2<i>H</i>‐Pyrrole Derivatives from an Aza‐Nazarov Reaction Cascade Involving Indole as the Neutral Leaving Group

Abstract

Abstract Trifluoromethyl‐substituted N ‐indolinyl‐1‐aza‐1,4‐dien‐3‐ones 4 , which are accessible in good yields from indolinylimino esters 6 in a two‐step procedure, undergo a novel type of aza‐Nazarov cyclization if treated with strong acids to give the hitherto unknown 3‐hydroxy‐5 H ‐pyrrole derivatives 8a – p . The solvent‐free polyphosphoric acid/acyl anhydride system as the acidic reaction medium is especially efficient and requires only short reaction times. According to quantum chemical calculations the key step in the reaction cascade is the cleavage of the N–N bond of the hydrazone fragment of a protonated N ‐(indolin‐1‐yl)‐1‐aza‐1,4‐dien‐3‐one intermediate such as 10 . This intermediate releases 3 H ‐indole as an unusual, but very efficient neutral leaving group. Several 1‐aza‐1,4‐dien‐3‐ones 4 and some 3‐hydroxy‐5 H ‐pyrrole derivatives 8 were characterized by X‐ray diffraction.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)