Chapter 5.2 - Five-Membered Ring Systems: Pyrroles and Benzo Analogs

Abstract

This chapter presents the synthesis and chemistry of pyrroles, indoles, and fused-ring systems. Pyrroles and especially indoles continue to draw a lot of attention from the scientific community because of their prevalence in natural products and wide range of biological and materials science applications. Multi-component reactions leading to pyrroles and pyrrole syntheses by transformation of other heterocycles are described in the chapter. Intramolecular ‘type a’ approaches to pyrroles mainly fall into three general categories: (1) the cyclocondensation reactions of γ-aminoketones; (2) the metal-mediated 5- endo cyclizations of homopropargyl amines; and (3) the metal-mediated 5- exo cyclizations of γ-aminoalkynes. Two established methods for preparing pyrroles via ‘type c’ processes include the intramolecular cyclization of enaminones and the ring-closing metathesis (RCM) of 4-aza-1,6-dienes (and subsequent oxidation). Several established pyrrole name reactions involve ‘type ac’ cyclizations including Knorr (α-aminocarbonyls + 1,3-dicarbonyls), Trofimov (oximes + alkynes), and Piloty–Robinson (hydrazines + aldehydes/ketones).