Synthesis of [6]helicene-based sulfonic acid, sulfonamide, and disulfonimides

Chiral disulfonimides: a versatile template for asymmetric catalysis

Since the emergence of pseudo-C2-symmetric chiral phosphoric acids (CPA), much work has been done to utilize these systems in stereoselective, organocatalytic processes. Despite the success in this field, reasonably basic substrates such as imines are often required to achieve appreciable activation. In order to access a wider variety of potential reaction partners, many related organocatalysts with enhanced Brønsted acidity have since been developed. Chiral disulfonimides (DSIs) have materialized as one such powerful class of organocatalysts and have been shown to expand the list of potential substrates to include aldehydes and ketones via Brønsted, Lewis, or bifunctional acid activation. This versatility renders DSIs amenable to an impressive scope of reaction types, typically with remarkable stereoselectivity induced by asymmetric counteranion-directed catalysis (ACDC). This review serves to provide a complete analysis of the successful applications, mechanistic insights, and unmet challenges exhibited to date in DSI-catalyzed and -assisted processes.

Mild, Modular, and Convergent Synthesis of Helical Naphtho[2,1-c]chromenes via a Multistep Cyclization/Aromatization Cascade Sequence

Tetracyclic 6H-naphtho[2,1-c]chromenes are expeditiously synthesized through a BF₃·OEt₂-mediated, three-step cascade reaction, creating new central pyran and aromatic rings. The cascade involves the addition of phenol-derived alkynyl substrates to BF₃-activated aldehydes followed by alkyne-Prins cyclization, Friedel-Crafts reaction, and final elimination. Aliphatic and electron-deficient aromatic aldehydes afford the products in 50-74% isolated yields, but benzaldehyde and tolualdehyde resulted in lower yields. X-ray analysis of a p-bromophenyl derivative (5aA) shows the two aromatic moieties are twisted by 28° to create a helical backbone.