Synthesis and Properties of Glycoazobenzene Macrocycles
Since their discovery more than 100 years ago, macrocycles have attracted much attention in chemistry. They define a distinct chemical space where molecular structures are located between small substances and macromolecules and applications at the junction between physics, chemistry and biology. Macrocycles have a wide range of applications in molecular recognition and self-organization, in catalysis and in drug discovery. Carbohydrate-containing macrocycles are found in nature in a large variety of forms, such as cyclodextrins and various antibiotically active substances. Carbohydrates are of great interest for the synthesis of macrocycles not only because of their biological activity, but also because of their good availability from natural sources and because of their complex but defined stereochemistry and multifunctionality, which allows a great structural diversity and broad modulation of the properties of the macrocycles derived from them. The main interest of this work was the synthesis and properties of photoswitchable glycoazobenzene macrocycles. Here, the azobenzene units serve as photoswitches, which can be reversibly converted from their thermodynamically more stable planar trans-isomer to the bent cis-isomer by irradiation with light. In the macrocycle, the energy of the light can thus be transformed into molecular motion. The temporal and spatial control of form and function by irradiation makes azobenzene conjugates ideal building blocks for incorporation into macrocyclic structures. While azobenzene glycoconjugates, in which the properties of carbohydrates are combined with those of the photoisomerizable azobenzene units, have been successfully developed in the past, the combination of azobenzene derivatives with carbohydrates in macrocyclic systems was unknown until now. The present work explores new territory in this field. The synthesis and photochromic properties of the first glycoazobenzene macrocycles are described in this thesis. The syntheses were designed in such a way that the different glycoazobenzene macrocycles could be obtained easily and as modular as possible from readily available building blocks in a few steps. Four different reactions were successfully used as key steps for macrocyclization in the synthesis, the thiourea bridging, the click reaction, the Sonogashira coupling and the Glaser coupling. The detailed investigation of the photochromic properties of the new macrocycles showed that the isomerization of the azobenzene units significantly and in a defined way changes the macrocycle structure and is associated with the switching of molecular properties such as solubility and chiroptic and physicochemical properties.