2 9 2 PRACTICAL ORGANIC CHEMISTRY 9,io-Dihydroanthracene-9,io-end o-a3-succinic Anhydride. (The Diels-Alder Reaction.) The Diels-Alder Reaction consists in the direct combination of a compound containing a conjugated diene system with a reagent which possesses a double or triple bond activated by suitable adjacent groups. Examples of such reagents are maleic anhydride, />-benzoquinone, acraldehyde and acetylene dicarboxylic esters. Combination always occurs at the 1,4 positions of the diene system: ,CH 2 HC O A CH CH Il Il CH^CH 8 thus butadiene combines with />-benzoquinone as shown above; in this case a second molecule of butadiene can similarly add on to the opposite side of the p-benzoquinone ring. The Diels-Alder Reaction usually occurs readily: it is of great value (a) for diagnosing the presence of a conjugated diene grouping, (6) for synthetic purposes in the preparation of cyclic systems. In the following preparation, this reaction is exemplified by the union of
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