Inherently Chiral Calix[4]arenes: A Chiral Sulfoxide as an Ortholithiation Director

Simultaneous construction of inherent and axial chirality by cobalt-catalyzed enantioselective C-H activation of calix[4]arenes

The simultaneous construction of multiple stereogenic elements in a single step is highly appealing and desirable in the field of asymmetric synthesis. Furthermore, the catalytic enantioselective synthesis of inherently chiral calix[n]arenes with high enantiopurity has long been a challenging endeavor. Herein, we report an enantioselective cobalt-catalyzed C-H activation/annulation for the efficient construction of inherently chiral calix[4]arenes bearing multiple C-N axially chiral element. By employing the benzamide tethered calix[4]arene as the substrate, the C-H annulation with alkynes can be successfully accomplished, leading to the generation of multiple stereogenic elements. A wide range of calix[4]arenes and alkynes are found to be well compatible, and exhibit good yields, high enantioselectivity and excellent diastereoselectivity. Notably, the gram-scale reaction, catalytic application, synthetic transformations, and chiral recognition further showcase the potential applications of this protocol.

Facile synthesis of a C4-symmetrical inherently chiral calix[4]arene

Inherently chiral calix[4]arenes with C₄-symmetry are extremely rare and difficult to synthesise, severely hampering any effort to expand on their potential as chiral supramolecular catalysts and building blocks. Herein we report a reaction of a tetracarbamate calix[4]arene with NBS which results in a high yield of an inherently chiral calix[4]arenes with C₄-symmetry. Furthermore, employing a chiral N-Boc proline moiety allows for separation of the diastereomers formed, thus obtaining the pure enantiomers after hydrolysis. The enantiomers could be assigned based on the CD spectra and DFT calculated values.

Attempted synthesis of a meta-metalated calix[4]arene

An evidence for the formation of a rare meta-metalated inherently chiral calix[4]arene is described. Our strategy involved using a mesoionic carbene to direct C–H activation, but proved to form an unexpectedly unstable intermediate that was identified through high-resolution mass spectrometry. On route to our target, a new optimized method to mononitrocalix[4]arenes was developed, including optimized and high yielding transformations to azide and 1,2,3-triazole derivatives which may have application in other areas of research.