Aldehydes, unsaturated examples

By catalytic reduction of a p-unsaturated ketones, prepared from aldehydes by the Claisen - Schmidt reaction (see under Aromatic Aldehydes), for example ... 

A variety of oxidants have been used in the Skraup/Doebner-von Miller reaction between anilines and a, 3-unsaturated aldehydes. For example, aniline 23 was reacted... 

Fermenting baker s yeast also catalyzes the 1,4-addition of a formal trifluoroethanol-d1-synthon to a,/i-unsaturated aldehydes, to give optically active l,l,l-trifluoro-2-hydroxy-5-alka-nones52. Presumably, the mechanism involves oxidation of the alcohol to the corresponding aldehyde followed by an umpolung step with thiamine pyrophosphate and Michael addition to the a,/i-unsaturated aldehyde. For example, l,l,l-trifluoro-2-hydroxy-5-hexanone (yield 26%, ee 93%) is thus obtained from trifluoroethanol and l-bnten-3-one. 

Decarbonylation is also observed upon photolysis of / ,y-unsaturated homoconjugated aldehydes and a-aryl aldehydes. For example, R-laurolenal produces two enantiomeric products in yields of 88% and 12%, respectively ... 

The Mannich reaction of an aldehyde enol (example Formula C in Figure 12.14) or a ketonic enol (example Formula C in Figure 12.15) often proceeds beyond the hydrochloride of a /l-aminocarbonyl compound or the Mannich base. The reason is that the secondary amine or its hydrochloride, which has previously been incorporated as part of the iminium ion, is relatively easy to eliminate from these two types of product. The elimination product is an a,fi-unsaturated aldehyde (example Formula E in Figure 12.14) or an a,/l-unsaturated ketone (example Formula D in Figure 12.15)—that is, an a,/l-unsaturated carbowyl compound. Figure 13.51 will show how the Mannich reaction of a carboxylated lactonic enol provides access to an a-methylene lactone, that is, an a,/l-unsaturated carboxyl compound. 

Epoxyannelation. The enolate of cyclohexanone reacts with this reagent (1) to form the a,(3-unsaturated epoxide 2, with displacement of dimethyl sulfide, and a mixture of two sulfur-containing products (3). The reaction to give an epoxide is general for enolates of unhindered, cyclic ketones, but is not observed with enolates of acyclic ketones. However, it is possible with enolates of aldehydes. An example is the conversion of phenylacetaldehyde into 4 and 5, in somewhat low yield. 

Reaction of (1) with an aldehyde or ketone followed by hydrolysis furnishes a y-hydroxy-a,)3-unsaturated aldehyde. For example, addition of (I) to propanal forms l,3-bis(methylthio>-l-hcxene-4-ol (4) in 89% yield hydrolysis with mercuric chloride gives franj-4-hydroxy-2-hcxenal (5) in 48% yield. 

Addition of 1,2-ethanedithiol to BF3 Et20-activated A(A -dialkylhydrazones 558 gave the 1,3-dithiolanes 559 in excellent yields (84-98%) (Equation 75). The former were obtained via reaction of A, A -dimethylhydrazine, 1-aminopyrrolidine, or (3 )-Tamino-2-(methoxymethyl)pyrrolidine with a variety of aldehydes including aromatic, aliphatic, and a,/3-unsaturated examples <1998TL7955>. 

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. ... 

Aldols can be further converted to other products in particular, they are a source of unsaturated aldehydes. For example, the reaction of ethanal gives 3-hydroxybutenal (acetaldol) ... 

As mentioned above, although some examples of intramolecular MBH reactions have been reported in the literature, this aspect is still in its infancy. Most known reports are based on the cyclizations of combinations of enone-enone, enone-acrylate, enone-aldehyde, unsaturated thioester-aldehyde, enone-allylic carbonate frameworks, etc. More recently, Krafft et al. have developed a novel, entirely organo-mediated intramolecular MBH reaction by using allyl chloride 277 as an alternative electrophile to afford densely functionalized cyclic enones 278. This reaction tolerates modification of the enone and the use of primary and secondary allylic chlorides and generates both five-and six-membered rings in excellent yields. Both mono- and disubstituted alkenes are formed with excellent selectivity in the absence of a transition metal catalyst (Scheme 1.100). ... 

The synthesis of racemic oc,P-unsaturated sulfoxides by the Homer-Wittig olefination of aldehydes is a well-established process [15]. In this procedure, deprotonation of a dialkyl arenesulfinylmethanephosphonate and reaction of the resulting anionic species with an aldehyde gives oc,P-unsaturated sulfoxides in high yield, but with variable (E) (Z) ratios according to the nature of the aldehyde. For example, reaction of a-phosphoryl sulfoxide (7) with benzaldehyde (8) gives the ct,P-unsaturated sulfoxide (9) in 70% yield with an ( ) (Z) ratio of 58 42 (Scheme 5.3) [16]. 

Propose a structural formula for the compound with the molecular formula CeHio02 that undergoes an aldol reaction followed by dehydration to give this a,/3-unsaturated aldehyde (See Examples 15.3,15.5)... 

Reactions with Aldehydes. The sulfonyl-stabilized phosphorane reagent has been used for Wittig alkenation of aldehydes. For example, condensation with isonicotinaldehyde gave the a,p-unsaturated sulfone as a single fisomer in 76% yield (eq 3). It is noteworthy that analogous reaction with (methylsulfonylmethy-lene)triphenylphosphorane produced the corresponding methyl-sulfonyl alkene as a 1 1 mixture of the E/Zisomers. ... 

This preparation is another example of the condensation of an aldehyde with malonic acid and pyridine to 3neld ultimately an ap-unsaturated acid (Doebner reaction). It is included here because, unlike the acids prepared from many of the lower aUphatic aldehydes, the product consists largely (about 95 per cent.) of the ap-isomeride and only about 5 per cent, of the PY-isomeride is present ... 

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