Catalytic Cascade Reactions Inspired by Polyketide Biosynthesis

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.

Synthesis of Enantioenriched Tetra-ortho-3,3′-substituted Biaryls by Small-Molecule-Catalyzed Noncanonical Polyketide Cyclizations

The arene-forming aldol condensation is a fundamental reaction in the biosynthesis of aromatic polyketides. Precisely controlled by the polyketide synthases, the highly reactive poly-β-carbonyl substrates are diverged into numerous aromatic natural products by selective cyclization reactions; a fascinating biosynthetic strategy that sparked our interest to investigate atroposelective aldol condensations. In this Account, we contextualize and highlight the ability of small-molecule catalysts to selectively convert noncanonical hexacarbonyl substrates in a double arene-forming aldol condensation resulting in the atroposelective synthesis of tetra-ortho-3,3′-substituted biaryls. The hexacarbonyl substrates were obtained by a fourfold ozonolysis enabling a late-stage introduction of all carbonyl functions in one step. Secondary amine catalysts capable of forming an extended hydrogen-bonding network triggered the noncanonical polyketide cyclization in order to form valuable biaryls in high yields and with stereocontrol of up to 98:2 er.

1 Biosynthesis of Aromatic Polyketides

2 Rotationally Restricted Aromatic Polyketides

3 3,3′-Substituted Binaphthalenes in Catalysis

4 Stereoselective Synthesis of Atropisomers

5 Synthesis of Enantioenriched Tetra-ortho-3,3′-Substituted Biaryls by the Atroposelective Arene-Forming Aldol Condensation

6 Conclusion

Catalytic Arene-forming Aldol Condensation: Stereoselective Synthesis of Rotationally Restricted Aromatic Compounds

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.