Facile Construction of Spirocyclopropanated Bi‐, Tri‐ and Tetracyclic Skeletons by Novel Cascades Involving Intra‐ and Intermolecular Heck Reactions of 2‐Bromo‐1,6‐enynes and Bicyclopropylidene

Acyclic 2‐bromo‐1,6‐enynes 5 ‐R, 9 ‐R and 11 ‐R with bulky substituents at the acetylenic terminus were co‐cyclized with the highly strained bicyclopropylidene ( 12 ) under palladium catalysis at 80 °C to give the cross‐conjugated tetraenes 13 ‐R, 18 ‐R and 19 ‐R in moderate‐to‐good yields (34−71%). Only the co‐cyclization of 5 ‐Ph gave rise to an additional product, which was identified as the 11‐membered ring 20 . At elevated temperatures (120−140 °C) the initially formed tetraenes underwent 6π‐electrocyclization to give spiro[cyclopropane‐1,4′‐bicylo[4.3.0]‐1(6),2‐dienes] 21 ‐R, 22 ‐R and 23 ‐R. This novel class of spirocyclopropanated oligocycles is also accessible by a one‐pot protocol. The highest yields for both the tetraenes and bicyclo[4.3.0]nonadiene and its heteroanalogues were obtained with bulky substituents at the alkyne terminus of the precursors. Heteroatom‐containing precursors 9 ‐R and 11 ‐R gave lower yields than their all‐carbon analogues 5 ‐R. The acyclic 2‐bromo‐1,8‐dien‐6‐ynes 28a , b , c upon palladium‐catalyzed co‐cyclization with bicyclopropylidene ( 12 ) at 110 °C gave spirocyclopropanated tricycles 31a , b and 32 , respectively, in moderate yields (14, 31 and 32%). These products were formed by two consecutive 6π‐electrocyclizations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)