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8.83 g (36.2 mmol) of 4-bromophenylalanine, 4.74 g (81.7 mmol) acetone and 7.00 g (37.43 mmol) of N-benzylmaleimide were heated under reflux for 68 hours. in 140 ml of toluene with the addition of molecular sieve A4. The solvent was removed in a vacuum and the residue was separated chromatographically (hexane/ethyl acetate 1:1+1% triethylamine). As the main product there were obtained 3.88 g of (3aRS,4RS,6aSR)-2-benzyl-4-(4-bromo-benzyl)-6,6-dimethyl-tetrahydro-pyrrolo[3,4-c]pyrrol-1,3-dione as a colourless oil which, after the addition of a small amount of methanol, crystallized overnight to colourless needles. M.p.: 142°-143° C. FAB-MS: 427 ([M+H]+, 58); 257 (66); 240 (8); 200 (6); 91 (58). 2.12 g of (3aRS,4SR,6aSR)-2-benzyl-4-(4-bromobenzyl)-6,6-dimethyl-tetrahydro-pyrrolo[3,4-c]pyrrole-1,3-dione were obtained as a byproduct as a colourless oil. FAB-MS: 427 ([M+H]+, 88); 257 (89); 91 (100).

1. temperatureunder reflux for 68 hours
2. customThe solvent was removed in a vacuum
3. customthe residue was separated chromatographically (hexane/ethyl acetate 1:1+1% triethylamine)