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J Am Chem Soc. 2016 Feb 17;138(6):1808-11. doi: 10.1021/jacs.5b12877. Epub 2016 Feb 4.

Unprecedented Intramolecular [4 + 2]-Cycloaddition between a 1,3-Diene and a Diazo Ester.

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Department of Chemistry, The University of Texas at San Antonio , San Antonio, Texas 78249, United States.
Department of Chemistry and Biochemistry, University of Maryland , College Park, Maryland 20742, United States.
Shanghai Engineering Research Center of Molecular Therapeutics and New Drug Development, School of Chemistry and Molecular Engineering, East China Normal University , Shanghai 200062, China.


Diazo compounds that are well-known to undergo [3 + 2]-cycloaddition provide the first examples of the previously unknown [4 + 2]-cycloaddition with dienes that occur thermally under mild conditions and in high yields. Reactions are initiated from reactants prepared from propargyl aryldiazoacetates that undergo gold(I)-catalyzed rearrangement to activated 1,3-dienyl aryldiazoacetates. These reactions proceed to mixtures of both [4 + 2]-cycloaddition and the 1,3-dienyl aryldiazoacetate after long reaction times. At short reaction times, however, both E- and Z-1,3-dienyl aryldiazoacetates are formed and, after isolation, thermal reactions with the E-isomers form the products from [4 + 2]-cycloaddition with ΔH(‡)298 = 15.6 kcal/mol and ΔS(‡)298 = -27.3 cal/(mol·°C). The Z-isomer is inert to [4 + 2]-cycloaddition under these conditions. The Hammett relationships from aryl-substituted diazo esters (ρ = +0.89) and aryl-substituted dienes (ρ = -1.65) are consistent with the dipolar nature of this transformation.

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