Synthesis, characterization, and metabolism studies of fluspidine enantiomers

The enantiomers of the potent σ1 ligand fluspidine (1) were prepared by using chiral preparative HPLC. Synthesis of racemic tosylate 2 and subsequent separation of enantiomers yielded (R)-2 and (S)-2 in excellent enantiomeric purities. The fluspidine enantiomers (R)-1 and (S)-1 were synthesized from (R)-2 and (S)-2 by nucleophilic substitution with tetra-n-butylammonium fluoride, affording (R)-1 with 99.6 % ee and (S)-1 with 96.4 % ee. Tosylates (R)-2 and (S)-2 can also serve as precursors for the radiosynthesis of enantiomerically pure radiotracers [(18) F](R)-1 and [(18) F](S)-1. The absolute configuration of the pure enantiomers was elucidated by comparison of their CD spectra with a calculated CD spectrum of a simplified model compound. In receptor binding studies, both enantiomers displayed very high σ1 receptor affinity and selectivity against the σ2 receptor. (R)-Fluspidine ((R)-1) is the eutomer, with a Ki value of 0.57 nM and a eudysmic ratio of 4. Incubation of (R)-1 and (S)-1 with rat liver microsomes led to the identification of seven and eight metabolites, respectively. Although the S-configured enantiomer formed additional metabolite (S)-1-3, it is metabolically more stable than (R)-1.