According to quantum chemical calculations (MIND013 method) on s-indacene l [ll-a [12]annulene perturbed by two o-bonds-substituents can be expected to have a strong influence on the bonding situation of the 1271 perimeter. The enthalpy of formation for the localized structure 1A with C,,-symmetry and the corresponding structure of the 2,4,6,8-tetraalkyl derivatives is calculated ca. 10 kcal.mol-' less than that for the delocalized structure 1B 1 A a with D2,-symmetry. For the I ,3,5,7-tetraalkyl-s-indacenes this energy gap should only be 2 kcal.mol-', whereas in the case of 4,8-diamino-s-indacenes a completely delocalized n-electron system should be preferred. Even nitrile groups in the 2-and 6-position can be expected to lower the energy difference between 1A and 1B to ca. 6 kcal . mol -I. A similarly significant alkyl group effect, changing the bonding situation of the 1271 perimeter of 1, has so far never been reported in the case of other [4n]-n-electron systems. This effect is due to enhanced coupling between the n-electron system and the substituents in the case of the delocalized structure 1B. The quantum chemical predictions could be experimentally established in the case of the stable 4,8-bis(dimethylamino)-s-indacene and the 4-dimethylamino-2,6,8-tri-tert-butyl d e r i~a t i v e . '~.~~ These results encouraged us to check the calculations also for pure alkyl derivatives of 1, to gain more detailed information about the 12n-electron system. 1,3,5,7-Tetra-tertbutyl-s-indacene 4 seemed to be especially suitable for this purpose since it should be additionally stabilized kinetically by the bulky alkyl groups compared to the thermally extremely unstable 1 . ['I In analogy to the synthesis of 4,8-bis(dimethylamino)-sindacene, 4 can be obtained as red needles (decomp.

The article contains sections titled: 1. Introduction and History 2. Properties 2.1. Physical Properties and Structure 2.2. Chemical Properties 2.3. Important Amino Acids 2.3.1. Proteinogenic Amino Acids 2.3.2. Other Important Amino Acids 3. Industrial Production of Amino Acids 3.1. General Methods 3.2. Production of Specific Amino Acids 3.2.1. l ‐Alanine 3.2.2. l ‐Arginine 3.2.3. l ‐Aspartic Acid and Asparagine 3.2.4. l ‐Cystine and l ‐Cysteine 3.2.5. l ‐Glutamic Acid 3.2.6. l ‐Glutamine 3.2.7. l ‐Histidine 3.2.8. l ‐Hydroxyproline 3.2.9. l ‐Isoleucine 3.2.10. l ‐Leucine 3.2.11. l ‐Lysine 3.2.12. d , l ‐Methionine and l ‐Methionine 3.2.13. l ‐Phenylalanine 3.2.14. l ‐Proline 3.2.15. l ‐Serine 3.2.16. l ‐Threonine 3.2.17. l ‐Tryptophan 3.2.18. l ‐Tyrosine 3.2.19. l ‐Valine 4. Biochemical and Physiological Significance 5. Uses 5.1. Human Nutrition 5.1.1. Supplementation 5.1.2. Flavorings, Taste Enhancers, and Sweeteners 5.1.3. Other Uses in Foodstuff Technology 5.2. Animal Nutrition 5.3. Pharmaceuticals 5.3.1. Nutritive Agents 5.3.2. Therapeutic Agents 5.4. Cosmetics 5.5. Agrochemicals 5.5.1. Herbicides 5.5.2. Fungicides 5.5.3. Insecticides 5.5.4. Plant Growth Regulators 5.6. Industrial Uses 6. Chemical Analysis 7. Economic Significance 8. Toxicology

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Two novel soluble polymer-bound oligo-L-leucines 2 and 5, which can be retained by a membrane reactor system, have been prepared and used as catalysts for the continuously operated asymmetric epoxidation of chalcone. The optimized batch reaction conditions yield epoxychalcone in high enantioselectivities (up to 94%) and conversions (over 99%) after 15 minutes.Epoxides are widely used compounds in organic synthesis. 1 For this reason several approaches to effect enantioselective epoxidation have been made. 2 Among the welldeveloped methods, the Juliá-Colonna epoxidation, which utilizes chiral polyamino acids (in particular poly-L-leucine) as heterogeneous catalysts, has emerged as the first reliable method for the asymmetric epoxidation of electron-deficient olefins, exhibiting exceptionally high chiral induction for chalcone. 3Degussa and their partners from the University of Liverpool have worked intensively on the improvement of heterogeneous Juliá-Colonna oxidation. A noteworthy modification includes the introduction of percarbonate in dimethoxyethane as a cheap oxidant/solvent system for the use in some polyamino acid-catalysed epoxidations. 4The poly-L-leucine catalyst remained insoluble under all the reaction conditions applied. The first homogeneous version of the Juliá-Colonna epoxidation of trans-chalcone was reported recently. 5 However, the conversion observed was only 39% after 1 hour and 80% after 24 hours with an ee of 95-98%.Since the chiral information has to be transferred from the catalyst to the olefinic substrate, homogeneous asymmetric catalysis is a more attractive method for the synthesis of chiral epoxides. However, up to now, no process has made the step from an academically promising method to an application on larger scale.The interest of Degussa in developing methods for asymmetric synthesis in a chemzyme membrane reactor 6 promoted further investigation of the homogeneous enantioselective epoxidation of a,b-unsaturated ketones.We reasoned that the preparation of polymer enlarged oligo(L-leucine)s which were soluble in common organic solvents and large enough to be retained in a membrane reactor could allow us to develop a continuously operated asymmetric epoxidation of trans-chalcone.There are two major approaches to synthesize homogeneously soluble polymer-enlarged catalysts, that is using either linear polymers or dendrimers as carriers.We have focused our work on linear polymers, because they demand less synthetic effort. Thus, we prepared two types of soluble polymer enlarged oligo(L-leucine)s 2 and 5 (Scheme 1, Scheme 2). The approach to catalyst 2 involved the co-polycondensation of 1 equivalent of commercially available O,O-bis(2-aminoethyl)-polyethyleneglycol 20000 1 with 16 equivalents of L-leucine-N-carboxyanhydride in CHCl 3 . 7 Scheme 1 Synthesis of homogeneously soluble oligo(L-leucine) 2.We found that the resulting polyethyleneglycol-supported oligo(L-leucine) 2 can act as an efficient homogeneous chiral catalyst in the epoxidation of trans-chalcone 6 employing the urea ...