Intramolecular approaches to pyrroles

Zinc(II) perchlorate-catalyzed reductions of 3-cyanoketones led to the formation of highly substituted pyrroles including pyrrole-3-carboxylates 07T461 and pyrrole-3-phosphonates 07T4156 . 

A gold(I)-catalyzed domino reaction sequence involving pentenynyl tosylamides led to the formation of 2,3-disubstituted pyrroles containing a quaternary center in the 2-substituent 07OL3181 . The mechanism of the reaction involved a 5-endo-dig cyclization followed by an aza-Claisen rearrangement. 

Electrophile-mediated cyclization reactions of alkynes tethered to pendant heteroatom nucleophiles is an emerging strategy for the synthesis of heterocycles. This methodology has now been applied to the synthesis of pyrroles. The iodocyclization of 3-aminoalkynes 1 led to the formation of dihydropyrrole 2 07TL7906 . Treatment of the latter with mesyl chloride in the presence of triethylamine then gave (i-iodopyrrolcs 3. 

Two different research groups independently reported metal-catalyzed cycloisomerizations of alkynyl-substituted pyridines leading to indolizines. Specifically, treatment of pyridine 7 with silver tetrafluoroborate gave indolizine 8 in excellent yield 07OL3433 . This reaction could also be mediated by copper(I) iodide 07JOC7783 . 

Palladium(II)-catalyzed cycloisomerization reactions of o-alkynyl benzylamines were utilized to prepare isoindole-1-phosphonates 9 07TL8599 07JOC5439 . 

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