Sandra Kaabel

Asymmetric Organocatalytic Wittig [2,3]-Rearrangement of Oxindoles

A highly enantioselective organocatalytic [2,3]-rearrangement of oxindole derivatives is presented. The reaction was catalyzed by squaramide, and this provides access to 3-hydroxy 3-substituted oxindoles in high enantiomeric purities.

Synthesis and Characterisation of Chiral Triazole‐Based Halogen‐Bond Donors: Halogen Bonds in the Solid State and in Solution

A general platform for the synthesis of various chiral halogen-bond (XB) donors based on the triazole core and the characterisation of factors that influence the strength of the halogen bond in the solid state and in solution are reported. The characterisation of XB donors in the solid state by X-ray crystallography and in solution by 1 H NMR titration can be used to aid the design of new XB donors. We describe the first example of a XB between iodotriazoles and thioureas in solution. In addition, the enantiodiscrimination of acceptors in solution through halogen-bond participation is described.

CaCl2, Bisoxazoline, and Malonate: A Protocol for an Asymmetric Michael Reaction

A mild protocol for the asymmetric Michael addition of dimethyl malonate to various α,β-unsaturated carbonyl compounds was developed. The salient feature of this methodology is that a cheap and environmentally friendly Lewis acid, CaCl2, was used as a catalyst. An aminoindanol- and pyridine-derived ligand provided in the presence of CaCl2 Michael adducts in moderate to high enantioselectivities. The scope of the reaction was demonstrated.

A quantitative method for analysis of mixtures of homologues and stereoisomers of hemicucurbiturils that allows us to follow their formation and stability

Hemicucurbiturils are a sub-group of macrocyclic compounds within the cucurbit[n]uril family. The ability of unsubstituted and substituted hemicucurbiturils to form host–guest complexes allows selective binding of anions, application in catalysis and chiral discrimination between enantiomeric guests. Herein we demonstrate the separation of hemicucurbituril homologues and diastereomers by reverse phase HPLC and the quantitative analysis of hemicucurbit[6]uril, (all-R,R)-cyclohexanohemicucurbit[6]uril, (all-R,S)-cyclohexanohemicucurbit[6]uril and (all-R,R)-cyclohexanohemicucurbit[8]uril. The applicability of the developed quantitative analysis is demonstrated by the estimation of the reaction yields of cyclohexanohemicucurbiturils and by following the depolymerisation of hemicucurbit[6]uril under acidic conditions. Analysis of the studied set of compounds – monomers, oligomers and macrocycles – reveals that the number of repeating units and UV extinction coefficients are not proportional; also, the UV absorbance of a macrocycle is 10-fold higher compared to its linear isomer. The possibility of a drastic difference in the ultraviolet absorbance of oligomeric analogues should also be considered in quantification of other classes of polymeric macrocycles.

Cucurbiturils: Synthesis, Structures, Formation Mechanisms, and Nomenclature

The cucurbituril (CB) family has grown exponentially during the past two decades due to its exceptional binding ability and wide possibilities for applications. This article focuses on the description of the currently known core cucurbiturils, with the objective of bringing their homologues and derivatives under a unified nomenclature. An overview of the CB formation mechanisms, which follow the principles of dynamic covalent chemistry, is given, and synthetic methods used in cucurbituril chemistry are covered. Based on their crystal structures, a comparison of the dimensions and shapes of cucurbituril derivatives is made, and their known solubilities in water are also listed.