Conjugated enals

Lopez J. C., Lukacs G. Pyranose-Derived Dienes and Conjugated Enals. Preparation and Diels-Alder Cycloaddition Reactions ACS Symp. Ser. 1992 494 33-49 Keywords carbohydrate, befera-Diels-Alder reactions, stereoselectivity... 

Whereas the Markovnikov addition of carboxylic acids to propargylic alcohols produces P-ketoesters, resulting from intramolecular transesterification [30, 31], the addition to propargylic alcohols in the presence of Ru(methallyl)2(dppe) 1 at 65 °C leads to hydroxylated alk-l-en-l-yl esters via formation of a hydroxy vinylidene intermediate [32, 33]. The stereoselectivities are lo ver than those obtained from non-hydroxylated substrates. These esters, which are protected forms of aldehydes, can easily be cleaved under thermal or acidic conditions to give conjugated enals, corresponding to the formal isomerization products of the starting alcohols (Scheme 10.6). 

J. C. Lopez, E. Lameignere, C. Bumouf, M. de Los Angeles Laborde, A. A. Ghini, A. Olesker, and G. Lukacs, Efficient routes to pyranosidic homologated conjugated enals and dienes from monosaccharides, Tetrahedron 49 1101 (1993). 

J. C. Lopez and G. Lukacs, Pyranose-derived dienes and conjugated enals—preparation and Diels-Alder cycloaddition reactions, in Cycloaddition Reactions in Carbohydrate Chemistry, 1992, pp. 33-49. 

The impossibility of a conjugated alkene in 15 as well as 13, makes this an exceptional case in aldols of aldehydes. Without the branch at the a-carbon, the product is more usually a conjugated enal.6 So the linear isomer 17 of 16 with the same base gives the enal 18 in good yield.7 The true product of the dimerisation is the anion 19. This is in equilibrium with the enolate that allows an ElcB elimination of water 20 to give the enal 18. 

Scheme 6. Preparation of 1,3-hydroxyesters and subsequent cleavage into conjugated enals.

Substituted allylsilanes on treatment with IOB.BF3 (2 equivalents) in dioxane afforded conjugated enals. 

The reaction of conjugate enals having a heteroatom at the / -position with olefins proceeds with the aid of RuH2(CO)(PPh3)3. The reaction of 5,6-dihydro-4H-pyran-3-carboxaldehyde with styrene gives the alkylation product in quan-... 

In contrast with the catalytic system based on RuCl(rf-C9H7)(PPh3)2 in micellar solutions [32], the reaction of secondary propargylic alcohols in 2-propanol/ H20 at 100 °C in the presence of 5 mol % of RuCl(Cp)(PMe3)2 leads to conjugated enals with E stereoselectivity (Eq. 12) [88]. 

The next example is particularly impressive. The enol partner is a symmetrical ketone that is very hindered—there is only one a hydrogen on either side. The electrophilic partner is a conjugated enal that is not enolizable but that might accept the nucleophile in a conjugate manner. In spite of these potential problems, the reaction goes in excellent yield. 

N-Acylaminodienes are more stable than the ccHresponding dienamines and are easily accessible in one synthetic operation from conjugated enals, (131) - (132) - (133), or alternatively from dienoic acids, (134) (135) (Scheme 34). ... 

Tandem reactions. The multiple activities of the Ru catalysts enable development of valuable tandem reactions. Further extension of the carbon chain of a conjugated enal obtained from a cross-metathesis reaction is readily achieved on treatment with diazoacetic esters to give alkadienoic esters. ... 

Carbonyl Activation. 1,3-Dioxolanation of conjugated enals is facilitated by TMSOTf in the presence of l,2-bis(trimethylsilyl-oxy)ethane. In particular, highly selective protection of sterically differentiated ketones is possible (eq 10). Selective protection of ketones in the presence of enals is also facilitated (eq 11)... 

Various products can be prepared by the reaction of Phl+OBFs" with unsaturated silylated substrates, such as silylalkenes and silylalkynes. ( )-Vinylsilanes react with Phl+OBFj" to give stable alkenyl(phenyl)iodonium tetrafluoroborates (Section 2.1.9.2), whereas (Z)-vinylsilanes 170 undergo dehy-drosilylation to afford alkynes in high yields (Scheme 3.72) [221-223]. AUylsilanes 171 are oxidized with Phl+OBFs" to give conjugated enals 172 [224],... 

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