Azabicyclo[3.1.0]hexane

3-Azabicyclo[3.1.0]hexane has attracted sustained interest in drug discovery over several decades. The bicyclic architecture imposes a well-defined envelope conformation on the five-membered ring, restricting the conformational freedom that limits monocyclic pyrrolidine analogues, and the fused cyclopropane introduces a distinct three-dimensional geometry that projects substituents at the 1- and 5-positions in a fixed spatial relationship that can be exploited to preorganise pharmacophoric groups for target engagement.  Azabicyclo[3.1.0]hexane derivatives have been extensively investigated for the treatment of central nervous system disorders, as antimicrobial agents, as vanilloid receptor ligands, and for the treatment of Hepatitis C virus infection, with the specificity or affinity for different targets varying substantially depending on the selection of substituents on the core scaffold. The scaffold has been studied as a conformationally constrained bioisostere for piperidine and pyrrolidine, and the cyclopropane ring provides an additional means of tuning electronic properties through its anisotropic, pseudo-aromatic character, which can influence pKa values and membrane permeability relative to purely saturated analogues. The conformationally constrained bicyclo[3.1.0]hexane scaffold offers advantages in terms of improving metabolic stability and limiting off-target effects when used as a cyclohexane isostere. Dopamine D3 receptor modulation, monoamine reuptake inhibition, glycine transporter 1 (GlyT1) inhibition, muscarinic receptor antagonism, and DPP-4 inhibition are among the target classes against which 3-azabicyclo[3.1.0]hexane derivatives have demonstrated activity in disclosed patent literature and published research, with major pharmaceutical organisations including GlaxoSmithKline, Boehringer Ingelheim, Johnson & Johnson, and DOV Pharmaceutical having developed compound series around the scaffold. A particular strength of the scaffold in CNS drug discovery is that the fixed geometry of the bicycle, combined with the nitrogen's lone pair orientation, can confer selectivity between closely related monoamine transporter subtypes, as demonstrated by the enantiomeric selectivity observed in dichlorophenyl-substituted series where each enantiomer shows a distinct SERT, NET, and DAT inhibition profile. Synthetic access has been achieved through cyclopropanation of pyrrolines, intramolecular alkylation, dearomative cyclisation, and more recently through photoredox-mediated and flow-chemistry approaches that have improved scalability.  

The 3-azabicyclo[3.1.0]hexane scaffold has achieved FDA approval as a pharmacophoric element in two clinically important antiviral peptidomimetics. In both cases, a proline residue at the P2 position of a protease-targeting peptide mimic is replaced with a conformationally locked bicyclic analogue to gain a decisive improvement in potency and target engagement. Boceprevir (Victrelis, Merck), FDA-approved in 2011 for hepatitis C genotype 1, incorporates a (1R,2S,5S)-6,6-dimethyl-3-azabicyclo[3.1.0]hexane-2-carboxylic acid unit at P2, where the fused cyclopropane locks the gem-dimethyl group at a fixed angle relative to the bicyclic ring, producing a 1000-fold increase in NS3 protease binding affinity over proline in an equivalent pentapeptide scaffold. Nirmatrelvir (Paxlovid, Pfizer), the oral SARS-CoV-2 3CL protease inhibitor, incorporates the same bicyclic unit at P2, where it replaced a leucine residue to reduce hydrogen bond donors, improve oral absorption, and maintain potent occupancy of the S2 subsite. Together, these approvals provide compelling clinical validation of the scaffold in direct-acting antiviral design.  

Our range of Azabicyclo[3.1.0]hexanes features novel substitutions and the incorporation of synthetically tractable functional groups such as boronic acids and esters, amines, carboxylic acids, ketones, hydroxyls and halogens to enable expedient synthetic strategies. The full range can be found using the substructure feature of our Search tools.