Metal-free access to β-carbolines via single-electron transfer catalyzed by a triaryl carbenium ion pair

The /3-carboline moiety exists in a wide range of drugs and natural prod-ucts, but synthesis often relies on noble metals and stochiometric oxi-dants. Here, we develop a metal-free strategy using a triaryl carbenium ion pair as a catalyst to access /3-carbolines from tryptamines and aro-matic aldehydes. This protocol exhibits excellent functional group tolerance, and its practicality is demonstrated with the preparation of Eudistomin U and fluorescent compounds on gram scales. Mechanistic studies reveal that the trityl radical is generated via a single-electron transfer process and subsequently trapped by an oxygen molecule. In addition, the in -situ-generated reactive oxygen-centered radical is also found to be essential for the efficient dehydroaromatization pro-cess to afford the desired products.

Automated summary

A metal-free synthesis of β-carbolines from tryptamines and aromatic aldehydes is achieved using a triaryl carbenium ion pair as a catalyst. This method exhibits excellent functional group tolerance and is demonstrated with the synthesis of Eudistomin U and fluorescent compounds on a large scale. Mechanistic studies reveal the formation of a trityl radical through a single-electron transfer process and its trapping by an oxygen molecule. In addition, an in-situ-generated reactive oxygen-centered radical is found to be essential for the efficient dehydroaromatization process.