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[摘要]：Palladium-catalyzed dicyanative [4 + 2] cycloaddition using various ene-enynes was investigated The key species in this process is a cyanoallene intermediate that is obtained by the cyanopalladation of conjugated enynes followed by 5-exo-cyclization To achieve an efficient [4 + 2] cycloaddition reaction, both the smooth generation of this species and critical control of regioselectivity in the 6-endo-cyclization step are quite important A study of the substrate scope revealed that the reaction is strongly affected by the steric bulk of the substituents on the enyne and alkene units and prefers to give trans-fused cycloadducts The stereochemistry of olefins was reasonably transferred to the corresponding products Further study proved that this transformation includes not a thermal [4 + 2] cycloaddition process via 1,2-dicyanoalkenes generated in situ but rather a palladium-mediated stepwise cyclization sequence to control a maximum of five contiguous stereogenic centers in a single operation An intermolecular version using methyl acrylate with conjugated cyclic enynes and TMSCN also gave the corresponding [4 + 2] cycloadducts in a regioselective manner |
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