[3+2]-Cycloaddition of Catalytically Generated Pyridinium Ylide: A General Access to Indolizine Derivatives

Pyridinium ylide is a versatile building block in synthetic organic chemistry, especially in the synthesis ofN-heterocycles. In this minireview, we have summarized the advances of [3+2]-cycloaddition of catalytically generated pyridinium ylide for the synthesis of indolizines (since 1970s). Accordingly, the ylide discussed in this article is generated in situ from carbene precursors andN-heteroarenes under catalytic conditions, including photocatalysis/thermocatalysis, and metal-catalysis. The metal-catalyzed version is the main focus, which is classified and summarized based on the type of dipolarophiles, alkyne and alkene, each class is further subdivided according to the metal catalysts used in these reactions. The analogous [3+2]-cycloaddition of vinyl metal carbene with pyridines, the formation of pyridinium ylide with other type of carbene precursors or with differentN-heteroarenes, have been also included herein separately.