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

Thermal Diels‐Alder Reactions with N ‐(2‐Alkenoyl)‐( S )‐proline Esters as Chiral Dienophiles Cyclopentadiene 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 (2 S ):(2 R ) = 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 ′‐〈(1 R ,2 R ,3 R ,4 S )‐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.