| Abstrakt: |
The outcomes and the mechanistic pathways of the palladiumcatalyzed Hecktype cascade oligocyclizations of various 2bromoalkenediynes were explored with respect to the lengths of the tethers between the multiple bonds and the nature of the substituent at the acetylenic terminus. Just like substrates containing two threeatom tethers, 2bromotridec1ene6,13diynes 10a,bwith one three and one fouratom tether undergo two consecutive intramolecular 5 and 6exodigcarbopalladations with subsequent 6πelectrocyclization and βhydride elimination to form tricycles 35a,bwith a central benzene ring in 67 and 61 yield, respectively, independent of the fact that 10acontains an electron donor and 10ban electron acceptor at the acetylene terminus. However, when 2bromotetradec1ene7,13diynes 22, 29on one side and 16, 27, 31on the other are subjected to Heck reaction conditions, tricycles with a central benzene ring are formed only, when the substituent at the acetylene terminus is not a methoxycarbonyl group as in 22, 29. Thus, the bisannelated benzene derivatives 36and 37are formed from 22and 29in 79 and 18 yield, respectively, whereas 16, 27and 31with their methoxycarbonyl substituents at the acetylenic end yield tetracyclic systems 38, 39and 40, consisting of a central fivemembered and two annelated sixmembered as well as an additional annelated threemembered ring, predominantly 54, 19 and 18 yield, respectively. The cascade reaction leading to the latter products must involve a 5exotrigcarbopalladation rather than 6πelectrocyclization as the third step. Apparently, the nature and the substitution pattern of the tether in the substrates 16, 22, 31linking the vinyl bromide moiety with the internal acetylene affect the yield of the tetracyclic product.© WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008 |
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