H. R. vaughn-wi~~iams
LlandaffCollege of Technology Cardiff, Wales
TWO Syntheses for the Organic Undergraduate
The ease with which cyclopentadienyl compounds can he synthesized from the reaction hetween 2,3-diketones and those ketones with t 7 - o methylene groups either side of the carbonyl group has made this a valuable synthesis for the organic student. A typical example of this type of reaction is the alkali induced condensation between henzil and dihenzyl ketone to give tetraphenyloyolopentadienone.
Further when the above dienone (I) is heated wit,h maleic anhydride i t undergoes the Diels-Alder reaction to give 4,5-phenanthrene (9.10) 3,B-diphenyldihydi-ophthalic anhydride (11)
maleic anhydride
Experimental benzil
dibenzylketone
Full details of this experiment are set out in "Selected Experiments in Organic Chemistry," by Helmkamp and Johnson (W. H. Freeman& Co.). I t was anticipated in our laboratories that phenanthraqninone could possibly replace henzil as the diketone in this type of reaction, and this has proved to be the case. The reaction between phenanthraquinone and dihenzyl ketone in the presence of alkali gives 3,4phenanthrene (9.10) 2,5-diphenylcyclopentadienone (I).
phenanthraquinone dibenzylketone
I
8,bPhenanth~ene(9.10) 8,6-diphayl cyclopatadienone. Phenanthraquinone (2.08 g), dibenzylketone (1.09 g), and ethanol (100 ml), are carefully heated in a 2.50-ml flask fitted with a water cooled reflux condenser. A solution of 10% ethmolic potassium hydroxide is prepared, and 5 ml of this solution are slowly added to the refluxing contents of the flmk. Same slight frothing is observed and the addition takes about 10-15 min. The flask is heated gently with a small flame for a further period of 15 min. The flask is then cooled in water and the contents filtered under reduced pressure. The yield of crude product is -2 g. 0.5 g of the impure product is heated with 100 ml of benzene until i t dissolves, the solution is filtered hot, and the filtrate cooled. The pure product (I) has a melting point 248-260T and is greenish-grey in color. 4,6-Phenanthrene (9.10) S,Gdiphenyldihydrophlhalie anhydride. Considerable care needs to be exercised in this experiment when handling the hot solvent. I t is advisable to use asbestos doves and an eyeshield. The dienone (I) (1 g) is heated with mdeic anhydride (1 g) in 40 ml of diethyleneglycol, in a conical flask under a fume hood. When the temperature reaches 210DC the mixture will become a clear yellow solution. The flask is set aside to cool, and the contents are diluted with 40 ml of ethanol and then filtered. The yield of crude material is -1 g. 0.5 g of the impure product is heated with 50 ml of diethyleneglywl and when all the product has dissolved the solution is cooled m d then filtered. The DUre product (11) is pale yellow in color and decomposes a t - 3 0 0 ~ 6
Volume 48, Number 4, April 1971
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259
