Diels-Alder Reaction of Chiral Substrates

Boron Lewis acids have also been found to be very useful in Diels-Alder reactions that employ chiral substrates. Anderson and coworkers have shown that using a combination of Bp3 OEt2 and trifluoroacetic acid, it is possible to obtain the cycloaddition products of nonactivated iminodienophiles such as (16) in good yields and high diastereoselectivity (Equation 11) [17]. 

Kim and co workers have shown that BE3 OEt2 can promote the cycloaddition of chiral acrylamides derived from derivatives of (S)-indoline-2-carboxylic acid, with cyclopentadiene to give the corresponding products in high diastereoselectivity (Equation 12) [18]. 

Lewis acids were shown to have a remarkable influence in the facial selectivity in the cycloadditions of sugar-derived dihydropyranones. Using BF3 OEt2 as a Lewis acid, the reaction between 2,3-dimethylbutadiene and (2S)-2-benzyloxy-2H-pyran-3-(6H)-one (17), affords a 98 2 ratio of (3S,4aR,8aS) versus (3S,4aR,8aR)-3-benzyloxy-6,7-dimethyl-4a,5,8,8a-tetrahydro-lH-2-benzopyran-4(3H)-one. The same reaction under the influence of SnCU gives a complete reversal in facial selectivity with a 24 76 ratio of products (Equation 13) [19]. 

Recently, Yamamoto and coworkers observed a unique form of chemoselectivity in the Diels-Alder reaction of a,P-unsaturated aldehydes and a,P-unsaturated ketones with cyclohexadiene. Upon subjecting a 1 1 mixture of acrolein and ethyl vinyl ketone toward cycloaddition with one equivalent of cyclopentadiene, the bulky tris-pentafluorophenyl boron Lewis acid (C6F5)3B gives primarily the cycloadduct of the a,p-unsaturated aldehyde, and on the contrary, Tf2NH gives mainly the cycloaddition adduct of the a,p-unsaturated ketone (Equation 14) [20]. 

Shipman and coworkers have shown that coordination of Bp3 OEt2 to 2-methylene aziridines (21) lead to an alternative precursor to 2-aminoallyl cations, which can be trapped with an appended 1,3-diene to effect an intramolecular [4 + 3] cycloaddition reaction that generates seven-membered ring systems such as (22) (Equation 16) [22]. 

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