Dibenzalacetone

Sufficient alcohol is used to dissolve the benzaldehyde rapidly, as well as to retain the benzalacetone in solution until it has had time to react with the second molecule of aldehyde. Lower concentrations of base slow up the formation of the dibenzalacetone and thus favor side reactions which tend to yield a sticky product. Higher concentrations of base give added difficulty in washing. These concentrations were suggested by, and are approximately the same as, those used in the preparation of benzalacetophenone described in Org. Syn. 7, r. 

Only temperatures between 20 and 250 were tried, it being expected that a change of temperature would have the same effect as it does in the preparation of benzalacetophenone mentioned above. 

Stirring is essential, as it makes considerable difference in the uniformity of the product. 

The benzaldehyde was U.S.P. quality which had been washed with sodium carbonate solution and distilled. Commercial C.P. acetone was used. The theoretical quantities are used, since an excess of benzaldehyde results in a sticky product, and an excess of acetone favors the production of benzalacetone. The mixture is prepared before addition in order to insure additions of equivalent quantities. 

Since the product is practically insoluble in water, large amounts can be used in the washing. Sodium compounds are probably the chief impurities. The dried product contains some sodium carbonate which results from the failure to completely remove the sodium hydroxide. There remain also the impurities insoluble in water. However, the product is pure enough for use in most reactions. 

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Phenacyl chloride Phenacyl bromide p-Bromophcnacyl bromide Benzalacetone Dibenzalacetone... 

In a 500 ml. wide-mouthed reagent bottle place a cold solution of 25 g. of sodium hydroxide in 250 ml. of water and 200 ml. of alcohol (1) equip the bottle with a mechanical stirrer and surround it with a bath of water. Maintain the temperature of the solution at 20-25°, stir vigorously and add one-half of a previously prepared mixture of 26-5 g. (25 -5 ml.) of purebenzaldehyde (Section IV,115) and 7 -3 g. (9-3 ml.) of A.R. acetone. A flocculent precipitate forms in 2-3 minutes. After 15 minutes add the remainder of the benzaldehyde - acetone mixture. Continue the stirring for a further 30 minutes. Filter at the pump and wash with cold water to eliminate the alkali as completely as possible. Dry the solid at room temperature upon filter paper to constant weight 27 g. of crude dibenzalacetone, m.p. 105-107°, are obtained. Recrystallise from hot ethyl acetate (2-5 ml. per gram) or from hot rectified spirit. The recovery of pure dibenzalacetone, m.p. 112°, is about 80 per cent. 

A large excess of acetone is taken, so as to diminish. the formation of dibenzalacetone. 

The lower the pressure used in the distillation the less decomposition occurs, and the less residue is left in the distilling flask. This residue contains some dibenzalacetone. 

Dibenzalacetone has been prepared by condensing benzal-dehyde with acetone using as condensing agents dry hydrogen chloride,1 io per cent sodium hydroxide solution,2 and glacial acetic acid with sulfuric acid.3 It has also been obtained by condensing benzalacetone with benzaldehyde in the presence of dilute sodium hydroxide.4 Straus and Ecker 5 were the first to record the use of ethyl acetate for crystallization. 

Both the cis- and the trans-disubstituted spiranes resulted, in different ratios, depending on the reaction conditions. Clearly, the trans spiranes are chiral. The first conjugate addition to the Michael acceptors 75a-c is intermolecular in nature and defines the sense of chirality at the first chiral center. Subsequent intramolecular ring closure to the spiranes 76 defines the cis or trans configuration of the product. When cyclohexane-1,3-dione (74a) was reacted with dibenzalacetone (75a) in the presence of ca 5 mol% (—)-quinine (3a, Scheme 4.3), a 2.5 1 trans/cis mixture resulted, with the trans isomer 76 having optical purity of ca 30% (Scheme 4.37) [61] (the absolute configuration of the predominant enantiomer was not assigned). 

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