The coordination of chiral ligands to Lewis acid metal derivatives, a useful strategy for enantioselective, electrophilic catalysis, generally leads to a lower level of catalytic activity than that of the original uncomplexed compound. Activation by further attachment of a proton or strong Lewis acid to the complex provides a way to overcome the deactivating effect of a chiral ligand. The research described herein has demonstrated that further enhancement of catalytic activity is possible by the judicious placement of fluorine substituents in the chiral ligand. This approach has led to a new, second-generation family of chiral oxazaborolidinium cationic species which can be used to effect many Diels-Alder reactions in >95% yield and >95% ee using catalyst loadings at the 1-2 mol % level. The easy recovery of the chiral ligand makes the application of these new catalysts especially attractive for large-scale synthesis.
A bidirectional total synthesis of chloptosin has been achieved in 16 operations (32 individual reactions) and 3% overall yield from the readily available materials. Palladium-catalyzed tryptophan synthesis, diastereoselective selenocyclization and oxidative deselenation successfully served as key steps in construction of the dimeric core amino acid. 2-Bromo-1-ethyl pyridinium tetrafluoroborate was efficiently employed in the peptide couplings with spatial encumbrance in this synthesis.
The first total synthesis of ent-(-)-azonazine has been accomplished using a hypervalent iodine-mediated biomimetic oxidative cyclization to construct the highly strained core. Based on the results from the completed synthesis, both the relative and absolute configurations of natural (+)-azonazine were revised.
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