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-c]­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-c]­quinolines in good overall yields. We also examined the photophysical
properties of this new series of polyheterocyclic compounds. Several
indolizino­[3,2-c]­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.