A straightforward procedure for the [2+2+2] cycloaddition of enediynes

Abstract

Enediynes undergo intramolecular [2+ 2+2] cycloaddition in the presence of cobalt(II) iodide (CoI2), manganese and an N-heterocyclic carbene (IPr) generated in situ from the corresponding imidazolium salt and butyllithium (BuLi). Polycyclic cyclohexadienes are obtained selectively. This new method represents an interesting alternative to those employing air-sensitive cyclopentadienylcobalt [CpCoL2 (L = CO, C2H4)] catalysts. Moreover, the N-heterocyclic carbene can be used catalytically, which is a significant improvement compared to the corresponding phosphine-based system which requires an excess of ligand. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.