Synthesis and evaluation of tetrahydroindazole derivatives as sigma-2 receptor ligands

A series of tetrahydroindazole derivatives were synthesized and evaluated for their affinities for both sigma-1 and sigma-2 receptors. These compounds are hybrid structures of a tetrahydroindazole substituted benzamide and a 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety or a 9-azabicyclo[3.3.1]nonan-3-yl-amine moiety. These newly synthesized hybrid analogs showed various affinities for sigma-2 receptor without any significant affinities for sigma-1 receptor. In particular, compounds 12, 15b, 15c, and 15d, demonstrated moderate affinity and excellent selectivity for sigma-2 receptor. It is interesting to note that 3-5 carbon units between the tetrahydroindazole substituted benzamide and the 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety are appropriate for sigma-2 receptor binding. No substitution on the 9-aza nitrogen is proper for sigma-2 affinity in the 2-(9-azabicyclo[3.3.1]nonan-3-yl-amino)-4-(3,6,6-trimethyl-4-oxo-4,5,6,7-tetrahydro-1H-indazol-1-yl)benzamide analogs.