Amino-oxetanes: a small ring earning its place in the clinic
Amides are among the most common functional groups in drug molecules, but medicinal chemists continually look for alternatives that can alter conformation, polarity and metabolic behaviour without sacrificing key interactions. Amino-oxetanes are an intriguing option. The oxetane ring combines low molecular weight, polarity and three-dimensionality, while a 3-amino-oxetane can retain both hydrogen-bond donor and acceptor capabilities found in an amide. But it is not simply an amide replacement in disguise: its preferred gauche conformation creates different torsion angles and exit vectors, potentially opening up chemical space that a planar amide cannot access. This chemistry has already reached the clinic. Rilzabrutinib, a reversible covalent BTK inhibitor approved by the FDA in August 2025, became the first fully synthetic oxetane-containing drug to receive FDA approval. The role of the oxetane emerged during lead optimisation. An earlier compound in the series incorporated an amino-oxetane as a capping group and showed a BTK IC₅₀ of 1.9 nM with high selectivity across a 254-kinase panel. The final rilzabrutinib molecule contains an oxetanylpiperazine motif, where the oxetane helped modulate the basicity of the adjacent amine while maintaining favourable solubility and permeability. Structural studies also showed that the oxetane contributes to the binding conformation of the inhibitor. More recently, a 2026 Journal of Medicinal Chemistry study put the amino-oxetane–amide comparison to a direct test. Across 12 matched molecular pairs, amino-oxetanes showed broadly comparable profiles to benzamides for properties including solubility, lipophilicity, clearance and permeability, while retaining both H-bond donor and acceptor capabilities. However, structural analysis revealed a significant difference: amino-oxetanes adopt a gauche conformation with exit vectors more closely resembling sulfonamides than benzamides. That distinction is important. Amino-oxetanes may not be perfect amide bioisosteres, but they offer something arguably more useful: a way to preserve selected properties of an amide while changing molecular topology and three-dimensional shape. As oxetanes move from an underused motif into approved medicines and increasingly systematic matched-pair studies, amino-oxetanes look less like a niche curiosity and more like a useful addition to the medicinal chemist's design toolkit. Explore our range of building blocks using the advanced search tools at https://lnkd.in/eVbfw6Ua To read more, take a look at the following interesting papers: 1. Do Amino-Oxetanes Resemble Amides? A Matched Molecular Pairs Property and Structural Comparison https://lnkd.in/dzuVasrr 2. Oxetanes in Drug Discovery Campaigns https://lnkd.in/gwAtieAi
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