Thermische Diels‐Alder‐Reaktionen mit N‐(2‐Alkenoyl)‐(S)‐prolinestern als chiralen Dienophilen in organischen und wäßrigen Reaktionsmedien

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Abstract
Thermal Diels‐Alder Reactions with N‐(2‐Alkenoyl)‐(S)‐proline Esters as Chiral DienophilesCyclopentadiene reacts with N‐acryloyl‐(S)‐proline benzyl and allyl ester in toluene at 0°C to give the bicycloheptene‐carboxylic acid amides with a stereoselection of (2S):(2R) = 81:19. In the reactions with the N,N′‐fumaroylbis(proline esters) the cycloadducts are formed in high yields and with isomer ratios of up to 97:3. The absolute configuration of N,N′‐〈(1R,2R,3R,4S)‐bicyclo[2.2.1]hept‐5‐ene‐2,3‐diyldicarbonyl〉bis‐[(S)‐proline benzyl ester] (6 a) has been determined by an X‐ray analysis. In the reactions of the acrylamides with cyclopentadiene and of the fumaramides with cyclopentadiene, isoprene and 2,3‐dimethylbutadiene the use of water/ethanol mixtures as solvent, if indicated in the presence of detergents, doubles the yields. However, a positive influence of the aqueous medium on the diastereomeric ratio can not be observed.

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