Thiophene bioisosteres of spirocyclic σ receptor ligands: relationships between substitution pattern and σ receptor...

On the basis of the 6',7'-dihydrospiro[piperidine-4,4'-thieno[3,2-c]pyran] framework, a series of more than 30 σ ligands with versatile substituents in 1-, 2'-, and 6'-position has been synthesized and pharmacologically evaluated in order to find novel structure-affinity relationships. It was found that a cyclohexylmethyl residue at the piperidine N-atom instead of a benzyl moiety led to increased σ(2) affinity and therefore to decreased σ(1)/σ(2) selectivity. Small substituents (e.g., OH, OCH(3), CN, CH(2)OH) in 6'-position adjacent to the O-atom were well tolerated by the σ(1) receptor. Removal of the substituent in 6'-position resulted in very potent but unselective σ ligands (13). A broad range of substituents with various lipophilic and H-bond forming properties was introduced in 2'-position adjacent to the S-atom without loss of σ(1) affinity. However, very polar and basic substituents in both 2'- and 6'-position decreased the σ(1) affinity considerably. It is postulated that the electron density of the thiophene moiety has a big impact on the σ(1) affinity.