Ionones have a broad spectrum of utilization in the flavor and fragrance industry because of their characteristic violet fragrance. Most often, the synthesis of ionones consists of two steps. First is the synthesis of pseudoionone, which is most often prepared by aldol condensation of citral with acetone in the presence of a basic catalyst. The second step is cyclization of the acquired pseudoionone in the presence of an acidic catalyst, which produces a mixture of ionones. The majority of works utilize alkaline hydroxides for the condensation and inorganic and organic acids for the cyclization. The ratio of the acquired α- and β-isomers depends on the selection of catalyst, reaction temperature and cyclization time.
The ionone synthesis consists of two steps. First, pseudoionone is prepared by the aldol condensation of citral with acetone in the presence of a basic catalyst (F-1). In the second step, pseudoionone is cyclicized in an acidic environment to α-ionone, β-ionone and γ-ionone, respectively (F-2). These isomers differ in the position of their double bond.
Pseudoionone is commonly prepared by condensation of citral with acetone. The procedure restrictively leads to two demanded isomers of pseudoionone, i.e. two 3-trans,5-cis and 3-trans,5-trans isomers. The condensation of citral with acetone is most often carried out in the presence of sodium, potassium or barium (II) hydroxide. Absolute alcohol or water with an excess of acetone is a suitable environment. Sodium methoxide, ethoxide, phenoxide, quaternary ammonium bases and other basic substances can catalyze the reaction well. The range of yields of pseudoionone is 70-80 percent. It was demonstrated that the condensation can be also catalyzed using ion-exchangers that guarantee pseudoionone production in high yields with a low quantity of waste substances in a period of time shorter than 6 h. Divinylstyrene copolymers with the active groups of -CH2-N(CH3)2 or -CH2-N+(CH3)3 activated by water solution of sodium hydroxide, Wofatit SBW and others were proved as suitable ion-exchangers.