Methods for the Construction of the Indolizine Nucleus*

Methods for the synthesis of various types of indolizines and a characteristic selection of their aza analogues are described. The reactions involved are classified as (a) condensation reactions, (b) 1,3-dipolar cycloadditions, and (c) 1,5-dipolar cyclizations. 1. Synthesis of Indolizines by Condensation Reactions 1.1. Reactions of 2-Methylpyridine and its Derivatives with Acid Anhydrides (Scholtz Reaction) 1.2. Cyclizations of Quaternary Pyridinium Salts (Tschitschibabin Reaction) 1.3. Cyclizations of 3-(2-Pyridyl)-1-propanols and their Derivatives 1.4. Reactions of Heteroaromatic Nitrogen Compounds with Acetylenic and Olefinic Compounds 1.5. Miscellaneous Condensations 2. Synthesis of Indolizines by 1,3-Dipolar Cycloadditions 2.1. Reactions of Pyridinium and Related Heteroaromatic Ylids 2.2. Reactions of N-Iminopyridinium Ylids 2.3. Reactions of Miscellaneous 1,3-Dipoles 3. Synthesis of Indolizines by 1,5-Dipolar Cyclizations