,γ-unsaturated α-ketoesters for the syntheses of multisubstituted 5,6-dihydroindolizines was developed.

Michael-Aldol Double Elimination Cascade to Make Pyridines: Use of Chromone for the Synthesis of Indolizines

A domino Michael-aldol double elimination route to indolizines having two different acyl groups at the C5 and C7 positions is described where chromone is employed as a two-carbon unit for the synthesis of a pyridine moiety for the first time. Various analogues were readily accessed in good yields under metal-free and eco-friendly conditions. Further manipulation of the resulting products allowed entry to novel indolizine-heterocycle adducts, which are difficult to access by other methods.

A regioselective [4 + 2] annulation approach to 5-acylindolizine-7-carbonitriles: generation of poly-substituted pyridines

The reaction of β-ketonitrile and N-substituted pyrrole-2-carboxaldehyde in the presence of piperidinium acetate in CH₃CN permitted regioselective access to 5-acylindolizine-7-carbonitrile through a domino Knoevenagel condensation-intramolecular aldol cyclization sequence. A wide range of new indolizine derivatives with a poly-functionalized pyridine moiety were readily synthesized in an atom-economical fashion, thereby expanding the indolizine chemical space for further applications.

Domino [4 + 2] Annulation Access to Quinone-Indolizine Hybrids: Anticancer N-Fused Polycycles

A highly efficient synthetic route to new quinone-indolizine hybrids was accomplished from quinones and N-substituted pyrrole-2-carboxaldehydes via a domino Michael addition-aldol condensation-aromatization sequence through which the central pyridine ring was constructed in atom-economical and environment-friendly manner. Post modification of the resulting products was also demonstrated, enabling further expansion of this heterocyclic chemical space. Biological evaluation of the quinone-indolizine hybrids revealed potent anticancer effects in human prostate adenocarcinoma cells (PC-3) and oral adenosquamous carcinoma cells (CAL-27).