Importance of cycloamylose substrate geometry and dynamic coupling in the deacylation of 3- and 4-nitrophenyl acetates

Comparison of the Conventional and Mechanochemical Syntheses of Cyclodextrin Derivatives

Many scientists are working hard to find green alternatives to classical synthetic methods. Today, state-of-the-art ultrasonic and grinding techniques already drive the production of organic compounds on an industrial scale. The physicochemical and chemical behavior of cyclodextrins often differs from the typical properties of classic organic compounds and carbohydrates. The usually poor solubility and complexing properties of cyclodextrins can require special techniques. By eliminating or reducing the amount of solvent needed, green alternatives can reform classical synthetic methods, making them attractive for environmentally friendly production and the circular economy. The lack of energy-intensive synthetic and purification steps could transform currently inefficient processes into feasible methods. Mechanochemical reaction mechanisms are generally different from normal solution-chemistry mechanisms. The absence of a solvent and the presence of very high local temperatures for microseconds facilitate the synthesis of cyclodextrin derivatives that are impossible or difficult to produce under classical solution-chemistry conditions. Although mechanochemistry does not provide a general solution to all problems, several good examples show that this new technology can open up efficient synthetic pathways.

Quinuclidine complex with alpha-cyclodextrin: a diffusion and 13C NMR relaxation study

The stability of an inclusion complex of quinuclidine with alpha-cyclodextrin in solution was investigated by NMR measurements of the translational diffusion coefficient. A 1:1 stoichiometry model yielded an association constant of 35 +/- 3 M(-1). The guest molecules exchange rapidly between the host cavity and the bulk solution. The reorientational dynamics of the guest and host molecules was studied using carbon-13 NMR relaxation at two magnetic fields. The relaxation of the host nuclei showed very little dependence on the guest-host concentration ratio, while the 13C spins in quinuclidine were sensitive to the solution composition. Using mole-fraction data, it was possible to extract the relaxation parameters for the bound and free form of quinuclidine. Relaxation rates of the guest molecule, free in solution, were best described by an axially symmetric model, while the data of the complex species were analyzed using the Lipari-Szabo method. Applying the axially symmetric model to the complexed quinuclidine indicated that the anisotropy of its reorientation in the bound form was increased.

Complex formation between alpha-cyclodextrin and 4-substituted phenols studied by potentiometric and competitive spectrophotometric methods

Stability constants for complex formation between alpha-cyclodextrin and the conjugate acid and base forms of nine phenols were measured in aqueous solution at 25 degrees. The potentiometric method, in which the apparent acid dissociation constant of the phenol is measured as a function of cyclodextrin concentration, was supplemented by a modified version of a competitive spectrophotometric methyl orange method. For all phenols, the 1:1 stability constant for the conjugate base form (K11b) was larger than K11a for the conjugate acid form. Finite K12b values were found for phenols whose 4-substituents could tolerate a positive charge by electron delocalization. Complex stability, as measured by K11a and K11b, increases with electron density and polarizability at the 4-substituent. It is concluded that the 4-substituent is the sole or predominant site of binding for both the conjugate acid and base forms of the phenols. The general result that K11b is greater than K11a for any phenol is accounted for by relative delocalization of charge in the anion and neutral species.