(3R,4S)-6-ethoxy-3-ethyl-2,3,4,5-tetrahydro-4-pyridineacetic acid ethyl ester

(4R,5R)-1-(3-benzyloxy-4-methoxyphenacyl)-5-ethyl-2-oxo-4-piperidineacetic acid ethyl ester

(4S,5R)-5-ethyl-2-oxo-4-piperidineacetic acid ethyl ester

(4S,5R)-(-)-5-ethyl-1-(3-hydroxy-4-methoxyphenethyl)-2-oxo-4-piperidineacetic acid ethyl ester