Aldehydes reactions with organoaluminum reagents

Yamamoto and Maruoka investigated the reaction of chiral acetals with organoaluminum reagents. Unprecedented regio- and stereochemical control was observed in the addition of trialkylaluminums to chiral a,/3-unsaturated acetals derived from optically pure tartaric acid diamide [83]. The course of the reaction seemed to be highly influenced by the nature of substrates, solvents, and temperature. These findings provide easy access to optically active a-substituted aldehydes (84), /3-substituted aldehydes (85), a-substituted carboxylic acids (86), or allylic alcohols (87). Because optically pure RJi)- and (5,5)-tartaric acid diamides are both readily available, this method enables the predictable synthesis of both enantiomers of substituted aldehydes, carboxylic acids, and allylic alcohols from a,/3-unsaturated aldehydes (Sch. 54). 

An even greater steric effect of the Lewis acid catalyst is observed with the organoaluminum reagents 4-6 in the Diels-Alder reactions of an aldehyde with a diene (equation III). 

The modified organoaluminum reagent, BINAL, can be used as a chiral Lewis acid catalyst in the asymmetric hetero-Diels-Alder reaction [41]. Reaction of various aldehydes with activated dienes under the influence of catalytic BINAL (5-10 mol%) at -20 °C gave, after exposure of the resulting hetero-Diels-Alder adducts to tri-fluroacetic acid, predominantly c/v-dihydropyrone in high yield with excellent enan-tioselectivity. 

An excellent catalytic system is based on (R)- and (5)-2,2 -dihydroxybinaphthoPThe newly designed enantiomerically pure ( + )-(/ )-3,3 -bis(triarylsilyl)binaphthol reacts with trimethyl-aluminum to produce a chiral organoaluminum reagent 15 which is a very efficient catalyst for the hetero-Diels-Alder reaction. [4 + 2] Cycloaddition of aliphatic and aromatic aldehydes 2 to dienes 14 in the presence of 10 mol% of 15 yields m-dihydropyrones 3 together with a few percent of the trans-isomers with a diastereoselectivity as high as d.r. 98.5 1.5. 

A number of organoaluminum reagents, especially Me2AlCl, have been found to be effective catalysts for reactions of enol silyl ethers with aldehydes.126 It is possible that in these reactions, aluminum enol-ates may be involved as the intermediates. Similarly, bismuth trichloride has been reported to be an efficient catalyst.127 Since BiCh is considered to be a weak Lewis acid, the involvement of a bismuth enolate is perhaps implicated. Enolate intermediates are definitely generated when tetraalkylammonium fluorides128 or tris(dialkylamino)sulfonium (TAS) difluorotrimethylsiliconates (104)129 are used as reagents to promote the reactions of enol silyl ethers. Their chemistry will therefore not be discussed in this chapter. 

Ene Reactions. In the presence of Lewis acids, isobutene adds to various enophiles to yield ene adducts. Enophiles such as alkoxyaldehyde (eq 13), dialkyl aminoaldehyde (eq 14), haloaldehyde (eq 15), and vinyl sulfoxides (eq 16) have been utilized. Chiral organoaluminum (eq 17) and organotitanium reagents (eq 18) have been reported to give high levels of asymmetric induction in the ene reaction of isobutene with activated aldehydes. 

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