Intramolecular Heck couplings and cycloisomerizations of bromodienes and enynes with 1 ',1 '-disubstituted methylenecyclopropane terminators: Efficient syntheses of [3]dendralenes

2-Bromoalka-l,n-dienes such as 9, 30 and 14 (n = 7) with tetrasubstituted methylenecyclopropane end groups, under palladium catalysis, underwent cyclization with cyclopropane-ring opening to give 2-ethenyl-3-methylene-1-cycloalkenes 41 (n = 6), 42 (n = 7), and 43, which are substituted monocyclic [3]dendralenes, in 65, 63 and 70 % yield, respectively. The same cross-conjugated trienes were isolated in good to excellent yields (77-92 %) from the corresponding 1,6- (10-H, 15) and 1,7-enynes (28-H) by a more atom-economic, palladium-catalyzed cycloisomerization. The vinylpalladium. halide intermediate generated by initial carbopalladation of the 1,6-enyne 10-H with in situ generated phenylpalladium. iodide also underwent the same cyclization cascade