Preparation of enantiomeric pure (−)-(3R,4S)-1-benzyl-3,4-epoxypiperidine and enriched (−)-(R)-1-Benzyl-3-hydroxy-1,2,3,6-tetrahydropyridine by kinetic separation of (±)-1-benzyl-3,4-epoxypiperidine under the action of chiral lithium amides