Atroposelective Synthesis of Planar-Chiral Indoles via Carbene Catalyzed Macrocyclization

Indole-based planar-chiral macrocycles are widely found in natural products and bioactive molecules. However, in sharp contrast to the preparation of indole-based axially chiral structures, the enantioselective catalysis of indole-based planar-chiral macrocycles is still a formidable task so far. Here we report an N-heterocyclic carbene (NHC)-catalyzed intramolecular atroposelective macrocyclization of 3-carboxaldehyde indole/pyrroles, featuring with broad substrate scope and good functional group tolerance, and allowing for a rapid access to diverse indole/pyrrole-based planar-chiral macrocycles with various tether-lengths (10-16 members) in good yields and with excellent enantioselectivities. Importantly, the indole-based macrocyclic structures with both planar and axial chirality were directly and efficiently obtained through this protocol with excellent enantioselectivities and diastereoselectivities. In addition, these synthesized planar-chiral macrocycles offer many possibilities for chemists to develop new catalysts or ligands, as well as new reactions.

Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-Catalyzed [2+2+2] Cycloaddition

The enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-based planar chiral cyclophanes was achieved for the first time by the rhodium-catalyzed intramolecular regio- and enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulvenes followed by stepwise oxidative transformations. The thus synthesized planar chiral bent cyclophanes, that possess bent p-terphenyl- and 9-fluorenone-cores, were converted to 9-fluorenol-based ones with excellent ee values of >99 % by diastereoselective 1,2-reduction. These 9-fluorenol-based cyclophanes exhibited high fluorescence quantum yields, which were significantly higher than that of an acyclic reference molecule (78-82 % vs. 48 %). The bending effect on the chiroptical property was also examined, which revealed that the anisotropy factors (g_abs values) for electronic circular dichroism (ECD) of these 9-fluorenol-based planar chiral bent cyclophanes increase as the tether length becomes shorter.