α‐Amino Acids and α,β‐Dipeptides Intercalated into Hydrotalcite: Efficient Catalysts in the Asymmetric Michael Addition Reaction of Aldehydes to N ‐Substituted Maleimides

β-Amino Acid Organocatalysts in the Asymmetric Michael Addition of Isobutyraldehyde to N-Substituted Maleimides

Asymmetric Michael additions of carbonyl compounds to N-substituted maleimides are among the most convenient reactions to prepare optically pure succinimide building blocks. Although a few β-amino acids were found to be highly efficient organocatalysts in the addition of α-branched aldehydes, the effect of their structure on the results of these reactions has not yet been investigated. In the present study, we disclose several unexpected and interesting structural effects of aliphatic and cycloaliphatic β-amino acids obtained in the enantioselective conjugate addition of isobutyraldehyde to N-benzylmaleimide. The dependence of the sense of the enantioselectivity on the bulkiness of the substituent on the β-carbon atom, the beneficial spatial arrangements of the functional groups in cis isomers with cyclohexane scaffold and the inversion of the enantioselectivity depending on the absence of a base additive observed with some trans isomers are unprecedented findings. The minor influence of the nitrogen substituent of the maleimide ring on both the reaction rate and the enantioselectivity was also evidenced using alicyclic β-amino acid prepared from an easily available terpene derivative.

Separation of amino acids by selective sorption through reconstructive intercalation in calcined MgAl-layered double hydroxide

Mg-Al layered double hydroxide (MgAl-LDH) exhibits selectivity in the intercalation of amino acids (AAs). When the MgAl-LDH derived mixed metal oxide was treated with different mixtures of AAs, preferential sorption of one AA over the other(s) was observed as indicated by XRD analysis of the products and HPLC analysis of the interlayer AA contents in the products. The order of preference was aspartic acid, glutamic acid (acidic AAs) > glycine, alanine (neutral AAs) > hystidine, and arginine (basic AAs). Among the acidic AAs, aspartic acid was preferred over glutamic acid and among the basic AAs, histidine was preferred over arginine. LDH shows equal preference for glycine and alanine. The selectivity can be explained on the basis of the isoelectric pH (pI) of the AA. A similar selectivity order was obtained when the mixtures of AAs were treated with nitrate-intercalated LDH (direct anion exchange) although the net AA intercalated is much lower due to competition with carbonate derived from atmospheric CO₂. The high selectivity observed in some cases (such as aspartic acid and glycine) would result in the quantitative separation of the individual AAs from their mixture.

Enantioselective Organocatalyzed Michael Addition of Isobutyraldehyde to Maleimides in Aqueous Media

Thiourea was introduced into (R,R)-1,2-diphenylethylenediamine as an organocatalyst to promote the reaction between isobutyraldehydes and maleimides. Enantioselective Michael addition reaction was carried out as an eco-friendly method using water as the solvent. As a result of the reaction between isobutyraldehyde and maleimide, ≥97% yield and 99% enantioselectivity were obtained at a low catalyst loading of 0.01 mol%. The solvent effect can be explained by theoretical calculations that indicate the participation of a transition state, in which the CF₃ substituent of the catalyst is a hydrogen bond activated by the surrounding water molecules. This discovery enabled the use of low catalyst loading in the organic reactions of chiral substances for pharmaceutical applications. Furthermore, a solvent effect for Michael reaction of the organocatalysts was proposed, and the organic reaction mechanisms were determined through quantum calculations.

Enantioselective Michael addition of aldehydes to maleimides catalysed by surface-adsorbed natural amino acids

Chiral hybrid materials obtained by adsorption of primary α-amino acids on the surface of inorganic oxides are economic, recyclable, highly enantioselective heterogeneous catalysts for the Michael addition of aldehydes to N-substituted maleimides.