The β-carboline moiety exists in a wide range of drugs and natural products, but synthesis often relies on noble metals and stochiometric oxidants. Here, we develop a metal-free strategy using a triaryl carbenium ion pair as a catalyst to access β-carbolines from tryptamines and aromatic 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 process to afford the desired products. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University Published version We are thankful for the financial support from the Start-up Grant of Nanjing Tech University (nos. 38274017103 and 38037037). T.-P.L. is thankful for the financial support from the Distinguished University Professor grant (Nanyang Technological University), AcRF Tier 1 grants from the Ministry of Education of Singapore (RG107/19, RG11/20, and RT14/20), and the Agency for Science, Technology and Research (A*STAR) under its MTC Individual Research Grants (M21K2c0114). Acknowledgment is also made to the Donors of the Science and Technology Innovation Research Fellowship in Nanjing for partial support of this research (no. 51237024) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (nos. KYCX20_1031 and KYCX21_1081).
