Cyclopropyl building blocks for organic synthesis. Part 113 - Cobalt(I)-mediated intramolecular [2+2+2] cocyclizations of (methylenecyclopropyl)diynes as an easy access to cyclopropanated oligocycles

Enediynes with methylenecyclopropane moieties attached either through the three-membered ring or the double bond smoothly under-went cobalt-mediated [2+2+2] cocyclizations to give cyclopropane-fused and spirocyclopropanated cobalt-complexed tricyclo[7.3.0.0(2,6)]dodeca-1,6-dienes (1,2,3,3a,4, 6,7,8-octahydro-as-indacenes) under relatively mild conditions in yields ranging from 31 to 94% (6 examples). Electron-withdrawing substituents at the acetylenic terminus of the precursor are essential for a successful oligocyclization. While diphenyloxyphosphinyl-substituted enediynes provided tricyclo[7.3.0.0(2,6)]dodeca-1,6-diene complexes as mixtures of two diastereomers in high yields, the analogous methoxycarbonyl-substituted precursors gave only one diastereomer. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)