Progress in synthesis of imidazo-, benzoimidazo- and phenanthroimidazophenanthridine skeletons

Imidazo-, benzoimidazo-, and phenanthroimidazophenanthridines have emerged as structural motifs of significant interest in organic chemistry due to their versatile applications. These nitrogen-containing heterocycles serve as versatile building blocks for functional materials including organic semiconductors and luminescent materials, while their structural analogs exhibit notable biological activities in natural products and pharmaceutical compounds. The extended π-conjugation systems inherent to these fused polycyclic architectures make them particularly valuable for developing advanced optoelectronic materials, especially high-performance fluorescent and phosphorescent emitters in organic light-emitting diodes (OLEDs). Since their initial synthesis was reported in 1938, considerable efforts have been devoted to developing efficient and practical synthetic strategies for imidazophenanthridines, benzoimidazophenanthridines, phenanthroimidazophenanthridines, and their derivatives. The objective of this spotlight review is to provide a concise overview of the advancements in the synthesis of these frameworks. It encompasses the synthesis of benzo[4,5]imidazo[1,2-f]phenanthridines, imidazophenanthridines, encompassing imidazo[1,2-f]phenanthridines, imidazo[1,5-f]phenanthridines, and zephycandidine A, and phenanthro[9′,10’:4,5]imidazo[1,2-f]phenanthridines. Furthermore, it explores the diverse range of substrates employed, the multifaceted mechanistic pathways involved in these reactions, and other pertinent information.