Efficient Preparation of (3S)-3-(4-Fluorobenzyl)piperidinium Mandelate

Methods for the preparation of (3S)-3-(4-fluorobenzyl)piperidine (2) and its mandelate salt (9) are described. The first generation synthesis started from 3-benzylpiperidone, and required Boc protection of the nitrogen for efficient separation of the enantiomers using chromatography on a chiral stationary phase. Subsequently, a resolution method using (R)-mandelic acid, produced high %ee salt 9 after recrystallization and eliminated the need for Boc protection. The third generation route, starting from pyridine-3-carboxaldehyde, led to a streamlined synthesis of racemate 2 and was optimized for producing multi-hundred gram quantities of the chiral salt.