Organic Peroxides in Radical Chemistry and Stereochemical Study of the Intramolecular Schmidt Reaction
Organic peroxides and organic azides are highly energetic functional groups and have been utilized in many synthetic applications for over a hundred years. The first part of this thesis focuses on organic peroxides as a source of radicals. After a general introduction to organic peroxides in radical chemistry in chapter 1, peroxyoxalates and diacyl peroxides move into the focus of this thesis (chapters 2-5). Di-tert-butyl peroxyoxalates (DTBPO) was found to be an ideal radical initiator in the key step of our short enantioselective synthesis of the natural product (+)-brefeldin C described in chapter 2.
Inspired by the thermal decomposition of DTBPO, peroxyoxalates were studied as a source of tertiary alkoxy radicals. An operationally simple method to access these radicals from tertiary alkyl hydroperoxides was developed using oxalyl chloride and is presented in chapter 3. The alkoxy radicals have been used in particular for the synthesis of various 4â€™ functionalized alcohols via 1,5-hydrogen atom transfer and subsequent trapping of the relocated radical with a suitable radical trap.
The one-pot procedure is fast, does not require workup after the reaction, and yields the functionalized alcohols in moderate to excellent yields. Unpublished results from work using this method and investigations on an analogous mechanism based on borinate radicals are compiled in chapter 4.
Due to the unstable nature of organic peroxides and the hazards associated with their handling, a safer approach to their synthesis and direct use in continuous flow was investigated in chapter 5. It has been found that clean dilauroyl peroxide can be formed in excellent yield and can be used directly as an initiator in a subsequent reaction that is connected in series.
In the second part of this thesis, the stereochemical challenges of the intramolecular Schmidt reaction and the strategies to address them are introduced in chapter 6.
Our investigations to control the stereochemistry of the triflate-mediated intramolecular Schmidt reaction are summarized in two draft manuscripts in chapters 7 and 8. Our group has developed a protocol to run the reaction under nonacidic conditions using azidotriflates: After an initial intramolecular SN2 reaction between the azide and the triflate moiety, an intermediate aminodiazonoium salt is formed, which undergoes a stereoselective 1,2-shift with concomitant N2 elimination. The substitution has been found to proceed highly stereospecifically. The formed iminium triflate is reduced diastereoslectively in a second step to the bicyclic amine. Remarkably, the chiral alcohol center controls the entire process and thus only one of the four possible diastereomers is obtained in a highly selective manner. The method has been used for the concise synthesis of a lehmizidine- and an indolizidine alkaloid that represent components of myrmicaria melanogaster ant venom.
It was further investigated whether the method is applicable to access bridgehead-functionalized azabicycles from the corresponding prefunctionalized azidoalcohols. The results of this study complement previous finding that have been conducted in two precedent PhD theses and are presented in the last chapter of this thesis.