Citral 2-butanone

Fig. 7. Methyl pseudoionones formed from reaction of citral with 2-butanone. Reaction at the 3-position of 2-butanone yields isomethylpseudoionone [111 7-41-5] whereas reaction at the 1-position gives normal-methylpseudoionone [26651-96-7].

In the condensation of 2-butanone with citral, if the reaction temperature is kept at 0—10°C, higher yields of the isomethyl pseudoionones, which are the more thermodynamically stable isomers, are obtained. The aldol iatermediates have more time to equilibrate to the more stable isomers at the lower temperature. The type of base used and a cosolvent such as methanol are also very important ia getting a high yield of the isomethyl pseudoionones (168). 

A Cfi + C4 synthesis of citral (32) dunethyl acetal consisted of the addition of the copper-lithium compound corresponding to 20 with the acetylenic acetal 60. The reaction occurred at - 35°C in 1 hour, and yielded the pure ( )-isomer in 92% yield. Another rather long syndiesis involved addition of the lithium derivative corresponding to 20 with 3,4-dichloro-2-butanone. Three conventional steps from die product of this Grignard reaction gave geranyl acetate (25 acetate). ... 

Citral undergoes base-catalyzed aldol condensation with 2-butanone to give the methylpseudoionones in excellent yield the C-3 C-1 ratio is 2 1 (equation 59). ... 

The aldehydes and ketones most commonly used are benzaldehyde, acetaldehyde, formaldehyde, and acetone. However, -butyraldehyde, n-propionaldehyde, salicylaldehyde, p-tolualdehyde, and 2-furaldehyde have also been employed. A number of unsaturated aldehydes, for example, 3,7-dimethyl-2,6-octadienal (citral) and cinnamaldehyde, have yielded cyclic acetals. Acetals have been prepared from 2-butanone, cyclohexanone, and glyoxal. ... 

Aldol condensation of citral with acetone followed by acid cyclization gives a mixture of ionones, of which the a-isomer is widely used for its violet odor (Fig. 10.1.9). However, the yff-ionone is more important because it is used in the synthesis of vitamin A. In a similar fashion, pseudoionone can be used in the synthesis of vitamin E. Condensation of citral with 2-butanone gives methylionones, also valued as perfume ingredients. The dihydropseudoionone geranyl acetone, a perfumery chemical having a magnolia-like odor, can be prepared from either geranyl chloride or linalool. 

Aldol condensation of citral with a ketone leads to materials known as -ionones, as shown in Fig. 8.62. Obviously, if acetone is used as the ketone, only one product, v t-ion-one [141-10-6] (47), is produced. If an asymmetric ketone, such as 2-butanone, is used, then two products will result, the so-called n-methyl- -ionone [26651-96-7] (330), R = Me) and the wo-methyl-vlj-ionone [1117-41-5] (331), R = Me). Treatment of these i (-ionones with an acid catalyst then gives a mixture of ionones, represented by... 

Structure (332) in which either R or R is H and one of the three dotted lines represents a double bond. As discussed above and shown in Fig. 8.10, dehydrolinalool (46) can be treated with an acetone equivalent, such as 2-methoxypropene, to yield il -ionone directly without going through citral, if the desired product is an ionone as opposed to a methylionone (402). Diketene (403) and acetoacetate esters (403) have also been used. Similarly, the methyl enol ether of 2-butanone can react with dehydrolinalool to give o-methyl-i >-ionone (404). 

A mixed aldol reaction between citral and 2-butanone yields two isomeric compounds. Write their structures. 

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