Synthesis of Tetrahydro‐β‐Carboline‐Based Spirocyclic Peptides via Cascade N ‐Boc Deprotection and Pictet‐Spengler Reaction of Tryptophan: Potent Anti‐Inflammatory and Anti‐Oxidant Activities

ABSTRACT Inflammation is a tightly regulated response that maintains tissue homeostasis; however, its dysregulation in the lung contributes to acute lung injury (ALI), characterised by excessive cytokine production, oxidative stress, epithelial‐endothelial barrier disruption, and impaired gas exchange. In the current study, we report the synthesis of 1,2,3,4‐tetrahydro‐β‐carboline (THβC)‐based spirocyclic peptides by an efficient cascade strategy involving N ‐Boc deprotection and Pictet‐Spengler cyclization, which was developed for tryptophan‐containing peptides using 9‐fluorenone, enabling the synthesis of THβC peptides. The reaction proceeds under mild conditions [C 6 F 5 OH (pentafluorophenol) catalyst, HFIP (1,1,1,3,3,3‐Hexafluoro‐2‐propanol) solvent] to generate a novel spiro‐β‐carboline scaffold, with broad substrate scope across aromatic, aliphatic, and alicyclic ketones. Further, the anti‐inflammatory potential of THβC derivatives was evaluated in LPS‐stimulated macrophages and lung epithelial cells. Initial screening demonstrated that compounds 3a, 3aa, 3c , 3e , and 3 g (20 μM) significantly reduced Il‐1β levels compared with the LPS control. Subsequent structure–activity relationship studies identified compounds 3aa and 3ag as potent inhibitors of Il‐1β expression, while compounds 3aa, 3ad, 3af and 3ag effectively suppressed Ccl‐2 and Cox‐2 expression. Based on their ability to attenuate intracellular ROS levels, compounds 3aa and 3ag were selected for further dose‐response evaluation and assessment in lung epithelial cells. These compounds significantly downregulated key inflammatory mediators in both macrophages and epithelial cells. Immunocytochemical analyses further revealed that compounds 3aa and 3ag reduced LPS‐induced VCAM expression and restored caveolin levels, indicating improved barrier integrity. Collectively, compounds 3aa and 3ag attenuate LPS‐induced inflammation and oxidative stress while preserving epithelial barrier integrity, supporting their potential as leads for pulmonary inflammatory disorders.