Synthesis of PD 158771 (Figure 1) involved 4-amino-cyclohexylacetic acid and its ethyl ester intermediates which required purification to obtain the desired correspondingtrans-isomer. HPLC methods were developed for the separation ofcis- andtrans-4-aminocyclohexylacetic acid and the corresponding ethyl esters following pre-column derivatization withNα-(2,4-dinitro-5-fluorophenyl)-L-alaninamide (Marfey's reagent). The chromatographic conditions were varied to achieve optimal separation.