Acrolein Lewis acid catalysis

Regioselectivities [7] and endo selectivity [8, 9] increase upon Lewis acid catalysis of Diels-Alder reactions (Scheme 9). Houk and Strozier [10] found that protonation on the carbonyl oxygen of acrolein amplifies the LUMO at the terminal and... 

All these features of Lewis acid catalysis can be explained by the effect the Lewis acid has on the LUMO of the dienophile.36,202,253 We shall take acrolein (298) as the simplest dienophile. The Lewis acid forms a salt (299) with the... 

The reaction did not require dry solvents or inert atmosphere and afforded the desired adducts in 42-98% yield. As expected, the more reactive acrolein and methyl vinyl ketone gave very good results with both acyclic and cyclic p-ketoesters and p-diketones and, surprisingly, acrylonitrile and methyl acrylate, reported to be totally inactive under Lewis acid catalysis, " afforded the corresponding adducts with a-acetylbutyrolactone in high yield (77 and 98% respectively). In the reaction with crotonaldehyde, a 1 1 mixture of... 

Compound 78 behaves as an excellent dienophile in the Diels-Alder reaction (Scheme 12). Reaction of 78 with acrolein at 70 °C for 4 days produces the hetero adduct 79 in 52% yield [28]. Treatment with cyclopentadiene at room temperature for 3 days leads to a 96 4 mixture of exo-HQ and endo 81, with an ee of 90% for 80. Considering that the optical purity of 78 is 96%, this translates into a selectivity 95% for the Diels—Alder reaction. Treatment of the mixture with LiAlH4 produces a 96.5 3.5 mixture of exo/endo-hydroxymQthy norbomenQ which, after recrystallization, furnishes pure (1/ , 2i )-2-hydroxybicyclo[2.2.1]hept-5-ene-2-methanol (82). The use of Lewis acid catalysis [TiCl2(0-/5 o-Pr)2] and low temperature ( — 20 °C) in the cycloaddition reaction increases the endo jexo selectivity, but also decreases both the 71-face selectivity and the yield. 

Chiral (helical) Lewis acids for asymmetric Diels-Alder catalysis are prepared from titanium tetraisopropoxide 5 and a chiral binaphthol ligand 4 [13]. The titanium reagent 6 plays an important role as chiral template for the fixation of a,fi-unsaturated aldehydes and thereby for the enantioface recognition of substrates. The asymmetric Diels-Alder reaction, e. g., of cyclopentadiene 7 and acrolein 8, is effected in the presence of catalytic (P)-6 (10 mol%), producing the endo adduct 9 (R =R2=H) in 88% ee (Scheme 1). 

Catalysis of Diels-Alder Reactions. EtAlCb is a useful Lewis acid catalyst for Diels-Alder reactions. It is reported to be more efficacious for the Diels-Alder reaction of Acrolein and butadiene than either AICI3 or Et2AlCl. It is a useful catalyst for intramolecular Diels-Alder reactions with a,p-unsaturated esters (eq 4) and aldehydes as dienophiles. It has also proven to be a very efficient catalyst for the inter- and intramolecular asymmetric Diels-Alder reaction of chiral a,p-unsaturated acyl sultams (eq 5) and has been used to catalyze a wide variety of Diels-Alder... 

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