The Use of Electrophilic Cyclization for the Preparation of Condensed Heterocycles

Abstract Condensed heterocycles are well-known for their excellent biological effects and they are undeniably important compounds in organic chemistry. Electrophilic cyclization reactions are widely used for the synthesis of mono-heterocyclic compounds. This review highlights the utility of electrophilic cyclization reactions as an effective generic tool for the synthesis of various condensed heterocycles containing functional groups that are able to undergo further chemical transformations, such as nucleophilic substitution, elimination, re-cyclization, cleavage, etc. This review describes the reactions of unsaturated derivatives of different heterocycles with various electrophilic agents (halogens, arylsulfanyl chlorides, mineral acids) resulting in annulation of an additional partially saturated heterocycle. The electrophilic reaction conditions, plausible mechanisms and the use of such transformations in organic synthesis are also discussed. The review mainly focuses on research published since 2002 in order to establish the current state of the art in this area. 1 Introduction 2 Electrophilic Cyclization Pathways Involving a Nitrogen Nucleo­philic Center 3 Electrophilic Cyclization Pathways Involving a Chalcogen Nucleophilic Center 3.1 Sulfur Centers 3.2 Oxygen Centers 3.3 Selenium Centers 4 Strategies and Mechanisms 5 Conclusion