Diels-Alder reactions organoaluminum reagents

In contrast, modest enantioselection has been observed in the asymmetric Diels-Alder reaction between cyclopentadiene (18) with methylacrylate and methylpropiolate catalyzed by chiral organoaluminum reagents 58 [59] (Equation 3.15) prepared from trimethylaluminum and (R)-(+)-3,3 -bis(triphenylsi-lyl)-l,l -bi-2-naphthol [60]. The reaction was highly cnJo-diastereoselective. 

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). 

Lewis acid, This organoaluminum reagent (MeA10) , a partial hydrolysis product of trimethylaluminum, is more efficient than conventional Lewis acids such as MCjAl, as catalyst for inducing Diels-Alder reactions and aminolysis of esters. 

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. 

ABSTRACT. With the aid of selected examples an overview is given of the development trends in highly discriminative reactions using novel Lewis acid catalysts. 1) Bulky organoaluminum reagent, methylaluminum 3,3 -bis(triphenylsilyl)-l,r-bi-2-naphthoxide, has been successfully utilized for enantioselective activation of carbonyl moiety. 2) A novel reactivity of acyloxyborane derivatives and their usefulness in organic synthesis is demonstrated. As an example, asymmetric Diels-Alder reaction catalyzed by new acyloxyboranes derived from chiral acids is described. 

Asymmetric Hetero-Diels Alder Reaction Catalyzed by Chiral Organoaluminum Reagent. Hetero-Diels-Alder reaction, an important organic transformation for the synthesis of a variety of heterocycles, was found to be catalyzed by the optically pure bulky aluminum reagent. ... 

Among several terpene derived chiral ketones examined, 3-d-bromocamphor was found to be most satisfactory and 3d-bromocamphor is responsible for the generation of (S)-catalyst. Combination of the racemic catalyst and chiral ketone in a 1 1 ratio gave a better results, suggesting that decomplexation of one enantiomeric organoaluminum reagent and the chiral ketone is more readily facilitated by the addition of aldehyde, thereby allowing the enantioselective activation of the aldehyde for the asymmetric hetero-Diels-Alder reaction. 

Maruoka K, Itoh T, Shirasaka T, Yamamoto H (1988) Asymmetric hetero-Diels-Alder reaction catalyzed by chiral organoaluminum reagent. J Am Chem Soc 110 310-312... 

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