When Indolizine Meets Quinoline: Diversity-Oriented Synthesis of New Polyheterocycles and Their Optical Properties

Fluorescence-based technologies play a pivotal role in various biomedical applications. Here we report an efficient route to a new class of fluorophores, indolizino­[3,2-<i>c</i>]­quinolines, via the oxidative Pictet–Spengler cyclization strategy. The condensation of several 2-methylpyridines with 2-bromo-2′-nitroacetophenone allowed for the rapid assembly of indolizines with a 2-nitrophenyl group at the C2 position. The subsequent reduction of the nitro group under mild conditions followed by oxidative Pictet–Spengler cyclization with various aryl aldehydes in the presence of a catalytic amount of FeCl₃ furnished the indolizino­[3,2-<i>c</i>]­quinolines in good overall yields. We also examined the photophysical properties of this new series of polyheterocyclic compounds. Several indolizino­[3,2-<i>c</i>]­quinolines were found to have unique and desirable optical properties, suggesting that these compounds may be suitable for use as prospective fluorescent probes in aqueous systems