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UW-Madison CHEM 345 - The Hexadehydro-Diels-Alder Reaction

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The hexadehydro-Diels-Alder reaction Extra Credit Paper Review 2 Victoria Chen Professor Schomaker Arynes are one of the most versatile substrates in chemistry. They can be used in pharmaceuticals, agrochemicals, dyes, polymers and other fine chemicals. Arynes are rings that contain conjugated double bonds (2) and a triple bond. Currently the methods of creating this highly strained molecule comes from removing two adjacent atoms or substrates from benzenoidprecursors. To do these reactions they typically require the use of a strong base and/or access to a1,2-disubstituted arene substance. In this paper, I learned about an alternative way to create arynes without using these substances but rather using the Diels-Alder reaction to trap the triple bound within the ring. Using ketotriyne, that could only undergo a hexadehydro-Diels-Alder (HDDA) reaction with a single regiochemical outcome, the chemists were able to produce the hexasubstituted, tetracyclic indenone derivative product with a 93% yield. In class, we learned about the basic Diels-Alder cycloaddition. Typically, if the diene originally contains two double bonds and the dienophile contains one double bond, this reaction forms a single double bond within a ring structure. However, in this article they used the Diels-Alder reaction to create a ring structure with a triple bond. This reaction used a diene with two triple bonded carbons and a dienophile with one triple bonded carbon. Using this reaction they could trap the triple bond with the ring itself.In this article, I found it interesting how two substituents could come together to force an unstable conformations. This principle never occurred to me. It would be like placing a magnet with the same poles facing each other between two books in order to keep the two of the same forces together even if they want to push each other apart. Another thing that I found interesting was how the chemists used a reaction that was very elementary and simple to fix a morecomplicated problem. They took a reaction that everyone knew and placed a spin on it to make the reaction work in the favor that they needed to in order to get the desired product. Within the paper itself it mentions a couple of future directions that could be taken in light of this discovery. The first is to look at the mechanism that this reaction uses to create the product. They could look at if the mechanism was a concerted or stepwise cycloaddition. The second is to look at different types of catalysts that could be used in this reaction. These catalystscould be lewis acids or transition metals. Another future direction to look for new modes of benzyne trapping and the feasibility of the bimolecular HDDA


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