The Origin of the Halogen Effect on Reactivity and Reversibility of Diels-Alder Cycloadditions Involving Furan

摘要

Diels-Alder reactions involving furan as a diene are valuable processes, yielding versatile oxanorbornenes that have been used in the syntheses of numerous complex molecular architectures such as prostaglandins and terpenoids. Furan substitution greatly affects the viability of these reactions. Halogen substituents have been found to increase rates and yields. In addition, halogen atoms can be easily removed or used as handles in carbon-carbon bond-forming reactions. Klepo and Jakopcic found that chlorine, bromine, or iodine substituents on furan increase cycloaddition yields and cycloadduct stability (see yields for conversion of 1 to 2). More recently, Padwa and co-workers found that conversions of furanylamides 3 b-d to 5 b-d proceed at higher rates and increased yields than the unsubstituted system 3 a to 5 a (Scheme 1).