Recent advances in the synthesis of indolizines and their π-expanded analogues

Abstract

Indolizine (pyrrolo[1,2-a]pyridine) is one of the five isomers of indole and it serves as a precursor for widespread indolizidine alkaloids. The straightforward synthesis of indolizines based on classical methodologies such as Scholtz or Chichibabin reactions has overshadowed numerous new strategies that have been revealed especially within the last ten years. The desire to achieve substitution patterns which were hard to build sparked the discovery of completely new pathways, e.g. transition metal-catalyzed reactions and approaches based on oxidative coupling. In this review, selected strategies toward indolizines published since 2005 are briefly summarized, commented upon, compared, and illustrated. The literature discussed here involves reactions based on either pyridine or pyrrole scaffolds, as well as selected methodologies leading to π-expanded indolizines.

Development and Cycloaddition Reactivity of a New Class of Pyridine‐Based Mesoionic 1,3‐Dipole

“…[11,16] Typical synthetic approaches to these products include cyclizations with synthetic pyridine or pyrrole derivatives, [17][18][19] substitution on preformed indolizines, [20] or 1,3-dipolar cycloaddition with pyridinium ylides. [11,16] Typical synthetic approaches to these products include cyclizations with synthetic pyridine or pyrrole derivatives, [17][18][19] substitution on preformed indolizines, [20] or 1,3-dipolar cycloaddition with pyridinium ylides.…”

Development and Cycloaddition Reactivity of a New Class of Pyridine‐Based Mesoionic 1,3‐Dipole

“…Indolizines are present in ar ange of pharmaceutically relevant products as well as electronic materials. [11,16] Typical synthetic approaches to these products include cyclizations with synthetic pyridine or pyrrole derivatives, [17][18][19] substitution on preformed indolizines, [20] or 1,3-dipolar cycloaddition with pyridinium ylides. [21] 2 can be considered as tabilized, mesoionic analogue to these latter 1,3-dipoles,and offers the ability to incorporate ar ange of aryl, heteroaryl and alkyl units into the 3-indolizine position from simple imines.Asan illustration of the utility of this approach, ester-substituted indolizines such as 5 (Scheme 3) has been shown by Lan and Yout ob es trong blue emitting materials of use in fluorescence imaging.…”

Iodine Mediated Base‐Controlled Regio‐Selective Annulation of 2‐(Pyridin‐2‐yl)acetate Derivatives with Acrylic Esters for the Synthesis of Indolizines

“…Considering the wide range of biological activities, developing efficient synthetic methods of indolizines has attracted wide attention in the fields of organic chemistry and chemical biology . The majority for the preparations of indolizines include Tschitschibabin reaction, 1,3‐dipolar cycloaddition, 1,5‐dipolar cycloaddition, I 2 ‐mediated oxidative cyclization, transition‐metal‐catalyzed intramolecular cyclization and intermolecular cyclization .…”