Organic Chemistry Seminar
[2+2] Photocycloaddition reactions belong undisputedly to the most important reaction classes in photochemistry. The creation of up to four new stereogenic centers in a single step and the further use of the formed cyclobutane rings – either directly or after appropriate ring opening – are hallmarks of this powerful transformation. In our group, we have studied two different approaches to achieve a catalytic enantioselective reaction course in [2+2] photocycloaddition reactions. The first approach is based on a triplet energy transfer by hydrogen-bonding chiral catalysts, which in turn are derived from a previously described template successfully employed in the total synthesis of (+)-meloscine. The second approach relies on chiral Lewis acids, which change the photophysical parameters of the substrate and allow a selective excitation in the chiral environment, which they provide.
A chiral xanthone turned out to be an efficient organocatalyst providing good turnover (10 mol%) and high enantioselectivities (>90% ee) in [2+2] photocycloaddition reactions while a related chiral thioxanthone allowed for enantioselective reactions promoted by visible light. Apart from this approach, we have also looked into the possibility of Lewis-acid mediated enantioselectivity in photochemical reactions. Chiral Lewis acids were developed for [2+2] photocycloaddition reactions and are currently being further explored.
The presentation discusses the background of the above-mentioned studies and provides the latest results of our research efforts in this area.