Pyridine analogues of spirocyclic σ₁ receptor ligands

Spirocyclic benzopyrans 2 interact with high affinity and selectivity with σ₁ receptors. Bioisosteric replacement of the benzene ring of the benzopyran substructure with the electron rich thiophene ring (3) led to increased σ₁ affinity. Herein the synthesis and pharmacological evaluation of electron deficient pyridine bioisosteres 4 are reported. Homologation of the aldehyde 6 to afford the pyridylacetaldehyde derivative 8 was performed by a Wittig reaction. Bromine lithium exchange of the bromopyridine 8, addition to 1-benzylpiperidin-4-one and cyclization led to the spirocyclic pyrranopyridine 10. Hydrogenolytic removal of the N-benzyl moiety of 10 provided the secondary amine 11, which allowed the introduction of various N-substituents (12a-d). Cyclization of the hydroxy acetal 9 with HCl led to various modifications of the substituent in 3'-position. Generally the σ₁ affinity of the pyridine derivatives is reduced compared with those of the benzene and thiophene derivatives 2 and 3. However, the relationships between the structure and the σ₁ affinity follow the same rules as for the benzene and thiophene derivatives. The most promising σ₁ ligand within this class of compounds is the pyranopyridine 15 with a double bond in the pyran ring revealing a Ki-value of 4.6 nM and a very high selectivity (>217-fold) over the σ₂ subtype.