# 13.9: Construction of Ring Systems by Cycloaddition

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Another example of a synthesis problem makes use of the cycloaddition reactions discussed here. Consider the synthesis of bicyclo[2.2.1]heptane, $$9$$, from compounds with fewer carbons. Whenever a ring has to be constructed, you should consider the possibility of cycloaddition reactions, especially [4 + 2] cycloaddition by the Diels-Alder reaction. A first glance at $$9$$, written in the usual sawhorse-perspective formula, might lead to overlooking the possibility of constructing the skeleton by [4 + 2] addition, because the compound seems only to be made up of five-membered rings. If the structure is rewritten as $$10$$, the six-membered ring stands out much more clearly: If we now try to divide the six-membered ring into  and  fragments, we find that there are only two different ways this can be done: The left division corresponds to a simple [4 + 2] cycloaddition, whereas the right division corresponds to a complex reaction involving formation of three ring bonds at once. Actual Diels-Alder reactions require diene and dienophile starting materials, and two possibilities, using 1,3-cyclopentadiene as the diene and ethene or ethyne as dienophile, follow: Either of the products can be converted to bicyclo[2.2.1]heptane by hydrogenation (Table 13-5): Neither ethene nor ethyne is a very good dienophile but [4 + 2] cycloadditions of either with 1,3-cyclopentadiene go well at temperatures of $$160$$-$$180^\text{o}$$ because 1,3-cyclopentadiene is a very reactive diene. Achieving the overall result of addition of ethene or ethyne to a less reactive diene could necessitate a synthetic sequence wherein one of the reactive dienophiles listed in Table 13-1 is used to introduce the desired two carbons, and the activating groups are subsequently removed. An example follows: Reactions that can be used to remove a $$\ce{-CO_2H}$$ group will be discussed in Chapter 18.