Speeding up Viedma Deracemization through Water-catalyzed and Reactant Self-catalyzed Racemization

Viedma deracemization is based on solution phase racemization, dissolution of racemic or scalemic conglomerates and crystal growth through autocatalytic cluster formation. With rate limiting racemization, its acceleration by appropriate catalysts may result in speeding up deracemization. A conglomerate-forming chiral compound may principally racemize directly, or via reverse of its formation reaction. For a hydrazine derivative, we investigated available racemization pathways in presence of pyrrolidine or thiourea amine as base catalysts: via Mannich or aza-Michael reaction steps and their reverse, or by enolization. Racemization by enolization was computationally found to dominate, both under water-free conditions and in presence of water, involving a multitude of different pathways. Faster racemization in presence of water resulted indeed in more rapid deracemization, when the base was pyrrolidine. Under water-free conditions, the role of water as enolization catalyst is assumed by chiral hydrazine itself – in autocatalytic racemization and in which both reactant and product are catalysts.     

Werner Complexes with ω‐Dimethylaminoalkyl Substituted Ethylenediamine Ligands: Bifunctional Hydrogen‐Bond‐Donor Catalysts for Highly Enantioselective Michael Additions

The racemic carbonate complex [Co(en)₂O₂CO]⁺ Cl^? (en=1,2‐ethylenediamine) and (S)‐[H₃NCH((CH₂)_nNHMe₂)CH₂NH₃]³⁺ 3 Cl^? (n=1–4) react (water, charcoal, 100 °C) to give [Co(en)₂((S)‐H₂NCH((CH₂)_nNHMe₂)CH₂NH₂)]⁴⁺ 4 Cl^? ( 3 a – d H⁴⁺ 4 Cl^?) as a mixture of Λ/Δ diastereomers that separate on chiral‐phase Sephadex columns. These are treated with NaOH/Na⁺ BAr_f^? (BAr_f=B(3,5‐C₆H₃(CF₃)₂)₄) to give lipophilic Λ‐ and Δ‐ 3 a–d ³⁺ 3 BAr_f^?, which are screened as catalysts (10 mol %) for additions of dialkyl malonates to nitroalkenes. Optimal results are obtained with Λ‐ 3 c ³⁺ 3 BAr_f^? (CH₂Cl₂, ?35 °C; 98–82 % yields and 99–93 % ee for six β‐arylnitroethenes). The monofunctional catalysts Λ‐ and Δ‐[Co(en)₃]³⁺ 3 BAr_f^? give enantioselectivities of <10 % ee with equal loadings of Et₃N. The crystal structure of Δ‐ 3 a H⁴⁺ 4 Cl^? provides a starting point for speculation regarding transition‐state assemblies.     

Enantioselective separation of all-E-astaxanthin and its determination in microbial sources

A method for the enantioselective separation of all-E-astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione), an important colorant in the feed industry, was developed. Different chiral stationary phases (CSPs) such as Pirkle phases (R,R Ulmo and l-leucine), modified polysaccharides and a beta-cyclodextrin have been investigated on their separation performance of astaxanthin enantiomers. Direct resolution was only achieved employing the Chiralcel OD-RH (cellulose-tris-3,5-dimethylphenyl-carbamate) under reversed phase conditions. The chiral separation of the enantiomeric forms of astaxanthin produced in microalgae and yeasts was reported. The yeast Xanthophyllomyces sp. produces astaxanthin predominantly in the R,R configuration, whereas in the green microalgae Scenedesmus sp. astaxanthin is built primarily in the S,S form. The separation method for the identification of astaxanthin enantiomers is of great interest since astaxanthin is used as functional food additive in human nutrition. Moreover the method may be used as a food chain indicator in farmed salmon.     

Crystal structure and quantum chemical calculations of (E)1-benzyl-3-((4-methoxyphenyl)imino)-5-methylindolin-2-one

The known Schiff base compound, (E)1-benzyl-3-((4-methoxyphenyl)imino)-5-methylindolin-2-one, was prepared as before by reacting 1-benzyl-5-methylindoline-2,3-dione with 4-methoxyaniline. The product was unambiguously characterized using elemental analysis, ¹H and ¹³C-NMR spectroscopy, and its new single-crystal X-ray structural analysis. Molecular orbital calculations were conducted in order to investigate the structures and relative stabilities of the (E) and (Z) isomers of 1-benzyl-3-([4 methoxyphenyl]-imino)-5-methylindolin-2-one. Specific attention was paid to the (E) isomer. The available crystallographic experimental data for the latter ensured also validation of the model structures computationally derived at the theoretical B3LYP/6-31G(d,p) level.