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Palladium-Catalyzed [4+2] Cross-Benzannulation Reaction of Conjugated Enynes with Diynes and Triynes

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journal contribution
posted on 25.06.1999, 00:00 by Vladimir Gevorgyan, Akira Takeda, Miyako Homma, Naoki Sadayori, Ukkiramapandian Radhakrishnan, Yoshinori Yamamoto
The enyne−diyne [4+2] cross-benzannulation proceeded smoothly in the presence of Pd(0) catalyst to give 1,2,4-trisubstituted benzenes 8aj, 1,2,3-trisubstituted benzenes 9ai, 1,2,3,5-tetrasubstituted benzenes 11ak, 1,2,3,4-tetrasubstituted benzenes 11l,m, and 1,2,3,4,5-pentasubstituted benzenes 11np in moderate to quantitative yields. In all cases the reaction was found to be regiospecific with regard to substitution at the benzene ring. The palladium-catalyzed [4+2] cycloaddition of monosubstituted conjugated enynes 3ac and 5a with unsymmetrical diynes 13aj and triynes 16ag also proceeded with perfect regiochemistry with regard to substitution at the benzene ring. However, since two differently substituted triple bonds are present in the enynophiles 13 and 16, the enyne preference toward either triple bond in the enynophiles became an issue, and in most cases, two regioisomeric products were obtained. The detailed deuterium-labeling studies were performed, and the mechanism of this regiospecific [4+2] cycloaddition between enynes and diynes was proposed.