# Photocatalyzed Ring Expansion of α‐Ketosulfonylaziridines: Ready Access to δ‐Sultams

## TL;DR

A new photocatalytic method expands aziridines into sultams with unique structures, offering a versatile way to create complex molecules.

## Contribution

A novel photocatalytic ring expansion reaction for synthesizing δ-sultams with unprecedented substitution patterns.

## Key findings

- The method achieves up to 85% yield and 2:1 diastereomeric ratio for trans-disubstituted products.
- The reaction shows good functional group tolerance and scalability up to 10-fold.
- Mechanistic studies confirm a radical–polar crossover pathway involving a carbonyl radical anion.

## Abstract

Herein, a novel photocatalytic ring expansion reaction of aziridines to δ‐sultams through intramolecular radical cyclization is reported. In particular, versatile N‐sulfonyl‐protected α‐ketoaziridines were envisioned to undergo a ring opening–expansion through a photoredox‐mediated radical–polar crossover (RPC) process to provide six‐membered sultams bearing an unprecedented 2,3‐disubstitution pattern. The generality of the reaction was explored with a wide variety of substrates, showing a good functional group tolerance. Moreover, the robustness of the method allows a 10‐fold upscaling, providing similar yield or diastereomeric preference for the trans‐disubstituted products. Mechanistic investigations supported the postulated radical–polar crossover pathway, involving aziridine ring opening by photocatalytic single‐electron reduction via a carbonyl radical anion, followed by intramolecular cyclization with the aryl unit of the N‐sulfonyl group and re‐aromatization. Thus, this strategy opens new possibilities for the construction of highly decorated heterocycles by photoredox‐mediated RPC ring expansion reactions.

A novel radical–polar crossover (RPC) photoredox‐catalyzed reaction for the synthesis of δ‐sultams from N‐sulfonyl α‐ketoaziridines through a controlled ring opening–expansion process is presented. This method allows the easy access to sultams with an unprecedented substitution pattern in up to 85% yield and 2:1 d.r., and highlights the potential of photocatalytic RPC strategies for the construction of value‐added heterocyclic molecules.