HFIP-assisted Brønsted acid-catalyzed ring opening of 1-azabicyclo[1.1.0]butane to access diverse C3-quaternary aza-azetidines and indole-azetidines

Nitrogen-based heterocycles represent 60% of small-molecule-approved drugs. Increasing demand for sp3-rich N-heterocyclic scaffolds as a bioisosteric replacement in drug discovery platforms has continued to drive the development of elegant methods for the synthesis of these important molecules. Among various N-heterocycles, azetidine has emerged as a valuable scaffold. Aza-azetidines, indole-azetidines, and spirocyclic azetidines with tertiary or quaternary C3-carbon make structural and/or functional parts of various important drug molecules. However, an efficient and catalytic synthetic strategy to access those important scaffolds is still missing in the literature. Herein, we report HFIP-assisted Brønsted acid-catalyzed strain-release driven ring opening of 1-azabicyclo[1.1.0]butane (ABB) to access aza-azetidines, indole-azetidines, and spirocyclic azetidines with quaternary C3-carbon in good yield. Detailed experimental and theoretical studies using DFT shed light on the reaction mechanism. We also find promising antibacterial activity in one of these indole-azetidine compounds against Staphylococcus aureus MTCC 1430.