3,4,5,6-Tetraphenyldihydrophthalic anhydride

The tetraphenyldihydrophthalic anhydride may be isolated at this point in practically quantitative yields by cooling the mixture, filtering with suction, and washing the solid with three 10-cc. portions of petroleum ether (b.p. 60-68°). The yield is 41-42 g. the product melts at 235-240°. 

The reaction has been shown to involve a stereospecific syn-addition with respect to the dienophile. For example, the reaction of 2,3-dimethylbuta-1,3-diene with maleic anhydride gives cis-l,2,3,6-tetrahydro-4,5-dimethylphthalic anhydride (Expt 7.20). An example of the use of a quinone as the dienophile is provided by the synthesis of cis-1,4,4a,9a-tetrahydro-2,3-dimethyl-9,l0-anthra-quinone which upon dehydrogenation (most simply by the action of oxygen upon its solution in alcoholic potassium hydroxide) yields 2,3-dimethylanthra-quinone (Expt 7.21). Tetraphenylcyclopentadienone (tetracyclone) readily undergoes the addition of dienophiles, such as maleic anhydride, to give an adduct, which then extrudes a molecule of carbon monoxide on heating, as in the preparation of 3,4,5,6-tetraphenyldihydrophthalic anhydride (Expt 7.22). 

Place a mixture of 7.0 g (0.018 mol) of tetraphenylcyclopentadienone (Expt 7.7) 1.9 g (0.019 mol) of maleic anhydride (Expt 5.218) and 5 ml of bromo-benzene in a 100-ml round-bottomed flask fitted with a reflux condenser. Reflux gently in a fume cupboard (carbon monoxide is evolved) for 1.5 hours during this period the dark brown colour of the cyclic ketone disappears. Cool to room temperature, add about 15 ml of light petroleum, b.p. 60-80 °C, break up the solid and filter. Wash with light petroleum and dry. The yield of crude product, m.p. 237-240 °C, is 7.5 g (90%). Dissolve the solid in hot benzene (120 ml), filter and add light petroleum, b.p. 60-80 °C (120 ml), to the hot filtrate and cool. Collect the solid which separates, wash with light petroleum and dry. The yield of pure tetraphenyldihydrophthalic anhydride, m.p. 235-240 °C, is 6.2 g (75%). The m.p. depends on the rate of heating. 

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