Laser controlled cycloaddition and reversion of crossdimers
To treat cataract, the major cause for blindness, the original eye lenses are removed and artificial lenses are implanted. With this procedure, the side effect of secondary cataract has become of interest to research. The treatment thereof calls for the development of a molecular integrated and cova...
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|Zusammenfassung:||To treat cataract, the major cause for blindness, the original eye lenses are removed and artificial lenses are implanted. With this procedure, the side effect of secondary cataract has become of interest to research. The treatment thereof calls for the development of a molecular integrated and covalently bound photolinker system for the controlled release of cytostatica. To load the polymeric intraocular lens, crossdimers of the photolinker coumarin together with the antimetabolite 5-fluorouracil are synthesized. After incorporation in the lens, the specific release via cycloreversion then is best realized with a two photon process which offers high spatial resolution, and which uses visual light to pass the cornea. The backbone of this work is the light driven, intermolecular and coumarin based crossdimerization and its reversion. To achieve this goal, the options given for photoreactions, like set up, substrates, and excitation are considered and their variables are evaluated. The choice of photolinker is narrowed down and a comparison between these absorbing substrates provided by kinetic measurements. A list of possible molecule types of the non-absorbing reaction partner for the crossdimerization is given, by which a huge variety of crossdimers can be build up. The commonly used lamp based light sources such as a Rayonet reactor are compared to the novel favored laser system. The sun is also utilized to project a large scale, energetically economic process. The laser, as a new tool for molecule synthesis, exhibits improvements to cycloaddition reactions in a number of experiments. Additives like sensitizers and triplet promoters are shown to be unnecessary with this high energy illumination. The laser fueled set up is analyzed and the photon distribution, together with the photon flux, is determined to be crucial. With parameters attuned to the lifetime of the excited state, the diffusion controlled dimerization reaction can be controlled and steered towards the principal generation of crossdimer. The reversibility of the dimerization reaction caused both by heat and light is demonstrated. Thermal cleavage is used in addition to laser synthesis in the specialization towards exclusive crossdimer synthesis. Light is primarily used in the customary laser based two photon absorption for specific single and two bond cleavage. Hereby the ever more popular two photon cycloreversion is measured of individual stereoisomers. All methods, developed for the dimer generation, belong to the general concept for the extended application of photoreaction in synthesis. Additional examples in form of a sequential photoreaction and photoremovable protective groups are presented accordingly.|