Amino Acid Esters as Chiral Auxiliaries in Lewis Acid Catalyzed Diels‐Alder Reactions
Cyclopentadiene, cyclohexadiene and alkyl‐substituted butadienes react with N‐acryloyl‐, N‐crotonoyl‐ and N‐(4‐nitro‐cinnamoyl)‐(S)‐proline benzyl and allyl ester in the presence of Lewis acids to give the cycloadducts in high yields and with stereoselectivities of up to 97:3 for the endo isomers. The sense of the asymmetric induction in these reactions can be directed by the use of differently coordinating catalysts (e.g. TiCl~4~: (2__R__):(2__S__) = 96:4; EtAlCl~2~: (2__R__):(2__S__) = 10:90 at 0°C). In the TiCl~4~‐catalyzed reactions with the fumaric acid derivatives the ratio of the diastereomers reaches values of up to 112:1 in the presence of 0.5–1 equivalent of the Lewis acid, thus indicating an efficient cooperation between the chiral auxiliaries. Higher catalyst/substrate ratios induce a reversal in the sense of the asymmetric induction. From the cycloadducts the chiral auxiliaries are removed by hydrogenolysis and subsequent hydrolysis of the proline amides. Alternatively, the cleavage of the amide bonds can be carried out by O‐alkylation with oxonium salts and subsequent hydrolysis of the iminium salts formed or by the application of an iodolactonization reaction.