The present invention relates to substantially pure piperidine derivative compounds of the formulae: ##STR00001## wherein R.sub.1 is hydrogen or hydroxy; R.sub.2 is hydrogen; or R.sub.1 and R.sub.2 taken together form a second bond between the carbon atoms bearing R.sub.1 and R.sub.2; R.sub.3 is --COOH or --COOR.sub.4; R.sub.4 has 1 to 6 carbon atoms; A, B, and D are the substituents of their respective rings each of which may be different or the same and are hydrogen, halogens, alkyl, hydroxy, alkoxy, or other substituents. A process of preparing such piperidine derivative compounds in substantially pure form is also disclosed.