Improved Synthesis of Pyrroles and Indoles via Lewis Acid‐Catalyzed Mukaiyama–Michael‐Type Addition/Heterocyclization of Enolsilyl Derivatives on 1,2‐Diaza‐1,3‐Butadienes. Role of the Catalyst in the Reaction Mechanism

The Mukaiyama–Michael‐type addition of various silyl ketene acetals or silyl enol ethers on some 1,2‐diaza‐1,3‐butadienes proceeds at room temperature in the presence of catalytic amounts of Lewis acid affording by heterocyclization 1‐aminopyrrol‐2‐ones and 1‐aminopyrroles, respectively. 1‐Aminoindoles have been also obtained by the same addition of 2‐(trimethylsilyloxy)‐1,3‐cyclohexadiene on some 1,2‐diaza‐1,3‐butadienes and subsequent aromatization. Mechanistic investigations indicate the coordination by Lewis acid of the enolsilyl derivative and its 1,4‐addition on the azo‐ene system of 1,2‐diaza‐1,3‐butadienes. The migration of the silyl group from a hydrazonic to an amidic nitrogen, its acidic cleavage and the final internal heterocyclization give the final products. Based on NMR studies and ab initio calculations, a plausible explanation for the migration of the silyl protecting group is presented.     

A Novel Assembly of Substituted Pyrroles by Acid‐Catalyzed Sequential Three‐Component Reaction of Amines,Alkynoates, and 1,2‐Diaza‐1,3‐dienes

A novel protocol for the assembly of polysubstituted pyrroles has been developed through the acid‐catalyzed, sequential three‐component reaction of primary aliphatic amines, alkynoates and 1,2‐diaza‐1,3‐dienes (DDs). This methodology proceeds with complete chemo‐/regioselectivity involving first formation of an enamino ester intermediate, in situ Michael addition with azo‐ene compounds and subsequent intramolecular ring closure.     

Enantioselective Synthesis of 4‐Substituted Dihydrocoumarins through a Zinc Bis(hydroxyamide)‐Catalyzed Conjugate Addition of Terminal Alkynes

A new enantioselective catalyst for the conjugate addition of terminal alkynes has been developed. Terminal alkynes react with 3‐alkoxycarbonylcoumarins in the presence of diethylzinc and bis(hydroxyamide) ligands to give chiral non‐racemic dihydrocoumarins substituted with an alkynyl group on the C‐4 position with good yields and enantiomeric excesses up to 95%.     

Indium(III) Chloride‐Catalyzed Propargylation/Amination/Cycloisomerization Tandem Reaction: One‐Pot Synthesis of Highly Substituted Pyrroles from Propargylic Alcohols, 1,3‐Dicarbonyl Compounds and Primary Amines

A convenient one‐pot propargylation/amination/cycloisomerization tandem process has been developed for the synthesis of highly substituted pyrroles derivatives from propargylic alcohols, 1,3‐dicarbonyl compounds and primary amines using indium(III) chloride as a multifunctional catalyst. This method provides a flexible and rapid route to substituted pyrroles.