Search

Aldol reactions belong to the most important methods for carbon–carbon bond formation and are also involved in one of the most astonishing biosynthetic processes: the biosynthesis of polyketides governed by an extraordinarily sophisticated enzymatic machinery. In contrast to the typical linear or convergent strategies followed in chemical synthesis, this late-stage catalysis concept allows Nature to assemble intermediates that are diversified into a broad range of scaffolds, which assume various crucial biological functions. To transfer this concept to small-molecule catalysis to access products beyond the natural systems, a stepwise approach to differentiate increasingly complex substrates was followed by investigating arene-forming polyketide cyclizations. An outline of our efforts to develop and apply these concepts are presented herein.

By taking inspiration from the fascinating biosynthetic machinery that creates aromatic polyketides, our group investigates analogous reactions catalyzed by small molecules. We are particularly captivated by the prospects of intramolecular aldol condensation reactions to generate different rotationally restricted aromatic compounds. In a first project of our independent research group, a highly stereoselective amine catalyzed synthesis of axially chiral biaryls, tertiary aromatic amides and oligo-1,2-naphthylenes has been developed. In this article, we outline the twists and turns for our escape from the aromatic flatland to structurally intriguing chiral arene scaffolds relevant for various fields of application.

Enantiopure (diastereomeric) doubly bridged biphenyl azepines prepared from (S)-3,3-dimethylbutan-2-amine and (S)-1-phenylpropylamine can be – as a function of the exocyclic side chain – either effective catalysts for the enantioselective epoxidation of unfunctionalized olefins or no catalysts at all. In the case of the (S)-1-phenylpropylamine derivative, an unwanted and unexpected Cope elimination derails the catalytic reactivity which can, however, be recovered by the addition of NBS prior to that of the substrate and other reagents.