In situvinylpyrrole synthesis. Diels-Alder reactions with maleimides to give tetrahydroindoles

A series of 108 tetrahydroindoles has been prepared by a one-pot synthesis from 2-alkylpyrroles, cyclic ketones, maleimides, and in acid catalyst. A 5-vinylpyrrole is formed by an acid-catalyzed condensation of a 2-alkyl-substituted pyrrole with a ketone, which is subsequently trapped in situ by a maleimide in a predominantly endo-addition Diels-Alder reaction. Isomerization of the double bond into the pyrrole ring gives a tetrahydroindole with predominant cis-fusion of the cycloalkane ring.