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the 1,4 addition of unsaturated compounds (dienophiles) to conjugated dienes. Diene synthesis results in the formation of six-membered rings with a double bond between the second and third carbon atoms of the initial diene. A typical example of diene synthesis is the preparation of tetrahydrophthalic anhydride (III) from butadiene-1,3 (I) and maleic anhydride (II):
The dienophiles are usually compounds with an ethylenic double or acetylenic triple bond, activated by an electronegative group, such as —CO—, —COOR, —CN, and —NO2. With such dienophiles, diene synthesis is effected by simply mixing or heating the reagents at 100°-120°C. If the dienophile lacks an activating group, drastic conditions are necessary to effect the diene synthesis. Also used as dienophiles are such carbonyl compounds as hexafluoroacetone, (CF3)2C=O; nitroso compounds, for example, nitrosobenzene, C6H5—N=O; and hexafluoroazomethane, CF3—N= N—CF3. Such heterocyclic compounds as furan can act as dienes.
The first examples of diene synthesis can be found in the works of V. N. Ipat’iev (1897) and S. V. Lebedev (1909) dealing with the study of the dimerization of isoprene. As a general reaction in organic chemistry, however, diene synthesis was discovered by the German organic chemists O. Diels and K. Alder in 1928. Diene synthesis is widely used in organic chemistry, for example, to synthesize steroids. The study of diene synthesis has made it possible to explain a number of processes in the polymerization of dienes.
B. L. DIATKIN