Advancements in the Preparation of 4H-Chromenes: An Overview

Functionalized 4H-chromenes represent one class of the most important structural scaffolds in both synthetic and medicinal chemistry. They often appear in an arrange of biological natural products, pharmaceutical agents and drug candidates. The significance has stimulated the development of efficient methodologies for the synthesis of such compounds. In this review, a comprehensive discussion on different synthetic approaches to 4H-chromenes has been presented with an emphasis on reaction features and mechanisms. Three main categories are outlined: 1) nucleophilic addition to 2H-chromene derivatives, 2) 4H-chromene ring formation involved cycloaddition or cyclization reactions, and 3) miscellaneous reactions.1. Introduction2. Nucleophilic Addition to 2H-Chromene Derivatives2.1. Nucleophilic Addition to 1-benzopyrylium Ion 2.2. Nucleophilic Addition to 2-iminochromenes3. Tandem Michael Addition/Cyclization/Dehydration from 2-hydroxystyrene Derivatives3.1. Cyclization on the 2-hydroxychalcone Moiety3.2. Cyclization on the Nucleophile Moiety4. Formal o-QMs-Based [4 + 2]-Cycloaddition Reaction4.1. Reactions Using Pre-Prepared o-QMs4.2. Reactions Using insitu Generated o-QMs5. Miscellaneous5.1. Phenols-Involved Reactions5.2. o-Hydroxy Styrenes-Involved Reactions5.3. Intramolecular Reactions5.4. Others6. Conclusions