Access to Indoles via Diels-Alder reactions of vinylpyrroles.

Abstract

The indole moiety is extremely common in biologically active natural and un-natural products. Exploration and discovery of methods for generating the indole nucleus provides new and potentially more efficient options for synthetic approaches to indole-containing compounds. Vinylpyrroles have electron-rich pi systems and perform well as dienes in normal electron-demand Diels-Alder reactions with sufficiently electron-deficient dienophiles. The resulting substituted dihydro- or tetrahydroindoles are dehydrogenated to the corresponding indoles. Tosylation of pyrrole promotes acylation under Friedel-Crafts conditions at the difficult-to-access 3-position, which after reduction and dehydration gives 3-vinylpyrroles. These partially deactivated stable crystalline pyrroles have sufficient electron density to provide a novel and advantageous [4+2] cycloaddition route to indoles. Due to the high reactivity and consequent tendency of pyrroles to undergo undesired side-reactions, protecting groups are often desirable in pyrrole chemistry. However, the requirement for a sufficiently electron-rich diene, along with the sensitive nature of pyrroles, restricts the use of traditional blocking groups. Methylthio-protected 2-vinylpyrroles are shown to act as effective dienes in Diels-Alder reactions, demonstrating new blocking group techniques for chemistry involving sensitive pyrroles.