Construction of Ring Systems by Cycloaddition Reactions

Another example of a synthesis problem makes use of the cycloaddition reactions discussed in this chapter. 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 [2] and [4] 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. l]heptane by hydrogenation (Table 13-5)  

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